Competitive growth of diverging columnar grains during directional solidification: A three-dimensional phase-field study

The competitive growth of two diverging grains in the directional solidification process was investigated through three-dimensional (3D) phase-field simulations. We explored the diverging grain boundary (GB) evolution and quantitatively analyzed the grain elimination in cases with different inclined angles of UO dendrites. It is found that the stochastic tertiary branching behavior resulted in a zigzag diverging GB. Previous two dimensional (2D) simulations about the competitive growth of diverging grains indicate a non-monotonic variation—that is, first increases and then decreases—of the grain elimination rate with the inclined angle of UO dendrites. The grain elimination in 3D diverging cases, however, shows a monotonic manner. As the spatial arrangement of FO dendrites relative to UO dendrites was a stagger configuration in 3D, not the face-to-face configuration in 2D, the competition of secondary arms at the GB region was not intense. Consequently, the liquid space size sandwiched by diverging grains became the leading factor influencing the grain elimination, and the grain elimination rate increased with the inclined angle of UO dendrites. Moreover, without the intense competition of secondary arms during the 3D diverging grain growth, the elimination of the UO grain was faster than those in 2D diverging grain growth and 3D non-uniplanar grain growth. These conclusions clarify the inconsistency between the previous 2D simulation research and the experimental research regarding the grain elimination in diverging competitive growth.