Ultrasonic study of the two-step martensitic phase transformation inNi2MnGa

The temperature dependence of the ultrasonic attenuation and velocity was measured in a ${\mathrm{Ni}}_2\mathrm{MnGa}$ single crystal in the temperature range from above a weakly first-order intermediate phase transition at $T_I\sim 265\mathrm{K}$ down to the martensitic phase-transformation temperature, $T_M\sim 220\mathrm{K}.$ The martensitic phase transformation occurs as a two-step process. The behavior of the velocity and attenuation in this premartensitic phase, $T_I>T>\mathrm{}{T}_M,$ 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 ${\mathrm{Ni}}_2\mathrm{MnGa}$ and martensitic phase transformations in other metals.