研究者詳細 - 髙田　尚記

論文 - 髙田　尚記

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Anomalous temperature-dependent strength of copper alloy manufactured by laser-beam powder bed fusion 国際誌 Open Access

Kim D., Takata N., Umeda J., Shimizu T., Kobashi M.

Additive Manufacturing Letters 11 巻 2024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

This study reports an anomalous temperature-dependent tensile behavior of laser-beam powder bed fusion (PBF-LB) processed Cu–Cr–Zr alloy. The yield strength of the alloy initially decreases as the temperature increases to 200±5 MPa and then increases to 350±11 MPa at 500°C before reducing to 234±6 MPa at 600°C. The microstructure consists of elongated Cu grains with a high concentration of Cr solute (∼1 mass%), resulting from rapid solidification during the PBF-LB process. Transmission electron microscopy for the specimens deformed at 500°C revealed the presence of numerous nanoscale Cr-rich particles embedded inside the supersaturated solid solution of the Cu matrix. Nanoscale particles can act as barriers to dislocation motion, leading to an increase in internal stress during plastic deformation at elevated temperatures. This work provides the high potential of post heat treatments for achieving superior mechanical performance using high solute supersaturation formed by the PBF-LB process.

DOI： 10.1016/j.addlet.2024.100236

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

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

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

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

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