Influence of Sn and Zr ternary alloying on β-phase stability, microstructural evolution and mechanical performance of solution-treated Ti-34Nb alloys for low-modulus biomedical implant applications

Metastable β-type titanium alloys are attractive candidates for load-bearing biomedical implants owing to their comparatively low elastic modulus and improved mechanical compatibility with human bone. In this study, the individual effects of 7 wt.% Sn and 7 wt.% Zr additions on the phase stability, microstructure, and mechanical performance of solution-treated and water-quenched (ST + WQ) Ti-34Nb alloys were systematically compared under identical processing conditions. The β-transus temperature of Ti-34Nb was determined by DSC to be 560 °C, which is consistent with the step-quench XRD interval of 550-600 °C and the equilibrium phase diagram, and decreased slightly to ~530 °C and ~520 °C, respectively, with the Sn and Zr additions. The XRD patterns showed that all the alloys exhibited the β phase with weak reflections present as a small fraction of athermal ω phase in the binary alloy, and no ω or α″ reflections were observed for the Sn- and Zr-containing alloys. Optical microscopy revealed progressive grain refinement from 450 ± 20 µm (Ti-34Nb) to 340 ± 30 µm (Ti-34Nb-7Sn) and 310 ± 30 µm (Ti-34Nb-7Zr). The Ti-34Nb alloy exhibited the highest hardness (202.58 ± 4.24 HV) and elastic modulus (48.76 ± 1.24 GPa), whereas Ti-34Nb-7Sn showed the lowest elastic modulus (42.51 ± 2.03 GPa) and the highest elongation (47.73 ± 2.09%), and Ti-34Nb-7Zr exhibited the highest yield and ultimate tensile strengths (754.16 ± 5.02 and 975.66 ± 6.05 MPa, respectively). This contrasting trend suggests that there are two different microstructural mechanisms which are responsible for localised hardness/modulus and bulk tensile strength. All alloys had ductile microvoid coalescence confirmed by fractographic analysis. The results illustrate that adding Sn and Zr can provide unique and tunable properties of strength, ductility, and stiffness in β-Ti alloys, and that the elastic modulus of Ti-34Nb-7Sn is about 60% less than commonly used biomedical titanium alloys, which supports the rational, composition-specific design of low-modulus β-Ti alloys for next-generation orthopaedic implants.