Ultrasonic investigation of multistage martensitic transformations in aged Ni-rich Ni–Ti shape memory alloys

In this work we study the influence of thermal treatment and thermal cycling on the temperature dependence of the elastic constants of polycrystalline Ni-rich Ni–Ti showing “unusual” multistage martensitic transformations. With this aim, the ultrasonic pulse echo technique has been applied and the temperature dependence of the longitudinal and shear elastic constant has been measured as a function of temperature after successive heat treatments of the sample. The ultrasonic measurements were complemented by differential scanning calorimetry (DSC). The multistage martensitic transformations could be observed for both DSC and ultrasonic pulse echo experiments.