Three-Dimensional Measurements of Microstructure Evolution in Martensitic NiTi Using High Energy Diffraction Microscopy

While micromechanical theories for shape memory alloys (SMAs) have been developed since the mid-20^th century, experiments to validate these theories on the microstructural scale are relatively new, challenging, and often limited to two-dimensional surface measurements. To address this open area, we utilize High-Energy Diffraction Microscopy (HEDM), an in-situ, non-destructive x-ray diffraction technique. This technique is perfectly suited for the study of microstructure evolution in SMAs, because it provides three-dimensional micromechanical information on the grain or even subgrain scale, including strain, crystallographic orientation, volume, center of mass, and topology. These measurements are essential to the understanding of SMA micromechanics, because they are not confined to surface measurements where the effects of free surface relaxation are unknown. We present a unique experiment on martensitic NiTi using HEDM. The results include microstructure evolution during detwinning/reorientation, three-dimensional maps of misorientation spread and estimates of lattice strain within each martensite domain, and the effects of cycling on the microstructure. These results can be used to inform and validate microstructural models of martensite deformation and potentially develop numerical schemes to estimate elastic properties of monoclinic martensite.