Microstructure of an experimental Ni base superalloy under various casting conditions

The effect of casting parameters such as melt superheat and solidification rate on the microstructure of an experimental Ni-based superalloy made by vacuum melting and investment casting methods was investigated. The results show that the as cast microstructure of this alloy consists of dendritic γ matrix, interdendritic γ/γ′, γ′ phase, MC carbides and minor phases in interdendritic regions. DSC analyses in addition to SEM and EDS proved that the nodular phase found in minor phases is σ phase, while the η phase was found to solidify in two forms; as plate-like and blocky shape. The volume fraction (Vf) of σ and η phases increases as the melt superheat increases and the solidification cooling rate decreases. Additionally, the Vf of interdendritic γ/γ′ increases with higher solidification rate and lower melt superheat. It was found that higher melt superheating temperature enlarges the size of the primary γ dendrites; however the higher solidification rate diminishes it.