On the onset of low-energy dislocation substructures in fatigue: Grain size effects

It has long been known that grain boundaries are effective sources and traps for dislocations. It is also known that geometrically necessary dislocations that have formed during cycling can rearrange into low-energy structures. Such structures have been evaluated in polycrystalline high-strength low-alloy steels of two grain sizes. A selected area channeling pattern technique was used in conjuction with transmission electron microscopy to follow dislocation structure evolution. With accumulated plastic strain, the coarsegrained (approximately 85 μm) microstructure developed more misorientations within single grains, while the fine-grained (approximately 6 μm) structure showed more homogeneous slip. This could be explained by considering two factors. The first is that coarse grains do not develop as much constraint during deformation as fine grains do. Therefore, multiple slip processes accommodate strain in coarse grains more easily, whereas whole grain rotations occur in fine-grained materials. The second factor is that the dislocation density was initially higher in the fine-grained structure. This would reduce the dislocation mean free path from the start of deformation, thereby reducing the ability to form lower-energy structures. Implications of these findings on fatigue crack initiation will also be discussed.