Kenta Yamanaka | Tohoku University (original) (raw)

Papers by Kenta Yamanaka

Intermetallics, 2020

The wettability and formation of intermetallic compounds (IMCs) formed between a Co-Cr-Mo-based a... more The wettability and formation of intermetallic compounds (IMCs) formed between a Co-Cr-Mo-based alloy and Sn-solder metal were investigated using the reflow soldering method. Soldering was conducted in an electric furnace in separate experiments at temperatures of 533, 553, 573, 623, and 673 K for 600 s. The morphology of the intermetallic layer formed at the Co-Cr-Mo-based/Sn-solder interface was characterized by scanning electron microscopy (SEM). The chemical composition and phase of the formed intermetallic layer were analyzed by electron probe microscopic analysis (EPMA) and X-ray diffraction analysis. Wettability analysis indicated that the soldering temperature influenced the wettability. With increasing soldering temperatures, the spreading factor increased, whereas the contact angle decreased. The intermetallic layers were found in triples in the intermediate zone between the solidified Sn-solder and the Co-Cr-Mo-based substrate. The thickness of the intermetallic layers increased in proportion with increasing soldering temperature. The EPMA analysis indicated only two IMCs formed at the interface of the joint at all of the investigated soldering temperatures, although three interfacial layers were observed by SEM analysis. The Sn-richer phase, Co(Cr,Mo)Sn 2 , formed adjacent to the Sn-solder matrix, whereas the Co(Cr,Mo)Sn was found near the Co-Cr-Mo-based substrate. The nanoindentation measurement revealed that the formed Co(Cr,Mo)Sn 2 and Co(Cr,Mo)Sn IMCs possessed lower hardness values compared to the Sn-Cu intermetallic systems.

High Temperature Materials and Processes

Metallic phase change materials (MPCMs) are attracting considerable attention for their applicati... more Metallic phase change materials (MPCMs) are attracting considerable attention for their application in thermal energy storage. Al–Si alloys are considered potential MPCMs; however, to develop storage systems/modules, it is crucial to fabricate corrosion-resistant materials for MPCMs. In this study, the corrosion behavior of Co−28Cr−6Mo−1.5Si (wt%) alloy was examined via immersion tests in commercial Al−Si alloy (ADC12) melt at 700°C for 10 h. The results were compared to those obtained for pure Al. Substrate thickness loss measurements revealed that the liquid metal corrosion was more severe in the Al−Si melt than that in pure Al, suggesting an increased reactivity due to Si addition. Interfacial analysis elucidated a direct reaction between the alloy substrate and molten Al in both cases. Furthermore, the formation of oxides such as Al2O3 and SiO2 did not contribute to corrosion resistance.

Metals

Ti–6Al–4V alloys undergo a multiple phase transformation sequence during electron beam powder bed... more Ti–6Al–4V alloys undergo a multiple phase transformation sequence during electron beam powder bed fusion (EB-PBF) additive manufacturing, forming unique dislocation substructures. Thus, determining the dislocation density is crucial for comprehensively understanding the strengthening mechanisms and deformation behavior. This study performed time-of-flight neutron diffraction (TOF-ND) measurements of Ti–6Al–4V alloys prepared via EB-PBF and examined the dislocation density in the as-built and post-processed states using convolutional multiple whole profile (CMWP) fitting. The present TOF-ND/CMWP approach successfully determined the bulk-averaged dislocation density (6.8 × 1013 m−2) in the as-built state for the α-matrix, suggesting a non-negligible contribution of dislocation hardening. The obtained dislocation density values were comparable to those obtained by conventional and synchrotron X-ray diffraction (XRD) measurements, confirming the reliability of the analysis, and indicati...

