THE INFLUENCE OF GRAIN-BOUNDARY GEOMETRY ON INTERGRANULAR CRACK-PROPAGATION IN NI3AL

The distribution of grain boundary types along intergranular cracks in Ni3Al was measured, by SIGMA value, and compared to the distribution in the bulk, using statistically significant sample sizes. It was found that low angle (SIGMA1) and symmetrical SIGMA3 boundaries (twins) are particularly strong, and all high angle boundaries, independent of their SIGMA values are weak. In particular, low SIGMA, high angle boundaries, as a group, are also weak. These results are in qualitative agreement with predictions based on the structural unit model and imply that the fracture strength of an intergranularly brittle polycrystalline aggregate can be increased only by increasing the fraction of low angle and symmetrical SIGMA3 boundaries.