The microstructural changes and their effect on CCGR after long time thermal exposure in DA718 and STD718

Samples of forged and heat treated DA718 and STD718 were examined to determine the effects of long time thermal exposures at 1200 and 1250 °F on microstructures and mechanical properties. In addition to the coalescence of γ′/γ″ and the amount increasing of δ phase, the existence of a bcc chromium enriched α-Cr phase was confirmed by XRD and SEM. The weight fraction of α-Cr and other phases were determined by chemical phase analyses methods. The results showed that the fractions of α-Cr in both DA718 and STD718 were less than 1% but more α-Cr particles precipitated in DA718 than in STD718 because the retained strain in DA718 promoted α-Cr precipitation. The results also showed that more α-Cr particles precipitated in Alloy 718 at higher exposure temperatures and for longer exposure times. The thermal exposure at 1200 and 1250 °F did not have any deleterious effects on the 1200 °F creep crack growth rate (CCGR).