Suppression of acoustic emission during superelastic tensile cycling of polycrystalline Ni$_{50.4}$Ti$_{49.6}$

TL;DR

This study examines acoustic emission during superelastic tensile cycling of NiTi alloy, revealing suppression of AE during reverse transformation and after initial cycles, linked to microstructural changes and elastic properties.

Contribution

It provides new insights into AE behavior during martensitic transitions in NiTi, highlighting how microstructural evolution affects acoustic signals during cyclic loading.

Findings

AE activity is high during initial forward martensitic transition.

AE is suppressed during reverse transition and subsequent cycles.

AE activity weakens after initial loading due to microstructural stabilization.

Abstract

We investigate acoustic emission (AE) that arises during the martensitic transition in a polycrystalline specimen of the prototypical superelastic/elastocaloric alloy Ni$_{50.4}$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 (IR) 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 much less 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 mechanically cycling due to increased retained martensite.