A scattering-based approach to in situ ultrasonic characterization of thermal phase transformations in NiTi shape memory alloys

NiTi shape memory alloys (SMAs) undergo solid-state, diffusionless phase transformations between different crystallographic structures when cooled, presenting a unique opportunity to investigate the ultrasonic response of evolving microstructures in situ. Unlike stress-induced transformations, which can be probed in the spatial domain via digital image correlation to examine heterogeneous nucleation of product phases, the spatial nature of phase transformations in response to temperature changes has been relatively unexplored. This study addresses these gaps by employing an ultrasonic immersion system with a temperature-controlled fluid bath, enabling spatially resolved measurements of ultrasonic properties, including scattered grain noise, throughout thermal cycling. The material system considered consists of aged Ni-rich NiTi, where Ni4Ti3 precipitates favor the formation of the intermediate R-phase by suppressing the direct B2 → B19' martensitic transformation. By analyzing scattering, wave speed, and attenuation throughout the B2 → R transition, this study investigates the influence of microstructural evolution on ultrasonic wave propagation in real time. The findings bridge the gap between ultrasonic scattering theory and experimental observation in phase-transforming systems, contributing to a more comprehensive understanding of wave propagation in materials undergoing structural transitions.