On the formation of martensite bands in pseudoelastic NiTi

Pseudoelastic NiTi is prone to the formation of martensite bands when subjected to tensile loading. It is now well understood that the stable growth of these martensite bands, promoted by the movement of meso-scale austenite-martensite interfaces at constant velocity, gives rise to the distinct stress-plateaus in stress-strain curves. Recent experimental and numerical work also helps to shed a light on the phase distributions and strain fields associated with the meso-scale interfaces. However, the early stages of martensite band formation – meso-scale martensite nucleation – are not well understood even though they promise to hold the key to explain why martensite bands form in the first place. In this contribution, we analyze high-resolution Digital Image Correlation data to document local strain fields during the early formation and evolution of, in the language of mechanics, a small disturbance that grows to form the nucleus of a martensite band in thin NiTi wires. Combining the experimental results with numerical modeling data, we propose a simple mechanism of nucleation and rapid lateral growth of a martensite nucleus that explains both, the non-linear response of NiTi prior to the onset of the stress plateau, and distinct elastic unloading effects that are directly related to strain softening effects and the macroscopically observed “nucleation peak” in pseudoelastic NiTi. A detailed analysis of our new model of martensite nucleation in tension leads to the most complete description, to date, of pseudoelasticity in NiTi as provided by martensite band nucleation and propagation under tensile loading.