MATEC Web of Conferences

Microstructural conversion mechanisms under hot forging process (at temperatures ranging from 750... more Microstructural conversion mechanisms under hot forging process (at temperatures ranging from 750 °C to 1050 °C and strain rates ranging from 10–3 s–1 to 1 s–1) of a Ti-5Al-2Sn-2Zr-4Mo-4Cr (Ti-17) alloy with a lamellar starting microstructure were experimentally identified in this work. After that, constitutive formulae for predicting the microstructural evolution were established followed by calculation using finite-element (FEM) analysis. In the α phase, a lamellae kinking is the dominant mode in the higher strain rate region and dynamic globularization frequently occurs at higher temperatures. On the other hand, continuous dynamic recrystallization is the dominant mode below the transition temperature, Tβ (880~890 °C) in the β phase. And, at conditions of lower strain rates and higher temperatures, dynamic recovery tends to be more active. For microstructural prediction, a set of constitutive equations modeling the microstructural evolution and forging properties are established ...

Additive Manufacturing Letters

Journal of Materials Science, 2022

Ti–26Nb–2Fe–(0, 2, 4, 6, 8)Sn alloys were prepared by arc melting and subjected to homogenization... more Ti–26Nb–2Fe–(0, 2, 4, 6, 8)Sn alloys were prepared by arc melting and subjected to homogenization, cold rolling, and solution treatment. The β phase stability of the alloys increased with the addition of Sn. Ti–26Nb–2Fe comprised ω + β phases, whereas a single β phase was detected in Ti–26Nb–2Fe–(2, 4, 6, 8)Sn. With an increase in Sn, the Young’s modulus first decreased from 83 GPa in Ti–26Nb–2Fe to 58 GPa in Ti–26Nb–2Fe–4Sn and increased to 63 GPa in Ti–26Nb–2Fe–8Sn. Sn suppressed twinning during tension. Although the work-hardening rate decreased with the decrease of twinning, Sn was beneficial for maintaining a low work-hardening rate and postponed necking. All the alloys exhibited remarkably high plasticity. A strong solid solution strengthening effect of Sn was not observed in the studied Ti–Nb–Fe–Sn alloys. Ti–26Nb–2Fe–4Sn with a good combination of high tensile strength (yield strength of 592 MPa and tensile strength of 622 MPa) and low Young’s modulus (58 GPa) exhibited significantly higher cell viability than that of the control group after a 7-day culturing, indicating that it is a suitable candidate for biomaterials.

Scientific Reports, 2020

In recent years, spherical powders with no or minimal internal pores fabricated by the plasma rot... more In recent years, spherical powders with no or minimal internal pores fabricated by the plasma rotating electrode process (PREP) have been highly recommended for powder-type additive manufacturing. Most research on PREP is aimed at establishing relationship between PREP parameters and powder size. However, almost no dedicated research on granulation behavior has been conducted so far. In the present study, PREP experiments of Ti64 and SUS316 alloys were carried out. Numerical modeling based on computational thermo-fluid dynamics was developed to analyze the granulation behavior. In particular, the roles of the additionally introduced gas blast and the morphology of the electrode end surface in fluid granulation were preliminarily investigated. The study showed that in addition to the electrode's rotating speed and diameter, manipulating the plasma arc current (i.e., the melting rate) could also be an effective way to control the PREP-powder size. According to the simulation, ther...

SSRN Electronic Journal, 2021

For components built by powder bed fusion with electron beam (PBF-EB), the resulting microstructu... more For components built by powder bed fusion with electron beam (PBF-EB), the resulting microstructure arising from non-equilibrium solidification–microsegregation and the formation of interdendritic phases significantly affects the mechanical properties and hot cracking resistance. Notably, the powder characteristics influence heat absorption and conduction, thereby altering the molten pool behavior and solidification parameters. However, the effect of powder feedstock on non-equilibrium solidification during PBF has not been widely investigated. In this study, a CoCrMo alloy was built using powders prepared by gas-atomization (GA) and plasma rotating electrode process (PREP). Under the given operating conditions, the samples built with the two powders were experimentally characterized and their compression properties were compared. By performing multi-scale numerical simulations, powder melting and solidification were visualized and analyzed to elucidate the mechanism through which the powder characteristics influence the non-equilibrium solidification behavior during PBF-EB. The study revealed that upon appropriated size control, compared to the GA powder, the PREP powder had a smaller specific surface area and higher sphericity; thus, the generated powder layer exhibited higher heat absorption and dissipation rates. Therefore, a high solidification rate is facilitated, thereby suppressing microsegregation. The findings contribute to PBF knowledge related to feedstock, thus proving to be an essential reference for selecting and optimizing metallic powders applicable to additive manufacturing.

