Thermodynamics of martensitic transformations affected by hydrostatic pressure

Abstract

The martensitic β₁→ β′₁ transformation in Ni₂MnGa Heusler alloy and B2 → R transformations in two Ti‒Ni‒Fe alloys have been studied and the latent heats of these transformations measured. The shifts of martensitic transformation temperatures under the action of applied hydrostatic pressure have been determined. The experimental data are analysed on the basis of Landau theory. To achieve this, a proper ferroelastic phase transition with two-component and three-component order parameters is considered. Mathematical expressions relating the pressure shifts of lability temperatures to the order parameters, softening elastic moduli and phases volumes are obtained. The volume changes accompanying the martensitic transformations in Ni₂MnGa, Ti_0.50Ni_0.47Fe_0.03 and Au_0.25Cd_0.475 are evaluated in three different ways.