Suppression of acoustic emission during superelastic tensile cycling of polycrystalline Ni50.4Ti49.6

We investigate acoustic emission (AE) that arises during the martensitic transition in a polycrystalline specimen of the prototypical superelastic/elastocaloric alloy ${\mathrm{Ni}}_{50.4}{\mathrm{Ti}}_{49.6}$ (at. %) driven using tensile strain. We use two independent AE sensors in order to locate AE events, and focus on contributions to the AE that arise away from the grips of the mechanical testing machine. Significant AE activity is present during the first mechanical loading primarily due to nucleation and growth of wide L\"uders-like bands during the forward martensitic transition (imaged using visible light and infrared radiation) that lead to persistent changes in intergranular interactions. AE activity is suppressed during the subsequent reverse martensitic transition on unloading, and in successive loading/unloading cycles, for which the L\"uders-like bands narrow and modify intergranular interactions to a much smaller extent. After the first loading, we find that the AE activity associated with the martensitic transition is weak, and we suggest that this is because the elastic anisotropy and strain incompatibility in Ni-Ti are low. We also find that the AE activity becomes weaker on mechanical cycling due to increased retained martensite.