Shape memory effect and aging behavior of Bi-added Ti-Cr alloys for biomedical applications

For the further development of biocompatible metastable β (bcc) Ti alloys, the purpose of this study is to evaluate the potential of bismuth (Bi) addition in terms of shape memory properties and phase stability. It was found that the shape memory effect appeared in Ti-5Cr-1.6Bi (mol%) alloy. However, permanent (unrecoverable) deformation due to dislocations or twinning was also introduced simultaneously from the early stage of deformation. Regarding the formation of isothermal ω phase and the resulting hardness change by aging, it was found that the hardness change was large and that the isothermal ω phase formed in Ti-5Cr-1.6Bi alloy, while age hardening was small and no isothermal ω phase formed in Ti-5Cr-6.1Bi alloy. These results indicate the suppression of not only athermal ω but also isothermal ω phase by Bi addition. However, considering the fact that the alloy becomes brittle when Bi addition is over 3 mol%, it can be concluded that 1-3 mol% Bi addition is worth for the improvement of shape memory effect, suppression of ω phase, X-ray imaging, magnetic resonance imaging, and biocompatibility in metastable β Ti alloys.