Crystal-plasticity modeling of the coupling between phase transformation and viscoplasticity in high-temperature shape memory alloys

Experimental studies on high-temperature shape memory alloys (HTSMAs) have proved that the shape memory effect is affected by thermally activated mechanisms. It is evidenced by a modification of their functional properties such as transformation temperatures, actuation strain, and transformation strain. The properties are affected by an increase in irrecoverable strains such as viscoplasticity, transformation-induced plasticity (TRIP) and their couplings, which also trigger an accumulation of retained phases. Understanding the underlying mechanisms responsible for, irrecoverable deformations, retained phases and the above effects, is a challenge that requires complicated in-situ experiments. However, interpreting the mechanisms through physics-based modeling studies can help address the challenge and understand the material’s behavior. Therefore, a crystal-plasticity constitutive model was formulated to account for relevant mechanisms/phenomena (in HTSMAs) such as phase-transformation, TRIP, and viscoplasticity. The coupling between viscoplasticity and phase-transformation leading to the change of properties and accumulation of retained martensite was also modeled, but via a phenomenological approach. The model was built following a multi-scale, macro-micro approach that bridges the atomic, micro, and macro scales, by accounting for a desired family of slip systems. The model was used to conduct a series of finite element analyses on single crystals and polycrystals of a Ni-Ti-Hf HTMSA, using parallel computing. The single crystal results and their trends, gave insights into the activation and evolution of underlying mechanisms while being sensitive to anisotropy. While the polycrystal results helped interpret the effect of grain distribution and the coupling on the entire response.