THE INFLUENCE OF THERMOMECHANICAL TREATMENT ON THE STRAIN BEHAVIOR OF A Fe–Ni–Co–Ti FERROMAGNETIC ALLOY NANOCOMPOSITE WITH SHAPE MEMORY EFFECT

Abstract

The paper presents the results of mechanical tests for a nanocomposite based on the ferromagnetic Fe−Ni−Co−Ti alloy with shape memory effect under uniaxial tension over a wide temperature range. The production of the nanocomposite was preceded by preliminary thermomechanical treatment (TMT), involving drawing, quenching, and ageing, for precipitation hardening. As a result of TMT, the nanocomposite acquired high superelastic strain and shape memory effect. It was experimentally established that the preliminary TMT with strain y = 7.4−22.5% and ageing at T = 650 °C for 5−10 min corresponded to the optimal combination of the maximum superelastic strain and the shape memory effect. This contributes to the deformation of the nanocomposite by channels of phase and twinning ductility in the test temperature range M_s < Т_test < A_f  (where  M_s is the onset temperature of forward martensitic transformation in cooling and A_f is the end temperature of backward martensitic transformation in heating). In the tensile diagram with compression y = 22.5%, a plateau with constant stress was found in the two-phase region M_f < T_test < M_s. A significant increase in the preliminary strain, more than 40%, substantially stabilizes the austenitic matrix, thus inhibiting martensitic transformation and reducing reversible effects because the austenite grain size refines when the lattice defect density increases. The austenitic grain size distributed was assessed applying chosen TMT conditions. When the austenite grain size increases, the degree of superelastic strain recovery becomes higher. The factors leading to quantitative increase of superelasticity were analyzed within different phenomenological models. The TMT has a critical role in the variation of structure and mechanical properties, in turn promoting inelastic effects at different temperatures.