2020 Volume 106 Issue 6 Pages 382-390
The upper and lower yield points of ferritic steel containing a small amount of carbon were discussed in terms of the critical stress for dislocation emission from a grain boundary, namely, “critical grain boundary shear stress”, on the assumption of the pile-up model. Considering some experimental results such as tensile testing, relaxation testing and nanoindentation testing on grain boundaries, we concluded that both upper and lower yield points could be similarly understood as a phenomenon of dislocation emission from dislocation sources existing at grain boundaries. The difference in stress between upper and lower yield points was explained in terms of the density of mobile dislocations, which determines the extent of stress concentration at grain boundary caused by pile-up of the dislocations. Slow cooling after annealing or aging at low temperature, by which Cottrell atmosphere is formed, leads to a significant decrement of the mobile dislocation density, and this results in an occurrence of the sharp upper yield point because of a reduced number of piled-up dislocations and insufficient stress concentration at grain boundaries.
