Solidification behavior of a Ni-based single crystal CMSX-4 superalloy solidified by downward directional solidification process

The solidification behavior of a CMSX-4 superalloy single crystal solidified using the downward directional solidification process has been investigated by using the directional solidification quenching technique. An analysis of transverse sections within the mushy zone yielded the solidification path, the evolution of solid volume fraction (fs) with the temperature decrease and the solidification sequence of eutectic pool. fs monotonically increased with decreasing temperature. Within the first 20% of the solidification interval, fs increased by about 60%. This increasing ratio then significantly reduced. It became flat at the final portion up to the freezing of (γ + γ′) eutectic. The Lever rule and Scheil model cannot describe the actual evolution of fs observed in the experiment; however, the Bower-Brody-Flemings model can, to some extent, depict this evolution. The solidification path was described as: primary crystallization of γ phase; termination of solidification with a eutectic reaction. The solidification sequence of the eutectic pool commenced with a fine (γ + γ′) structure in the center of interdendritic regions or adjacent to γ phases, then progressed spatially and developed a petal-like coarse (γ + γ′) structure. Following this sequence, the γ′ particles precipitate from the γ phase between the impinging coarse petal-like γ′ phases.