Superelasticity and its cyclic stability in [001]-oriented single crystals and oligocrystals of FeMnAlNi alloy in compression

Abstract

Superelasticity and stability of superelasticity in isothermal loading/unloading cycles under compression up to 3.5 – 4% at room temperature were studied in [001]-oriented Fe43.5 Mn34 Al15 Ni7.5 (at.%) single crystals and Fe42.5 Mn34 Al15 Ni7.5 Ti1 (at.%) oligocrystals (polycrystals with a large grain size of ~1000 μm) at the α-γʹ-martensitic transformation. Using dynamic mechanical analysis and in situ cooling/heating in a microscope column, it was found that after quenching from 1473 K with water and ageing at T = 473 K for 3 hours, cooling-induced martensite in these crystals was not observed. Stress-induced martensite developed during deformation under a compressive load, wherein the critical stresses necessary for the stress-induced α-γʹ-martensitic transformation changed only slightly when the test temperature increased. The α values characterizing this growth were 0.56 and 0.33 MPa/K for the [001]-oriented single crystal and oligocrystal, respectively. Superelasticity in these crystals was observed in a wide temperature range from 203 to 473 K. The maximum superelasticity achieved at room temperature was 6.2% and 5% in single-and oligocrystals, respectively. Single crystals exhibited good stability in loading/unloading cycles, in contrast to oligocrystals. Almost complete degradation of the superelasticity loop was observed after 40 and 20 cycles in single and oligocrystals, respectively. Degradation of the functional properties under cyclic influences was associated with the appearance of residual martensite. In single crystals, large martensite lamellae were optically observed predominantly in one system. In oligocrystals, several martensite systems interact, which leads to greater energy dissipation and a faster degradation of functional properties compared to single crystals.