The role of residual dislocation arrays in slip induced cavitation, migration and dynamic recrystallization at grain boundaries

Abstract

Experiments with bicrystals of nickel were designed to study whether different reactions between dislocations at grain boundaries can lead to different effects. The experiments were carried out at 573 K in low cycle fatigue. The large majority of the bicrystals were Σ9, and a few were Σ21. The results can be broadly classified into two groups. In one the dislocations activated in the adjacent crystals were symmetrical and in the other they were nonsymmetrical. The symmetrical case consisted of two types, screw-screw (S-S) where one the dislocations was of opposite sign, and symmetrical-mixed (M-M) where both had an edge component similarly tilted with respect to the boundary plane. The S-S case left no residuals in the boundary plane and produced no effects at the boundary. The M-M case gave rise to a low energy array of edge residuals in the boundary, which led to onset of dynamic recrystallization, but did not produce cavitation nor boundary migration. In the nonsymmetrical group three cases were examined, screw-mixed (S-M), edge sessile-rigid (ES-R, where edge dislocations with their Burgers vector normal to the boundary plane were activated in only one crystal) and edge glissile-rigid (EG-R, Burgers vector parallel to the boundary plane). The S-M reaction led to heavy cavitation and migration (they were inversely related), while the EG-R reaction produced only migration. The ES-R case gave evidence of all three effects to some degree. In summary nonsymmetrical reactions appeared to promote migration and cavitation, while the symmetrical reactions produced predominantly dynamic recrystallization.