Elastic strain and strength-elongation performance of medium-entropy Zr-Nb-Ti-O alloys for bone implants

Beta-type Zr-Nb-Ti (ZNT) medium-entropy alloys (MEAs) are receiving increasing research interest as orthopedic implants due to their appropriate mechanical properties, corrosion resistance, and biocompatibility. However, improvements in their elastic admissible strain, strength, and ductility are still required to ensure their high performance in clinical applications. In this study, a series of (ZrNbTi)_100-xO_x (x = 0, 0.5, 1.0, and 1.5; denoted ZNTO₀, ZNTO_0.5, ZNTO_1.0, and ZNTO_1.5) MEAs were fabricated by arc melting followed by cold-rolling and annealing. Their microstructures, mechanical properties, wear and corrosion resistance, and biocompatibility were systematically studied. The addition of oxygen could simultaneously enhance strength and ductility owing to interstitial solid-solution strengthening and strain-hardening. ZNTO_0.5, ZNTO_1.0, and ZNTO_1.5 showed significantly improved elastic admissible strain and strength-elongation product compared to ZNTO₀; in particular, ZNTO_1.5 exhibited the best combination of mechanical properties with an admissible strain of ∼1.5 %, an ultimate strength of ∼1150 MPa, and an elongation of ∼22 %. The wear and corrosion resistance of the ZNTO_x MEAs increased with increasing oxygen content. The ZNTO_x MEAs showed better corrosion resistance than those of Ti-6Al-4V and Co-Cr-Mo alloys due to formation of surface passivation film composed of ZrO₂, Nb₂O₅, and TiO₂ oxides. The ZNTO_x MEAs also showed cell viability of >97 % toward MG-63 cells. Overall, the ZNTO_1.5 MEA has significant potential as an orthopedic implant material due to its comprehensive mechanical properties, high wear and corrosion resistance, and adequate biocompatibility. STATEMENT OF SIGNIFICANCE: This work reports on ZNTO_x (x = 0, 0.5, 1.0, and 1.5) medium-entropy alloys (MEAs) with a comprehensive combination of biomechanical, corrosion, and biocompatibility properties. The addition of O to ZNT MEAs can significantly improve their elastic admissible strain, strength-elongation product, and wear and corrosion resistance. The ZNTO_x MEAs showed better corrosion resistance in Hanks' solution than Ti-6Al-4V and Co-Cr-Mo alloys and cell viability of >97 % toward MG-63 cells. The results demonstrate that the ZNTO_1.5 MEA has significant potential as an orthopedic implant material due to its best combination of elastic admissible strain and strength-elongation product, effective wear and corrosion resistance, and adequate biocompatibility.