研究者詳細 - 髙田　尚記

Image-based finite element analysis of anomalous thermal conductivity in Al-10%Si-0.4%Mg alloy additive-manufactured by laser powder bed fusion 査読有り Open Access

Suzuki A., Miyasaka T., Takata N., Kobashi M.

Keikinzoku/Journal of Japan Institute of Light Metals 72 巻 ( 5 ) 頁： 164 - 171 2022年

記述言語：日本語掲載種別：研究論文（学術雑誌）出版者・発行元：Keikinzoku/Journal of Japan Institute of Light Metals

Laser powder bed fused Al-10Si-0.4Mg (mass%) alloy exhibits low and anisotropic thermal conductivity. To elucidate dominant microstructural features generating the characteristic thermal conductivity, thermal conduction analysis was performed by an image-based finite element method (FEM) using scanning electron microscope (SEM) images. The serial sectioning images of as-fabricated and 300ºC/2 h heat treated Al-10Si-0.4Mg alloys were obtained by focused ion beam SEM (FIB-SEM) and were used for constructing three-dimensional models. The homogenization method was applied to calculate average thermal conductivity of these models. When the thermal conductivity of α-Al phase was set at the thermal conductivity of pure aluminum, calculated thermal conductivity of as-fabricated model was much higher than experimentally measured thermal conductivity of as-fabricated Al-10Si-0.4Mg alloy, indicating that α-Al phase in the as-fabricated Al-10Si-0.4Mg alloy had much lower thermal conductivity than pure aluminum. In addition, the anisotropy of thermal conductivity appeared only in the as-fabricated model in which the α-Al/Si interfacial thermal resistance was taken into account. These results were used for discussing dominant contributors of low and anisotropic thermal conductivity of laser additive manufactured Al-10Si-0.4Mg alloy.

DOI： 10.2464/jilm.72.164

その他リンク： https://www.jstage.jst.go.jp/article/jilm/72/5/72_7205-T06/_article/-char/ja/

Metal-like ductility and high hardness in nitrogen-rich HfN thin films by point defect superstructuring 査読有り国際共著国際誌 Open Access

Lorentzon M., Sangiovanni D.G., Takata N., Zhu T., Hahn R., Palisaitis J., Hultman L., Birch J., Ghafoor N.

Communications Materials 6 巻 ( 1 ) 2025年12月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Communications Materials

The strength of refractory ceramics is much limited by their brittleness. Counterintuitively, we observe simultaneous high hardness and metallic-like ductility in highly overstoichiometric single-crystal HfN1.22 and HfN1.33 films grown by ion-assisted reactive magnetron sputtering. Here, we show by electron microscopy, diffraction methods, and ab-initio calculations the existence of a superstructure ordering of metal vacancies and nitrogen interstitials into sub-nanometer hyper-overstoichiometric and quasi-stoichiometric domains that self-organize into a checkerboard pattern superimposed onto the NaCl-structured lattice. A high intrinsic dislocation density and low dislocation nucleation barrier on {111} < 011> slip systems enable extraordinary ductility and strain hardening at >50% strain, in room temperature uniaxial compression of HfN1.22 micropillars. The films simultaneously exhibit a high hardness – tunable up to 28 GPa via control of stoichiometry – and a remarkable fracture resistance due to dislocation-mediated stress dissipation. The presented findings provide a route for synthesizing nitride superstructured films with unique hardness/toughness combinations.

DOI： 10.1038/s43246-025-00768-5

Inhomogeneous α-Al/Mg<inf>2</inf>Si two-phase microstructures with chemical fluctuation produced by laser-beam powder bed fusion 査読有り国際誌 Open Access

Otani Y., Takata N., Suzuki A., Kobashi M., Umeda J.

Materials and Design 252 巻 2025年4月

担当区分：責任著者記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Materials and Design

This study investigated the additive manufacturing of high-strength Al–Mg–Si ternary alloy with refined α-Al/Mg2Si two-phase microstructures by laser-beam powder bed fusion (PBF-LB). Although PBF-LB processing produced crack-free samples with high relative densities (>99.5 %), the scanning laser irradiation caused significant Mg vaporization, reducing the Mg content of the sample from 11.3 to 7.6 %. Laser-induced vaporization caused micron-scale chemical fluctuations in the melt-pool structure, resulting in the development of inhomogeneous microstructures. Refined cellular microstructures with many columnar α-Al phases surrounded by numerous Mg2Si nano-particles were observed in most parts of the melt-pool structure, corresponding to hypo-eutectic compositions in the Al–Mg–Si ternary system. However, primary solidified Mg2Si phases were observed in some parts of the melt-pool boundaries with local Mg-rich compositions (hyper-eutectic compositions). The PBF-LB manufactured alloy specimens with a total composition of Al–7.6Mg–5.4Si (wt%) exhibited a high tensile strength, which reduced significantly with increasing testing temperature, and low ductility (529 MPa and < 2 %, respectively) at room temperature. Moreover, the specimens underwent mechanical deterioration at elevated temperatures, owing to a significant coarsening of metastable microstructural factors (nanoscale Mg2Si precipitates or their related metastable phases and atomistic clusters) that contribute towards the high room-temperature strength.

DOI： 10.1016/j.matdes.2025.113787

Fe/polymer joining via Fe/TiB<inf>2</inf> composite structures via in-situ laser-induced reaction of Fe-Ti-B system: Effect of powder composition 査読有り国際誌 Open Access

Zhou S., Omiya K., Ueda Y., Suzuki A., Takata N., Kobashi M.

Journal of Advanced Joining Processes 10 巻 2024年11月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of Advanced Joining Processes

Achieving strong direct joining between steel and polymers through mechanical interlocking is crucial for developing multi-material structures, particularly in the automotive and aerospace industries. This study synthesized micro-scale structures on a pure Fe substrate (simulating interstitial-free (IF) steel) for mechanical interlocking with thermoplastic parts. Numerous submillimeter-scale Fe/TiB2 composite particles were in-situ synthesized by laser scanning on the Fe-Ti-B powder mixture and well-bonded with the Fe substrate. The effects of powder composition (TiB2 volume fraction) on the morphology, microstructure, and joint strength with PA6 were investigated. A TiB2 volume fraction over 60 % was essential for the formation of the composite particles promoted by a TiB2 skeletal structure. Higher TiB2 volume fractions increased the area fraction of the composite particles and decreased the bonding ratio (adhesive) of the particles with the substrate due to poor adhesiveness at the edge of the laser-scanning line. A wider high-temperature region was generated at a higher TiB2 volume fraction, suggesting that the reaction heat to form TiB2 assisted the bonding of the particles with the substrate at the edge of the laser scanning line. The Fe/PA6 joint strength increased to approximately 30 MPa with increasing the TiB2 volume fraction to 100 % and showed a linear correlation with the product of particle area fraction and bonding ratio. A higher TiB2 volume fraction was preferable for enhancing the joint strength via the micro-scale structures synthesized by laser scanning on the Fe-Ti-B powder mixture. A combination of the micro-structuring process using a high fraction of TiB2 with advanced joining technologies will contribute to manufacturing high-strength Fe/polymer hybrid parts.

DOI： 10.1016/j.jajp.2024.100249

Pressure-less melt infiltration characteristics of Mg melt into Ti powder preforms with various pore structures: Toward the application to the binder jetting additive manufacturing 査読有り国際誌 Open Access

Li Z., Suzuki A., Takata N., Kobashi M.

Journal of Materials Processing Technology 331 巻 2024年10月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of Materials Processing Technology

Binder jetting (BJT) additive manufacturing enables the fabrication of complex-shaped metallic parts but requires sintering as post-processing to obtain dense parts. The sintering inevitably causes the shrinkage from green body to dense products, resulting in low dimensional accuracy. In contrast, pressure-less melt infiltration (PMI) fills pores in the green body with a liquid metal driven by capillary pressure and fabricates a dense metal matrix composite without severe shrinkage. To consider the applicability of pressure-less melt infiltration to the post-processing of binder jetting additive manufacturing, it is required to clarify the relationship between pressure-less melt infiltration process conditions and the relative density of the composites. In this study, the liquid Mg/solid Ti system was selected because of good wettability and absence of Mg-Ti intermetallic, and the conditions of Ti powder preforms (corresponding to the green body) were focused on. Ti powder preforms with various powder sizes (2300, 410, 140, and 20 μm) and different shapes (irregular and spherical) were cold-pressed at 76, 102, 127, 153, and 178 MPa to fabricate preforms with various relative densities, pore sizes, and pore morphologies to response the similar characteristics in BJT preforms. Mg melt infiltrated into the preforms under constant conditions to fabricate Mg/Ti composites. The structures of the preforms and composites were characterized by sample size measurement, optical and scanning electron microscopy, and X-ray computed tomography. It was revealed that the relative density of the composite increased with decreasing Ti powder size and using spherical Ti powder, whereas the compact pressure exhibited a slight effect. Furthermore, defect volume increased with increasing infiltration height but decreased around the top surface of the sample. Minute observations around defects in the composites revealed that defects were formed at larger pores, and Mg melt was infiltrated into pores above the defects. These results indicated that infiltration velocity varied depending on the flow channel structure, and gas was trapped at larger pores (far from the outside of the sample) where infiltration velocity was low. Using smaller Ti powder decreased the variation in pore size in the preform, resulting in improving the relative density of the composites by suppressing the gas trapping. This study provides a new insight into the pressure-less melt infiltration process as a post-processing of binder jetting additive manufacturing.

DOI： 10.1016/j.jmatprotec.2024.118509

Achieving exceptional strain-hardening ability in nanocrystalline Ni–W coatings through compositionally modulated multilayer approach 査読有り国際共著国際誌

Padmaganesan H.T., Lavakumar B., Choshi A., Takata N., Wasekar N.P., Prasad M.J.N.V.

Materials Science and Engineering: A 912 巻 2024年10月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Materials Science and Engineering: A

Nanocrystalline Ni–W coatings show exceptional strength; however, they exhibit poor toughness due to limited ductility and high residual stresses which impede their use in practical applications as a potential hard chrome replacement. In the present study, compositionally modulated multilayer (CMM) coatings with alternate layers of Ni-1 at% W (soft) and Ni-15 at% W (hard) of three different layer thicknesses of 1, 0.5, and 0.1 μm are developed by pulse reverse electrodeposition to achieve the strength-ductility synergy, thereby enhanced wear resistance. Microcompression test results reveal that all the CMM coatings exhibit remarkably higher strain-hardening ability than the homogenous coatings. Significant fluctuations in the strain-hardening rate and a positive shift in the onset stress for the work-hardening stages were observed with the layer thickness. This suggests a strong interplay between the hard and soft layers on the deformation characteristics of multilayer coatings. Detailed electron microscopy of the cross-section of the deformed micropillars shows substantial evidence of co-deformation of both layers. A mechanism is proposed to explain the enhanced strain-hardening in multilayers involving the strength incompatibility arising in the layers leading to lateral constraints, resulting in strain gradient plasticity, stabilizing of local shear by the soft layers, and the barrier effect of the interfaces.

DOI： 10.1016/j.msea.2024.146971

Enhancing Capillary Pressure of Porous Aluminum Wicks by Controlling Bi-Porous Structure Using Different-Sized NaCl Space Holders 査読有り国際誌

Shen H., Suzuki A., Takata N., Kobashi M.

Materials 17 巻 ( 19 ) 2024年10月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Materials

Capillary pressure and permeability of porous media are important for heat transfer devices, including loop heat pipes. In general, smaller pore sizes enhance capillary pressure but decrease permeability. Introducing a bi-porous structure is promising for solving this trade-off relation. In this study, the bi-porous aluminum was fabricated by the space holder method using two different-sized NaCl particles (approximately 400 and 40 μm). The capillary pressure and permeability of the bi-porous Al were evaluated and compared with those of mono-porous Al fabricated by the space holder method. Increasing the porosity of the mono-porous Al improved the permeability but reduced the capillary pressure because of better-connected pores and increased effective pore size. The fraction of large and small pores in the bi-porous Al was successfully controlled under a constant porosity of 70%. The capillary pressure of the bi-porous Al with 40% large and 30% small pores was higher than the mono-porous Al with 70% porosity without sacrificing the permeability. However, the bi-porous Al with other fractions of large and small pores did not exhibit properties superior to the mono-porous Al. Thus, accurately controlling the fractions of large and small pores is required to enhance the capillary performance by introducing the bi-porous structure.

DOI： 10.3390/ma17194729

Mechanical inhomogeneity of melt-pool structure in additive manufactured Al alloy: micropillar compression test approach 査読有り

Design of Al-Fe based alloys strengthened by Al6Fe metastable phase formed in laser powder bed fusion 査読有り

N. Takata, W. Wang, Y. Cheng, T. Miyawaki, A. Suzuki, M. Kobashi, M. Kato

Proceedings of the 44th Risø International Symposium on Materials Science 頁： 369 - 374 2024年9月

D. Kim, A Choshi, N Takata, A. Suzuki, M. Kobashi

Proceedings of the 44th Risø International Symposium on Materials Science 頁： 293 - 296 2024年9月

Introducing Hatch Spacing into Deposited Energy Density toward Efficient Optimization of Laser Powder Bed Fusion Process Parameters 査読有り国際誌

Kunieda M., Suzuki A., Takata N., Kato M., Kobashi M.

