부산대학교 도서관

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