Full-field investigation of in-phase and out-of-phase thermomechanical loading paths in high-temperature shape memory alloys

This study investigated the evolution of strain and temperature fields of a NiTiHf High-Temperature Shape Memory Alloy (HTSMA) undergoing complex thermomechanical loads involving simultaneous stress and temperature changes. In situ Digital Image Correlation (DIC) was used to track strain due to combined phase transformation and plastic yielding. Two types of loadings were performed: out-of-phase (OP) involving a simultaneous increase of stress and decrease of temperature and in-phase (IP) involving a simultaneous increase of stress and temperature. Five cycles were performed for each path. Due to a temperature gradient in the samples during the experiments, transformation nucleated in regions with the adequate temperature at a given stress level. Results showed a smooth transformation behavior for out-of-phase loadings along with a concentration of residual strain where transformation fronts meet one another. Due to the simultaneous high stress and high temperature for in-phase paths, creep was identified as a significant deformation mechanism for those paths. Furthermore, DIC allowed us to identify twinning/detwinning regions within the samples. Partial phase diagrams of the alloy were generated using the test results. Those experiments demonstrated the feasibility of characterizing HTSMAs using thermo-mechanical cycling to gain insights into the couplings between thermally- and mechanically-induced solid-state phase transformations for combined loading paths. • Proportional/non-proportional loading paths are performed in a NiTiHf alloy. • Coupling of Digital Image Correlation and Thermal Imaging to show strain localization. • Thermal gradient drives the propagation of martensitic transformation. • New material response is unveiled for initially self-accommodated martensite. • Martensite stabilization and creep dislocations shift transformation boundaries.