A novel equiatomic TiNbTaZrMo high-entropy alloy (HEA) was developed as a new metallic biomaterial. The constituent elements of the HEA were biocomparable, and the HEA was designed based on parameters such as the mixing enthalpy (ΔHmix), the omega parameter (Ω), the delta parameter (δ), and the valence electron concentration (VEC) theory. The bcc solid solution phases with the different lattice constants were obtained in as-cast and annealed states. The HEA showed considerable strength with deformability and superior biocompatibility comparable to pure Ti. This study demonstrated the possibility of using HEAs as a new class of 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.