Precipitation of a chromium-rich α-phase in Alloy 693 and its effect on tensile properties

Alloy 693 is a newly developed Ni-base superalloy. It contains Ni, Cr and Al as major alloying elements with minor additions of Ti and Nb. The alloy is designed to achieve an excellent combination of mechanical strength, corrosion and oxidation resistance properties at elevated temperatures. Al and Ti induce precipitation of the strengthening γ′-phase particles. During coarsening of these particles, ductility of the alloy was observed to decrease along with its strength. The concomitant decrease in ductility was found to be primarily associated with the precipitation of a Cr-rich α-phase particles, which acted as preferential sites for crack initiation prior to yielding due to their brittle nature. Factors responsible for the formation of α-phase have been discussed.