Effect of secondary dendrite orientations on competitive growth of converging dendrites of Ni-based bi-crystal superalloys

The microstructure evolution of bi-crystal samples during directional solidification was characterized to understand the effect of secondary dendrite orientation on the competitive growth of converging dendrites of Ni-based superalloys. The results showed that the secondary dendrite orientation affected obviously the needed critical withdrawal rate in which the bi-crystal coexisted during competitive growth. And, the needed lowest critical withdrawal rate was obtained when the secondary dendrite orientation was perpendicular to the grain boundary of bi-crystal samples. When the secondary dendrite orientation was increased to 45°, the needed critical withdrawal rate for competitive growth was largest. Because the secondary dendrite orientation effected the spatial arrangement of dendrites with respect to the grain boundary in three dimensions space, the insertion between unfavorably oriented dendrites and favorably oriented dendrites and the corresponding lateral motion were believed as the main mechanism for overgrowth based on the experimental investigation.