Effect of grain boundary precipitations on fatigue cracking path of Alloy 617 in a superheated steam environment at 750 °C

Abstract

Crack growth tests were carried out in a superheated steam environment under a cyclic loading frequency condition in which crack propagation is expected with a transgranular and intergranular (IG) mixed mode. The oxide and precipitations that formed near the crack tip were analyzed by a transmission electron microscope. The crack propagated transgranularly through the grains, then turned to the IG path up to the crack tip. Cr-rich oxide, Al oxide, and a Cr-depleted zone beneath the oxide formed around the IG crack. Comparisons were made with a fully developed IG crack. The tendency to form more Cr oxide than Al oxide was also observed. No formation of a Cr-depleted zone was confirmed in the vicinity of the Cr oxide in the crack. A more detailed analysis of the grain boundary precipitation behavior, such as nucleation, coherency, and growth, is necessary. However, it was suggested that the depletion zone that developed near the grain boundaries could be the preferred pathway of the crack after its transition to the IG path.