Nippon Kinzoku Gakkaishi/Journal of the Japan Institute of Metals 88 巻 ( 9 ) 頁： 145 - 152 2024年9月

記述言語：日本語掲載種別：研究論文（学術雑誌）出版者・発行元：Nippon Kinzoku Gakkaishi/Journal of the Japan Institute of Metals

The optimization of processing parameters is indispensable for the laser powder bed fusion (L–PBF) process. The deposited energy density (DED) is one of the process indexes for the L–PBF process and has a simplified formula of P·v−0.5, where P is the laser power, and v is the scan speed. This parameter describes the change in the relative density and the melt pool morphology with laser power and scan speed well, whereas it does not include the effect of other processing parameters, e.g., hatch spacing (S). In the present study, an attempt was made to incorporate the effect of S into DED. Al–12Si (mass%) alloy cube samples were fabricated by L–PBF under various P, v, and S, for evaluating the relative density and the melt pool morphology. The melt pool depth and width of L–PBF–manufactured Al–12Si alloy increased linearly with P·v−0.5 and did not exhibit a clear correlation with S. Based on the experimental observation, the effect of hatch spacing on DED was estimated to be S−0.5, and a new index of P·v−0.5·S−0.5 was proposed. This index described the change in the relative density of the L–PBF–manufactured Al–12Si alloy with laser conditions (P, v, and S) well when the thermal conduction mode melting was dominant. This study also indicated the limitation of the applicability of P·v−0.5·S−0.5 under the keyhole or transition mode melting.

DOI： 10.2320/jinstmet.JA202401

Variation in microstructural features of melt-pool structure in laser powder bed fused Al–Fe–Cu alloy at elevated temperatures 査読有り国際誌 Open Access

Cheng Y., Miyawaki T., Wang W., Takata N., Suzuki A., Kobashi M., Kato M.

Journal of Materials Research and Technology 32 巻頁： 4048 - 4058 2024年9月

担当区分：責任著者記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of Materials Research and Technology

The present study was undertaken to understand the effect of annealing on the microstructural features of the melt-pool structure and the associated multiscale mechanical properties of the Al–2.5%Fe–2%Cu alloy manufactured by laser powder bed fusion (LPBF). Microstructural characterizations and tensile tests were conducted for the LPBF-built specimen and those subsequently annealed at various temperatures ranging from 200 to 500 °C. Nanoindentation hardness mapping was used to evaluate the mechanical inhomogeneity of the melt-pool structure and its changes by annealing at different temperatures. The LPBF-manufactured sample exhibited a melt-pool structure containing numerous particles of the Al23CuFe4 phase (oC28, orthorhombic structure) formed because of local melting and rapid solidification during the LPBF process. The relatively coarsened cellular structure localized along the melt-pool boundary resulted in local soft regions. The local vulnerability contributed to the direction dependence of the tensile ductility. A slight variation was observed in the inhomogeneous microstructure of the samples annealed at 200 or 300 °C. The formation of numerous Al23CuFe4 nanoprecipitates in the α-Al supersaturated solid solution prevented strength loss after post-heat treatments. In addition, considerable coarsening of the intermetallic phase after annealing at 500 °C eliminated the melt-pool structure. The tensile performance of the specimens demonstrated a ductile fracture mode, wherein ductile fracture occurred in the α-Al matrix with low hardness while the harder θ-Al13Fe4 stable phase was embedded within it. The anisotropy in the mechanical properties was less pronounced owing to the significant microstructural changes.

DOI： 10.1016/j.jmrt.2024.09.013

Effect of heat treatments on inhomogeneous deformation of the melt-pool structure of Al–Si alloy manufactured via laser powder bed fusion 査読有り国際誌 Open Access

Otani Y., Takata N., Suzuki A., Kobashi M., Kato M.

Materials Science and Engineering: A 909 巻 2024年9月

担当区分：責任著者記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Materials Science and Engineering: A

Laser powder bed fusion (L-PBF) is an additive manufacturing technology widely applied to the manufacture of metallic parts. The L-PBF processed aluminum-silicon (Al–Si) cast-type alloy components exhibit an anisotropic tensile ductility. In this study, we investigated the influence of heat treatments on the microstructural features of an Al–12%Si binary alloy fabricated via L-PBF and the associated inhomogeneous deformation of its melt-pool structure, which contributes to the resulting anisotropic tensile ductility. The mechanical inhomogeneity of the melt-pool structure and the change induced by annealing at various temperatures (300 and 530 °C) were characterized using microscale digital-image correlation analyses of scanning electron microscopy images obtained in situ during tensile deformation and nanoindentation hardness mapping. In the specimen fabricated via L-PBF, the high strain was concentrated at the locally coarsened microstructure (the soft region) along the boundaries of the melt pools rather than at the refined solidification microstructure (the hard region) within the melt pools. The localized strain varied depending on the geometrical relation between the orientation of the melt-pool boundary and the tensile direction, thus contributing to the anisotropic tensile ductility. This tendency appeared less pronounced in the specimen annealed at 300 °C, which exhibited a slightly homogenized melt-pool structure. The reduced strain localization is associated with a reduced difference in local strength between the melt-pool boundary (soft region) and the melt-pool interior (hard region). The slight homogenization of the melt-pool structure emphasized the effect of grain morphologies in the α-Al matrix on the inhomogeneous deformation within melt pools. In the specimen annealed at 530 °C, which exhibited a homogenized α-Al/Si two-phase microstructure, the uniformly distributed Si phase would be responsible for the homogenous deformation, resulting in an isotropic tensile ductility. This study advances our understanding of the correlation between the melt-pool structure and the deformation behavior of Al–Si alloys processed using L-PBF, which provides new insights for controlling the ductility by post-processing.

DOI： 10.1016/j.msea.2024.146808

Thermal stability of refined Al<inf>6</inf>(Fe, Mn) phase formed in laser powder bed fusion process 査読有り国際誌

Wang W., Xu Y., Deguchi M., Takata N., Suzuki A., Kobashi M., Kato M., Mitsuhara M., Nakashima H.

Journal of Alloys and Compounds 992 巻 2024年7月

担当区分：責任著者記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of Alloys and Compounds

The addition of Mn in Al–Fe alloys stabilizes the Al6Fe metastable phase (orthorhombic structure, oC28) owing to the partial occupation of Fe sublattice sites by Mn atoms (formation of Al6(Fe, Mn) phase), thereby achieving superior high-temperature strength of Al–Fe–Mn ternary alloys that are manufactured via laser powder bed fusion (L-PBF) process. This study aimed to fundamentally understand the thermal stability of the refined Al6(Fe, Mn) phase formed in the L-PBF fabricated Al–2.5%Fe–2%Mn alloy in terms of a thermodynamic approach and the kinetics of morphological changes in the Al6(Fe, Mn) phase at elevated temperatures ranging from 200 to 500 ℃. The L-PBF processed samples showed a high hardness of approximately 125 HV due to the formation of numerous dispersoids of the Al6(Fe, Mn) phase with sizes of several tens of nanometers in the α-Al matrix containing concentrated solute elements of Fe and Mn. The hardness and microstructure were almost unchanged even after exposure to a high temperature of 200 ℃ for an extended period of 1000 h. Upon exposure to 300 ℃, nucleation and growth of the Al6(Fe, Mn) phase occurred locally, particularly at grain boundaries in the α-Al matrix. Such a local growth contributed to a reduction in hardness upon thermal exposure. Simultaneously, numerous nanoscale precipitates were formed in the α-Al matrix, resulting in a suppressed reduction in hardness. Contrary to the result of the thermodynamic calculations, the θ-Al13Fe4 phase was scarcely found even after long-term exposure to 500 ℃. The θ-phase formation was accompanied by the dissolution of the refined Al6Fe metastable phase for strengthening, which significantly reduced the hardness during thermal exposure. The suppressed θ-phase formation could be associated with the thermodynamically stable Al6(Fe, Mn) phase developed during the L-PBF process, which predominantly contributed to the high microstructural stability of the L-PBF fabricated Al–Fe–Mn alloy at elevated temperatures.

DOI： 10.1016/j.jallcom.2024.174593

Elucidating dominant flow channel size for capillary performance of open-cell porous wicks 査読有り国際誌

Shen H., Suzuki A., Takata N., Kobashi M.

International Journal of Heat and Mass Transfer 223 巻 2024年5月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：International Journal of Heat and Mass Transfer

The capillary pressure (ΔPcap) and permeability (K) of porous wicks, both of which depend on the size of the flow channel, are crucial for the performance of heat transfer devices including loop heat pipes. However, the flow channel in actual porous wicks shrinks at the connective part between adjacent pores called the throat and does not have a constant size. The dominant flow channel size (average size or throat size) for ΔPcap and K remains unclear. To elucidate the dominant flow channel size, porous aluminum with constant pore size and various throat sizes was controllably fabricated by the space holder method using sodium chloride (NaCl) particles. Two types of pore size (simple pore size reflecting the space holder size and connectivity-reflected pore size) were characterized by X-ray computed tomography. The ΔPcap and K were evaluated by capillary rise tests using water. The average simple pore size was almost constant, but the average connectivity-reflected pore size changed depending on the porosity controlled by the amount of the NaCl space holder because the throat was enlarged, and the connectivity was improved by increasing porosity. It was clarified that ΔPcap changed depending on the average connectivity-reflected pore size regardless of the constant simple pore size. In contrast, K was determined by neither the simple nor connectivity-reflected pore sizes and could be dominated by local size around the throat. These results provide an insight into the design of optimal porous structure appropriate for porous wicks applied to heat transfer applications.

DOI： 10.1016/j.ijheatmasstransfer.2024.125217

Four-dimensional electrochemical impedance spectroscopy: Role of microstructure on corrosion behaviour of Al-Si alloys additive-manufactured by laser powder bed fusion 査読有り国際誌

Hoshi Y., Miyazawa R., Otani Y., Kim D., Takata N., Kobashi M.

Corrosion Science 232 巻 2024年5月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Corrosion Science

Four-dimensional (4D) electrochemical impedance spectroscopy was applied to clarify the role of microstructures on the corrosion behaviour of Al-Si alloys additively manufactured by laser powder bed fusion in the immersion of NaCl solution. 3D complex impedance plots, comprising real, imaginary, and time axes, indicate that the impedance increases gradually at initial stage and then does not show apparent time variation. These behaviours, following surface morphology analysis, are strongly related to the remaining fine silicon particles on the surface due to the preferential Al dissolution, which suppress the further dissolution from underlying along with the protective oxide film formation.

DOI： 10.1016/j.corsci.2024.112018

Production of sub-micron-sized high-entropy alloy particles and nanoparticles via pulsed laser ablation of CrMnFeCoNi targets in water 査読有り国際共著国際誌

Ninomiya R., Kim D., Takata N., Lyth S.M., Ishikawa K., Miyajima Y.

Applied Physics Letters 124 巻 ( 20 ) 2024年5月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Applied Physics Letters

High-entropy alloys (HEAs) are a class of materials known for their unique properties, including high strength, excellent wear resistance, and good corrosion resistance. Sub-micron- and nanosized HEA particles were fabricated via pulsed laser ablation in liquid using a Cantor alloy target. The Cr20Mn20Fe20Co20Ni20 target was immersed in pure water and ablated using a focused nanosecond-pulsed Nd: YAG laser. A dark solution containing HEA particles was obtained which was stable for about one week before agglomeration and precipitation was observed. The diameters of the obtained particles ranged from several tens of nanometers to several hundred nanometers. Increasing the laser power resulted in higher particle concentration and an increase in the intensity of UV-vis absorption spectra. Electron diffraction was used to confirm that the composition of the particles was close to that of the Cantor alloy, although the concentrations of Cr and Mn were slightly deficient. There was also a weak dependence of the composition on laser power, and all the particles also contained oxygen. Selected area electron diffraction revealed that the composition varied spatially within some particles and that they are mainly polycrystalline. This work shows that HEA particles can be quickly, safely, and effectively manufactured using liquid-based laser ablation, opening the pathway for mass manufacture and disruptive applications in, e.g., catalysis or tribology.

DOI： 10.1063/5.0200341

Effect of Cr content on thermally activated deformation in single-crystal micropillars of Fe–Cr binary alloys 査読有り国際共著国際誌

Zhu T., Takata N., Kim D., Kobashi M., Yoshino M., Tsukamura M.

Materials Science and Engineering: A 900 巻 2024年5月

担当区分：責任著者記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Materials Science and Engineering: A

This study identifies the role of solute Cr in the thermal activation process in the plastic deformation of α-(Fe, Cr) ferrite (BCC solid solutions) using micropillar compression tests for micron-sized single-crystals fabricated on various Fe–Cr binary alloys that were solution-treated at 1573 K. Fe–18%Cr single-crystal micropillars with a mean diameter d of ∼2 μm exhibited a relatively higher strain rate sensitivity (m) of 0.12 for the stress required for slip initiation. Larger Fe–18%Cr micropillars (d = 5 μm) exhibited a lower m value of 0.04. The corresponding activation volume of 50 b3 was equivalent to the millimeter-sized pure Fe and Fe–18%Cr alloy specimens (60–70 b3), indicating the prevalence of a general mechanism involving dislocation motion (kink-pair nucleation of screw dislocations). This mechanism controls the thermal activation process of plastic deformation in the Fe–18%Cr alloy, which has sufficient sources for dislocation multiplication. In the Fe–25%Cr and Fe–40%Cr single crystal micropillars, a less pronounced size-dependent m value (0.01–0.03) was determined, almost independent of specimen size. The corresponding activation volumes of large micropillars (74–77 b3) were similar to those of millimeter-sized specimens, which indicates a mild effect of solute Cr atoms on the thermal activation process of dislocation motion in Fe–Cr binary alloys at ambient temperature. Initial dislocations, rather than Cr content in Fe–Cr binary alloys, led to the low changes in size-dependent activation volume of Fe–Cr alloys with higher Cr content. Planar faults (Cr-enriched regions) were observed in Fe–40%Cr alloys, which may become sufficient sources for dislocation nucleation. However, the influence of Cr-enriched regions on dislocation motion remains unclear.

