The Effect of Loading Orientation on the Stress Corrosion Cracking of Al-Mg Alloys

The effect of loading orientation (S-L versus T-L) on intergranular stress corrosion cracking (IGSCC) behavior was assessed in AA5083 and AA5456 in two tempers (-H116, -H131) at nitric acid mass loss test (NAMLT) levels of 24 mg/cm² and 40 mg/cm². Regardless of temper or composition, loading in the T-L orientation demonstrated reduced susceptibility to IGSCC as a result of delamination toughening caused by fissure propagation in the L-T plane. Electron backscatter detection analysis confirmed these fissures to be intergranular, and the fissure size/density increased in plane strain constraint conditions toward the center of the specimen. Occurrence of these out-of-plane fissures was validated by testing on NAMLT of 10 mg/cm² and 67 mg/cm² AA5456-H116, although delamination propagation occurred only under full immersion in 0.6 M NaCl, and not testing (fatigue or slow rising displacement) conducted in moist air. These delamination phenomena are understood further through application of a hydrogen-enhanced decohesion-based micromechanical model assessing H effects at the crack tip.