In the case of the cracking of grain boundary by cyclic creep, it may be considered that grain orientation relations and external stress condition determine whether the grain boundaries are cracked or not. Since the grain orientation relations of adjacent grains at the grain boundary play an important role in grain boundary deformations and cracking during cyclic creep, the analysis of grain orientations is essential to this study.
In order to establish the properties of grain boundaries at which cracks are formed in cyclic creep at 0.7$T_m$, groups of grains adjacent to the free surface of polycrystalline Al-4%Mg solid solution alloy have been analyzed by X-ray back Laue reflection method prior to testing. And subsequently the grain boundaries have been observed with constant creep strain interval during testing to check the grain boundary cracks.
It has thus been found that cracked boundaries are associated with large geometrical factor, and the strong traces of the dominant slip system near the intersection of the boundary with the free surface.
