Atomic-level study on the interaction of plastic slip with S3{112} tilt grain boundary and {112} twins in bcc metals

Abstract

The Σ3{112} tilt grain boundary (GB) is found in many grains in bcc polycrystalline metals due to its low energy and high stability. Moreover, it is the coherent boundary of the {112} twin. This paper studies the interaction of a pileup of 1/2⟨111⟩ dislocations with the {112} GB, extendable to the coherent {112} twin boundary (TB). The results are applied to the interaction of the pileup of dislocations with the {112} twin. The interacting dislocation is transformed into a GB dislocation (or TB dislocation) that acts as a source of disconnections responsible for the shear-coupled GB migration leading to twin growth or shrinkage when the interface is a TB. While a single dislocation cannot be transmitted through the interface, the stress field of the pileup facilitates the transmission if the tensile part of the dislocation core is closer to the interface than the compression part. The {112} twin is found to create barriers to the motion of 1/2⟨111⟩ crystal dislocations, and the strength of the barrier depends on crystallographic parameters. The results obtained in the slip-TB interaction prove that there is no transmission of dislocations through the twin. Thus, under twinning shear stress, all twins are strong obstacles for the glide of dislocations. Under antitwinnning shear stress, twins with thickness less than a few nanometers (5.6 nm in Fe) are annihilated by the interaction with a pileup of dislocations, contributing to softening, whereas thicker twins block the propagation of dislocations and confine dislocations inside the twin, which contributes to hardening.