Superelasticity in high-temperature Ni–Mn–Ga alloys

Abstract

The stress–strain behavior of two single crystalline Ni–Mn–Ga alloys with martensitic transformation temperatures above the Curie temperature has been studied during uniaxial compression along the [001] and [110] axes. The superelastic effect associated to the reversible stress-induced martensitic transformation has been obtained in these high-temperature shape memory alloys. The experimental values of the critical stress to induce the martensitic transformation, σc, depend linearly on the test temperature. This dependence is described well by the thermodynamics of the stress-induced martensitic transformation through a Clausius–Clapeyron-type equation. However, the experimental values of slopes dσc/dT obtained in the two compression directions can only be interpreted well if it is assumed that different martensitic phases are formed for each compression axis. Good agreement with the thermodynamic model exists if tetragonal martensites with c/a<1 and >1 are induced in the [001] and [110] axes, respectively.