An ab initio study of the structural and mechanical alterations of Ti-Nb alloys

This article describes a systematic theoretical investigation of the role of Nb substitution on the structural and mechanical properties of Ti-Nb alloys. The aim is to understand the origin of the low-rigidity of some of these materials. This quality makes these materials suitable for metallic implants. The mechanical stability conditions in conjunction with the calculated elastic constants of Ti-Nb predict the destabilization of α′ and ω structures, while the β-phase can be stabilized for Nb content above 10 at. %. The evaluated Young's moduli (E) follow the sequence of Eω > Eα′ > Εα″ > Εβ, revealing high Eω and Eα′ values (greater than 120 GPa), while the Eβ value converges to approximately 87 GPa. The averaged E, estimated from a weighted average of Eω, Eα′, Εα″, and Εβ ab initio values, reproduces the experimental Ti-Nb Young's modulus w-shaped curve. Young's modulus surface reveals highly anisotropic E values for all Ti-Nb phases. Eβ exhibits values under 30 GPa along the [100] direction for Nb compositions larger than 12 at. %, suggesting that the orientational growth of a Ti-Nb alloy is important for the design of low-rigidity alloys, especially at small Nb concentrations. These results can be used as a guide for the design of novel low-rigidity alloys for biomedical applications.