Two-way shape memory effect and its cyclic stability in [001]-oriented CoNiGa single crystal

Two-way shape memory effect (TWSME) and its stability under cooling/heating cycling through the martensitic transformation (MT) temperatures in stress-free condition were investigated in mono-phase [001]-oriented single crystals of the ferromagnetic Co49Ni21Ga30 (at %) alloy. TWSME is induced by thermomechanical training– cooling under different external compressive stress and subsequent stress-free heating. It is shown that the TWSME strain εTWSME depends on the applied stress in the training cycle. The maximum of the εTWSME equal to 4.2(±0.2)% is achieved after training under σext=125 MPa and then decreases with increase of σext. The value of εTWSME = 4.2(±0.2)% is equal to the theoretical transformation strain for [001]-orientation at B2-L10 MT in compression. In crystals after training under σext=125 MPa with maximum εTWSME, the stability of the TWSME strain and martensitic start temperature Ms is observed up to 150 cooling/heating cycling. From 150 to 300 thermocycle, TWSME strain decreases from 4.1(±0.2)% to 3.5(±0.2)%, and the Ms temperature decreases from 251 to 242 K.