Effect of secondary arm orientation on unusual overgrowth at converging grain boundary during directional solidification in 3D

The competitive growth of two converging grains with multiple dendrite rows during directional solidification in three dimensions (3D) was investigated through phase-field simulations. We studied the effect of secondary arm orientation on the overgrowth of favorably oriented (FO) dendrite by unfavorably oriented (UO) dendrite, i.e. the so-called unusual overgrowth. It is observed that, when the initial spatial arrangement of FO and UO dendrites varies, the effect of secondary arm orientation on the unusual overgrowth will also be different. In addition to the unusual overgrowth similar to that previously illustrated in two dimensions (2D) and thin samples, we have observed a more complicated unusual overgrowth manner when the initial spatial arrangement between the FO dendrites and UO dendrites was the stagger configuration and there is a rotation of the secondary arm in either the FO grain or the UO grain. In these cases, the spatial arrangements of some UO dendrites with respect to FO dendrites can be automatically adjusted from the initial stagger configuration to the face-to-face configuration. Such an adjustment results in the interaction of FO dendrite not only with the UO dendrite penetrating into the liquid gap but also with the face-to-face UO dendrite. Hence, the unusual overgrowth of FO dendrites occurs easier than in the cases with only face-to-face interaction, i.e., the cases in 2D and thin samples. These findings illustrate the inconsistency between previous simulations and experiments regarding the effect of secondary arm orientation on the unusual overgrowth.