Ultrasonic study of the two-step martensitic phase transformation in Ni2MnGa

The temperature dependence of the ultrasonic attenuation and velocity was measured in a Ni2MnGa single crystal in the temperature range from above a weakly first-order intermediate phase transition at T-I similar to 265 K down to the martensitic phase-transformation temperature, T-M similar to 220 K. The martensitic phase transformation occurs as a two-step process. The behavior of the velocity and attenuation in this premartensitic phase, T-I > T > T-M, is consistent with neutron-scattering data and shows that the premartensitic phase is a more ordered modulated phase than the parent phase. While precursor effects are observed above T-I, the martensitic phase transformation occurs abruptly. These results are compared with neutron-scattering results, transmission electron microscopy, and previous ultrasonic studies in Ni2MnGa and martensitic phase transformations in other metals.