SLIP NUCLEATION AT GRAIN BOUNDARIES

Abstract

This project has investigated the phenomenon of slip nucleation at grain boundaries in the pre-macroyield regime. Fe-3.9% Si alloy was chosen as the material for the study due to its high sensitivity to dislocation etching. The specimens were annealed at high temperatures to produce a range of grain sizes and pulled in tension to obtain the yield stress-grain size relationship. Then, a new batch of coarse-grained Fe-3.9%Si specimens were deformed in tensile to progressively higher and higher stress levels below their respective yield stresses and the slip activities occurring at each stress level were studied in details by means of the dislocation etch pit technique. The results show that slip activities in Fe-3.9Si could be detected even at about 70% of the yield stress. At this stress level, about 10% of the grains were found to exhibit slip traces connected to the grain boundaries. The arrangement of dislocations in the slip traces suggests that slip dislocations are generated at the grain boundaries, and that the density of the slip sources varies from boundary to boundary in the material. With increasing stress level, while more slip sources were activated along previously yielded grain boundaries, an increased number of other grain boundaries also showed evidence of slip generation. In some cases, the slip traces have traversed across the host grain and were stopped at the opposite grain boundary. The percentage of grains showing slip activity increases with the normalised applied stress (with respect to the yield stress) but is independent of the grain size. As the applied stress was increased to near the yield stress, more grains yielded and they form clusters. Yielding occurred when the clusters of yielded grains spread and/or join up across the entire width of the specimen gage. Even at yielding, about 50% of the grains remained free of observable slip activities. A model is proposed in which slip nucleation is viewed as facilitated by thermally activated pile-ups of grain boundary dislocations (GBDs), the formation of which is assisted by a high incompatibility stress between the adjoining grains. According to the model, the density of the dislocation sources at grain boundaries is independent of the grain size but it is controlled by factors influencing the formation of pile-ups in the grain boundary, including load level, boundary diffusivity, boundary structure and the boundary plane orientation. The model predicts that slip nucleation at grain boundaries at room conditions occurs progressively from boundary to boundary over the tensile test, with boundaries having a high incompatibility strain and structural grain boundary dislocations (GBDs) with a large glide component showing earlier signs or slip nucleation than those with a larger climb component. Boundaries with GBDs or largely climb character are non-ideal sources of slip nucleation at room conditions. The predictions of the model in general agree with the experimental results obtained in the present work, except the boundary structural effect due to the lack of needed experimental facilities. It is suggested that future work be carried out to attest this aspect of the model's predications.