Strain-induced dislocation emission from grain boundaries in stainless steel

Abstract

The uniaxial tensile straining of stainless steel 304 sheet material and transmission electron microscope observations of representative electron-transparent thin sections prepared from variously strained samples showed that dislocation profiles extending from the grain boundaries, and associated with ledges on the boundary plane, increase in frequency (the number of profiles per unit length of grain boundary plane) with increasing strain. Because of the nature of these profiles, gleaned from numerous observations, the majority are considered to be emission profiles, particularly at low plastic strains ( ϵ p ⩽ 2%). Consequently, grain boundaries in stainless steel are concluded to be the principal sources for initial dislocations. These conclusions were supported by the in situ straining of thin microtensile specimens of stainless steel 304 and direct observations of dislocation emission from grain boundaries in a high voltage electron microscope. In these observations, dislocation profiles resembling dislocation pile-ups were observed to form at grain boundary ledges, and ledges were observed to form by the glide motion of dislocations in the grain boundary plane. Grain boundary dislocations moving in the interface plane were observed in some cases to dissociate apparently into lattice dislocations which were emitted from the grain boundary to form profiles gliding on the {111} planes. Although in situ high voltage electron microscopy experiments proved to be extremely difficult, unpredictable and lacking in the ability to record elegant and sequential images attesting to the dynamic features of dislocation emission from grain boundaries in response to an applied strain, the results obtained confirm the feasibility of the experiments and provide direct evidence and corroboration for conclusions drawn from standard post-deformation experiments.