| Uniaxial experiments of a nearly equiatomic NiTi alloy are conducted in the temperature and deformation regime in which it exhibits the shape memory effect and pseudoelasticity. These remarkable characteristics arise from the diffusionless transformation between two solid state phases, austenite and martensite. Through a systematic series of experiments, the loading rate and the choice of ambient medium are found to have a significant influence on the apparent mechanical response. Discrete local measurements of strain and temperature show this to be caused by a complex interaction between the temperature sensitivity of the transformation stresses, self-heating or self-cooling due to the latent heat of transformation, and the prevailing heat transfer conditions of the experiment.;A full field monitoring scheme, using NiTi strip specimens, is developed to track the nonuniform strain and temperature fields during stress-induced phase transformations. Synchronized histories of deformation, obtained by photographs of the disturbance of a brittle coating on the surface, and temperature, obtained by infrared thermal imaging, lead to a clear explanation of the observed events. During loading (or unloading), nucleation of the new phase is a distinct event requiring a higher (lower) stress than that required to propagate the transformation. This nucleation peak, coupled with the local temperature changes, govern the number of nucleations. It is also shown that under steady-state conditions coexisting fronts all propagate at a speed that is inversely proportional to the number of traveling fronts.;Nucleation in thin NiTi strips occurs in a sharp band, 55... |
