Analysis of orientation clustering in a directionally solidified nickel-based ingot

Orientation imaging microscopy was used to study the field of lattice orientations in a directionally solidified ingot of nickel-based alloy in order to understand the evolution; of grain structure and microtexture as a function of distance from the chilled surface. The lattice field is organized into equiaxed and randomly oriented grains at the chill plane. However, with increasing distance-away from the chill plane, the microstructure organizes into increasingly coarse and ramified grain clusters in the region of columnar growth. The clusters comprise sets of grains connected by small-angle grain boundaries. Thus, the clustering of neighboring dendrites reflects a sharing of low-angle misorientations swing to the development of a strong [100] fiber texture and its concomitant, a profound change in misorientation distribution. The absence of any preferred orientation within a single cluster suggests that the clusters consist of separate grains. The radius of gyration and the fractal dimension have been used to characterize clustering in the microstructure. Clustering was found to increase with distance from the chill plane as the strength of the fiber texture increases. A critical misorientation angle was found at which the largest cluster percolates the microstructure. This critical angle decreases with increasing strength of the fiber texture.