DEFORMATION EVOLUTION DURING INITIATION OF TRANSGRANULAR STRESS-CORROSION CRACKING

Face centered cubic metals and alloys have multiple slip systems and a re characterized by high dislocation velocities. Nevertheless, these m aterials suffer from transgranular stress corrosion cracking (T-SCC), that occurs by environmentally-induced cleavage. Since plasticity prec edes fracture in all T-SCC phenomena, the evolution of deformation pat terning during TSCC is an important element of the local microfracture mode. Experimental observations show that the presence of the SCC-cau sing environment during straining is promoting localized plastic defor mation at the near-surface region and producing an entirely different deformation pattern compared with that developing in laboratory air. T he deformation evolving in the presence of the SCC electrolyte is high ly localized, exhibiting closely spaced, coarse slip bands. The amount of localized strain developing at the nearsurface region prior to nuc leation of stress corrosion cracks is equivalent to the strain require d for ductile fracture of the material in air, suggesting the existenc e of a fundamental fracture criterion. The above phenomenology of the deformation evolution is considered in relation to T-SCC initiation an d propagation. The T-SCC is suggested to be a macroscopically brittle but microscopically ductile fracture occurring by localized plastic fl ow. An environment-induced deformation localization mechanism is descr ibed where the role of the environment involves generation of vacancie s and subsequent dislocation nucleation from the near-surface region a t loads well below those required for normal yielding. The evolution o f the localized deformation pattern during T-SCC is suggested to be an outcome of nonuniformity and periodicity in the dissolution process.