Optimizing the cell compatibility and mechanical properties in TiZrNbTa medium-entropy alloy/ β-Ti composites through phase transformation

Medium-entropy alloys (MEAs) typically exhibit outstanding mechanical properties, but their high Young's modulus results in restricted clinical applications. Mismatched Young's modulus between implant materials and human bones can lead to "stress shielding" effects, leading to implant failure. In contrast, a- Ti alloys demonstrate a lower Young's modulus compared to MEAs, albeit with lower strength. In the present study, based on the bimodal grain size distribution (BGSD) strategy, a series of high-performance TiZrNbTa/Ti composites are obtained by combining TiZrNbTa MEA powders with nano-scale grain sizes and commercially pure Ti (CP-Ti) powders with micro-scale grain sizes. Concurrently, Zr, Nb, and Ta that are a- Ti stabilizer elements diffuse into Ti, inducing an isomorphous transformation in Ti from the high Young's modulus alpha-Ti phase to the low Young's modulus a-Ti phase at room temperature, optimizing the mechanical biocompatibility. The TiZrNbTa/ a-Ti composite demonstrates a yield strength of 1490 +/- 83 MPa, ductility of 20.7 % +/- 2.9 %, and Young's modulus of 87.6 +/- 1.6 GPa. Notably, the yield strength of the TiZrNbTa/ a-Ti composite surpasses that of sintered CP-Ti by 2.6-fold, and its ductility outperforms TiZrNbTa MEA by 2.3-fold. The Young's modulus of the TiZrNbTa/ a-Ti composite is reduced by 28 % and 36 % compared to sintered CP-Ti and TiZrNbTa MEA, respectively. Additionally, it demonstrates superior biocompatibility compared to CP-Ti plate, sintered CP-Ti, and TiZrNbTa MEA. With a good combination of mechanical properties and biocompatibility, the TiZrNbTa/ a-Ti composite exhibits significant potential for clinical applications as metallic biomaterials.