Corrosion Science, 2021

Abstract Co-based alloys are currently being used in a wide range of high temperature application... more Abstract Co-based alloys are currently being used in a wide range of high temperature applications owing to their high resistance to oxidation and corrosion. However, their oxidation-induced degradation could still occur during the long-term exposure to high temperature. Thus, the continuous development of oxidation-resistant Co-based alloys is of crucial importance. In this research, the influence of Si addition on the oxidation behavior of Co–Cr–Mo–xSi alloys under the isothermal oxidation treatment at 700 °C in air was investigated. The Si concentration (x) was varied from 0.1 to 5.0 wt.%. Surface morphologies and chemical compositions of the oxide films formed were analyzed by using SEM-EDS and XPS. The chemical compositions obtained from the surface analysis revealed that Si has played a role in the stabilization of Cr oxides on the surface of Co–Cr–Mo–xSi alloys. With increasing Si concentration, Co-oxide formation on the alloy surface was suppressed by the presence of Cr-oxide due to the selective oxidation of Cr atoms. Furthermore, SiO2 was both found along the grain boundaries and interfaces between the outmost oxide layer and matrix. It was also found that the oxide thickness was reduced with increasing Si concentration. This was a result of the formation of stable Cr-oxide and SiO2 sub-layer that became a barrier inhibiting the inward and outward diffusion of O and Cr.

Materials Science and Engineering: A, 2019

Abstract This paper presents microstructures of Inconel 718 fabricated by electron beam melting (... more Abstract This paper presents microstructures of Inconel 718 fabricated by electron beam melting (EBM), a powder bed fusion (PBF), additive manufacturing (AM) process, under various conditions with an extended range of electron beam power (P) and scanning speed (V). An adaptive offset method (AOM) was used to optimize the beam scanning line offset adaptively to fabricate blocks without macroscopic defects. In the AOM, the line offset is changed depending on the geometry of melt-pools of adjacent scanning lines. The AOM is valid as long as melt pool depth is larger than layer thickness, and it greatly broadens the process window for building dense and even parts. The broadened process window extends the range of the solidification condition and the variation of the resultant microstructures. Fully columnar grains and mixtures of columnar and equiaxed grains were formed. Mixtures of columnar grains and equiaxed grains were formed under two different types of extreme conditions. The first type is with low line energy/small line offset, which causes equiaxed grains associated with a small number of defects. The second type is with high line energy/large line offset, which causes equiaxed grains via the columnar-to-equiaxed transition (CET). The fully columnar grains are preferably oriented to direction in the build direction. The blocks with the different microstructure exhibited similar strengths but different elongations. A processing map was established by plotting the points indicating the build condition and resultant microstructure in the P–V space as a guide for controlling microstructure.

Materials Science and Engineering: A, 2019

In the present study, low-cost high-entropy Al (5,10) Cr 12 Fe 35 Mn (28,23) Ni 20 alloys contain... more In the present study, low-cost high-entropy Al (5,10) Cr 12 Fe 35 Mn (28,23) Ni 20 alloys containing 5 and 10 at. % Al were designed based on the thermodynamic principles. The chemical compositions of the studied high entropy alloys (HEAs) were selected to obtain face centered cubic solid solutions with good cold deformability. The alloys were produced using arc melting and were subsequently subjected to cold rolling to reduce their thickness up to 90%, to study their structure stability and strengthen them. Elemental mapping of the as-cast and deformed structures was conducted using electron microprobe analysis. Subsequently, the mechanical properties of the as-cast and heavily deformed alloys were determined using micro hardness measurements and tensile and compression tests. The microstructure evolution was studied using X-ray diffraction and electron backscatter diffraction. It was observed that the thermodynamic parameters of HEAs could be used to enhance their mechanical properties, and the experimental results were in agreement with those observations. After substituting Mn with Al in Al 5 Cr 12 Fe 35 Mn 28 Ni 20 , the yield stress of Al 10 Cr 12 Fe 35 Mn 23 Ni 20 increased by ~14%. Moreover, the alloy maintained good cold workability and its tensile ductility exceeded 40%. Cold rolling the alloy to 90% increased its yield stress more than four times. Despite the heavy deformation caused by cold rolling (2.3 true strain), deformation twinning was induced in these alloys.