DOI： 10.1016/j.msea.2024.146499

Effect of low-temperature annealing on electric resistivity and mechanical anisotropy of Al-Si binary alloy fabricated by laser-powder bed fusion 査読有り国際誌

Konishi Y., Hamasaki Y., Sasaki T., Adachi H., Ishikawa K., Kim D., Takata N., Miyajima Y.

Journal of Alloys and Compounds 982 巻 2024年4月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of Alloys and Compounds

Al-12mass%Si binary alloy was fabricated by laser-powder bed fusion (L-PBF). The effect of low temperature annealing (LTA) less than 573 K on electrical and mechanical properties, microstructure, and residual stress of L-PBF Al-12mass%Si was investigated. Due to LTA, electrical resistivity at 77 K and 293 K changed from 34 nΩm down to 10 nΩm and from 68 nΩm down to 45 nΩm, respectively. The lattice constant of α-Al phase increased around 0.0005 nm by LTA according to X-ray diffraction. The change was associated with the decrease in the concentration of solid-solution Si of around 3.4 mass% by electrical resistivity measurement and 3 mass% by X-ray diffraction, when the annealing temperature was 473 K. Whereas, Vickers hardness increases around 145 HV compared with that for the as-built value of 141 HV, and then, decreases down to 126 HV and saturates. TEM/EDS observations confirmed the precipitation of fine Si within the α-Al phase. The decrease in annealing temperature increases the time to reach the Vickers hardness peak. 0.2% proof stress obtained by compression tests shows anisotropy, which can be attributed to the anisotropy on residual stress, which still exists after LTA.

DOI： 10.1016/j.jallcom.2024.173737

Microstructural origins of high strength of Al–Si alloy manufactured by laser powder bed fusion: In-situ synchrotron radiation X-ray diffraction approach 査読有り国際誌

Takata N., Liu M., Hirata M., Suzuki A., Kobashi M., Kato M., Adachi H.

Journal of Materials Science and Technology 178 巻頁： 80 - 89 2024年4月

担当区分：筆頭著者,　責任著者記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of Materials Science and Technology

The microstructural factors contributing to the high strength of additive-manufactured Al–Si alloys using laser-beam powder bed fusion (PBF-LB) were identified by in-situ synchrotron X-ray diffraction in tensile deformation and transmission electron microscopy. PBF-LB and heat treatment were employed to manufacture Al–12%Si binary alloy specimens with different microstructures. At an early stage of deformation prior to macroscopic yielding, stress was dominantly partitioned into the α-Al matrix, rather than the Si phase in all specimens. Highly concentrated Si solute (∼3%) in the α-Al matrix promoted the dynamic precipitation of nanoscale Si phase during loading, thereby increasing the yield strength. After macroscopic yielding, the partitioned stress in the Si phase monotonically increased in the strain-hardening regime with an increase in the dislocation density in the α-Al matrix. At a later stage of strain hardening, the flow curves of the partitioned stress in the Si phase yielded stress relaxation owing to plastic deformation. Therefore, Si-phase particles localized along the cell walls in the cellular-solidified microstructure play a significant role in dislocation obstacles for strain hardening. Compared with the results of the heat-treated specimens with different microstructural factors, the dominant strengthening factors of PBF-LB manufactured Al–Si alloys were discussed.

DOI： 10.1016/j.jmst.2023.08.050

CALPHAD-aided design of high-strength Al-Si-Mg alloys for sufficient laser powder bed fusion processability 査読有り国際誌

Otani Y., Saki K., Takata N., Suzuki A., Kobashi M., Kato M.

Journal of Alloys and Compounds 977 巻 2024年3月

担当区分：責任著者記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of Alloys and Compounds

This study proposes a design concept for an Al-Si-Mg ternary alloy to achieve both high strength and laser-based powder bed fusion (PBF-LB) processability using calculation of phase diagrams (CALPHAD). The thermodynamic calculations using the CALPHAD method evaluated the Al-10Si-4.5Mg (wt%) alloy composition for strengthening by both the Si and Mg2Si phases and high PBF processability by reducing the risk of hot tearing behind the small freezing range of 27 °C and crack susceptibility index (|dT/d(Fs1/2)|) of 77 °C. The PBF-LB Al-10Si-4.5Mg alloy sample (processed by laser power of 128 W and laser scanning velocity of 1000 mm/s) exhibited a fine three-phase microstructure and exceptional hardness of 199 HV. No hot tearing occurred, whereas many cracks preferentially propagated along the melt pool boundaries during the PBF-LB process. The severe lateral cracking resulted in a maximum relative density of 96 % in the Al-10Si-4.5Mg alloy samples. Preferential cracking could be responsible for the localization of the relatively coarse particles of the brittle Mg2Si phase in the cellular microstructures of the melt pool boundaries. Understanding the lateral cracking mechanism can provide new insights into alloy modification using the controlled solidification path to avoid a specific two-phase eutectic reaction (L→α-Al+Mg2Si) to improve the PBF processability. A modified alloy composition of Al-10Si-3Mg (wt%) was assessed using the CALPHAD approach. PBF-LB processing using the modified alloy powder alleviated cracking and improved the maximum relative density to 98 % for the PBF-LB manufactured samples. This result accentuated the validity of the design strategy for Al-Si-Mg ternary alloy utilizing the CALPHAD method.

DOI： 10.1016/j.jallcom.2024.173449

Microstructural factors controlling crack resistance of Zn–Al–Mg alloy coatings prepared via hot-dip galvanizing process: Combined approach of in-situ SEM observation with digital image correlation analysis 査読有り国際誌

Kim D., Takata N., Yokoi H., Suzuki A., Kobashi M.

Journal of Materials Research and Technology 29 巻頁： 1535 - 1541 2024年3月

担当区分：責任著者記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of Materials Research and Technology

To understand the microstructural factors controlling deformability during the forming process, a comprehensive analysis using in-situ scanning electron microscopy (SEM) with digital image correlation (DIC) for hot-dip Zn–6 %Al–3 %Mg alloy coatings on steel sheets in bending deformation was performed. The Zn–Al–Mg alloy coating exhibited two major microstructural constituents: the primary solidified Al (fcc) phase with dendritic morphologies and a fine ternary eutectic (TE) microstructure of the Zn (hcp), Al, and Zn2Mg phases. Major cracks were initiated on the tensile-strained coating surface, where the strain was localized in the TE regions, whereas the softer dendritic Al phases prevented the crack propagation. Microcracks were initiated in the binary eutectic (BE) Zn/Zn2Mg phases (Al-poor region) localized around the dendritic Al phases, whereas they were arrested in the TE region containing fine granular Al phases. The dendritic Al and fine granular Al phases embedded in the TE microstructure contributed to the high crack resistance.

DOI： 10.1016/j.jmrt.2024.01.235

Laser-beam powder bed fusion of Al–Fe–Cu alloy to achieve high strength and thermal conductivity 査読有り国際誌

Cheng Y., Miyawaki T., Wang W., Takata N., Suzuki A., Kobashi M., Kato M.

Additive Manufacturing Letters 8 巻 2024年2月

担当区分：責任著者記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Additive Manufacturing Letters

Laser-beam powder bed fusion (PBF-LB) technique was used to produce an Al–2.5 %Fe–2 %Cu ternary alloy, featuring a two-phase eutectic composition of α-Al/Al23CuFe4 in non-equilibrium solidification, as determined by thermodynamic calculations. The specimen manufactured by PBF-LB exhibited a high tensile strength exceeding 350 MPa and a low thermal conductivity of approximately 140 W m−1 K−1. Subsequent annealing at 300 °C improved the thermal conductivity to 175 W m−1 K−1 without compromising the strength. This improvement was attributable to forming numerous Al23CuFe4 nanoprecipitates, which consumed solute elements. By appropriately managing the factors contributing to strengthening, a superior strength–conductivity balance can be achieved by implementing post-heat treatments.

DOI： 10.1016/j.addlet.2023.100191

Controlling the addition of solute Ti in Al-Mg-Zn-Cu-Ni alloy for enhanced high-temperature creep properties 査読有り国際誌

Li R., Kondo M., Suzuki T., Suzuki A., Takata N.

Materials Science and Engineering: A 889 巻 2024年1月

担当区分：最終著者,　責任著者記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Materials Science and Engineering: A

This study aims to fundamentally understand the effect of 0.1 at% Ti addition (for introducing solute Ti with low diffusivity into the α-Al matrix) on the microstructure and high-temperature mechanical performance of a heat-resistant Al–5Mg–3.5Zn–2Cu–2Ni (at.%) quinary alloy subjected to heat treatments. The precipitation morphologies of the T-Al6Mg11Zn11 phase are systematically characterized in terms of the distribution of solute Ti, which is controlled by a peritectic reaction during solidification. High-temperature strength and creep properties (200 °C/105 MPa) are examined using the Ti-added alloy specimen and compared with the results of Ti-free quinary alloy specimens. In the Ti-added alloy, solute Ti is distributed within the dendritic α-Al phases formed through the peritectic reaction. The precipitation morphologies vary depending on solute Ti contents in the α-Al matrix after isothermal aging at 200 and 300 °C. Numerous fine T-phase precipitates are homogeneously dispersed in the α-Al matrix containing high solute Ti content. Fine precipitation morphologies appear stable even after long-term exposure at 200 °C. Ti addition extends creep rupture life (at 200 °C) without the loss of static high-temperature strength and room-temperature ductility. The improved creep properties could be responsible for the reduction in creep rate by the refined precipitation morphologies in the α-Al matrix containing higher solute Ti contents. Local coarsening of the precipitates occurs owing to the solute Ti during creep (under stress). The mechanism is discussed in terms of the effect of Ti addition on the phase equilibria between the α-Al and other intermetallic phases.

DOI： 10.1016/j.msea.2023.145859

Austenite Reversion Behavior of Maraging Steel Additive-manufactured by Laser Powder Bed Fusion 査読有り国際誌

Takata N., Ito Y., Nishida R., Suzuki A., Kobashi M., Kato M.

ISIJ International 64 巻 ( 2 ) 頁： 303 - 315 2024年

担当区分：筆頭著者,　責任著者記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：ISIJ International

This study was set to fundamentally investigate the characteristics of austenite reversion occurring in maraging steels additive-manufactured by laser powder bed fusion (L-PBF). The maraging steel samples manufactured under different L-PBF process conditions (laser power P and scan speed v) were subjected to heat treatments at 550°C for various durations, compared with the results of the austenitized and water-quenched sample with fully martensite structure. The L-PBF manufactured samples exhibited the martensite structure (including localized austenite (γ) phases) containing submicron-sized cellular structures. Enriched alloy elements were detected along the cell boundaries, whereas such cellar structure was not found in the water-quenched sample. The localized alloy elements can be rationalized by the continuous variations in the γ-phase composition in solidification during the L-PBF process. The precipitation of nanoscale intermetallic phases and the following austenitic reversion occurred in all of the experimental samples. The L-PBF manufactured samples exhibited faster kinetics of the precipitation and austenite reversion than the water-quenched sample at elevated temperatures. The kinetics changed depending on the L-PBF process condition. The enriched Ni element (for stabilizing γ phase) localized at cell boundaries would play a role in the nucleation site for the formation of γ phase at 550°C, resulting in enhanced austenite reversion in the L-PBF manufactured samples. The variation in the reaction kinetics depending on the L-PBF condition would be due to the varied thermal profiles of the manufactured samples by consecutive scanning laser irradiation operated under different P and v values.

DOI： 10.2355/isijinternational.ISIJINT-2023-045

Combustion Foaming of Porous (Al, Fe)3Ti with L12 Ordered Structure 査読有り国際誌

Suzuki, A; Fujishiro, T; Takata, N; Kobashi, M

MATERIALS TRANSACTIONS 65 巻 ( 4 ) 頁： 414 - 421 2024年

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Materials Transactions

The combustion foaming process enables the fabrication of closed-cell porous intermetallics by utilizing the combustion reactions generating a large amount of heat. A closed-cell porous Al3Ti (D022 crystal structure) fabricated by the combustion foaming process has a lightweight, high stiffness, high melting temperature, and good oxidation resistance but exhibits brittleness due to the poor deformability of Al3Ti. It is known that the addition of third elements (X = Cr, Mn, Fe, etc.) changes the crystal structures from D022 to L12, resulting in an improvement in the deformability in the case of dense materials. In the present study, an attempt was made to fabricate the closed-cell porous (Al, Fe)3Ti with an L12-ordered crystal structure through the combustion foaming. The effect of the amount of the exothermic agent, which was added to control the reaction heat of combustion foaming, on the porosity, pore morphology, microstructure, and constituent phases was investigated. The closed-cell porous L12-(Al, Fe)3Ti without any intermediate phases was successfully fabricated when the exothermic agent was added over 5 vol%. The porosity reached the maximum at approximately 80% when the exothermic agent of 10 vol% was added. The TiB2 particles, which were formed by the reactions of the exothermic agent, were aggregated in the porous sample with the exothermic agent of 5 vol% but uniformly dispersed in the porous samples foamed with the exothermic agent over 10 vol%. Based on the results above and the temperature measurements during the combustion foaming, it is important for fabricating highly porous L12-(Al, Fe)3Ti with the uniform microstructure to control the maximum temperature just above the liquidus temperature of (Al, Fe)3Ti. This study provides new insights into the hierarchical control of porous materials with the desired pore structures and cell wall materials.

