Re-precipitation mechanisms of the γ′ phase with sphere, near-sphere, cubic, octets and finally-dendrite in as-cast Ni-based superalloys

The morphology and size distribution of γ′ phases have a significant impact on the thermal processing and manufacturing of components made of superalloys and their service performance under adverse conditions. The homogenization treatment of as-cast superalloys ingot is the first modulation of the morphology, size distribution and other characters of γ′ phase. Therefore, investigation on the re-precipitation mechanism of γ′ phase is of critical theoretical value and engineering significance. In this research, the morphology evolution and size distribution of re-precipitation γ′ phase in an as-cast Ni-based superalloy during homogenizing treatment were investigated extensively and systematically. The re-precipitation mechanism of γ′ phase with sphere, near-sphere, cubic, octets and finally-dendrites was identified under various cooling conditions. The γ′ phase size is increased while the nucleation density is decreased with the cooling conditions changing from air cooling (AC) to furnace cooling (FC). The γ′ phase stability during growth was discussed and the reprecipitation γ′ phase is generated from a refine spherical shape at fast cooling condition (AC); while irregularity growth even splitting happened at slow cooling condition (FC). The findings help understand the re-precipitation mechanism of the γ′ phase of Ni-based superalloys and provide a basis for controlling of the microstructure and tailoring of the properties of the alloys.