Orientations of Transgranular Stress Corrosion Cracks in Austenitic Steels tested in MgCl 2 Solutions

Orientations of stress corrosion cracks have been determined in single crystals of Type 316, Type 310 and a few experimental steels. The cracks are more irregular in Type 316 than in Type 310 which shows a tendency to form {100} facets and {100} crack planes. The electrochemical behaviour of crystal slices did not vary systematically with orientation and, although differences were detected, it is considered that faster repassivation on {100} crystal faces is not the explanation of the observed crack plane orientation. Cross slip of dislocations was identified from slip-traces at the surface and indicates a localisation of slip to surface regions which could be associated with local stress relaxation. Such processes would increase the aspect ratio if they occurred in the bulk and increased the number of dislocations operating in a given crack length. Toward the crack tips fairly regular slip steps of about 30nm spaced at 1μm were observed but smaller steps (10nm) and spacings (70nm) could be detected on shadowed carbon replicas at some cracks; however, the crack advance between slip steps is considerable. Crack growth rates were accelerated at positions of relatively easy slip, for example, at faces where the screw dislocation emitted from the crack tip had relatively thin sections to traverse to emerge as edge dislocations on the adjacent face. A number of factors are considered to contribute to {100} crack face formation but none. are proven exclusive to the others.