DOI： 10.2320/matertrans.MT-M2023213

In-Situ SEM Observation and DIC Strain Analysis for Deformation and Cracking of Hot-Dip ZnMgAl Alloy Coating 査読有り国際誌

Takata N., Yokoi H., Kim D., Suzuki A., Kobashi M.

Corrosion Science and Technology 23 巻 ( 2 ) 頁： 113 - 120 2024年

担当区分：筆頭著者,　責任著者記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Corrosion Science and Technology

An attempt was made to apply digital image correlation (DIC) strain analysis to in-situ scanning electron microscopy (SEM) observations of bending deformation to quantify local strain distribution inside a ZnMgAl-alloy coating in deformation. Interstitial-free steel sheets were hot-dipped in a Zn-3Mg-6Al (mass%) alloy melt at 400 oC for 2 s. The specimens were deformed using a miniature-sized 4-point bending test machine inside the SEM chamber. The observed in situ SEM images were used for DIC strain analysis. The hot-dip ZnMgAl-alloy coating exhibited a solidification microstructure composed of a three-phase eutectic of fine Al (fcc), Zn (hcp), and Zn2Mg phases surrounding the primary solidified Al phases. The relatively coarsened Zn2Mg phases were locally observed inside the ZnMgAl-alloy coating. The DIC strain analysis revealed that the strain was localized in the primary solidified Al phases and fine eutectic microstructure around the Zn2Mg phase. The results indicated high deformability of the multi-phase microstructure of the ZnMgAl-alloy coating.

DOI： 10.14773/cst.2024.23.2.113

Formation Behavior of Fe–Zn Intermetallic Layers at the Interface between Fe–Mn and Pure Zn Melt at 460°C 査読有り国際誌 Open Access

Yoneda S., Takata N.

ISIJ International 64 巻 ( 9 ) 頁： 1457 - 1463 2024年

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：ISIJ International

The effect of Mn on the alloying reaction during hot-dip galvanization was investigated. The microstructure of the Fe–Zn intermetallic layers consisted of ζ, δ, and Γ phases for both pure Fe and Fe–2Mn (wt.%) alloy. The intermetallic layers grew thicker with increasing dipping time, and the growth rate of each layer was similar for both substrates. In the case of Fe–2Mn, the formation of the δ1p phase was observed after dipping for 2 s. However, δ1p formation was delayed for pure Fe, indicating that Mn may promote nucleation of the δ1p phase. It is known that the δ1p phase nucleates in the Fe-saturated ζ phase. The Fe content at the ζ/δ1p interface was found to be lower for the Fe–2Mn alloy by electron probe microanalysis, suggesting that the supersaturation of Fe for the nucleation of δ1p is decreased by Mn addition and Mn may stabilize the δ1p phase. Once δ1p became a continuous layer, the growth rates of the δ1p layer on pure Fe and Fe–2Mn were similar. Mn could affect only the nucleation of δ1p during the initial stage of the alloying reaction.

DOI： 10.2355/isijinternational.ISIJINT-2024-096

Evaluation of reactive wetting kinetics of carbon fibers by molten Al-Ti alloy and its application to the fabrication of Al/carbon fiber composites 査読有り国際誌

Suzuki A., Ishiguro R., Miwa T., Takata N., Kobashi M., Furukawa Y.

Journal of Alloys and Compounds 968 巻 2023年12月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of Alloys and Compounds

Wetting and interfacial reactions between molten Al and carbon fibers (CFs) are crucial for the fabrication of high-thermal conductive Al/CF composites through liquid-state processes including casting. The formation of Al4C3 which has a low thermal conductivity and high reactivity with water needs to be suppressed while improving wettability. The addition of Ti into molten Al forms TiC and contributes to significant improvement in wettability. To control a thin morphology of TiC, the kinetics of reactive wetting need to be clarified. In the present study, the reactive wetting kinetics between molten Al or Al-Ti alloy and pitch-based CF were investigated using the dipping coverage method combined with microstructural observations. The addition of Ti into molten Al drastically accelerated the wetting and reduced the apparent activation barrier for reactive wetting, which was related to the change in the interfacial products from the Al4C3 phase to the TiC phase. The Al4C3 phase grew while eroding the CFs, whereas the TiC phase formed in a thin-layered morphology around CFs and suppressed the diameter reduction of CF. The reduction in the apparent activation barrier and the morphology of carbides indicated a change in the dominant phenomena for reactive wetting from Al4C3 growth to TiC formation (controlled by diffusion in liquid Al). Based on the kinetics evaluations, CF were hybridized with pure Al and Al-Ti alloy through a casting process, and their thermal conductivities were evaluated. The addition of CF into pure Al degraded the thermal conductivity due to the eroded CF and coarsened Al4C3 phase, whereas the addition of CF into Al-Ti alloy enhanced the thermal conductivity. This study provides new insights into reactive wetting phenomena related to the fabrication of high-thermal conductive metal matrix composites.

DOI： 10.1016/j.jallcom.2023.172168

Wear properties of aluminum alloys fabricated by laser powder bed fusion 査読有り国際共著国際誌

Gong S., Takata N., Kobashi M., Shin S.E.

Tribology International 187 巻 2023年9月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Tribology International

The production of an aluminum alloy using the laser powder bed fusion (LPBF) process results in a unique microstructure that differs from that of the same alloy when cast; this microstructure is advantageous for wear resistance. This study compared the wear resistance of Al-12Si and Al-2.5Fe (mass%) binary alloys produced by LPBF process with their cast equivalents, to analyze their suitability for lightweight wear-resistant parts. The wear resistance of the LPBF-produced alloys was significantly improved owing to the increase in hardness induced by the rapid cooling rate of the LPBF process and the dense dispersion of the fine secondary phase. Thus, Al alloys fabricated using LPBF can provide excellent wear resistance and possess the potential for application as industrial materials.

DOI： 10.1016/j.triboint.2023.108769

B2-structured Fe<inf>3</inf>Al alloy manufactured by laser powder bed fusion: Processing, microstructure and mechanical performance 査読有り国際共著国際誌

Vilardell A.M., Pelcastre L., Nikas D., Krakhmalev P., Kato M., Takata N., Kobashi M.

Intermetallics 156 巻 2023年5月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Intermetallics

Prealloyed Fe3Al was successfully manufactured by laser powder bed fusion. The best set of process parameters led to parts with a relative density of 99.5 %, a surface roughness, Sa, of 31.5 ± 5.6 μm and a hardness of 319 ± 14 HV0.1. Its microstructure as well as its mechanical properties at room and high temperatures were analyzed. The results of the chemical composition showed minor variations in aluminum content oscillating between 21 and 28 at.% along the melt pool. Additionally, elongated grains were observed to grow parallel to the building direction, as well as the development of a weak {001} texture along the building direction. The mechanical properties were influenced by the temperature. Compression tests showed a loss in strength with the increase in temperature, from a yield strength of 621 ± 40 MPa at room temperature to 89 ± 20 MPa at 650 °C. Reciprocating sliding wear tests showed that fragmentation of the intermetallic at room temperature occurs, whereas plastic deformation dominated at higher temperatures. For all temperatures, tribochemical wear was also present due to the oxidation of wear debris.

DOI： 10.1016/j.intermet.2023.107849

Effect of low-temperature annealing on electrical resistivity and mechanical properties of laser-powder bed fused AlSi10Mg alloy 査読有り国際誌

Miyajima Y., Nakamura Y., Konishi Y., Ishikawa K., Wang W., Takata N.

Materials Science and Engineering: A 871 巻 2023年4月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Materials Science and Engineering: A

The AlSi10Mg sample was additive-manufactured using laser powder bed fusion (L-PBF), and subsequent low-temperature annealing (LTA) at 413K or 473 K was applied. Vickers hardness tests show the peak at around 160 HV by 473 K annealing and around 145 HV by 413 K annealing. The flow stress of tensile and compression deformations are also affected by LTA, and peak aging condition gives the highest 0.2% proof stress. 0.2% proof stress obtained by tensile tests is around 270 MPa, whereas that obtained by compression tests is between 326 MPa and 386 MPa. Electrical resistivity at 293 K changes from approximately 80 nΩm down to around 60 nΩm, whereas that at 77 K changes from around 40 nΩm down to around 20 nΩm. The estimated concentration of solute Si in the as-built specimen is between 2.5 mass% and 2.9 mass%, corresponding to the previous results using energy-dispersive X-ray spectroscopy. The lattice constant obtained from XRD increased from 0.4043 nm up to 0.4048 nm by LTA. Both electrical resistivity and XRD measurement indicate a decrease in the concentration of solute atoms. Microstructure observations revealed numerous nano-sized precipitates of Si phase with a plate-like or granular morphologies in α-Al grains due to LTA.

DOI： 10.1016/j.msea.2023.144876

Design of Al–Fe–Mn alloy for both high-temperature strength and sufficient processability of laser powder bed fusion 査読有り国際誌

Wang W., Takata N., Suzuki A., Kobashi M., Kato M.

Additive Manufacturing 68 巻 2023年4月

担当区分：責任著者記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Additive Manufacturing

This paper presents a novel design concept for Al alloys with both sufficient laser powder bed fusion (L-PBF) processability and high-temperature mechanical performance using an Al–Fe–Mn ternary alloy with a refined α-Al/Al6M (M = Fe, Mn) two-phase microstructure. The near-eutectic (liquid → α-Al(fcc) + Al6M) composition of Al–2.5Fe–2Mn (mass%) was designed to avoid the formation of coarse primary intermetallics (having a detrimental effect on the PBF processability for densification) based on non-equilibrium liquidus projection thermodynamic calculations. The designed Al–2.5Fe–2Mn alloy powder exhibited sufficient L-PBF processability, and fully dense centimeter-sized samples (relative densities of above 99%) were successfully prepared. The L-PBF Al–2.5Fe–2Mn alloy samples exhibited a significantly refined solidification microstructure consisting of Al6M and α-Al phases in the melt-pool regions. Fe and Mn alloying elements contributed to the formation of nanoscale Al6M-phase particles and a high solid solution concentration in the α-Al matrix. The strength of the L-PBF Al–2.5Fe–2Mn alloy was slightly reduced at higher temperatures, but remained above 240 MPa at an elevated temperature of 300 °C. This superior high-temperature mechanical performance was attributed to the high thermal stability of the refined α-Al/Al6M two-phase microstructure and the formation of nanosized Al6M-phase precipitates.

DOI： 10.1016/j.addma.2023.103524

Microstructural origin of anisotropic tensile ductility of Al-Si alloy manufactured by laser powder bed fusion 査読有り国際誌

Otani Y., Takata N., Suzuki A., Kobashi M., Kato M.

Scripta Materialia 226 巻 2023年3月

担当区分：責任著者記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Scripta Materialia

Aluminum alloys fabricated by laser powder bed fusion (L-PBF) exhibit anisotropic tensile ductility. To identify microstructural origins in melt-pool (MP) structures (formed by the L-PBF process) contributing to the anisotropic ductility, the local strain distribution inside the MP structure in deformation was quantified by a combination of digital image correlation (DIC) strain analysis and in-situ SEM observations of tensile tests. It was found that higher strain was localized in locally coarsened microstructures (softer regions) along melt-pool boundaries (MPBs). The strain localization played a significant role in crack initiation (and propagation) around MPBs, contributing to fracture. The strain localization at MPBs changed depending on the geometrical relation of the MPBs with the loading direction (LD), which was controlled by the LD with respect to the building direction of L-PBF samples. The three-dimensional MPB distribution related to the LD would be responsible for the anisotropic ductility.

DOI： 10.1016/j.scriptamat.2022.115259

Effects of Mn and Cu Additions on Solidification Microstructure and High-Temperature Strength of Cast AlFe Binary Alloy 査読有り国際誌

Okano N., Takata N., Suzuki A., Kobashi M.

Materials Transactions 64 巻 ( 2 ) 頁： 492 - 499 2023年

担当区分：責任著者記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Materials Transactions

In order to investigate the effects of Mn and Cu additions on solidification microstructure and high-temperature strength of cast AlFe alloys, we have fabricated various AlFe-based alloys with compositions of Al1%Fe, Al1%Fe1%Mn, Al1%Fe1%Cu, and Al1%Fe1%Cu1%Mn (mol%) solidified at different cooling rates (0.3 K·s11 and 145 K·s11). In the Al1%Fe binary alloy, the coarsened ª-Al13Fe4 phase with a needle-shaped morphology was often observed in the furnace-cooled sample (0.3 K·s11), whereas the cast sample (145 K·s11) exhibited several elongated ¡ phases surrounded by fine ¡/Al6Fe eutectic microstructure. Such a solidification microstructure was observed in the cast Al1%Fe1%Cu alloy, whereas the Al23CuFe4 phase was locally formed in the finally solidified zone in the furnace-cooled sample. In the Al1%Fe1%Mn alloy, the Al6(Fe, Mn) phase was formed regardless of the cooling rate. Finer ¡/Al6(Fe, Mn) two-phase eutectic microstructure was almost entirely occupied in the cast sample. The fine eutectic microstructure was observed in the cast Al1%Fe1%Cu1%Mn alloy as well. Compression tests for cast alloy specimens revealed that the Al1%Fe1%Cu1%Mn alloy exhibited the highest strength level among the studied alloy specimens, indicating the combined addition of Mn and Cu elements could be effective in improving the high-temperature strength of the cast AlFe alloys.

DOI： 10.2320/matertrans.MT-LA2022010

Austenite Reversion Behavior of Maraging Steel Additive-manufactured by Laser Powder Bed Fusion 査読有り

Takata N., Ito Y., Nishida R., Suzuki A., Kobashi M., Kato M.

