Influence of casting conditions on the solution treated microstructure of Ni-base superalloy

Abstract

Abstract Ni-base superalloy specimens were cast under different superheat levels, low and high, and various cooling rates. Solution treatment was carried out for 2 h at 1120 °C or 1180 °C then air cooled. The solution treatment at 1120 °C represents the standard solution treatment (SST) while that at 1180 °C represents the modified solution treatment (MST). DSC measurements, SEM and EDS emphasized that the α nodular and interdendritic γ/γ' phases found in as cast structure were entirely removed after solution treatment either at 1120 °C or 1180 °C. The volume fraction of η phase in microstructure of all specimens decreased with MST in comparison with SST. After SST and MST, needle like η phase had disappeared except in alloy H1 that was produced under lower cooling rate and high superheat, with SST. In addition to needle like shape, platelet η phase was found in the microstructure of H1 alloy after SST. The microstructure of other alloys contains η phase only with platelet shape. Generated metastable nano-scale γ' particles precipitated in alternating layers with η phase at the expense of transformed η phase. At the platelet η phase circumference, new γ phase with high Cr content was formed at the expense of the dissolved η phase during solution treatment.