Materialia, 2019

The microstructural evolution of commercially pure titanium (CP-Ti) was investigated during addit... more The microstructural evolution of commercially pure titanium (CP-Ti) was investigated during additive manufacturing (AM) with electron beam melting (EBM). Abnormal grain growth (AGG) was observed throughout all but the top of the specimen, with a mixture of fine and very coarse (hexagonal close-packed) grains, resulting in relatively large scattering in the Vickers hardness at a given build height. AGG proceeded by the growth of some grains to a few hundred micrometers in size at the expense of the surrounding substructured fine grains, which were created during EBM but prior to the occurrence of AGG. A small amount of-phase particles were also produced during rapid cooling after solidification, probably contributing the occurrence of AGG by pinning the grain boundaries. The observations highlight the evolution of a unique, process-specific microstructure during EBM fabrication. Improvement of the scan strategy as well as modification of the chemical composition are needed to obtain a homogeneous microstructure.

The Proceedings of Ibaraki District Conference, 2016

JOURNAL OF THE JAPAN WELDING SOCIETY, 2017

Materials Letters, 2017

CoCrFeNiTi-based high-entropy alloy with superior tensile strength and corrosion resistance achie... more CoCrFeNiTi-based high-entropy alloy with superior tensile strength and corrosion resistance achieved by a combination of additive manufacturing using selective electron beam melting and solution treatment,

To obtain an intrinsic stress-strain response with no effects on stress due to the friction betwe... more To obtain an intrinsic stress-strain response with no effects on stress due to the friction between the sample and the anvils or on the temperature increase due to adiabatic heating, a method of compensating for a stress-strain response was proposed. Using the stress-strain response that was compensated for, a processing map, consisting of both power dissipation efficiency and flow instability map, was constructed. On the basis of our observations using EBSD, it was found that the processing map after the compensation can be effectively used to well predict the optimum hot forging conditions.

Intermetallics, 2020

The wettability and formation of intermetallic compounds (IMCs) formed between a Co-Cr-Mo-based a... more The wettability and formation of intermetallic compounds (IMCs) formed between a Co-Cr-Mo-based alloy and Sn-solder metal were investigated using the reflow soldering method. Soldering was conducted in an electric furnace in separate experiments at temperatures of 533, 553, 573, 623, and 673 K for 600 s. The morphology of the intermetallic layer formed at the Co-Cr-Mo-based/Sn-solder interface was characterized by scanning electron microscopy (SEM). The chemical composition and phase of the formed intermetallic layer were analyzed by electron probe microscopic analysis (EPMA) and X-ray diffraction analysis. Wettability analysis indicated that the soldering temperature influenced the wettability. With increasing soldering temperatures, the spreading factor increased, whereas the contact angle decreased. The intermetallic layers were found in triples in the intermediate zone between the solidified Sn-solder and the Co-Cr-Mo-based substrate. The thickness of the intermetallic layers increased in proportion with increasing soldering temperature. The EPMA analysis indicated only two IMCs formed at the interface of the joint at all of the investigated soldering temperatures, although three interfacial layers were observed by SEM analysis. The Sn-richer phase, Co(Cr,Mo)Sn 2 , formed adjacent to the Sn-solder matrix, whereas the Co(Cr,Mo)Sn was found near the Co-Cr-Mo-based substrate. The nanoindentation measurement revealed that the formed Co(Cr,Mo)Sn 2 and Co(Cr,Mo)Sn IMCs possessed lower hardness values compared to the Sn-Cu intermetallic systems.

