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 gamma matrix, interdendritic gamma/gamma', gamma' 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 sigma phase, while the eta phase was found to solidify in two forms; as plate-like and blocky shape. The volume fraction (V-f) of sigma and eta phases increases as the melt superheat increases and the solidification cooling rate decreases. Additionally, the Vf of interdendritic gamma/gamma' increases with higher solidification rate and lower melt superheat. It was found that higher melt superheating temperature enlarges the size of the primary gamma dendrites; however the higher solidification rate diminishes it. (C) 2010 Elsevier B.V. All rights reserved.