Quantitative Measures of Microstructural Phase Transformation in Nickel-Titanium

Martensitic transformation in superelastic Nickel-Titanium has been successfully quantified and analyzed at the microstructural length scale for the first time. A novel approach enables the determination of full-field, in situ surface displacements that are used as an indicator of the extent of microstructural martensitic transformation. The resulting high spatial-resolution displacement fields provide point-by-point information on the evolution of local deformation within individual grains as the macroscopic transformation front progresses, and enable analysis of the martensite phase fraction inside grains that results from interactions between the macroscopic front and microstructural features. The heterogeneous structure of the martensite band, including areas of elevated strain from plastic deformation and areas of low strain from retained austenite, will be given special attention. The high-spatial resolution strain data is related to the underlying crystallographic information, provided by Electron Backscatter Diffraction (EBSD), in order to quantify and elucidate microstructural effects on both small-scale and meso-scale phase transformation. Ultra high-resolution experiments focusing on individual microstructural elements, such as single grains, will also be discussed.