Tetsu-To-Hagane/Journal of the Iron and Steel Institute of Japan 109 巻 ( 3 ) 頁： 201 - 214 2023年

担当区分：筆頭著者,　責任著者記述言語：日本語掲載種別：研究論文（学術雑誌）出版者・発行元：Tetsu-To-Hagane/Journal of the Iron and Steel Institute of Japan

This study was set to fundamentally investigate the characteristics of austenite reversion occurring in maraging steels additive-manufactured by laser powder bed fusion (L-PBF). The maraging steel samples manufactured under different L-PBF process conditions (laser power P and scan speed v) were subjected to heat treatments at 550°C for various durations, compared with the results of the austenitized and water-quenched sample with fully martensite structure. The L-PBF manufactured samples exhibited the martensite structure (including localized austenite (γ) phases) containing submicron-sized cellular structures. Enriched alloy elements were detected along the cell boundaries, whereas such cellar structure was not found in the water-quenched sample. The localized alloy elements can be rationalized by the continuous variations in the γ-phase composition in solidification during the L-PBF process. The precipitation of nanoscale intermetallic phases and the following austenitic reversion occurred in all of the experimental samples. The L-PBF manufactured samples exhibited faster kinetics of the precipitation and austenite reversion than the water-quenched sample at elevated temperatures. The kinetics changed depending on the L-PBF process condition. The enriched Ni element (for stabilizing γ phase) localized at cell boundaries would play a role in the nucleation site for the formation of γ phase at 550°C, resulting in the enhanced austenite reversion in the L-PBF manufactured samples. The variation in the reaction kinetics depending on the L-PBF condition would be due to the varied thermal profiles of the manufactured samples by consecutive scanning laser irradiation operated under different P and v values.

DOI： 10.2355/tetsutohagane.TETSU-2022-066

Introducing Hatch Spacing into Deposited Energy Density toward Efficient Optimization of Laser Powder Bed Fusion Process Parameters 査読有り

Kunieda M., Suzuki A., Takata N., Kato M., Kobashi M.

Materials Transactions 64 巻 ( 6 ) 頁： 1099 - 1106 2023年

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Materials Transactions

The optimization of processing parameters is indispensable for the laser powder bed fusion (L-PBF) process. The deposited energy density (DED) is one of the process indexes for the L-PBF process and has a simplified formula of P v 0.5, where P is the laser power, and v is the scan speed. This parameter describes the change in the relative density and the melt pool morphology with laser power and scan speed well, whereas it does not include the effect of other processing parameters, e.g., hatch spacing (S). In the present study, an attempt was made to incorporate the effect of S into DED. Al12Si (mass%) alloy cube samples were fabricated by L-PBF under various P, v, and S, for evaluating the relative density and the melt pool morphology. The melt pool depth and width of L-PBF-manufactured Al12Si alloy increased linearly with P v 0.5 and did not exhibit a clear correlation with S. Based on the experimental observation, the effect of hatch spacing on DED was estimated to be S 0.5, and a new index of P v 0.5 S 0.5 was proposed. This index described the change in the relative density of the L-PBF-manufactured Al12Si alloy with laser conditions (P, v, and S) well when the thermal conduction mode melting was dominant. This study also indicated the limitation of the applicability of P v 0.5 S 0.5 under the keyhole or transition mode melting.

DOI： 10.2320/matertrans.MT-ME2022002

Improved high-temperature creep properties of heat-resistant Al-Mg-Zn-Cu-Ni quinary alloy by adding a trace amount of Ti 査読有り Open Access

Kondo M., Suzuki T., Li R., Takata N.

Keikinzoku/Journal of Japan Institute of Light Metals 73 巻 ( 6 ) 頁： 260 - 265 2023年

記述言語：日本語掲載種別：研究論文（学術雑誌）出版者・発行元：Keikinzoku/Journal of Japan Institute of Light Metals

The present study investigated the effect of trace titanium (Ti) addition on high-temperature creep properties of a heat-resistant quinary aluminum (Al) alloy, with a composition of Al-5Mg-3.5Zn-2Cu-2Ni (mol%). Both the quinary alloy and Ti-added alloy were solution-treated at 480°C and then aged at 200°C for 10 hours. The creep tests were conducted at 200°C under 105 MPa. The addition of trace Ti element reduced the minimum creep rate and delayed the onset of the tertiary creep stage (creep acceleration), resulting in the extended creep rupture life of the Al-5Mg-3.5Zn-2Cu-2Ni quinary alloy. The creep rupture life of the Ti-added alloy was higher than that of the A7075 alloy but lower than that of the A2618 alloy, whereas the initial creep rate (up to 0.2% in strain) of the Ti-added quinary alloy was lower than that of the A2618 alloy. The superior creep properties were presumably due to the enhanced stability of the fine precipitation morphologies of the T-Al6Mg11Zn11 phase by solute Ti inside the α-Al matrix.

DOI： 10.2464/jilm.73.260

Influence of Dislocation Substructure on Size-Dependent Strength of High-Purity Aluminum Single-Crystal Micropillars 査読有り

Takata N., Uesugi S., Zhu T., Takeyasu S., Suzuki A., Kobashi M.

Materials Transactions 64 巻 ( 8 ) 頁： 1952 - 1958 2023年

担当区分：筆頭著者,　責任著者記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Materials Transactions

In order to understand the influence of dislocation substructures on the size-dependent strength (smaller is stronger) of micron-sized metals, we have fabricated single-crystal cylindrical micropillars with various diameters approximately ranging from 1 to 10µm, which were prepared on the surface of the fully annealed sample and the subsequently cold-rolled samples of high-purity aluminum (Al). The annealed micropillars exhibited a size dependence of the resolved shear stress required for slip. The shear stress (¸i) normalized by shear modulus (G) and the specimen diameter (d) normalized by Burgers vector (b) followed the correlation of ¸i/G = 0.33(d/b)10.63. The size-dependent strength was reduced by cold-rolling, resulting in lower power-law exponents (0.2630.31) for the correlation in the cold-rolled specimens. The fine dislocation substructures introduced by the cold-rolling could be associated with the reduced size-dependent strength, which can be rationalized using the stochastic model of the dislocation source length in an assumption of homogenously distributed dislocations existing in the experimental micropillars. The inhomogeneous dislocation substructure with various dislocation cell sizes would contribute to a variation in the measured strength depending on the location, likely due to the probability of exiting the dislocation cell walls (local variation in dislocation density) in the micropillars fabricated on the cold-rolled Al samples. [doi:10.2320/matertrans.MT-L2023004]

DOI： 10.2320/matertrans.MT-L2023004

Investigation of dominant contributors to high strength of additively manufactured Al-Fe alloy by using single-crystal micropillar compression tests 査読有り

Choshi A., Zhu T., Takata N., Suzuki A., Kobash M.

Keikinzoku/Journal of Japan Institute of Light Metals 73 巻 ( 11 ) 頁： 523 - 529 2023年

担当区分：責任著者記述言語：日本語掲載種別：研究論文（学術雑誌）出版者・発行元：Keikinzoku/Journal of Japan Institute of Light Metals

In this study, we aim to address the dominant microstructural parameter of the high strength of Al-Fe alloy components additive-manufactured by the laser powder bed fusion (L-PBF) process. This study systematically investigated the compression response of single-crystal micropillars with a fixed diameter of approximately 3 μm prepared on the surfaces of the L-PBF built Al-2.5%Fe binary alloy sample, and the subsequently heat-treated samples (300°C and 500°C), together with the slowly solidified Al-2.5%Fe alloy sample for comparison. The single-crystal micropillars of the L-PBF built sample exhibited a high yield strength of approximately 250 MPa and pronounced strain hardening. The compressed single-crystal specimens exhibited a uniform deformation indicating the activated multi-slip systems. Such a trend was observed in the 300°C heat-treated sample. However, the relatively low yield strength and strain hardening rate were found in single-crystal micropillars of the 500°C heat-treated sample as well as the slowly solidified sample. In the compressed micropillars, a single-slip system was dominantly activated. These results indicated that numerous nano-sized Al 6Fe-phase particles (contained in the specimens of the L-PBF built sample and the 300°C heat-treated sample) could contribute to high yield strength and the enhanced activation of multi-slip systems resulting in the pronounced strain hardening.

DOI： 10.2464/jilm.73.523

Design of heat-resistant Al–Mg –Zn–Cu–Ni quinary alloy: Controlling intermetallic phases and mechanical performance at elevated temperature 査読有り国際誌

Li R., Takata N., Suzuki A., Kobashi M.

Materials Science and Engineering: A 857 巻 2022年11月

担当区分：責任著者記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Materials Science and Engineering: A

Owing to good formability, wrought Al alloys are used in a wide range of industries, including turbochargers in automobile engines. The development of Al alloys with increased high-temperature strength is necessary to improve the fuel efficiency of engines. In this study, a modified heat-resistant Al–Mg–Zn–Cu–Ni quinary alloy with superior high-temperature strength and creep rupture life at 200 °C (potential temperature in service for compressor radial impellers) was designed. The thermodynamic assessments provided a modified alloy composition of Al–5Mg–3.5Zn–2Cu–2Ni (at %) exhibiting α-Al phase in equilibrium with T-Al6Mg11Zn11 and Al3(Ni, Cu)2 phases at elevated temperatures. The experimental alloy showed granular T and Al3(Ni, Cu)2 phases often located on grain boundaries in the α-Al matrix, although the undissolved Al3Ni phase (formed in solidification) locally remained. Apart from the T phase, the η-Zn2Mg phase containing Cu element preferentially precipitated on grain boundaries. A large number of fine precipitates were dispersed homogeneously in the grain interior after the aging at 200 °C or 300 °C. Compared to the base alloy with a ternary composition of Al–5Mg–3.5Zn (at %) and conventional Al alloys, the designed quinary alloy (pre-aged at 200 °C for 10 h) showed a high strength at elevated temperatures above 200 °C. The creep-rupture life at 200 °C/105 MPa was remarkably extended to 665 h by the addition of Cu and Ni elements into the base ternary alloy. The rupture life was more than ten times longer than one (57 h) of the ternary alloys. The alloy modification based on the thermodynamic calculations provides an effective approach for designing lightweight Al alloys with superior high-temperature strength and creep rupture life.

DOI： 10.1016/j.msea.2022.144055

Effect of solute Ni on thermal activation process of plastic deformation in Fe–18Cr single-crystal micropillars 査読有り国際誌

Zhu T., Li H., Takata N., Kobashi M., Yoshino M.

Materials Science and Engineering: A 857 巻 2022年11月

担当区分：責任著者記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Materials Science and Engineering: A

The aim of the present study was to identify the effect of solute Ni element on the plastic deformation and its thermal activation process of 18Cr ferritic stainless steels. Single-crystal micropillars with different diameters of approximately 2 and 5 μm were fabricated on the surface of Fe–18%Cr–Ni alloy sheets containing different contents of Ni (0–2 mass%). Compression tests were performed on the micropillars at different initial strain rates to investigate the strain rate sensitivity (m) of stress for the slip initiation and the specimen-size dependence of the m value. In micropillars with a small diameter of 2 μm, the Fe–18%Cr binary alloy specimen exhibited a relatively higher strain-rate sensitivity of initial slip stress (m = 0.12), whereas m was reduced to 0.01 by adding 2% Ni. The same trend was observed in micropillars with a larger size of approximately 5 μm. The experimental results were utilized to evaluate the activation volume of plastic deformation. The activation volumes of the Fe–18%Cr alloy and alloy containing 1% Ni followed the specimen-size dependence trend of activation volume in bcc metals; however, the alloy containing 2% Ni exhibited larger activation volumes, suggesting a different thermal activation process presumably due to the significant interaction that occurred between dislocations and solute Ni atoms in bcc solid-solutions.

DOI： 10.1016/j.msea.2022.144076

Machine-learning assisted optimization of process parameters for controlling the microstructure in a laser powder bed fused WC/Co cemented carbide 査読有り国際誌

Suzuki A., Shiba Y., Ibe H., Takata N., Kobashi M.

Additive Manufacturing 59 巻 2022年11月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Additive Manufacturing

A Single laser scan was performed on a sintered WC/Co composite and the process conditions were optimized to realize a WC/Co two-phase microstructure in a WC/Co composite fabricated by laser powder bed fusion (L-PBF). Laser irradiation on the sintered WC/Co composites generated two different microstructural regions. One is composed of WC and Co phases (WC/Co two-phase region), and the other includes W2C and W3Co3C phases (WC decomposition region). In the L-PBF-fabricated WC/Co composites, these two microstructural regions are distributed in a complex manner and their fractions varies depending on the laser parameters. Using a convolutional neural network model that is trained with the micrographs of single-laser-scanned WC/Co composites, the fractions of the two microstructural regions in the WC/Co composites fabricated by the L-PBF under various laser conditions were quantified. Multiple linear regression analyses revealed that the laser power and spot size contribute more than the scan speed, which has been verified by numerical analysis based on the moving heat source model. A support vector machine (SVM) suggested that low laser power and low scan speed are required for suppressing the WC decomposition in the consolidated samples. The WC decomposition has been successfully suppressed in the samples fabricated under SVM-recommended conditions.

DOI： 10.1016/j.addma.2022.103089

Controllable tensile performance of additively manufactured Al–Fe alloy 査読有り国際誌

Qi X., Takata N., Suzuki A., Kobashi M., Kato M.