High Temperature Materials and Processes

Metallic phase change materials (MPCMs) are attracting considerable attention for their applicati... more Metallic phase change materials (MPCMs) are attracting considerable attention for their application in thermal energy storage. Al–Si alloys are considered potential MPCMs; however, to develop storage systems/modules, it is crucial to fabricate corrosion-resistant materials for MPCMs. In this study, the corrosion behavior of Co−28Cr−6Mo−1.5Si (wt%) alloy was examined via immersion tests in commercial Al−Si alloy (ADC12) melt at 700°C for 10 h. The results were compared to those obtained for pure Al. Substrate thickness loss measurements revealed that the liquid metal corrosion was more severe in the Al−Si melt than that in pure Al, suggesting an increased reactivity due to Si addition. Interfacial analysis elucidated a direct reaction between the alloy substrate and molten Al in both cases. Furthermore, the formation of oxides such as Al2O3 and SiO2 did not contribute to corrosion resistance.

Metals

Ti–6Al–4V alloys undergo a multiple phase transformation sequence during electron beam powder bed... more Ti–6Al–4V alloys undergo a multiple phase transformation sequence during electron beam powder bed fusion (EB-PBF) additive manufacturing, forming unique dislocation substructures. Thus, determining the dislocation density is crucial for comprehensively understanding the strengthening mechanisms and deformation behavior. This study performed time-of-flight neutron diffraction (TOF-ND) measurements of Ti–6Al–4V alloys prepared via EB-PBF and examined the dislocation density in the as-built and post-processed states using convolutional multiple whole profile (CMWP) fitting. The present TOF-ND/CMWP approach successfully determined the bulk-averaged dislocation density (6.8 × 1013 m−2) in the as-built state for the α-matrix, suggesting a non-negligible contribution of dislocation hardening. The obtained dislocation density values were comparable to those obtained by conventional and synchrotron X-ray diffraction (XRD) measurements, confirming the reliability of the analysis, and indicati...

MATEC Web of Conferences

Microstructural conversion mechanisms under hot forging process (at temperatures ranging from 750... more Microstructural conversion mechanisms under hot forging process (at temperatures ranging from 750 °C to 1050 °C and strain rates ranging from 10–3 s–1 to 1 s–1) of a Ti-5Al-2Sn-2Zr-4Mo-4Cr (Ti-17) alloy with a lamellar starting microstructure were experimentally identified in this work. After that, constitutive formulae for predicting the microstructural evolution were established followed by calculation using finite-element (FEM) analysis. In the α phase, a lamellae kinking is the dominant mode in the higher strain rate region and dynamic globularization frequently occurs at higher temperatures. On the other hand, continuous dynamic recrystallization is the dominant mode below the transition temperature, Tβ (880~890 °C) in the β phase. And, at conditions of lower strain rates and higher temperatures, dynamic recovery tends to be more active. For microstructural prediction, a set of constitutive equations modeling the microstructural evolution and forging properties are established ...

Additive Manufacturing Letters

Journal of Materials Science, 2022

Ti–26Nb–2Fe–(0, 2, 4, 6, 8)Sn alloys were prepared by arc melting and subjected to homogenization... more Ti–26Nb–2Fe–(0, 2, 4, 6, 8)Sn alloys were prepared by arc melting and subjected to homogenization, cold rolling, and solution treatment. The β phase stability of the alloys increased with the addition of Sn. Ti–26Nb–2Fe comprised ω + β phases, whereas a single β phase was detected in Ti–26Nb–2Fe–(2, 4, 6, 8)Sn. With an increase in Sn, the Young’s modulus first decreased from 83 GPa in Ti–26Nb–2Fe to 58 GPa in Ti–26Nb–2Fe–4Sn and increased to 63 GPa in Ti–26Nb–2Fe–8Sn. Sn suppressed twinning during tension. Although the work-hardening rate decreased with the decrease of twinning, Sn was beneficial for maintaining a low work-hardening rate and postponed necking. All the alloys exhibited remarkably high plasticity. A strong solid solution strengthening effect of Sn was not observed in the studied Ti–Nb–Fe–Sn alloys. Ti–26Nb–2Fe–4Sn with a good combination of high tensile strength (yield strength of 592 MPa and tensile strength of 622 MPa) and low Young’s modulus (58 GPa) exhibited significantly higher cell viability than that of the control group after a 7-day culturing, indicating that it is a suitable candidate for biomaterials.

