Ashley N. Bucsek | Searcharxiv

arXiv2018

Three-dimensional in situ characterization of phase transformation induced austenite grain refinement in nickel-titanium

Near-field and far-field high-energy diffraction microscopy and microcomputed tomography X-ray techniques were used to study a bulk single crystal nickel-titanium shape memory alloy sample subjected to thermal cycling under a constant applied load. Three-dimensional in situ reconstructions of the austenite microstructure are presented, including the structure and distribution of emergent grain boundaries. After one cycle, the subgrain structure is significantly refined, and heterogeneous Σ3 and Σ9 grain boundaries emerge. The low volume and uneven dispersion of the emergent Σ boundaries across the volume show why previous transmission electron microscopy investigations of Σ grain boundary formation were inconsistent.

cond-mat.mtrl-sci↗

arXiv2018

Ferroelastic twin reorientation mechanisms in shape memory alloys elucidated with 3D X-ray microscopy

Three-dimensional (3D) X-ray diffraction methods were used to analyze the evolution of the load-induced rearrangements of monoclinic twin microstructures within bulk nickel-titanium specimens in 3D and across six orders of magnitude in length scales: changes in lattice plane spacings and orientations at the nanoscale, growth and nucleation of martensite twin variants at the microscale, and localization of plastic strain into deformation bands at the macroscale. Portions of the localized deformation bands were reconstructed in situ and in 3D. Analyses of the data elucidate the sequence of twin rearrangement mechanisms that occur within the propagating localized deformation bands, connect these mechanisms to the texture evolution, and reveal the effects of geometrically necessary lattice curvature across the band interfaces. The similarities between shear bands and localized deformation bands in twin reorientation are also discussed. These findings will guide future researchers in employing twin rearrangement in novel multiferroic technologies, and they demonstrate the strength of 3D, multiscale, in situ experiments to improve our understanding of complicated material behaviors and to provide opportunities to advance our abilities to model them.

cond-mat.mtrl-sci↗