Behavior of the elastic properties near an intermediate phase transition in Ni2MnGa.

Abstract

An ultrasonic pulse echo technique was employed to measure the single-crystal elastic constants and their temperature dependence in the vicinity of an intermediate phase transition at 265 K in a ${\mathrm{Ni}}_{2}$MnGa single crystal. The measured elastic constants at 300 K (in units of ${10}^{11}$ dyn/${\mathrm{cm}}^{2}$) are as follows: ${\mathit{C}}_{44}$=10.3, ${\mathit{C}}^{\ensuremath{'}}$=0.445, and ${\mathit{C}}_{\mathit{L}}$=25.0, where ${\mathit{C}}^{\ensuremath{'}}$=1/2(${\mathit{C}}_{11}$-${\mathit{C}}_{12}$) and ${\mathit{C}}_{\mathit{L}}$=1/2(${\mathit{C}}_{11}$+${\mathit{C}}_{12}$+2${\mathit{C}}_{44}$). The present ultrasonic results differ from a earlier ultrasonic study and provide the first direct determination of the elastic constant ${\mathit{C}}^{\ensuremath{'}}$. All three elastic constants soften as the premartensitic transformation temperature is approached, and the ultrasonic attenuation also increases dramatically as the intermediate phase transition is approached. The behavior of the elastic constants and the ultrasonic attenuation is consistent with inelastic neutron-scattering results and confirms the existence of the intermediate phase. The ultrasonic results are compared with the neutron data, with earlier ultrasonic results in ${\mathrm{Ni}}_{2}$MnGa, and with studies of the martensitic phase transition in NiAl alloys. \textcopyright{} 1996 The American Physical Society.