Materials Science and Engineering: A 855 巻 2022年10月

担当区分：責任著者記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Materials Science and Engineering: A

Achieving both high strength and ductility is challenging in additive manufacturing of Al alloys using the laser powder bed fusion (L-PBF) process by altering the laser conditions to control the microstructural characteristics. The current work focuses on the influence of varying laser energy density (ED), which is adjusted by different sets of laser power (P) and scan speed (v), on the microstructure-tensile property relationships of the L-PBF-fabricated Al–2.5%Fe alloy under varying laser conditions (P = 204–230 W, v = 450–600 mm/s) with a fixed powder layer thickness (t = 30 μm) and hatch distance (h = 30 μm). With increasing ED (= P/v·t·h), a slight increase in the average size of the columnar α-Al grains was found in the experimental samples concurrently with a higher fraction of <001>-oriented grains. Nanosized particles of the Al6Fe phase with an increased tendency to interconnect were observed in the matrix inside the melt pool, and the θ phase formed within the submicron-sized cellular structure along the melt pool boundaries (MPBs). A low tensile ductility, induced by the local microstructural inhomogeneities across MPBs, was found in all specimens tensile-deformed along the building direction, which was characterized by a preference fracture along the MPBs. At higher applied ED, the tensile ductility of the experimental Al–2.5Fe alloy was significantly improved with only a slight decrease in tensile strength. The difference in concentration of Fe across the MPBs in the matrix was also moderately diminished, which is consistent with the reduced local difference in microhardness across the MPBs under higher ED conditions. Therefore, the present results demonstrate that local microstructural inhomogeneities across MPBs can be controlled by the laser conditions, thereby producing both high strength and good ductility in the L-PBF processed Al–Fe alloys.

DOI： 10.1016/j.msea.2022.143893

Enhancing the solid-state joinability of A5052 and CFRTP via an additively manufactured micro-structure 査読有り国際誌

Iwata K., Suzuki A., Kim S.G., Takata N., Kobashi M.

Journal of Materials Processing Technology 306 巻 2022年8月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of Materials Processing Technology

An Al alloy (A5052) sheet was joined with a carbon fiber-reinforced thermoplastic (CFRTP) sheet by hot pressing without melting the CFRTP sheet (that is, solid-state joining). A micro-structure was manufactured via laser irradiation of Al-Ti-C powders bedded on the A5052 sheet. The laser irradiation to Al-Ti-C powders generates numerous particle-shaped protrusions additively on the A5052 sheet, which facilitate the infiltration of CFRTP and mechanically interlock with the CFRTP sheet. To determine the optimal conditions for joining A5052 and CFRTP consisting of a polyamide 6 (PA6) matrix, the effects of joining temperature during hot pressing on the interfacial structure and joint strength were investigated. The maximum joint strength was attained at the joining temperature of 210 °C. At lower temperatures, PA6 was not sufficiently softened to infiltrate the micro-structure, resulting in lower joint strengths. At higher temperatures, carbon fibers (CFs) were exposed on the CFRTP surface and inhibited the penetration of PA6 and CFs into the micro-structure, thereby lowering the joint strength. To infiltrate PA6 and CFs into the micro-structure, hot pressing was performed in a gradient temperature field. The proposed process enabled appropriate infiltration of CFs and PA6 into the micro-structure and led to high and stable joint strength. It is expected that using the additively manufactured micro-structure approach proposed herein, the operating windows of various cutting-edge joining processes can be expanded to more moderate and lower energy conditions.

DOI： 10.1016/j.jmatprotec.2022.117629

Microstructural Variations in Laser Powder Bed Fused Al-15%Fe Alloy at Intermediate Temperatures 査読有り国際誌

Wang, WY; Takata, N; Suzuki, A; Kobashi, M; Kato, M

MATERIALS 15 巻 ( 13 ) 2022年7月

担当区分：責任著者記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Materials

The samples of the Al–15Fe (mass%) binary alloy that were additively manufactured by laser powder bed fusion (L-PBF) were exposed to intermediate temperatures (300 and 500◦C), and the thermally induced variations in their microstructural characteristics were investigated. The L-PBF-manufactured sample was found to have a microstructure comprising a stable θ-Al13Fe4 phase localized around melt-pool boundaries and several spherical metastable Al6Fe-phase particles surrounded by a nanoscale α-Al/Al6Fe cellular structure in the melt pools. The morphology of the θ phase remained almost unchanged even after 1000 h of exposure at 300◦C. Moreover, the nanoscale α-Al/Al6Fe cellular structure dissolved in the α-Al matrix; this was followed by the growth (and nucleation) of the spherical Al6Fe-phase particles and the precipitation of the θ phase. Numerous equiaxed grains were formed in the α-Al matrix during the thermal exposure, which led to the formation of a relatively homogenous microstructure. The variations in these microstructural characteristics were more pronounced at the higher investigated temperature of 500◦C.

DOI： 10.3390/ma15134497

PubMed

その他リンク： https://www.mdpi.com/1996-1944/15/13/4497

Fast scanning calorimetry study of Al alloy powder for understanding microstructural development in laser powder bed fusion 査読有り国際誌

Naoki Takata, Mulin Liu, Hongmei Li, Asuka Suzuki, Makoto Kobashi

Materials and Design 219 巻 2022年7月

担当区分：筆頭著者,　責任著者記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Materials and Design

We examined the variation in the solidification microstructure of AlSi10Mg alloy powder over a wide range of cooling rates controlled by using the fast scanning calorimetry (FSC) technique. Two exothermic peaks derived from the primary solidification of the α-Al phases and the α-Al/Si eutectic reaction were detected. These exothermic peaks became broader and shifted to lower temperatures at higher cooling rates. Focus ion beam (FIB) milling enables microstructural characterization of AlSi10Mg powder solidified at controlled cooling rates above 102 °C·s−1 in the FSC equipment. The sample solidified at 4 × 104 °C·s−1 exhibited a fine microstructure consisting of a primary α-Al phase surrounded by lamellar-shaped Si phases. The fraction of the Si phase continuously decreased with increasing cooling rate, indicating higher content of solute Si element in the α-Al phase solidified at a higher cooling rate. The secondary dendrite arm spacing (λ) of the primary α-Al phase decreased in the sample solidified at a higher cooling rate (dT/dt). The relation follows a general equation of λ = A (dT/dt)-n (A: 38.4, n: 0.33) in wide range of cooling rate (10-1 ∼ 104 °C·s−1). The results were utilized to discuss the cooling rate of Al alloys during the laser powder bed fusion (L-PBF) process.

DOI： 10.1016/j.matdes.2022.110830

その他リンク： https://www.sciencedirect.com/science/article/pii/S026412752200452X

Negative strain rate sensitivity of yield strength of Al-Si alloy additive-manufactured using laser powder bed fusion 査読有り国際誌

Takata N., Liu M., Suzuki A., Kobashi M., Kato M.

Scripta Materialia 213 巻 2022年5月

担当区分：筆頭著者,　責任著者記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Scripta Materialia

This article reports an unexpected finding of the mechanical behavior of Al-12%Si alloy manufactured using the laser powder bed fusion (L-PBF) process. The as-manufactured specimen has a negative strain rate sensitivity of flow stress at an early stage of tensile deformation (below 3% in strain). The post heat-treated specimens exhibited a positive strain-rate sensitivity and none of these specimens exhibited a serrated stress-strain curve, indicating a mechanism different from the Portevin–LeChatelier effect. Transmission electron microscopy revealed the dynamic precipitation of the nano-sized Si-phase inside the supersaturated solid solution of the α-Al matrix in the as-manufactured specimen deformed at ambient temperatures. The dynamically precipitated Si phase interacted with the introduced dislocations, enhancing the flow stress during the early stages of plastic deformation. The dynamic precipitation could contribute to the negative strain-rate sensitivity of the flow stress at an early stage of tensile deformation (including the yield strength).

DOI： 10.1016/j.scriptamat.2022.114635

計算状態図を基にした耐熱アルミニウム鍛造合金の設計と組織制御招待有り査読有り

高田尚記, 鈴木飛鳥, 小橋眞

まてりあ 61 巻 ( 4 ) 頁： 195 - 201 2022年4月

High-temperature strength sustained by nano-sized eutectic structure of Al-Fe alloy manufactured by laser powder bed fusion 招待有り査読有り国際誌

Wang, WY; Takata, N; Suzuki, A; Kobashi, M; Kato, M

MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING 838 巻 2022年3月

担当区分：責任著者記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Materials Science and Engineering: A

This study investigated the high-temperature strength of a hypereutectic Al–Fe binary alloy (Al–15%Fe) additive-manufactured by laser powder bed fusion (L-PBF). The L-PBF Al–15%Fe alloy exhibited superior yield strength to the other existing L-PBF processed alloys due to α-Al/Al6Fe eutectic structure with a nanoscale cellular morphology. The present results demonstrated the feasibility of using L-PBF processed Al–Fe alloys as high-temperature lightweight materials.

DOI： 10.1016/j.msea.2022.142782

Influence of Stabilizing Elements on Ductile-Brittle Transition Temperature (DBTT) of 18Cr Ferritic Stainless Steels 査読有り国際誌

Masataka Yoshino, Tetsuya Tagawa, Hongmei Li, Naoki Takata

ISIJ International 62 巻 ( 4 ) 2022年1月

Change in microstructural characteristics of laser powder bed fused Al–Fe binary alloy at elevated temperature 査読有り国際誌

Qi X., Takata N., Suzuki A., Kobashi M., Kato M.

Journal of Materials Science and Technology 97 巻頁： 38 - 53 2022年1月

担当区分：責任著者記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of Materials Science and Technology

The present study addressed the change in the microstructure of Al–2.5 wt% Fe binary alloy produced using laser powder bed fusion (L-PBF) technique by thermal exposure at 300 °C, and the associated mechanical and thermal properties were systematically examined as well. Multi-semi-cylindrical patterns corresponding to melt pools in the microstructure were macroscopically observed for the as-manufactured sample. No change in the melt-pool morphology was observed after thermal exposure for 1000 h. Inside the melt pools, a large number of the nanoscale metastable Al6Fe phase particles were uniformly distributed inside columnar grains of the α-Al matrix containing concentrated solute Fe in supersaturation. The sequential formation and coarsening of stable θ-Al13Fe4 phases were observed upon exposure to a 300 °C environment, but a considerable amount of nano-sized metastable Al6Fe phases remained even after 1000 h. Furthermore, the thermal exposure continuously reduced the concentration of solute Fe atoms in the α-Al matrix. No significant grain growth was found in α-Al matrix after 1000 h owing to the pinning effect of the dispersed fine particles on grain boundary migration. These results demonstrate a sluggish change in microstructural morphologies of the Al–2.5 wt% Fe alloy. The quantified microstructural parameters addressed dominant strengthening contributions by the solid solution of Fe element and Orowan strengthening mechanism by fine Al–Fe intermetallics in the L-PBF-produced alloy. The high strength level was sustained even after being exposed to 300 °C for long periods. The superior balance of mechanical properties and thermal conductivity can be achieved in the experimental alloys by taking advantage of the various microstructural parameters related to the Al–Fe intermetallic phases and α-Al matrix.

DOI： 10.1016/j.jmst.2021.04.038

Solidification microstructure and room-temperature fracture toughness of Al-based cast alloys prepared through eutectic reactions in Al-Mg-Zn ternary system 査読有り Open Access

Okano N., Aikawa M., Takata N., Suzuki A., Kobashi M.

Keikinzoku/Journal of Japan Institute of Light Metals 72 巻 ( 1 ) 頁： 79 - 87 2022年

担当区分：責任著者記述言語：日本語掲載種別：研究論文（学術雑誌）出版者・発行元：Keikinzoku/Journal of Japan Institute of Light Metals

This study was set to examine the fundamentals of solidification microstructure and room-temperature fracture toughness of different eutectic aluminum (Al) alloys strengthened by T-Al6Mg11Zn11 and/or β-Al3Mg2 intermetallic phases. The thermodynamic calculations for an Al-Mg-Zn ternary system assessed three alloy compositions of Al-22.5Mg-23.5Zn, Al-35.5Mg-2Zn and Al-34.5Mg-5Zn (mol%) corresponding to α-Al/T, α-Al/β and α-Al/T/β eutectic compositions, respectively. The designed alloys were prepared by different cast process for changing the cooling rate in solidification. The Al-22.5Mg-23.5Zn and Al-35.5Mg-2Zn alloys exhibited a number of fibrous α phase surrounded by of T-phase and β-phase matrix, respectively. The morphologies of eutectic microstructures were refined by high cooling rate in solidification. The three-phase microstructure of α, T and β phases was often observed in slowly solidified Al-34.5Mg-5Zn alloy, whereas only the α/β two-phase eutectic microstructure was observed in the rapidly solidified alloy sample. The indentation fracture method using Vickers indentation test was applied to measure the room-temperature fracture toughness of the experimental alloys prepared by mold-casting. The α/β two-phase eutectic alloys exhibited a low fracture toughness of 1.1 MPa·m0.5 in comparison with the α/T two-phase eutectic alloy. The lower fracture toughness would be responsible for the brittleness of β-phase matrix in the α/β two-phase eutectic alloys.

DOI： 10.2464/jilm.72.79

Meso-structural Control on Metal Surfaces Based on Powder Metallurgy and Its Application to Metal/Polymer Joining 招待有り査読有り

Suzuki A., Takata N., Kobashi M.

