Negative mixing enthalpy medium-entropy Ti–Zr–Nb–Al alloys with ultrahigh elastic admissible strain, strength-elongation product, and biocompatibility for bone implant applications

Beta-type Ti-Zr-Nb (TZN) medium-entropy alloys (MEAs) are being extensively investigated as orthopedic implants due to their metallic mechanical properties, corrosion resistance, and biocompatibility. However, improvements in elastic admissible strain, strength, and ductility are still needed to ensure their high performance in clinical applications. In this study, we designed a series of negative mixing enthalpy MEAs by adding 0, 3, 5, 7, and 10 at.% aluminum (Al) to equimolar TZN alloy (denoted TZNA_x (x = 0, 3, 5, 7 and 10)). These alloys were fabricated by arc melting followed by cold-rolling and annealing, and their microstructure, mechanical properties, wear and corrosion resistance, and biocompatibility were systematically studied. In general, the TZNA_x MEAs showed significantly improved elastic admissible strain and strength-elongation product compared with those of CP-Ti and Ti64; in particular, the TZNA₅ exhibited the best combination of mechanical properties with an elastic modulus of ∼54.6 GPa, a yield strength of ∼1040 MPa, an ultimate strength of ∼1132 MPa, and an elongation of ∼21 %. The wear resistance of the TZNA_x MEAs increased with increasing Al content. The TZNA_x MEAs showed an extremely low corrosion rate of 124-587 nm /year in Hanks' solution due to the formation of a surface passivation film composed of TiO₂, ZrO₂, Nb₂O₅ and Al₂O₃ oxides. The TZNA_x MEAs also showed a relative cell viability of more than 95 % toward MG-63 cells. Overall, the TZNA₅ MEA has significant potential as an orthopedic implant material due to its exceptional and comprehensive mechanical properties, high wear and corrosion resistance, and adequate biocompatibility. STATEMENT OF SIGNIFICANCE: This work reports on negative mixing enthalpy Ti-Zr-Nb-Al medium-entropy alloys (MEAs) with superior elastic admissible strain, strength-elongation product, and biocompatibility. The TZNA₅ MEA exhibited the best combination of biomechanical properties. The wear resistance of the TZNA_x MEAs increased with increasing Al content. The TZNA_x MEAs showed high corrosion resistance in Hanks' solution due to the formation of a surface passivation film composed of TiO₂, ZrO₂, Nb₂O₅ and Al₂O₃ oxides. The TZNA_x MEAs showed a relative cell viability of more than 95 % toward MG-63 cells. Overall, the TZNA₅ MEA has significant potential as an orthopedic implant material due to its exceptional and comprehensive mechanical properties, high wear and corrosion resistance, and adequate biocompatibility.