Design of metastable β-type Ti–Nb–Mo alloys with low elastic modulus for biomedical applications: Evaluation of phase stability, mechanical properties, and corrosion resistance

The development of metastable β-type Ti alloys containing non-toxic elements and exhibiting low elastic modulus is desirable for biomedical implant applications. The design of Ti–11Nb–9.96Mo and Ti–14.8Nb–8.72Mo alloys in the current study utilised a mixed strategy in terms of electronic parameters (Bo–Md), electron-to-atom ratio (e/a), and molybdenum equivalence (MoE) to position the alloys in the marginal β-region with lower elastic modulus. Both alloys had a β + α″ microstructure, and Rietveld refinement of X-ray diffraction patterns showed a β-dominant structure with a higher α″ fraction in Ti–11Nb–9.96Mo than in Ti–14.8Nb–8.72Mo, demonstrating marginal β-phase stability. Optical microscopy revealed finer grains in Ti–11Nb–9.96Mo and coarser grains in Ti–14.8Nb–8.72Mo, whereas scanning electron microscopy revealed fine intragranular acicular structures, providing morphological evidence of α″ martensite in both alloys. Ti–11Nb–9.96Mo possessed greater yield strength (625.59 ± 8.56 MPa) and ultimate tensile strength (709.6 ± 11.15) MPa, whereas Ti–14.8Nb–8.72Mo was more ductile with a lower elastic modulus (57.72 ± 1.38 GPa). Electrochemical testing in phosphate-buffered saline indicated that both alloys had better corrosion resistance than commercially pure Ti and Ti–6Al–4 V, and Ti–14.8Nb–8.72Mo had the lowest corrosion current density of (8.21 ± 0.07) × 10 −8 A/cm 2 and the noblest corrosion potential (−0.1748 ± 0.0170 V). Overall, designing Ti–Nb–Mo alloys in the marginal β-stability regime enabled reduced elastic modulus, while the alloys exhibited favourable strength and corrosion resistance, supporting their potential for biomedical implant applications.