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 = 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 (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 = 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 T₁ on crystal axis orientation, stress state and the magnitude of mechanical hysteresis is discussed.