Characteristics of grain boundaries on intergranular fractured Al-Zn-Mg alloy aged at peak condition

In this paper, the phenomenon of intergranular fracture in a peak-aged Al-6.7%Zn-2.8%Mg-0.4%Cu alloy has been investigated by an electron backscattered diffraction (EBSD) technique and the effect of crystallographic orientation on the crack propagation was clarified. Individual grain boundaries were divided into four groups based on their difference in Schmidt factor (S.F.) and stress transfer factor (N_ij). The difference in S.F. higher than 0.05 corresponds with the groups A and B and lower that 0.05 is characteristic for the groups C and D. N_ij factor higher than 0.5 is specific for the groups A and C and the groups B and D having this factor lower than 0.5. It has been found that grainboundary fracture occurs at the boundaries belonging to the group D in 46% of all cases, in 24% corresponding with the group B and in 23% with the group C. Preferential deformation in the vicinity of grainboundaries belonging to the group B has been also verified by an atomic force microscopy (AFM) technique as the same as a case in the peak-aged Al-Mg-Si alloy.