INVESTIGATION OF HYSTERESIS DURING THERMOELASTIC MARTENSITIC TRANSFORMATIONS IN Ti50Ni49.9Mo0.1 ALLOY UNDER MECHANICAL LOADING CONDITIONS

10.25712/ASTU.1811-1416.2024.04.005

Abstract

The paper analyzes the physical nature of hysteresis, its relationship with the processes of relaxation of micro stresses and stabilization of the martensitic phase, as well as the effect of mechanical stress on the production of energy and energy dissipation. Deformation hysteresis in cycles of martensitic transformations in Ti₅₀Ni_49.9Mo_0.1 alloy under the action of mechanical stresses was investigated. Deformation hysteresis indicates that during the accumulation and return of deformation in the cycle of martensitic transformations, energy dissipation occurs, indicating the course of irreversible processes. Despite the reversible nature of the accumulation and return of deformation, in a thermodynamic sense, these processes are irreversible. During cooling and heating, energy is dissipated, accompanied by the production of entropy due to heat fluxes at the interface. An important factor of hysteresis is the production of entropy during reversible martensitic transformations. Entropy is produced both by irreversible processes within the system and at the interface with the medium. When cycling martensitic transformations, saturation of the phase hardening is observed, which leads to a decrease in the area of the hysteresis loop and its stabilization. Thus, after stabilization, hysteresis is caused only by heat dissipation at the internal boundaries and acoustic emission, whose contribution to energy dissipation is small and may not be taken into account. A decrease in the hysteresis area during cycling of martensitic transformations may be associated with a decrease in entropy production due to a decrease in the number of boundaries in the transformation cycle. This is confirmed by a decrease in the volume of the phase involved in martensitic transformations, which is evident from the high-temperature return of residual deformation and X-ray studies. The increase in the area of the stabilized hysteresis loop with an increase in load is explained by an increase in energy dissipation during the interaction of martensitic boundaries with accumulated phase hardening defects.