Ni-Mn-Ga high temperature shape memory alloys: Function stability in β and β + γ regions

Record-breaking values of the tensile superelastic strain (about 12%) have been found previously in Ni-Mn-Ga single crystalline alloys at 400 °C which placed such materials ahead of known high temperature shape memory alloys (HTSMAs) promising in the automotive or aerospace industries operating in the range of 400–500 °C and above. This paper addresses two main issues that commonly affect Ni-Mn-Ga HTSMAs and limit their application, namely cycling stability of the transformation temperatures and thermomechanical actuation. These issues have been studied systematically by using six different Ni-Mn-Ga HTSMAs. The results show initial transformation temperatures up to 500 °C, which evolve, together with transformation strains, during more than 300 thermal cycles with and without mechanical loading. The specific evolution of a given sample depends on the microstructure, heat treatment prior to the cycling and whether the initial state of austenite is a single (β) or a dual phase (β + γ). The cycling protocol employed can be considered as an innovative training procedure to achieve the stabilization of the functionality and longer lifetime of these materials.