Orientation dependence and tension/compression asymmetry of shape memory effect and superelasticity in ferromagnetic Co 40Ni 33Al 27, Co 49Ni 21Ga 30 and Ni 54Fe 19Ga 27 single crystals

Abstract

In the present study the effects of crystal axis orientation, stress state (tension/compression) and test temperature on shape memory effect and superelasticity of Ni 54Fe 19Ga 27(I), Co 40Ni 33Al 27(II), Co 49Ni 21Ga 30(III) (numbers indicate at.%) single crystals were investigated. The shape memory effect, the start temperature of superelasticity T 1 and the mechanical hysteresis Δ σ were found to be dependent on crystal axis orientation and stress state. Superelasticity was observed at T 1 = A f ( A f, 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 (Δ σ ≤ 30 MPa). An increase in Δ σ 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 T 1 = A f + 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 T 1 on crystal axis orientation, stress state and the magnitude of mechanical hysteresis is discussed.