Scientific Reports, 2020

In recent years, spherical powders with no or minimal internal pores fabricated by the plasma rot... more In recent years, spherical powders with no or minimal internal pores fabricated by the plasma rotating electrode process (PREP) have been highly recommended for powder-type additive manufacturing. Most research on PREP is aimed at establishing relationship between PREP parameters and powder size. However, almost no dedicated research on granulation behavior has been conducted so far. In the present study, PREP experiments of Ti64 and SUS316 alloys were carried out. Numerical modeling based on computational thermo-fluid dynamics was developed to analyze the granulation behavior. In particular, the roles of the additionally introduced gas blast and the morphology of the electrode end surface in fluid granulation were preliminarily investigated. The study showed that in addition to the electrode's rotating speed and diameter, manipulating the plasma arc current (i.e., the melting rate) could also be an effective way to control the PREP-powder size. According to the simulation, ther...

SSRN Electronic Journal, 2021

For components built by powder bed fusion with electron beam (PBF-EB), the resulting microstructu... more For components built by powder bed fusion with electron beam (PBF-EB), the resulting microstructure arising from non-equilibrium solidification–microsegregation and the formation of interdendritic phases significantly affects the mechanical properties and hot cracking resistance. Notably, the powder characteristics influence heat absorption and conduction, thereby altering the molten pool behavior and solidification parameters. However, the effect of powder feedstock on non-equilibrium solidification during PBF has not been widely investigated. In this study, a CoCrMo alloy was built using powders prepared by gas-atomization (GA) and plasma rotating electrode process (PREP). Under the given operating conditions, the samples built with the two powders were experimentally characterized and their compression properties were compared. By performing multi-scale numerical simulations, powder melting and solidification were visualized and analyzed to elucidate the mechanism through which the powder characteristics influence the non-equilibrium solidification behavior during PBF-EB. The study revealed that upon appropriated size control, compared to the GA powder, the PREP powder had a smaller specific surface area and higher sphericity; thus, the generated powder layer exhibited higher heat absorption and dissipation rates. Therefore, a high solidification rate is facilitated, thereby suppressing microsegregation. The findings contribute to PBF knowledge related to feedstock, thus proving to be an essential reference for selecting and optimizing metallic powders applicable to additive manufacturing.

Corrosion Science, 2021

Abstract Co-based alloys are currently being used in a wide range of high temperature application... more Abstract Co-based alloys are currently being used in a wide range of high temperature applications owing to their high resistance to oxidation and corrosion. However, their oxidation-induced degradation could still occur during the long-term exposure to high temperature. Thus, the continuous development of oxidation-resistant Co-based alloys is of crucial importance. In this research, the influence of Si addition on the oxidation behavior of Co–Cr–Mo–xSi alloys under the isothermal oxidation treatment at 700 °C in air was investigated. The Si concentration (x) was varied from 0.1 to 5.0 wt.%. Surface morphologies and chemical compositions of the oxide films formed were analyzed by using SEM-EDS and XPS. The chemical compositions obtained from the surface analysis revealed that Si has played a role in the stabilization of Cr oxides on the surface of Co–Cr–Mo–xSi alloys. With increasing Si concentration, Co-oxide formation on the alloy surface was suppressed by the presence of Cr-oxide due to the selective oxidation of Cr atoms. Furthermore, SiO2 was both found along the grain boundaries and interfaces between the outmost oxide layer and matrix. It was also found that the oxide thickness was reduced with increasing Si concentration. This was a result of the formation of stable Cr-oxide and SiO2 sub-layer that became a barrier inhibiting the inward and outward diffusion of O and Cr.

