Shape-Memory alloys (SMAs) are materials that recover their original shape when subjected to stress or thermal cycling. Twinning is an essential microstructural process in SMAs. In Ni-Ti alloys, it is well-known that there is a twin formation hierarchy whereby some twin systems are observed more frequently than others. In this study, we used Molecular Dynamics (MD) simulations to assess the formation hierarchy by looking at the twins' surface energy and mobility. Therefore, we performed atomistic simulations of Compound, Type I and Type II twin boundaries and studied their interface structure with the topological model. The atomistic structure of the Type II twins is still debated due to their irrational twin plane orientation at the continuum scale. Our simulations show a ledged/terraced geometry consistent with previous experimental analyses.

Moreover, we assessed the disconnections gliding mechanism by modelling the atomic system with different orientations of the twin plane. As a result, our simulations show that the twin-interface disconnections gliding mechanism does not change with varying orientations of the twin plane, showing dislocation gliding on the terrace planes. Additionally, our MD simulations reveal the propagation mechanism of the different twin structures in Ni-Ti at T=0K. Interestingly, we found that irrational twin planes (i.e. Type II twins), the most observed ones in experiments, are not the lowest surface energy twinning systems. Instead, they require almost zero stress to glide at T=0K. On the other hand, other twinning systems (Compounds and Type I twins), which are less frequently observed, are lower energy but can require thermal activation. Our work thus suggests a novel selection mechanism for twin systems in SMAs, whereby interface mobility plays a key role.