Design and fabrication of Ti–Zr-Hf-Cr-Mo and Ti–Zr-Hf-Co-Cr-Mo high-entropy alloys as metallic biomaterials

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.