Hot tearing in directionally solidified Ni-based superalloys

The hot tearing mechanism has been studied in directionally solidified (DS) Ni-based superalloys with columnar grain structures. Castability tests and optical metallography of quenched samples were employed to explore the effect of microstructural evolution, grain boundary characteristics on the susceptibility of hot cracking formation during DS. Gleeble tests were performed on DS samples at temperatures above the solidus. The effect of major alloying elements, such as Ti, Ta and Hf, as well as minor elements, such as Zr and B, has been examined. It is found that in most of the Ni-based superalloys studied there exists a critical temperature range Delta T-CTR, within which any opening of the grain boundary during DS can hardly be compensated for by feeding of liquid due to the high solid fraction. Significant change of the liquid fraction within Delta T-CTR resulted in bad castability. Dramatic influence of the interaction of Zr and B on the hot tearing susceptibility was also found, with the effect being particularly strong if Zr concentration was high. The present results lead to two factors influence the hot tearing susceptibility within Delta T-CTR: (1) the strains and strain rates developed; and (2) the grain boundary cohesion. High strains and strain rates promote hot tearing, while stronger solid skeleton resulted from more bridging of the secondary dendrite arms generally leads to better hot tearing resistance.