HIGH-TEMPERATURE SUPERELASTICITY IN [001]-ORIENTED NiMnGaFe SINGLE CRYSTALS UNDER COMPRESSION

The high-temperature superelasticity, the temperature dependence of critical stresses for martensite formation, and the yield strength of martensite and austenite in [001]-oriented NiMnGaFe single crystals under compression have been studied. In as-grown and quenched single crystals the yield strength of martensite in single crystals decreased by 640–700 MPa with increases in test temperature, while the strain hardening coefficient increases an order. At high temperatures, this results in the yield strength of martensite being achieved, the plastic deformation occurring during the martensitic transformation, and the superelasticity range ending. Annealing at 1273 K for 4 hours with following quenching leads to an increase in the yield strength of martensite by 250–350 MPa over the entire temperature range, compared to as-grown single crystals. This causes an expansion of the superelasticity range in quenched single crystals from ΔT = 115 K (308–423 K) to ΔT = 155 K (318–473 K).