Unraveling factors affecting the reversibility of martensitic phase transformation in FeNiCoAlTi shape memory alloys: Insights from HR-EBSD and acoustic emission analysis

The reversibility of the martensitic phase transformation is crucial for the functionality of shape memory alloys. The easier the martensite is pinned, the faster occurs the degradation of the shape memory effect during cyclic loading. This study investigates various factors influencing the stabilization of martensite during deformation of single-crystalline states of the FeNiCoAlTi system. In order to investigate early pinning mechanisms in more detail a 〈011〉 single crystal with expected lower reversibility of the martensitic phase transformation and, consequently, lower superelasticity was chosen. Complementary in situ methods, such as digital image correlation and acoustic emission, were used to characterize the evolution of the martensitic phase transformation and the role of dislocations during deformation. Thereby, experimental evidence was elaborated pointing at mechanisms that so far have been underrated in terms of functional degradation. Beside interaction of martensitic variants and dislocation activity, detwinning, associated with high acoustic energy, was identified as an additional significant factor contributing to reduced reversibility and superelasticity. In addition, high-resolution electron backscatter diffraction measurements were applied for the first time to identify residual stresses in the austenitic matrix, which significantly contribute to the reverse transformation. Micro-mechanical experiments using pillar compression tests were carried out to study the influence of residual back stresses of the austenitic matrix on the reversibility of the martensitic phase transformation. A reduced reversibility was found in the case of the absence of back stresses. The findings from this study foster the understanding of pinning mechanisms during loading of the FeNiCoAlTi shape memory alloy eventually enabling targeted optimization for enhanced superelastic material behavior.