Insight into how Sn content and annealing temperature dictate microstructural characteristics to regulate the superelasticity in Ti-Zr-Nb-Sn alloys

Abstract

Toxic element-free β-type Ti-Zr-Nb-Sn shape memory alloys offer exceptional potential for advanced biomedical applications, yet achieving optimal superelasticity requires precise control of alloy composition and microstructure, which is particularly sensitive to the interplay between Sn content and annealing conditions. This study systematically investigates the effects of Sn content (3.5–5 at%) and annealing temperature (850–1100 °C) on the microstructure, texture evolution, and superelastic behavior of cost-effective Ti-20Zr-9Nb-xSn alloys. A strong {001}_β<110>_β recrystallization texture, vital for maximizing the transformation strain of β → α″, is found to develop under a precise synergy of Sn content and annealing treatment. We found that its formation is governed by two factors: (i) a low valence electron-to-atom (e/a) ratio (4.0–4.15), which ensures low β-phase stability and promotes unconventional deformation textures, and (ii) the achievement of a critical, composition-dependent β grain size during annealing. Excessive Sn promotes Zr₅Sn₃-type second phase formation, which retards recrystallization via Zener pinning, thereby weakening {001}_β<110>_β texture development and necessitating higher annealing temperatures. The Ti-20Zr-9Nb-5Sn alloy annealed at 950 °C exhibited a maximum recovery strain of 4.8 %, demonstrating that tailored thermomechanical processing can simultaneously optimize microstructure and transformation conditions for optimizing superelasticity. Above all, this study, for the first time, proposes a new superelastic region in the conventional $\overline{B_o}-\overline{M_d}$ diagram for predicting novel superelastic β Ti-Zr-based alloys. The presented linking of Sn content and annealing temperature to functional performance establishes fundamental guidelines for optimizing alloy composition and thermomechanical processing, providing a foundation for developing high-performance Ti-Zr-Nb-Sn superelastic alloys for potential biomedical applications.