The Mechanism of Orientation Dependence of Cyclic Stability of Superelesticity in NiFeGaCo Single Crystals Under Compression

Using single crystals of the Ni49Fe18Ga27Co6 (at.%) alloy, oriented along [001]- and [123]-directions, cyclic stability of superelasticity is investigated in isothermal loading/unloading cycles at T = A(f) +(12-15) K (100 cycles) under compressive stress as a function of given strain per cycle, presence of disperse gamma-phase particles measuring 5-10 mu m, austenitic (B2 or L2(1)) and stress-induced martensitic crystal structure (14M or L10). It is shown that single-phase L2(1)-crystals demonstrate high cyclic stability during L21-14M-transitions with narrow hysteresises Delta sigma < 50 MPa in the absence of detwinning of the martensite. During the development of L2(1)-14M stress-induced transformation, the reversible energy Delta Grev for these crystals exceeds the dissipated energy Delta G(irr), and Delta G(rev)/Delta G(irr) = 1.7-1.8. A significant degradation of superelasticity is observed in [123]-oriented crystals during the development of L2(1)-14M-L1(0)-transformations followed by detwinning of the L10-martensite crystals and heterophase (B2+gamma) single crystals, irrespective of their orientation during the B2-L10-transition. In the latter case, martensitic transformations are characterized by a wide stress hysteresis Delta sigma ae<yen> 80 MPa and the dissipated energy exceeds the reversible energy Delta G(rev)/Delta G(irr) = 0.5. The empirical criterion, relying on the analysis of the reversible-to-irreversible energy ratio, Delta G(rev)/Delta G(irr), during stressinduced martensitic transformations, can be used to predict the cyclic stability of superelasticity in NiFeGaCo alloys subjected to different types of heat treatment.