CRYSTALLOGRAPHIC FEATURES OF INTERGRANULAR CRACK INITIATION IN FATIGUED COPPER POLYCRYSTALS

In order to study the mechanism of grain boundary (GB) cracking in fatigued polycrystalline copper, specimens were fatigued in symmetrical push-pull at an intermediate constant plastic strain range at room temperature in dry air. The intergranular cracks were examined under the scanning electron microscope. Many GB cracks were found to have been formed by the impingement of persistent slip bands (PSBs) against the grain boundaries (PSB-GB cracks). The orientations of the grains adjacent to the cracks were determined by electron back-scattering patterns. The misorientations of the cracked boundaries were calculated and the boundary plane orientations were also determined. High-energy grain boundaries were found to be preferred sites for cracking. The activated slip systems in the component grains adjacent to the cracks were determined and analyzed. With these data, the cracking stresses due to the interaction between the PSBs and the boundaries were calculated for the observed PSB-GB cracks in a pile-up type dislocation model in a three-dimensional analysis. The results confirmed that, with reasonable assumptions, the estimated minimum theoretical shear stresses which are required to act in the PSBs for causing PSB-GB cracks were always smaller than the real shear stresses operating in the PSBs.