Nickel-titanium B19’ martensite is a strongly plastically anisotropic material with only one available slip system, which is the [100](001) M slip. Despite this, B19’ martensite polycrystals can be homogeneously plastically formed, reaching up to very high plastic strains. The absence of other slip systems is compensated by plastic twinning, in particular by the frequently appearing irreversible (20-1) M twins. However, these twins act on the same (010) M lattice plane as the plastic slip, and thus, do not seem to be a very suitable complement to the slip in terms of the Von Mises criterion. In fact, exactly the same strains as by the (20-1) M twins can be achieved also by the [100](001) M slip itself, and thus, a question arises, whether they can be understood as plastic twins in the conventional sense.

Interfacial stress fields play a critical role governing the hysteresis and functional fatigue of shape memory alloys. These stress fields manifest at austenite-martensite interfaces (i.e., habit planes) as a consequence of geometric incompatibility between the austenite and martensite phases. As the material approaches transformation, these interfacial stress fields act as an energy barrier, requiring extra energy to be driven into the system to overcome it, resulting in a hysteresis. In addition, increasing the energy in the system also increases dislocation generation, resulting in functional fatigue. In this research, we employ dark-field X-ray microscopy (DFXM), a high-resolution diffraction microstructure imaging technique, to characterize austenite-martensite interfaces and interfacial stress fields during mechanical cycling in a CuAlNi shape memory alloy. The results show, in 3D, the emergence and evolution of individual austenite-martensite interfaces and spatially mapped orientation and elastic strain, including the interfacial elastic strain fields at austenite-martensite interfaces. These findings will contribute to a better understanding of the origins of hysteresis and functional fatigue by investigating interfacial stress fields and dislocation generation at phase interfaces and their effects on macroscopic behavior.

The crystallographic theory of martensite (CTM) forms the foundation of our understanding of stress-induced reversible martensitic phase transformation cycling, relying on assumptions that exclude precipitates, grain boundaries, plasticity, and strained lattices. An ongoing challenge persists in adapting our understanding of CTM-based micromechanical theory (e.g., habit plane variant, or HPV, prediction and the origins of hysteresis and, subsequently, functional fatigue) to real, engineering-grade SMAs. Due to the complexity, elucidating the micromechanical phenomena requires novel high-resolution 3D in-situ characterization techniques.

NiTi shape memory alloys (SMA) are attractive functional materials that are widely used in different fields of engineering and medicine for production of various shape-memory devices. The severe plastic deformation (SPD) is one of the most effective methods for the formation of an ultrafine-grained structure (UFG) in NiTi SMA, which is characterized by the best combination of functional properties. The development of SPD methods continues in the direction of the investigation of new approaches allowing to produce bulk samples with a nanocrystalline structure. Severe torsion deformation (STD) is a promising SPD method that allows for high plastic strain accumulation without significant dimensional changes. It was established that application of STD provides simultaneous improvement of strength and ductility of studied alloys. However, STD has never been applied to NiTi SMA. Therefore, in this study, STD was applied to NiTi SMA at various deformation temperatures from 350 to 600°C in order to maximize the number of turns and accumulate significant strain. Deformation in this temperature range leads to the formation of a dynamically polygonised dislocation substructure, that must provide conduction of the highest number of turns and facilitate the formation of an UFG structure. The goal of this work is to evaluate the possibility of applying the torsional deformation to bulk NiTi samples at low temperatures (in the range of dynamic polygonization) to accumulate high strains and refine the structure.