Journal of the Society of Powder Technology, Japan 59 巻 ( 10 ) 頁： 511 - 517 2022年

記述言語：日本語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of the Society of Powder Technology, Japan

This paper introduces meso-scale structure synthesis on metal surfaces through reaction processes from raw metal powders. The mesostructures are applied to metal/polymer anchor joining. The first topic is the synthesis of Fe-based porous structure on Fe substrate through the combustion reaction of the Fe-Ti-B system. The structure could be controlled in view of the phase equilibria at the maximum temperature of the reaction. A Fe/epoxy resin joint via the porous structure exhibits high tensile strength in a direction perpendicular to the joint interface. The second topic is the fabrication of mesostructure protruded on an Al alloy surface by laser scanning on Al-Ti-C powders. The mesostructural morphology is related to the formation of TiC and Al3Ti phases, which can be understood based on the solidification sequence after the laser irradiation. An Al alloy/CFRTP joint via the mesostructure exhibits high joint strength compared with previously reported similar anchor joints.

DOI： 10.4164/sptj.59.511

Influence of Stabilizing Elements on Ductile-Brittle Transition Temperature (DBTT) of 18Cr Ferritic Stainless Steels 査読有り国際誌

Yoshino M., Tagawa T., Li H., Takata N.

ISIJ International 62 巻 ( 4 ) 頁： 788 - 798 2022年

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：ISIJ International

The influence of different stabilizing elements (Nb, Ti and Zr) which are added to stabilize interstitial elements in the ferrite matrix on the ductile-brittle transition temperature (DBTT) of 18Cr ferritic stainless steels was systematically investigated at different temperatures ranging from −25°C to 75°C. Hot-rolled sheets of 18 mass% Cr ferritic stainless steels containing a trace amount (0.1 atomic%) of Nb, Ti or Zr were studied. The added stabilizing elements formed precipitates of carbides/nitrides in all the steels tested in the present study. The particle diameter of the precipitates observed in the Nb-added steel was much smaller than those in the other steels. The measured DBTTs of the Base steel and the Nb-added, Ti-added and Zr-added steels were 39°C, −3°C, 39°C and 11°C, respectively. Although initiation of cleavage cracks in each steel occurred independent of the kind of stabilizing element, the Nb-added steel exhibited a relatively higher Charpy impact value due to ductile plastic deformation following cleavage crack initiation, resulting in a lower DBTT (superior impact toughness). The occurrence of plastic deformation in the Nb-added steel is considered to be due to the fine carbide and/or nitride precipitates which were homogenously distributed in the material. Detailed analyses of the fracture surfaces were also performed for the other steels, and the results were used as a basis for discussing the effects of each stabilizing element on the DBTT of 18Cr ferritic stainless steels in terms of the precipitation morphologies observed in the ferrite matrix.

DOI： 10.2355/isijinternational.ISIJINT-2021-506

Design and controlling microstructure of cast-type aluminum eutectic alloys with an allowable service temperature of 300°C 招待有り査読有り Open Access

Takata N., Okano N., Aikawa M., Okano T., Suzuki A., Kobashi M.

Keikinzoku/Journal of Japan Institute of Light Metals 72 巻 ( 6 ) 頁： 381 - 387 2022年

Effect of heat treatment on solute Si content in Al-12%Si alloy additive-manufactured by laser powder bed fusion 査読有り Open Access

Takata N., Liu M., Suzuki A., Kobashi M., Kato M., Iwai S., Adachi H.

Keikinzoku/Journal of Japan Institute of Light Metals 72 巻 ( 5 ) 頁： 178 - 184 2022年

担当区分：筆頭著者,　責任著者記述言語：日本語掲載種別：研究論文（学術雑誌）出版者・発行元：Keikinzoku/Journal of Japan Institute of Light Metals

We have investigated microstructural characteristics and solute Si element in the α-Al (fcc) matrix of Al-12%Si binary alloy additive-manufactured by laser powder bed fusion (L-PBF) process in terms of different locations in the melt-pool structure (inside melt-pool or around melt-pool boundaries). Their variations by post heat-treatments at various temperatures were examined. The fraction of Si phase and lattice parameter of α-Al phase were quantified by XRD measurements combined with sample rotation and oscillation systems. In the L-PBF manufactured sample, the columnar α -Al phases contained highly concentrated Si element in solution (above 3 mol%) inside melt-pools, whereas the measured Si content was approximately 2 mol% in the relatively coarsened α-Al phase along melt-pool boundaries. The location dependence of solute Si content would be associated with a continuous change in growth rate of solid phase in solidification by local heating in the L-PBF process. The solute Si content was significantly reduced by heat-treatments and its location dependence appeared less pronounced. The as-manufactured sample exhibited a lower lattice parameter of α-Al phase with concentrated Si in solution in comparison with the heat-treated samples. The fraction of Si phase became higher in the heat-treated samples. These results were rationalized by microstructural change at elevated temperatures.

DOI： 10.2464/jilm.72.178

その他リンク： https://www.jstage.jst.go.jp/article/jilm/72/5/72_7205-T08/_article/-char/ja/

Observation of compressive deformation behavior of Al-12%Si alloy lattice structures additive-manufactured by laser powder bed fusion using mechanical stress-induced luminescent material 査読有り Open Access

Kato M., Suzuki A., McMillan M., Liu X., Takata N., Kobashi M.

Keikinzoku/Journal of Japan Institute of Light Metals 72 巻 ( 5 ) 頁： 246 - 250 2022年

Influence of Added Fourth Elements on Precipitation in Heat-Resistant AlMgZn Ternary Alloys 査読有り国際誌

Ishii H., Takagi R., Takata N., Suzuki A., Kobashi M.

Materials Transactions 63 巻 ( 4 ) 頁： 513 - 521 2022年

担当区分：責任著者記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Materials Transactions

DOI： 10.2320/matertrans.MT-L2021021

その他リンク： https://www.jstage.jst.go.jp/article/matertrans/63/4/63_MT-L2021021/_article/-char/ja/

Aluminum matrix composites reinforced with multi-walled carbon nanotubes and C<inf>60</inf> manufactured by laser powder bed fusion 査読有り国際共著国際誌

Yoo S., Shin S.E., Takata N., Kobashi M.

Journal of Materials Science 2022年

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of Materials Science

Carbon nano-materials are widely used as reinforcing materials for aluminum-based composites due to their excellent metallurgical properties. This study presents an approach for manufacturing Al/C composites utilizing a high energy ball milling and laser powder bed fusion (LPBF) process. We prepared Al/C60 and Al/MWCNT composites under 25 different conditions to observe their microstructure and mechanical property trends according to laser power and scan speed. The relative density of LPBFed composites increased as the energy density increased due to the oxide layer present on the aluminum surface and the high laser reflectance. It was found that, as the energy density increased, the increase in nanohardness and elastic modulus of the LPBFed composites was due to the high relative density. However, the high laser power condition of 179 W resulted in the softening of materials due to grain coarsening, resulting in a decrease in nanohardness and elastic modulus. Under the same laser conditions, the nanohardness of the LPBFed Al/2 vol.% MWCNT composites was approximately 0.5 GPa higher than that of the LPBFed Al/1 vol.% C60 composites, which is explained by the high relative density and grain refinement strengthening.

DOI： 10.1007/s10853-022-07119-6

その他リンク： https://link.springer.com/article/10.1007/s10853-022-07119-6

Optimizing Laser Conditions and Controlling Microstructure/Properties of Laser Powder Bed Fused Al-Si Alloys 査読有り Open Access

Suzuki A., Takata N., Kobashi M., Kato M.

Funtai Oyobi Fummatsu Yakin/Journal of the Japan Society of Powder and Powder Metallurgy 69 巻 ( 10 ) 頁： 417 - 425 2022年

記述言語：日本語掲載種別：研究論文（学術雑誌）出版者・発行元：Funtai Oyobi Fummatsu Yakin/Journal of the Japan Society of Powder and Powder Metallurgy

In the laser beam powder bed fusion (PBF-LB) processes, laser parameters need to be customized for fabricating dense components. Al-Si alloys are representative Al alloys used for the PBF-LB processes. The Al-Si alloys fabricated by the PBF-LB processes exhibit characteristic microstructures and mechanical properties, which are affected by the laser parameters. This paper describes the effect of laser conditions on the relative density, microstructure, and mechanical properties of Al-12Si (mass%) alloy fabricated by the PBF-LB process. The first topic is a machine-learning-assisted exploration of laser parameters (laser power and scan speed) for fabricating dense Al-12Si alloy parts. Using a neural network model, the laser condition range for fabricating dense Al-12Si alloy parts is predicted. A methodology for accurately predicting the laser condition range for fabricating dense components is introduced. The second topic is controlling the microstructures and mechanical properties of the dense Al-12Si alloys. Variations in α-Al crystal grain size/orientation, the width of primary α-Al phase, and solute Si content in α-Al matrix with laser conditions are introduced. The changes in mechanical properties with laser conditions and correlation with microstructural features are also described.

DOI： 10.2497/jjspm.69.417

Influence of added fourth elements on creep properties of heat-resistant Al-Mg-Zn ternary alloys at 200°C 査読有り Open Access

Kondo M., Suzuki T., Li R., Takata N.

Keikinzoku/Journal of Japan Institute of Light Metals 72 巻 ( 6 ) 頁： 473 - 481 2022年

記述言語：日本語掲載種別：研究論文（学術雑誌）出版者・発行元：Keikinzoku/Journal of Japan Institute of Light Metals

The effect of Cu or Ni addition on creep properties of a heat-resistant aluminum (Al) alloy (with a composition of Al-5Mg-3.5Zn (mol%)) strengthened by precipitates of T-Al6Mg11Zn11 intermetallic phase was investigated. The quaternary alloys with various Cu contents (0.25-3 mol%) or Ni contents (0.5-2 mol%) were solution-treated at 470 °C and then aged at 200°C for 10 hours. The creep tests were conducted using the heat-treated specimens at 200°C under 105 MPa. Both Cu and Ni additions increased the creep rupture time. The 1 mol% Cu added alloy exhibited the longest creep rupture time due to the enhanced precipitation of the refined intragranular phases (T or MgZn2 phases). However, more than 2 mol% Cu addition led to the transition of T phase to S-Al2CuMg phase formed on grain boundaries. The phase transition might have a detrimental effect on creep rupture time. In the Ni-added alloy specimen, the creep rupture time became longest when 1 mol% Ni was added. The added Ni element promoted the formation of the Al3Ni phase on the grain boundaries, resulting in the suppression of connecting cavities formed on grain boundaries in creep deformation.

DOI： 10.2464/JILM.72.473

Enhancement in strength and ductility of laser powder bed fused Al–12Si alloy by introducing nanoscale precipitates 査読有り国際誌

Liu M., Takata N., Suzuki A., Kobashi M., Kato M.

Additive Manufacturing Letters 1 巻頁： 100008 2021年12月

担当区分：責任著者記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Additive Manufacturing Letters

In order to manage both the strength and ductility of Al–Si alloys additively manufactured by laser powder bed fusion (L-PBF), an attempt was made to introduce nanoscale precipitates within the α-Al (fcc) supersaturated solid solution using the L-PBF manufactured Al–12%Si alloy by artificial aging treatments. The aging treatment at 120°C improved both the tensile strength and ductility (total elongation) of the L-PBF manufactured specimen. The simultaneous strength–ductility enhancement is due to the homogeneously distributed nanoscale Si precipitates that enhanced the strain hardening. These results provide new insights for overcoming the strength-ductility trade-off in the L-PBF manufactured Al alloys.

DOI： 10.1016/j.addlet.2021.100008

Precipitation morphology and kinetics of T-Al6Mg11Zn11 intermetallic phase in Al–Mg–Zn ternary alloys 査読有り国際誌

Takata N., Takagi R., Li R., Ishii H., Suzuki A., Kobashi M.

Intermetallics 139 巻 2021年12月

担当区分：筆頭著者,　責任著者記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Intermetallics

This study was undertaken to understand the fundamentals of T-Al6Mg11Zn11 intermetallic phase precipitation in heat-resistant Al–Mg–Zn ternary alloys, with the goal of further strength improvements. The thermodynamic assessment involved three alloy compositions, Al–4Mg–4.5Zn, Al–5Mg–3.5Zn, and Al–7Mg–1.5Zn (at%), with the T phase in a fixed fraction (approximately 6–7.5%) in equilibrium with the α-Al phase at 300 °C. The alloy compositions were set on different tie-lines between the α-Al and T phases in the two-phase region, resulting in varied compositions of each phase. In the three experimental alloys aged at 300 °C, the T phase precipitated on the grain boundaries of the α-Al matrix, and granular precipitates were homogeneously dispersed intragranularly. There was no significant difference in the precipitation morphology. It was demonstrated that the lattice parameters of the T and α-Al phases could be controlled based on the alloy composition and applied heat treatments. However, the lattice parameters have no significant effect on the precipitation morphologies of the thermodynamically stable T phase in the Al–Mg–Zn ternary alloys. The Zn-rich Al–4Mg–4.5Zn alloy exhibited a pronounced softening behavior after 10 h of aging due to the enhanced coarsening of precipitates in the α-Al matrix. The thermal instability of the precipitates was responsible for the transformation sequence from the initially formed metastable phases (η-Zn2Mg associated phases) to the stable T phase. These results provided a fundamental insight that alloy strengthening by introducing thermodynamically stable intermetallic phases is essential for sustaining the strength of heat-resistant Al–Mg–Zn ternary alloys after long-term treatment at elevated temperatures.

DOI： 10.1016/j.intermet.2021.107364

Control of microstructural characteristics and mechanical properties of AlSi12 alloy by processing conditions of laser powder bed fusion 査読有り国際誌

Suzuki A., Miyasaka T., Takata N., Kobashi M., Kato M.

