The cyclic stress-strain response and corresponding dislocation structures of a fatigued [¯345]/[¯117] bi-crystal were investigated for shear strain amplitudes that ranged from 2.8 x 10-4 to 6.45 x 10-3. Like polycrystals, the bi-crystal exhibited very high saturation stresses, and no obvious plateau, but there was a bulge in the cyclic stress-strain curve. The dislocation structures in the fatigued bi-crystal were also similar to those usually observed in cyclically deformed polycrystals and in multiple-slip single crystals. However, the dislocation structures in the 2 component crystals were different. At low strain amplitudes, the prevailing dislocation structure in the [¯345] component consisted of loop patches while the typical dislocation structure in the [¯117] crystal consisted of labyrinth-like loop patches plus persistent slip band ladder structures. Upon increasing the strain amplitude above 8.9 x 10-4, cell structures and labyrinth structures were found in the [¯345] and [¯117] components, respectively. The labyrinth structure was found to contain (001) and (210) walls that were perpendicular to each other. It was also found that the grain boundary acted as an obstacle to the motion of dislocations. Dislocation-free zones, which had previously been found beside grain boundaries in fatigued Cu polycrystals, were not observed in the present fatigued bi-crystals. The formation of labyrinth structures was believed to be caused by the interaction of dislocations with 2 orthogonally oriented Burgers vectors. The occurrence of multiple slip in the 2 components, due to the constraining effect of the grain boundary, was suggested to have produced the observed fatigue deformation behaviors and dislocation structures.

Y.M.Hu, Z.G.Wang: Acta Materialia, 1997, 45[7], 2655-70