Cyclic degeneration of elastocaloric effect for NiTi shape memory alloy: Experimental observation and constitutive model

Based on the cyclic tension-unloading tests with various peak strains (4%-12%), the effect of loading level on the cyclic deformation and evolution of elastocaloric effect (eCE) for NiTi shape memory alloy (SMA) wires was investigated. Experimental results show that the transformation ratchetting and degeneration of eCE simultaneously occur during the cyclic deformation, and these two phenomena aggravate with the increase of loading level. Although the refrigeration capacity (maximum temperature drop in the reverse phase transformation process) exhibits a positive relativity with loading level in the 1st loading cycle, it changes non-monotonically with the variation of loading level in the steady-state cycle, which can be attributed to the interaction between the phase transformation and plasticity deformation. Moreover, it is found that the phase transformation between austenite (A) and martensite (M) phases occurs as a two-step process involving an intermediate phase, rhombohedral (R) phase. Then, a thermo-mechanically coupled cyclic constitutive model is constructed based on the fundamental laws of irreversible thermodynamics. Besides the thermo-elastic deformation, four different inelastic deformation mechanisms, i.e., R phase transformation (RT), martensite transformation (MT), plasticity in A and R phases (ARP), transformation-induced plasticity (TRIP) and their interactions are considered in the proposed model. The volume fractions of R phase, M phase, martensite affecting zone (MAZ) and the free zone (FZ) of mixed A + R phases are introduced. The thermodynamic driving forces for the four considered inelastic deformation mechanisms, i.e., RT, MT, ARP and TRIP are derived by the energy dissipation inequality and a new proposed Helmholtz free energy. Internal heat generation caused by the mechanical dissipation and the latent heat of phase transformation, and the heat balance equation are obtained by applying the conservation law of energy. The forward and reverse inheritances of dislocation (i.e., plastic deformation) caused by the moving martensite interfaces during MT are addressed. Adopting a first-order approximation of the constitutive equations and lumped analysis of heat transfer, the proposed constitutive model for a material point is extended to describe the overall responses of the whole wire. Finally, the predicted results for the cyclic deformation of NiTi SMA and correspondent evolution of eCE are compared against the experimental ones to validate the capability of the proposed model.