Medium-entropy Zr–Nb–Ti alloys with low magnetic susceptibility, high yield strength, and low elastic modulus through spinodal decomposition for bone-implant applications

Medium-entropy Zr-Nb-Ti (ZNT) alloys are being extensively investigated as load-bearing implant materials because of their exceptional biocompatibility and corrosion resistance, and low magnetic susceptibility. Nevertheless, enhancing their yield strength while simultaneously decreasing their elastic modulus presents a formidable obstacle, significantly constraining their broader utilization as metallic biomaterials. In this study, three medium-entropy ZNT alloys, i.e., Zr₄₅Nb₄₅Ti₁₀, Zr_42.5Nb_42.5Ti₁₅, and Zr₄₀Nb₄₀Ti₂₀ (denoted ZNT₁₀, ZNT₁₅, and ZNT₂₀, respectively), were designed based on the miscibility gap in the ZNT phase diagram and prepared by annealing of cold-rolled ingots. Their microstructures, mechanical properties, wear resistance, corrosion resistance, magnetic susceptibility, and biocompatibility were systematically studied. Spinodal decomposition occurred in the cold-rolled ZNT₁₀ and ZNTi₁₅ after annealing at 650 °C for 2 h and resulted in nanoscale Zr-rich β₁ and (Nb, Ti)-rich β₂ phases, which significantly improved their yield strength and reduced their elastic modulus. The wear resistance of the alloys decreased with an increase in Ti content. Dense ZrO₂, Nb₂O₅, and TiO₂ oxide layers were formed during the polarization process in Hanks' solution, which enhanced the corrosion resistance of the alloys. These ZNT alloys exhibited significantly lower magnetic susceptibility than medical Ti alloys. The ZNT alloys showed a cell viability of more than 94 % toward MG-63 cells after culturing for 3 d Overall, the spinodal ZNT₁₅ showed the best combination of mechanical properties, wear resistance, corrosion resistance, low magnetic susceptibility, and sufficient biocompatibility among the three alloys. STATEMENT OF SIGNIFICANCE: This work reports on medium-entropy Zr-Nb-Ti (ZNT) alloys with heterostructure. Spinodal decomposition significantly improved their mechanical strength and reduced the elastic modulus of the alloys. The wear resistance of the ZNT alloys decreased with an increase in Ti content. Dense ZrO₂, Nb₂O₅, and TiO₂ oxide layers were formed during the polarization process in Hanks' solution, which enhanced the corrosion resistance of the alloys. The ZNT alloys exhibited significantly lower magnetic susceptibility than medical Ti alloys. The ZNT alloys showed a cell viability of >94 % toward MG-63 cells after culturing for 3 d The results demonstrate that spinodal ZNT alloys have enormous potential as bone-implant materials due to their outstanding overall mechanical properties, high corrosion resistance, wear resistance, low magnetic susceptibility, and sufficient biocompatibility.