Systematic study of (TiZr)xNby(TaMo)z medium entropy alloys for biomedical implants

In this study, four (TiZr)xNby(TaMo)z medium entropy alloys (MEAs) have been investigated as biomedical implants systematically, including their phase structure, microstructure, mechanical properties, corrosion resistance, in vitro biocompatibility and in vivo implantation. With the addition of Ta and Mo, the yield strength of the alloys increased from 678 ± 17 MPa to 821 ± 14 MPa, and their elastic modulus rose from 52 ± 2 GPa to 59 ± 2 GPa. The strength-elastic modulus performance of the alloys was superior to that of most β-titanium alloys. The pseudo-binary solid solution strengthening theory exhibited that the strength enhancement of the alloys was attributed to the high modulus mismatch between Mo, Ta and other elements. And the bonding force of transition group elements based on Coulomb's law and d-electron alloy theory could explained the change in elastic modulus precisely. The alloys with corrosion current densities ranging from 3.277 × 10−7 A/cm2 to 5.105 × 10−7 A/cm2 performed better corrosion resistance than pure Ti, and their oxide films were composed of oxides of each constituent element. In addition, the alloys exhibited comparable in vitro biocompatibility to pure Ti and Zr. Finally, the alloys with lowest elastic modulus were implanted in the mid-femur of Sprague Dawley (SD) rats for 12 weeks, no toxic tissue response was observed and the new bone formed around them. In summary (TiZr)xNby(TaMo)z alloys with low elastic modulus and good strength might be used as materials for bone nails, intramedullary nails and bone plates in the future.