Mapping thermal phase transformations in NiTi using immersion ultrasonic testing

Nickel-titanium (NiTi) shape memory alloys display reversible, thermoelastic martensitic transformations in response to applied stress or cooling. Methods such as digital image correlation (DIC) have been used to map the transformations under loading, providing spatially resolved information about strain accumulation. However, thermal transformations are monitored using differential scanning calorimetry (DSC), which probes only a small volume and cannot investigate spatial evolution. This study proposes a novel approach using ultrasonic testing to monitor the R-phase transformation in situ in Ni-rich NiTi through measurements of longitudinal wave speed and attenuation in a temperature-controlled immersion water bath. By mapping the spatial evolution of acoustic properties during transformation, this technique aims to track transformation fronts and identify local variations in behavior on the bulk scale. This methodology could extend beyond shape memory alloys to other materials systems where understanding thermal phase transformations is crucial for performance optimization, or where the temperature dependence of acoustic properties can reveal information about microstructure.