Competitive converging dendrites growth depended on dendrite spacing distribution of Ni-based bi-crystal superalloys

The microstructure evolution and the dendrite spacing distribution were characterized by the minimum spanning tree method to investigate the relationship between the competitive converging dendrites growth and the dendrite spacing distribution in Ni-based bi-crystal superalloys with different withdrawal rates. The results indicated that the overgrowth was closely related with the dendrite spacing distribution determined by the withdrawal rate. As a selection criterion, the relative spacing at the grain boundary with respect to the spacing inside the favorable oriented (FO) grain had been identified. In particular the FO grain had been overgrown if its minimum spacing at the grain boundary had been smaller than the minimum spacing inside the FO grain. On the contrary the FO grain overgrew the unfavorable oriented (UO) grain if the maximum dendrite spacing at the grain boundary was larger than that inside the FO grain. Furthermore, the overgrowth speed was correlation with the ratio between the number of dendrite spacings at the grain boundary out of the stable range inside the FO grain and the total number of dendrite spacings at the grain boundary. The mechanism of competitive converging dendrites growth was experimentally validated that the dendrite spacing was adjusted to a stable range by the development of new FO dendrites or the elimination of existing FO dendrites.