Orientation dependence and tension/compression asymmetry of shape memory effect and superelasticity in ferromagnetic Co40Ni33Al27, Co49Ni21Ga30 and Ni54Fe19Ga27 single crystals

In the present study the effects of crystal axis orientation, stress state (tension/compression) and test temperature on shape memory effect and superelasticity of Ni54Fe19Ga27(I), Co40Ni33Al27(II), Co49Ni21Ga30(III) (numbers indicate at.%) single crystals were investigated. The shape memory effect, the start temperature of superelasticity T1 and the mechanical hysteresis Δσ were found to be dependent on crystal axis orientation and stress state. Superelasticity was observed at T1 = Af (Af, reverse transformation-finish temperature) in tension/compression for [0 0 1]-oriented Ni-Fe-Ga crystals and in compression for [0 0 1]-oriented Co-Ni-Ga crystals, which all displayed a small mechanical hysteresis (Δσ ≤ 30 MPa). An increase in Δσ of up to 90 MPa in the Co-Ni-Al and the Co-Ni-Ga crystals lead to stabilization of the stress-induced martensite, and an increase in to T1 = Af + Δ. The maximal value of Δ (75 K) was found in [0 0 1]-oriented Co-Ni-Al crystals in tension. A thermodynamic criterion describing the dependencies of the start temperature of superelasticity T1 on crystal axis orientation, stress state and the magnitude of mechanical hysteresis is discussed.