Takeshi Nagase

The design and development of TiZrHfAl medium entropy alloy (MEA), and the TiZrHfAlNb0.2 and TiZrHfAlV0.2 high entropy alloys (HEAs) is described. The combination of 4th subgroup elements (Ti, Zr, and Hf) with Al is discussed based on the periodic table and taxonomy of HEAs. The alloys were designed using alloy parameters for HEAs, predicted ground state diagrams from the Materials Project, and the calculation of phase diagrams (CALPHAD). Rapid solidification was effective to suppress the formation of intermetallic compounds, resulting in BCC/B2 phase formation. Significant differences in the constituent phases and Vickers hardness between ingots and melt-spun ribbons were found among the TiZrHfAl MEA, TiZrHfAlNb0.2, and TiZrHfAlV0.2 HEAs.

The ingots of CuxZnMnNi (x = 1,2) medium-entropy (ME) brasses were fabricated using metallic mold-casting process without a vacuum chamber. The molten metal was obtained by high-frequency melting of the mixture of pure Cu, pure Ni, and pre-alloy ingots of Mn-Cu and Zn-Ni using silica-based crucible in Ar flow. The metallic mold-casting ingots were obtained using centrifugal casting in air atmosphere. The composite of body-centered-cubic (BCC) and face-centered-cubic (FCC) phases were obtained in the ingots of equiatomic CuZnMnNi ME brass, while a near-single FCC phase was obtained in the ingots of non-equiatomic Cu2ZnMnNi ME brass, where the identification of the constituent phases was mainly performed by XRD analysis. The ingots showed superior deformability and high 0.2% proof stress during compression test conducted at room temperature.

BioHEAs, specifically designed high entropy alloy (HEA) systems for biomedical applications, represent a new era for biometals. However, recent challenges are (1) the poor shape customizability, and (2) the inevitable severe segregation due to the intrinsic fact that HEA is an ultra-multicomponent alloy system. To achieve shape customization and suppression of elemental segregation simultaneously, we used an extremely high cooling rate (~10 K/s) of the selective laser melting (SLM) process. We, for the first time, developed pre-alloyed Ti Nb Ta Zr Mo BioHEA powders and SLM-built parts with low porosity, customizable shape, excellent yield stress, and good biocompatibility. The SLM-built specimens showed drastically suppressed elemental segregation compared to the cast counterpart, representing realization of a super-solid solution. As a result, the 0.2% proof stress reached 1690 ± 78 MPa, which is significantly higher than that of cast Ti Nb Ta Zr Mo (1140 MPa). The SLM-built Ti Nb Ta Zr Mo BioHEA is promising as a next-generation metallic material for biomedical applications. 7 1.4 0.6 0.6 1.4 0.6 1.4 0.6 0.6 1.4 0.6 1.4 0.6 0.6 1.4 0.6

Deformation behavior of equiatomic HfNbTaTiZr high entropy alloy single crystals and polycrystals were investigated. The single crystalline specimens could be obtained from the coarse-grained polycrystals annealed just below the melting point. 1/2<111> screw dislocations played an important role in the deformation behavior, similar to body centered cubic (bcc) metals. Moreover, a yield stress anomaly was found to appear in the alloys solutionized at 1473 K. The yield stress decreased rapidly with increasing temperature up to 673 K, while the stress increased at 873 K. Further increase in temperature resulted in a decrease in yield stress. It is also noted that the microstructure of the alloys depended strongly on annealing temperature. At 773–1073 K, phase separation into two bcc phase took place, while the hexagonal close-packed (hcp) phase was precipitated at 773–973 K. The ω phase which is typical in β-titanium alloys was also observed at 673 K. The strain-rate sensitivity of the deformation behavior and micro-Vickers hardness after the heat treatment suggest that the dynamic precipitation of the hcp phase is responsible for the yield stress anomaly.

Applying empirical alloy parameters (including Mo equivalent), the predicted ground state diagram, and thermodynamic calculations, noble nonequiatomic Ti–Zr–Hf–Nb–Ta–Mo high-entropy alloys for metallic biomaterials (BioHEAs) were designed and newly developed. It is found that the Moeq and valence electron concentration (VEC) parameters are useful for alloy design involving BCC structure formation in bio medium-entropy alloys and BioHEAs. Finally, we find a Ti Zr Hf Nb Ta Mo (at.%) BioHEA that exhibits biocompatibility comparable to that of CP–Ti, higher mechanical strength than CP–Ti, and an appreciable room-temperature tensile ductility. The current findings pave the way for new Ti–Zr–Hf–Nb–Ta–Mo BioHEAs development and are applicable for another BioHEA alloys system. 28.33 28.33 28.33 6.74 6.74 1.55

In this study, we sucsess the fabrication of dense compornent of Ti-20at.%X (X = Cr and Nb) alloys by Selected laser melting (SLM) pwocess, from a mixture of poweder element powders. The volume rasio of pore and non-molten particles is dependent of the enegy density. The difficulty of fabrication of Ti-X alloy comporment is dependent of melting temperature of X element. Thus, Ti-20at.%Cr alloys, which has the lowest melting temperature of X is easier to monufacture of dense comporment. The Ti-20at.%Cr alloys and Ti-20at.%Nb comprise β-Ti single-phase components without any non-molten particles and macroscopic defects. In addtion, the {001}〈100〉 crystallographic texture of these Ti-Cr and Ti-Nb alloys can be controlled effectively by optimizing the SLM parameters. This means that the SLM is key techmelogy of controlling of Young’s modulus and shape at the same time because Young's modulus of be-ta phase in Ti alloys is strongly related to the crystal orientation.

