Experimentally reconstructing the interplay of slip and microstructure in polycrystalline shape-memory alloys

• We reconstruct the geometrically necessary martensitic microstructure and plastic slip in shape-memory alloys • Stress-induced martensitic transformation initiates in thin ”shoots” that are transmitted between grains • Slip is strongly localized through ”bridges” passing through clusters of transformation-saturated grains • At the sub-grain scale, ”zig-zagging” slip localization is coupled to the development of Type I twins Shape-memory alloys (SMAs) such as NiTi (nitinol) can recover large strains through a reversible stress- or temperature-induced martensitic transformation, but cyclic transformation degrades reversibility. Recent evidence has linked this functional fatigue to the emission of dislocations from the fine-twinned martensitic microstructure that forms near the phase boundary, but the precise mechanism of slip-microstructure coupling is widely debated. This creates a mesoscale gap in the understanding of SMAs and their fatigue: multiscale simulation is prohibitively expensive, while experimental methodologies that can spatially resolve fine microstructure and individual dislocations (e.g., TEM) cannot capture bulk mechanical behavior. In this work, we develop a new method to reconstruct the geometrically necessary martensitic transformation and plastic slip in polycrystalline SMAs from high-resolution, full-field deformation maps. Stress-induced martensitic transformation initiates in thin ”shoots” passing through well oriented grains and saturates when poorly oriented grains have fully transformed. Localized networks of coupled slip and reorientation form microstructural bridges that propagate transformation through clusters of saturated grains. When loading in tension, this coupling of reorientation and slip underpins the motion of macroscopic Lüders-like localization bands. Contrary to recent theories focusing on dislocation emission from a Type II twin interface, we observe that the most intense slip localization events are coupled to the development of Type I twins.