Stress-Induced Martensitic Phase Transformation in Nitinol Under Hard Cyclic Loading

This paper describes an experimental study of stress-induced martensitic phase transformation in the shape memory alloy Nickel-Titanium, also known by the trade name Nitinol. The rich local thermo-mechanical interactions that underlie phase transformation are examined using three-dimensional Digital Image Correlation (to determine local strain fields) and infrared imaging (to determine local thermal fields). We quantify the complex local interactions between released/absorbed latent heat and the extent of transformation, and explore the characteristics of the phase fronts. There exists a remarkable amount of memory in the pattern of the martensitic formation from cycle to cycle. The initial pattern in which the martensite nucleates and propagates in the first cycle strongly dictates how the martensite nucleates and propagates in future cycles. The presence of this cycle-to-cycle strain pattern memory indicates that the local elastic stress fields in the martensite are driven by a dislocation structure and martensitic nuclei that largely stabilize during the first loading cycle.