In Al_xCoCrFeNi high entropy alloys (x = 0.3–0.5), the NiAl phase with the B2 structure is precipitated rapidly along the fcc grain boundaries. During recrystallization after conventional cold rolling, the NiAl precipitates effectively suppress the grain growth, which results in the ultrafine-grained microstructure. It should be noted that no severe plastic deformation is necessary to obtain the microstructure. The volume fraction of the NiAl precipitates increases with increasing x. As a result, the average grain size of the fcc matrix (<italic>d</italic>_m) after the recrystallization decreases with increasing x, and therefore, a minimum <italic>d</italic>_m of 0.5 μm can be obtained at x = 0.5. The grain refinement by the NiAl precipitates is consistent with the Zener-Smith model. At x = 0.5, the alloy with <italic>d</italic>_m = 0.5 μm exhibits a yield stress of 1163 MPa and an elongation of 24% at room temperature.

Fast electron irradiation can induce the solid-state amorphization (SSA) of many intermetallic compounds. The occurrence of SSA stimulated by fast electron irradiation was found in the Al_0.5TiZrPdCuNi high-entropy alloy (HEA). The relationship between the occurrence of SSA in intermetallic compounds under fast electron irradiation and the empirical alloy parameters for predicting the solid-solution-formation tendency in HEAs was discussed. The occurrence of SSA in intermetallic compounds was hardly predicted, only by the alloy parameters of <italic>δ</italic> or Δ<italic>H</italic>_mix, which have been widely used for predicting solid-solution formation in HEAs. All intermetallic compounds with Δ<italic>H</italic>_mix ≤ -35 kJ/mol and those with <italic>δ</italic> ≥ 12.5 exhibit the occurrence of SSA. This implies that the intermetallic compounds with a largely negative Δ<italic>H</italic>_mix value and a largely positive <italic>δ</italic> parameter are favorable for the occurrence of SSA.

The Ti–Ag alloy system is an important constituent of dental casting materials and metallic biomaterials with antibacterial functions. The binary Ti–Ag alloy system is characterized by flat liquidus lines with metastable liquid miscibility gaps in the phase diagram. The ternary Ti–Ag-based alloys with liquid phase separation (LPS) were designed based on the mixing enthalpy parameters, thermodynamic calculations using FactSage and Scientific Group Thermodata Europe (SGTE) database, and the predicted ground state diagrams constructed by the Materials Project. The LPS behavior in the ternary Ti–Ag–Nb alloy was investigated using the solidification microstructure analysis in arc-melted ingots and rapidly solidified melt-spun ribbons via trans-scale observations, combined with optical microscopy (OM), scanning electron microscopy (SEM) including electron probe micro analysis (EPMA), transmission electron microscopy (TEM), and scanning transmission electron microscopy (STEM). The solidification microstructures depended on the solidification processing in ternary Ti–Ag–Nb alloys; macroscopic phase-separated structures were observed in the arc-melted ingots, whereas fine Ag globules embedded in the Ti-based matrix were observed in the melt-spun ribbons.

© 2020 Acta Materialia Inc. TiZrHfYLa0.2 high-entropy alloys (HEAs) with dual hexagonal-closed-packed (HCP) structures were designed based on the concept of liquid phase separation (LPS) and segregation for enhancing the immiscibility of the constituent elements. The LPS leads to a particular solidification microstructure on the free surface side and Cu-hearth contacted area in the ingots. The dual HCP structures with equi-axis Ti–Zr–Hf dendrite and Y-La-rich interdendrite were observed at most regions of the arc-melted ingots. The mixing enthalpy among the constituent elements and predicted phase diagrams constructed by the Materials Project were effective for the alloy design of the HEAs with dual HCP structures.

The liquid phase separation (LPS) behavior in Co-Cr-based high-entropy alloys (HEAs) is an important target for the development of Co-Cr-based HEAs for metallic biomaterials (BioHEAs). The solidification microstructure in Ag-Co-Cr-Fe-Mn-Ni-Ag, Co-Cr-Cu-Fe-Mn-Ni-Cu, and Co-Cr-Cu-Fe-Mn-Ni-B HEAs, which were designed as the combination of the equiatomic CoCrFeMnNi with Ag, Cu, and the interstitial element of B, was investigated as the fundamental research of LPS in Co-Cr-based HEAs. Ingots of equiatomic AgCoCrFeMnNi, equiatomic CoCrCuFeMnNi, non-equiatomic CoCrCuxFeMnNi (x = 2, 3), and CoCrCuxFeMnNiB0.2 (x = 1, 2, 3) with a small amount of B were fabricated using the arc-melting process. A macroscopic phase-separated structure was observed in the ingots of the equiatomic AgCoCrFeMnNi and CoCrCuxFeMnNiB0.2 (x = 2, 3) HEAs. The addition of a small amount of B enhanced the LPS tendency in the Co-Cr-Fe-Mn-Ni-Cu HEAs. The LPS behavior was discussed through the heat of mixing and computer coupling of phase diagrams and thermochemistry (CALPHAD).