Materials Science and Engineering: A, 2019

Abstract This paper presents microstructures of Inconel 718 fabricated by electron beam melting (... more Abstract This paper presents microstructures of Inconel 718 fabricated by electron beam melting (EBM), a powder bed fusion (PBF), additive manufacturing (AM) process, under various conditions with an extended range of electron beam power (P) and scanning speed (V). An adaptive offset method (AOM) was used to optimize the beam scanning line offset adaptively to fabricate blocks without macroscopic defects. In the AOM, the line offset is changed depending on the geometry of melt-pools of adjacent scanning lines. The AOM is valid as long as melt pool depth is larger than layer thickness, and it greatly broadens the process window for building dense and even parts. The broadened process window extends the range of the solidification condition and the variation of the resultant microstructures. Fully columnar grains and mixtures of columnar and equiaxed grains were formed. Mixtures of columnar grains and equiaxed grains were formed under two different types of extreme conditions. The first type is with low line energy/small line offset, which causes equiaxed grains associated with a small number of defects. The second type is with high line energy/large line offset, which causes equiaxed grains via the columnar-to-equiaxed transition (CET). The fully columnar grains are preferably oriented to direction in the build direction. The blocks with the different microstructure exhibited similar strengths but different elongations. A processing map was established by plotting the points indicating the build condition and resultant microstructure in the P–V space as a guide for controlling microstructure.

Materials Science and Engineering: A, 2019

In the present study, low-cost high-entropy Al (5,10) Cr 12 Fe 35 Mn (28,23) Ni 20 alloys contain... more In the present study, low-cost high-entropy Al (5,10) Cr 12 Fe 35 Mn (28,23) Ni 20 alloys containing 5 and 10 at. % Al were designed based on the thermodynamic principles. The chemical compositions of the studied high entropy alloys (HEAs) were selected to obtain face centered cubic solid solutions with good cold deformability. The alloys were produced using arc melting and were subsequently subjected to cold rolling to reduce their thickness up to 90%, to study their structure stability and strengthen them. Elemental mapping of the as-cast and deformed structures was conducted using electron microprobe analysis. Subsequently, the mechanical properties of the as-cast and heavily deformed alloys were determined using micro hardness measurements and tensile and compression tests. The microstructure evolution was studied using X-ray diffraction and electron backscatter diffraction. It was observed that the thermodynamic parameters of HEAs could be used to enhance their mechanical properties, and the experimental results were in agreement with those observations. After substituting Mn with Al in Al 5 Cr 12 Fe 35 Mn 28 Ni 20 , the yield stress of Al 10 Cr 12 Fe 35 Mn 23 Ni 20 increased by ~14%. Moreover, the alloy maintained good cold workability and its tensile ductility exceeded 40%. Cold rolling the alloy to 90% increased its yield stress more than four times. Despite the heavy deformation caused by cold rolling (2.3 true strain), deformation twinning was induced in these alloys.

Materialia, 2019

The microstructural evolution of commercially pure titanium (CP-Ti) was investigated during addit... more The microstructural evolution of commercially pure titanium (CP-Ti) was investigated during additive manufacturing (AM) with electron beam melting (EBM). Abnormal grain growth (AGG) was observed throughout all but the top of the specimen, with a mixture of fine and very coarse (hexagonal close-packed) grains, resulting in relatively large scattering in the Vickers hardness at a given build height. AGG proceeded by the growth of some grains to a few hundred micrometers in size at the expense of the surrounding substructured fine grains, which were created during EBM but prior to the occurrence of AGG. A small amount of-phase particles were also produced during rapid cooling after solidification, probably contributing the occurrence of AGG by pinning the grain boundaries. The observations highlight the evolution of a unique, process-specific microstructure during EBM fabrication. Improvement of the scan strategy as well as modification of the chemical composition are needed to obtain a homogeneous microstructure.

The Proceedings of Ibaraki District Conference, 2016

JOURNAL OF THE JAPAN WELDING SOCIETY, 2017

Materials Letters, 2017

CoCrFeNiTi-based high-entropy alloy with superior tensile strength and corrosion resistance achie... more CoCrFeNiTi-based high-entropy alloy with superior tensile strength and corrosion resistance achieved by a combination of additive manufacturing using selective electron beam melting and solution treatment,

To obtain an intrinsic stress-strain response with no effects on stress due to the friction betwe... more To obtain an intrinsic stress-strain response with no effects on stress due to the friction between the sample and the anvils or on the temperature increase due to adiabatic heating, a method of compensating for a stress-strain response was proposed. Using the stress-strain response that was compensated for, a processing map, consisting of both power dissipation efficiency and flow instability map, was constructed. On the basis of our observations using EBSD, it was found that the processing map after the compensation can be effectively used to well predict the optimum hot forging conditions.