Carbide Precipitation in Nickel Base Superalloys 718 and 625 and Their Effect on Mechanical Properties

Abstract

In addition to the precipitation of the strengthening intermetallic phases, namely, the metastable y" and y' phases, carbide phases also precipitate during age hardening heat treatments of Alloy 718 and Alloy 625. A detailed investigation has been carried out on the precipitation of primary and secondary carbide phases in these two alloys with reference to their identity, distribution and morphology. It has been found that the predominant carbide phase in Alloy 718 is a niobium rich MC phase, while in the case of Alloy 625, mainly M,,C, and, to some extent, M,C type carbides precipitate. Carbide precipitation during ageing has been seen to be confined almost exclusively to the austenite grain boundaries in both the alloys. NbC formation at the grain boundaries in Alloy 718 is associated with the appearance of distinct intermetallic precipitate ( y" and y' ) free zones near these boundaries whereas in Alloy 625, no well defined intermetallic precipitate free zones (PFZ) have been observed. The precipitation of different types of grain boundary carbides is discussed in terms of the differences in the chemistry of these alloys. The presence or absence of PFZ at grain boundaries is discussed in the light of solute depletion /vacancy depletion mechanisms.