Design and theoretical assessment of Ti-Mo-Nb-Sn alloys for biomedical applications: a review

Biomaterials include a wide range of materials, including metals, ceramics, polymers, and composites. Of these, metallic materials are the most widely used, with titanium and its alloys being chosen for the medical field, particularly in orthopedics and dentistry, owing to their favorable weight-to-strength ratio, high mechanical strength, corrosion resistance, and biocompatibility. However, titanium in its pure form does not always provide an optimal compromise between strength, elastic modulus, and microstructural stability, so alloying with biocompatible elements is used to fine-tune its mechanical and biological properties. First- and second-generation titanium implant alloys nonetheless present recognized limitations, including an elastic modulus considerably higher than that of cortical bone, which produces stress shielding, and the potential toxicity of the Al and V alloying elements used in Ti-6Al-4V. These shortcomings have driven sustained interest in metastable β-type titanium alloys stabilized by non-toxic elements such as Mo, Nb, Ta, Zr, and Sn. This review reorganizes current knowledge of the Ti-Mo-Nb-Sn system around the requirements such an alloy must satisfy, the individual and combined roles of Mo, Nb, and Sn as β-stabilizers, the compositions reported in the literature, the structure-to-property relationships that govern their mechanical behavior, and the methods used to assess their corrosion resistance and biocompatibility, concluding with the principal research gaps and future perspectives for this alloy family. The reviewed studies reported elastic moduli of about 49–65 GPa for single-β compositions, with yield strengths comfortably surpassing 200 MPa, with values of 400–975 MPa. Data regarding the biological profile exists for only one composition, and more information is inferred from related ternary alloys. Windows of 4–6 Mo, 11–20 Nb, and 6–9 Sn wt.% looking promising, but fatigue, wear resistance and long-term biological data are missing.