Understanding Grain Growth in 3D: Microstructural Evolution of Nickel during Annealing using Three-dimensional X-ray Microscopy

This thesis characterizes and quantifies microstructural evolution in nickel. The orientation maps of nickel were collected using a non-destructive, synchrotron based 3D characterization technique called Near-field High Energy X-ray Diffraction Microscopy (nf-HEDM) by the Suter group at Carnegie Mellon University. Six snapshots of the nickel sample were collected after being annealed at 800°C in 30 minute intervals. The temporal changes in crystallographic orientation, grain size and misorientation in the microstructure were measured over the different anneal states. The observed microstructure was also modified to represent a more isotropic system by merging twin related domains into a single grain. Subsequently the microstructures were also meshed and smoothed to determine grain boundary properties as a function of the five grain boundary parameters. Various morphological and topological features such as grain size, number of neighbors, difference between nearest and average of second nearest neighbors, grain curvature and integral mean grain curvature were analyzed with respect to grain growth. It was observed that a grain’s volume change is best predicted by the difference between the number of neighbors a grain has and the average number of neighbors the neighboring grains have as well as curvature. However, the correlation between neighborhood topology and curvature becomes significant only when adjacent twin related domains are merged. A study of the influence of grain boundary curvature on grain boundary velocity revealed that they did not show a positive linear correlation. Grain boundary velocity and curvature also varied with crystallographic parameters. An isotropic simulation used to predict grain growth of the merged twin microstructure did not reproduce the observed microstructure.