© 2019 The Authors Novel TiZrHfCr0.2Mo and TiZrHfCo0.07Cr0.07Mo high-entropy alloys for metallic biomaterials (bio-HEAs) were developed based on the combination of Ti–Nb–Ta–Zr–Mo alloy system and Co–Cr–Mo alloy system as commercially-used metallic biomaterials. Ti–Zr-Hf-Cr-Mo and Ti–Zr-Hf-Co-Cr-Mo bio-HEAs were designed using (a) a tree-like diagram for alloy development, (b) empirical alloy parameters for solid-solution-phase formation, and (c) thermodynamic calculations focused on solidification. The newly-developed bio-HEAs overcomes the limitation of classical metallic biomaterials by the improvement of (i) mechanical hardness and (ii) biocompatibility all together. The TiZrHfCr0.2Mo and TiZrHfCo0.07Cr0.07Mo bio-HEAs showed superior biocompatibility comparable to that of commercial-purity Ti. The superior biocompatibility, high mechanical hardness and low liquidus temperature for the material processing in TiZrHfCr0.2Mo and TiZrHfCo0.07Cr0.07Mo bio-HEAs compared with the Ti–Nb–Ta–Zr–Mo bio-HEAs gave the authenticity of the application of bio-HEAs for orthopedic implants with multiple functions.

© 2020 The Japan Institute of Metals and Materials. CoCr and CoCrMo-based alloys are commercially used in the industry especially for high wear resistance and superior chemical and corrosion performance in hostile environments. These alloys were widely recognized as the important metallic biomaterials. Here, the first development of CoCrMoFeMnW and CoCrMoFeMnWAg high-entropy alloys (HEAs) based on CoCrMo metallic biomaterials is reported. Ingots of six-component Co2.6Cr1.2Mo0.2FeMnW0.27 (Co41.5Cr19.1Mo3.2Fe16Mn16W4.3, at%) HEAs with a minor · phase and of seven-component Co4.225Cr1.95Mo0.2FeMnW0.2Ag0.5 (Co46.6Cr21.5Mo2.2Fe11Mn11W2.2Ag5.5, at%) and Co2.6Cr1.2Mo0.1FeMnW0.1Ag0.18 (Co42.1Cr19.4Mo1.6Fe16.2Mn16.2W1.6Ag2.9, at%) HEAs without an · phase were fabricated. The alloy was designed by a taxonomy of HEAs based on the periodic table, a treelike diagram, predicted phase diagrams constructed by Materials Project, and empirical alloy parameters for HEAs. The · phase formation prevented the formation of solid solutions in CoCrMo-based HEAs without a Ni element. The · phase formation in as-cast ingots was discussed based on the composition dependence and valence electron concentration theory.

Nobel non-equiatomic Ti-Nb-Ta-Zr-Mo high-entropy alloys (HEAs) for metallic biomaterials (bio-HEAs) were designed and developed. The pseudo-binary phase diagrams focusing on solidification were constructed by thermodynamic calculations. The shifting the alloy composition of the equiatomic TiNbTaZrMo bio-HEA can realize the drastic improvement of the deformability. Notably, the non-equiatomic Ti, Zr-rich composition stimulated the molecular interaction between biological cells and bio-HEA, indicating the possibility of the proposed non-equiatomic Ti-Nb-Ta-Zr-Mo HEAs as an advanced biomaterial for bone tissue engineering applications. This is the first achievement for the alloy design including the control of alloy composition for the development of new bio-HEAs.

Cu-Zn-Mn-Ni-Sn high-entropy (HE) and Cu-Zn-Mn-Ni medium-entropy (ME) brasses were designed and the ingots were fabricated by conventional casting process via metallic mold casting. The ingots of the equiatomic CuZnMnNi ME brass showed dendrite structures composed of body-centered-cubic (BCC) and face-centered-cubic (FCC) phases. A single FCC phase was obtained in the ingots of non-equiatomic Cu ZnMnNi (x = 2, 3, and 4) ME brasses. The addition of Sn to Cu ZnMnNi (x = 1, and 2) ME brasses significantly affected the solidification microstructure and the constituent phases. The ingots of the CuZnMnNiSn HE brass showed only sharp peaks corresponding to BCC phase in XRD patterns. The Sn addition to Cu ZnMnNi (x = 1, and 2) ME brasses led to superior mechanical strength in the ingots. x x 0.2 x