Investigations of the β-Ti shape memory alloys via neutral element stabilization and their microstructures, phase constituents, and phase transformation

Abstract

Due to the global aging population, shape memory alloys (SMAs) have become an important area of focus in the biomedical and biomaterials fields, particularly for their promising applications in medical devices. While Ni–Ti SMAs have been widely used in biomedical applications due to their excellent shape deformation and recovery strains, concerns about their hypersensitivity remain. As a result, this study shifts its focus from Ni–Ti alloys to biocompatible β-Ti SMAs. Specifically, this work investigates Ti-based SMAs modified with neutral elements, examining 3 alloy systems: (i) Ti–Zr–Hf, (ii) Ti–Zr–Sn, and (iii) Ti–Zr–Hf–Sn. Among the alloys tested, the Ti–38Zr–10Hf alloy stood out as the most promising, exhibiting the lowest phase transformation temperature (T_p) of 802 K, making it an ideal candidate for further Sn addition. In terms of phase stabilization, the β-phase could not be stabilized at room temperature (RT) with just the addition of Zr and Hf (set (i) alloys). However, both the Ti–Zr–Sn (set (ii)) and Ti–Zr–Hf–Sn (set (iii)) alloys successfully stabilized the β-phase at RT. Notably, the addition of neutral elements did not significantly affect the lattice constant of the β-phase, suggesting that these alloys have high potential for lattice deformation strain. This indicates their suitability for SMA applications. Additionally, the micro Vickers hardness of the Sn-added alloys showed a clear minimum near the martensite transformation start temperature (M_s), which is a typical phenomenon in SMAs. The lowest hardness was observed when Sn was added in the range of 5–6 mol%. These findings suggest that β-Ti SMAs with neutral element additions may hold promise for future biomedical applications.