Rate and thermal sensitivities of unstable transformation behavior in a shape memory alloy

A special plasticity-based constitutive model with an up–down–up flow rule used within a finite element framework has previously been shown to simulate the inhomogeneous nature and the thermo-mechanical coupling of stress-induced transformation seen in a NiTi shape memory alloy. This paper continues this numerical study by investigating the trends of localized nucleation and propagation phenomena for a wider range of loading rates and ambient thermal conditions. Local self-heating (due to latent heat of transformation), the inherent Clausius–Clapeyron relation (sensitivity of the materialʹs transformation stress with temperature), the size of the specimenʹs nucleation barriers, the loading rate, and the nature of the ambient environment all interact to create a variety of mechanical responses and transformation kinetics. The number of transformation fronts is shown to increase dramatically from a few fronts under nearly isothermal conditions to numerous fronts under nearly adiabatic conditions. A non-dimensional film coefficient and non-dimensional conductivity are identified to be the major players in the range of responses observed. It is shown that the non-dimensional film coefficient generally determines the overall temperature response, and therefore force–displacement response, of a transforming specimen; whereas, the non-dimensional conductivity is the more important player in determining the number of nucleations, and therefore the number of transformation fronts, that may occur.