Insights on the Possibility of Interstitial Oxygen in NiTi Shape Memory Alloys

It is generally believed that oxygen in NiTi alloys exists only as Ti₄Ni₂O. In this work, the possibility of interstitial oxygen has been investigated by experiment and modelling. The sintered elemental NiTi alloys with different oxygen contents were prepared to investigate the relationship between the Ni equivalent (Ni_eq) and peak transformation temperature (M_p). The results showed at the same Ni_eq, the M_p was considerably higher than the value reported in the literature. The NiTi matrix with different oxygen contents was observed using atomic probes. Oxygen was found to be uniformly distributed throughout the matrix. The number of independent O atoms and Ti–O clusters increased with increasing oxygen content. Density functional theory was used to calculate the solid solution energy and the formation energy of NiTi superlattice with interstitial oxygen. Oxygen could exist stably in the interstices. The formation energy of interstitial oxygen in NiTi[O] was always higher than that of Ti₄Ni₂O, which suggested Ti₄Ni₂O may be the final equilibrium state. However, interstitial oxygen is still an important form existed in the non-equilibrium state, such as powder metallurgical NiTi. The results of this study provide a new basis for predicting the M_p of Ni–Ti–O systems.