Abstract
Transgranular stress corrosion cracking of notched, solution treated and quenched Mg-8.6 wt % Al alloy rods tested in both salt solution and distilled water environments at 20 C (68 F) has been studied by direct electron diffraction techniques conducted on actual stress corrosion fracture surfaces, optical fractography, scanning electron fractography, and replica electron fractography. The studies revealed that considerable deformation twinning accompanied the formation of a stress corrosion fracture surface and that opposite faces of a stress corrosion crack contained matching faceted (stepped) topography. The detailed faceted topographical patterns were consistent with their origin being associated with underlying deformation twins formed prior to the passage of the crack tip and, frequently, deformation twins were observed adjacent and parallel to the stress corrosion fracture surface. All observations were consistent with a stress corrosion crack propagation process involving the formation of deformation twins at the crack tip and subsequent propagation of the crack along twin boundary interfaces. This process appears to be in fair accord with the known effects of heat treatment and grain size on both the extent of deformation twinning and the occurrence of transgranular stress corrosion cracking in magnesium alloys.
