Directional Effects in the Stress Corrosion Cracking of an Aluminum Alloy

Studies of the relative effects of grain morphology and preferred orientation on the directional susceptibility to intergranular stress corrosion cracking of a 7075-T651 aluminum alloy plate have been conducted. The role of grain morphology was found to be of paramount importance in controlling crack propagation. It was found that a slow rate of propagation along grain boundaries parallel to the applied stress accounts for the low susceptibility of longitudinally stressed specimens. The results suggest that the threshold stress for short transverse specimens approximates the low stress necessary to crack grain boundaries normal to the applied stress, and the threshold stress for longitudinal specimens approximates the high stress needed to propagate cracks along boundaries parallel to the applied stress. Preferred orientation was found to be of secondary importance when compared to the effect of grain morphology. A high degree of preferred orientation, rather than any specific slip plane orientation, was found to increase the susceptibility of specimens with the same grain morphology. Pitting corrosion was found to occur to a much greater extent on unstressed than on stressed (undergoing stress corrosion cracking) specimens. This phenomenon is explained by what is termed an "internal cathodic protection mechanism".