Metallography of environment sensitive fracture

Abstract

Environment sensitive cracks initiate at structurally related sites—grain, twin or interphase boundaries, slip steps or persistent slip bands. That is so even if cracks are initiated from nonstructually dependent pits and whether the mechanism of cracking is essentially dissolution controlled or due to the localized embrittlement of the metal. The subsequent growth of such cracks often, but not invariably, shows the same structural dependence as the initiation stage. The metallography of intergranular, transgranular, and mixed-mode cracking is considered in the context of various metal/environment combinations, together with its implications for some aspects of the mechansims of environment sensitive cracking. Finally, attention is drawn to the importance of the coalescence of environment sensitive cracks, manifest at the microscopic as well as the macroscopic level, in relation to crack growth kinetics and the prediction of the remaining safe life of components.