The Influence of Thermomechanical Treatment on the Strain Behavior of the Fe–Ni–Co–Ti Ferromagnetic Alloy Nanocomposite with Shape Memory Effect

The paper describes mechanical tests of a nanocomposite produced from the ferromagnetic shape memory Fe−Ni−Co−Ti alloy 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. The TMT imparted high superelastic strain and shape memory effect to the nanocomposite. The preliminary TMT with strain ψ = = 7.4−22.5% aged at T = 650°C for 5−10 min was experimentally found to correspond to the optimal combination of the maximum superelastic strain and shape memory effect. This contributed to the phase and twinning plastic deformation of the nanocomposite over the test temperature range Ms < Ttest < Af (where Ms is the start temperature of forward martensitic transformation on cooling and Af is the finish temperature of reverse martensitic transformation on heating). A plateau with constant stress was found in the two-phase Mf < Ttest < Ms region on the tensile curve at drawing strain ψ = 22.5%. A significant increase in the preliminary strain (more than 40%) substantially stabilized the austenitic matrix, thus inhibiting the martensitic transformation and reducing reversible effects because the austenite grain size refines when the lattice defect density increases. The austenite grain size distribution was assessed versus the chosen TMT conditions. When the austenite grain size increased, the superelastic strain recovery became higher. The factors leading to greater superelasticity were analyzed within different phenomenological models. The TMT has a crucial role in the variation of structure and mechanical properties, in turn promoting inelastic effects at different temperatures.