Effect of crystal grain orientation on grain boundary fracture in peak-aged polycrystalline Al-Mg-Si alloys

Grain boundary fracture was studied in tensile test for peak-aged balanced and excess Si Al-Mg₂Si alloys. The route of crack propagation for intergranular fracture in these alloys was considered by Schmid factor (S.F.) and stress transmission factor (N_ij) by crystal orientation to tensile axis. The texture of these alloys before tensile test was recrystallized textures, and there were no significant difference of initial texture between these alloys. Fractured grain boundaries has been classified into the following four groups: • Group A: Larger difference in S.F. than 0.05 and higher N_ij than 0.5. • Group B: Larger difference in S.F. than 0.05 and lower N_ij than 0.5. • Group C: Smaller difference in S.F. than 0.05 and higher N_ij. than 0.5. • Group D: Smaller difference in S.F. than 0.05 and lower N_ij than 0.5. The cracking was influenced by the grain boundary with larger difference in S.F. and lower N_ij in the balanced alloy. The propagation of cracks was influenced by grain boundaries of group B, C, or D with the angles from 60° to 90° to the tensile axis. The cracking and propagation of cracks occurred all together in excess Si alloy at the same time as beginning of tensile test, and grain boundaries in this alloy mostly fractured in the group D with the angle of 30°-90° to the tensile axis.