Additive Manufacturing 48 巻 2021年12月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Additive Manufacturing

To elucidate the possibility of controlling the microstructure and mechanical properties of Al–Si alloys additive-manufactured by laser powder bed fusion (L–PBF), the relationship among the laser conditions, microstructural characteristics (crystal grain, cellular microstructure, and solute Si in α-Al matrix), and tensile properties of L–PBF manufactured AlSi12 alloy samples was investigated. The crystal grain size and fraction of <001> oriented α-Al crystal grains toward the building direction increased as the laser power became higher and scanning velocity became slower. The width of cellular microstructure (sub–cell size) where the primary α-Al phase was surrounded by α-Al/Si eutectic microstructure increased as the laser power became higher and scanning velocity became slower. The refinement of microstructure was attributed to the solidification under high cooling rate, which was achieved by low laser power and high scanning velocity. The area fraction of Si phase analyzed from FE–SEM images tended to decrease as the laser power became higher and scanning velocity became slower, suggesting the possibility of solute Si concentration in α-Al matrix controlled by laser conditions. The STEM/EDS analysis revealed high concentration of solute Si in the α-Al matrix (above a solubility limit of 1.5 mol% in the Al–Si binary system) of L–PBF manufactured AlSi12 alloy samples and its concentration became higher in the sample manufactured using a higher laser power. The tensile tests of as–built AlSi12 specimens fabricated under various laser conditions were also performed. High strength and high ductility tended to be achieved when high power laser was applied in slow laser scanning velocity. The 0.2% proof stress was relatively higher when the laser scanning velocity was faster. The mechanical properties correlated with crystal grain size and sub–cell size rather than solute Si in the α-Al matrix. These results provide new insights to control mechanical performance of AlSi12 alloy via L–PBF processing parameters.

DOI： 10.1016/j.addma.2021.102383

Precipitation Hardening at Elevated Temperatures above 400 degrees C and Subsequent Natural Age Hardening of Commercial Al-Si-Cu Alloy 査読有り国際誌

Li Ruoqi, Takata Naoki, Suzuki Asuka, Kobashi Makoto, Okada Yuji, Furukawa Yuichi

MATERIALS 14 巻 ( 23 ) 2021年12月

担当区分：責任著者記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Materials

The precipitation of intermetallic phases and the associated hardening by artificial aging treatments at elevated temperatures above 400◦ C were systematically investigated in the commer-cially available AC2B alloy with a nominal composition of Al–6Si–3Cu (mass%). The natural age hardening of the artificially aged samples at various temperatures was also examined. A slight increase in hardness (approximately 5 HV) of the AC2B alloy was observed at an elevated temperature of 480◦ C. The hardness change is attributed to the precipitation of metastable phases associated with the α-Al15 (Fe, Mn)3 Si2 phase containing a large amount of impurity elements (Fe and Mn). At a lower temperature of 400◦ C, a slight artificial-age hardening appeared. Subsequently, the hardness decreased moderately. This phenomenon was attributed to the precipitation of stable θ-Al2 Cu and Q-Al4 Cu2 Mg8 Si6 phases and their coarsening after a long duration. The precipitation sequence was rationalized by thermodynamic calculations for the Al–Si–Cu–Fe–Mn–Mg system. The natural age-hardening behavior significantly varied depending on the prior artificial aging temperatures ranging from 400◦ C to 500◦ C. The natural age-hardening was found to strongly depend on the solute contents of Cu and Si in the Al matrix. This study provides fundamental insights into controlling the strength level of commercial Al–Si–Cu cast alloys with impurity elements using the cooling process after solution treatment at elevated temperatures above 400◦ C.

DOI： 10.3390/ma14237155

Controlling WC/Co two-phase microstructure of cemented carbides additive-manufactured by laser powder bed fusion: Effect of powder composition and post heat-treatment 査読有り国際誌

Ibe H., Kato Y., Yamada J., Kato M., Suzuki A., Takata N., Kobashi M.

Materials and Design 210 巻 2021年11月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Elsevier Ltd.

In this study, an attempt is made to control the WC-Co two-phase microstructures by laser powder-bed fusion (LPBF) process using cemented carbide powders with different compositions of WC-25Co (mass%) and WC-17Co (mass%) and post heat-treatment at 1380 °C. The as-built cemented carbide samples had different microstructures from those of the cemented carbides fabricated by conventional manufacturing methods. The microstructures of the as-fabricated WC-25Co and WC-17Co cemented carbides were characterized by two types of regions. Region 1 consisted of W2C and W3Co3C phases, including numerous pores and defects. Region 2 consisted of WC and Co phases. Post heat-treatment of the LPBF-built WC-25Co sample at 1380 °C led to the formation of a defect-free WC-Co two-phase microstructure. In contrast, the W3Co3C phase remained in the LPBF-built WC-17Co sample even after heat treatment. The addition of carbon to the WC-17Co powder prevented the formation of the W3Co3C phase in the cemented carbide processed by LPBF and subsequent heat treatments. The addition of carbon to the WC-17Co powder had a slight effect on the hardness of as-fabricated samples, but reduced the hardness of heat-treated samples. The reduced hardness was attributed to the increased volume fractions of the WC and W3Co3C phases.

DOI： 10.1016/j.matdes.2021.110034

その他リンク： https://www.sciencedirect.com/science/article/pii/S026412752100589X

Effect of added stabilizing elements on thermal activation process of plastic deformation in 18Cr ferritic stainless steel 査読有り国際誌

Li H., Takata N., Kobashi M., Yoshino M.

Materials Science and Engineering A 824 巻 2021年9月

担当区分：責任著者記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Materials Science and Engineering A

The effects of a trace amount (~0.1 at.%) of added stabilizing elements (Nb, Ti, and Zr) on the tensile properties of 18Cr ferritic stainless steel sheets at strain rates ranging from 10−3–102 s−1 (from ordinary quasi-static deformation to dynamic deformation) were investigated. The additional Ti or Zr elements formed coarse carbide or nitride precipitates, which consumed interstitial solute elements (C and/or N) in the ferrite matrix. In the Nb-added steel, however, only fine NbN precipitates with a few hundred nanometers in size were observed locally, implying a small amount of solute Nb in the ferrite matrix. Ti or Zr additions had only a slight effect on the deformation resistance of 18Cr ferritic steel sheets and strain rate sensitivity (m) of the flow stress, however, the Nb addition increased the deformation resistance and significantly reduced the strain rate sensitivity. The measured activation volume (v*) values of the Ti- and Zr-added steels (62 b3 and 66 b3) were almost equivalent to one of the 18Cr base steel (64 b3). This result suggested that a general thermally activated process for screw dislocation motion is controlled by double-kink nucleation, which is slightly affected by Ti or Zr additions. The high v* value (106 b3) of the Nb-added steel indicated that the thermally activated process of plastic deformation would be associated with the substitutional solute Nb atoms (act as short-range obstacles) interacting with the moving dislocations. These results showed that the Nb element played a distinct role in plastic deformation from the other stabilizing elements of Ti and Zr.

DOI： 10.1016/j.msea.2021.141866

Dual plateau stress of C15-type topologically close-packed lattice structures additive-manufactured by laser powder bed fusion 査読有り国際誌

Liu X., Suzuki A., Takata N., Kobashi M., Kato M.

Scripta Materialia 202 巻 2021年9月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Scripta Materialia

Imitating crystal structures offers new avenues to design lattice structures composed of unit cells with nodes (act as atoms) and struts (act as atomic bonds). We found out unique dual plateau regions in the stress–strain curves of lattice structures inspired by topologically close-packed (TCP) structure of C15 Laves phase (AB2-type intermetallic compounds) and manufactured via the laser powder bed fusion. X-ray computed tomography of compressively deformed lattice structures as well as finite element analysis revealed that the existence of two kinds of units with different relative density caused the dual plateau regions. In addition, the C15 lattice structures exhibited a hybrid deformation mode of bending- and stretch-dominated deformations and can absorb high total energy of 5.0 MJ·m−3 in two stress levels.

DOI： 10.1016/j.scriptamat.2021.114003

Control of surface micro− structure for Al alloy/polymer joining fabricated by laser process using Al− Ti− C powders: Effect of powder composition 査読有り国際誌

Seung−Gwang Kim, Asuka Suzuki, Naoki Takata, Makoto Kobashi

Journal of Advanced Joining Processes 4 巻頁： 100068 2021年7月

Thermal activation process of plastic deformation in Fe–18Cr single-crystal micropillars with high-density dislocations 査読有り国際誌

Hongmei Li, Tianqi Zhu, Naoki Takata, Makoto Kobashi, Masataka Yoshino

Materials Science and Engineering: A 819 巻 ( 5 ) 頁： 141459 2021年5月

Processability and Optimization of Laser Parameters for Densification of Hypereutectic Al–Fe Binary Alloy Manufactured by Laser Powder Bed Fusion 査読有り国際誌

Wenyuan Wang, Naoki Takata, Asuka Suzuki, Makoto Kobashi, Masaki Kato

crystals 11 巻 ( 3 ) 頁： 320 2021年3月

In‐Situ X‐ray Observations and Microstructural Characterizations for Understanding Combustion Foaming and Reaction Processes to Synthesize Porous Al3Ti Composites from Al–Ti–B4C Powders 査読有り国際誌

Takamasa Inukai, Asuka Suzuki, Naoki Takata, Makoto Kobashi, Yuji Okada, Yuichi Furukawa

Advanced Engineering Materials 頁： 2001284 2021年3月

Managing both high strength and thermal conductivity of a laser powder bed fused Al–2.5 Fe binary alloy: Effect of annealing on microstructure 査読有り国際誌

Xing Qi, Naoki Takata, Asuka Suzuki, Makoto Kobashi, Masaki Kato

Materials Science and Engineering: A 805 巻頁： 140591 2021年2月

Understanding and suppressing shear band formation in strut-based lattice structures manufactured by laser powder bed fusion 査読有り国際誌

Xiaoyang Liu, Takafumi Wada, Asuka Suzuki, Naoki Takata, Makoto Kobashi, Masaki Kato

Materials & Design 199 巻頁： 109416 2021年2月

Joint strength of Fe/epoxy resin hybrid structure via porous Fe/TiB2 composite layer synthesized by in-situ reaction process 査読有り国際誌

Suzuki Asuka, Noritake Kazuki, Takata Naoki, Kobashi Makoto

JOURNAL OF MATERIALS PROCESSING TECHNOLOGY 288 巻 2021年2月

Critical resolved shear stress of activated slips measured by micropillar compression tests for single-crystals of Cr-based Laves phases 査読有り国際共著国際誌

Yunlong Xue, Naoki Takata, Hongmei Li, Makoto Kobashi, LiangYuan

Materials Science and Engineering: A 806 巻 ( 4 ) 頁： 140861 2021年1月

Effect of Trace Solute Titanium on Plastic Deformation of α-(Fe, Cr) Single-Crystal Micropillars Fabricated from 18Cr Ferritic Stainless Steel 査読有り国際誌

Hongmei Li, Tianqi Zhu, Naoki Takata, Makoto Kobashi, Masataka Yoshino

Materials Science and Engineering: A 803 巻頁： 140455 2021年1月

Microstructure of Al-2.5 Fe Binary Alloy Fabricated by Laser Powder Bed Fusion 査読有り国際誌

Xing Qi, Naoki Takata, Asuka Suzuki, Makoto Kobashi, Masaki Kato

Materials Science Forum 1016 巻頁： 1175 - 1180 2021年1月

Inhomogeneous Microstructure and its Thermal Stability of AlSi10Mg Lattice Structure Manufactured via Laser Powder Bed Fusion 査読有り国際誌

Mu Lin Liu, Naoki Takata, Asuka Suzuki, Makoto Kobashi

Materials Science Forum 1016 巻頁： 826 - 831 2021年1月

Mechanical behavior of in-situ alloyed Ti6Al4V(ELI)-3 at.% Cu lattice structures manufactured by laser powder bed fusion and designed for implant applications 査読有り国際共著

A.M. Vilardell, A. Takezawa, A. du Plessis, N. Takata, P. Krakhmalev, M. Kobashi, M.Albu, G. Kothleitner, I. Yadroitsava, I. Yadroitsev

Journal of the Mechanical Behavior of Biomedical Materials 113 巻頁： 104130 2021年1月

Influence of added fourth elements on precipitation in heat-resistant Al-Mg-Zn ternary alloys 査読有り

Ishii H., Takagi R., Takata N., Suzuki A., Kobashi M.

Keikinzoku/Journal of Japan Institute of Light Metals 71 巻 ( 7 ) 頁： 275 - 282 2021年

担当区分：責任著者記述言語：日本語掲載種別：研究論文（学術雑誌）出版者・発行元：Keikinzoku/Journal of Japan Institute of Light Metals

In the present study, we investigated the effects of Cu or Ni additions on the precipitation of intermetallic phases in the heat-resistant aluminum alloy with a ternary composition of Al-5Mg-3.5Zn (mol%). The quaternary alloys with 1 mol%Cu or 1 mol%Ni content were solution-treated at 480 °C and subsequently aged at 300 °C for different periods. Both Cu and Ni additions have a slight effect on the age-hardening of these alloys at 300 °C. The added Cu element partitioned into not only T-Al6Mg11Zn11 phase but η-Zn2Mg phase precipitated in the α-Al matrix. The observed Cu enrichment in the precipitates of T phase indicated high stability of T phase in the Al-Mg-Zn-Cu quaternary system, which was different from the results of thermodynamic calculations using the existing database. The added Ni element enhanced the formation of fine Al3Ni phase located at grain boundaries and slightly influenced the precipitation of T phase in grain interior. These results provided new insights to design novel heat-resistant Al alloys using the Al-Mg-Zn-Cu-Ni system.

DOI： 10.2464/jilm.71.275