A UNIFIED STRUCTURAL MECHANISM FOR INTERGRANULAR AND TRANSGRANULAR CORROSION CRACKING

a unified, structural mechanism is proposed to explain the phenomena of intergranular and transgranular stress corrosion cracking. the dependence of failure on composition and stress in copper alloys andin stainless steels is analyzed in terms of two general conditions 1. a chemically reactive path and 2. a mechanism of concentrating stress across the reactive path. intergranular fracture in a chemical environment is considered in terms of the normal stress distribution at the head of a group of dislocations piled up against a grainboundary. the available data are consistent with the proposed model with respect to 1. a grain size dependence 2. stacking fault energy, which defines the number of coherent twin boundaries per grain and thereby the stress concentration at the boundary and 3 the existence of an endurance stress, approximately equal to the macroscopic yield stress, below which the stress concentration at the boundary is insufficient to cause fracture. the nucleating sites for transgranular fracture in single crystals, and in polycrystalline aggre gates that fail transgranularly, are shown to be cottrell-lomer barriers. a detailed crystallographic mechanism of fracture is proposed which dependson 1 orientation with respect to applied stress and 2. the strength of the barrier, which is related to the stacking fault energy.