Giant elasticity in the Ni-Mn-Ga single crystalline FSMA martensites

The phenomenon of the giant elastic completely reversible linear strain is found at constant temperatures in Ni-Mn-Ga martensites. Single crystals of some off-stoichiometric Ni-Mn-Ga compositions possess rubber-like behavior in the martensite phase with completely elastic strain reaching over 10% at some 90MPa compressive stress at room temperature. The observed phenomenon is different from the conventional (two-phase) superelasticity, since it is found completely in the martensite phase and no intermartensitic reaction is observed during the load-unload cycling. The main features of the observed rubber-like behavior are: 1) complete elasticity and giant values of the strains achieved; 2) the linear character of stress-strain dependence; 3) excellent stability to the mechanical cycling, stress rate and temperature variation, in contrast to the conventional two-phase superelasticity due to the stress induced martensite transformation. Current work is directed to study the structural mechanisms of the found phenomenon by means of the neutron diffraction in-situ under compressive stress cycling