High-temperature shape memory loss in nitinol: a first principles study

We have performed first-principles calculations to investigate the possibility of shape memory loss in a member of the binary smart alloy family – NiTi. A detailed analysis of the transition kinetics and dynamical pathway reveals the possibility of the B19′ phase of NiTi losing its shape memory when subjected to high stress conditions and is heated above a critical temperature, Tc. The B19′ phase is predicted to transform to P-NiTi, which is also predicted to be dynamically stable and temperature-quench recoverable. It is found that the B2(B33) → B19′ transition is dominated by the β shearing mode with pronounced distortion in the (001) planes and significant volume reduction. Furthermore, the B19′ → P transition is dominated by the γ shearing mode with pronounced distortion in the (010) planes and slight volume expansion. The cumulative effect of both processes activates the lowering and eventual breaking of symmetry in the precursor phases and drives the permanent deformation and shape memory loss. We further show that the P-NiTi structure is stabilized (over B19′ structure) by kinetics. The findings of this study will stimulate further studies on how to retain and improve the shape memory feature in NiTi and other binary smart alloys to prevent property failure when used in the fabrication of devices operated in the high temperature and pressure regime.