Directional Anisotropy of Crack Propagation Along $\Sigma$3 Grain Boundary in BCC Fe

TL;DR

This study uses molecular dynamics simulations to reveal that crack propagation along the $ ext{Sigma}$3 twin boundary in BCC Fe exhibits directional anisotropy, with different ductile or semi-brittle behaviors depending on crack orientation.

Contribution

It demonstrates the directional dependence of crack growth along the coherent twin boundary in BCC Fe, linked to dislocation asymmetry, which was not previously characterized.

Findings

Crack grows differently along opposite directions of the boundary.

Crack behavior varies between ductile and semi-brittle modes.

Anisotropy is attributed to dislocation twinning-antitwinning asymmetry.

Abstract

Crack growth behaviour along the coherent twin boundary (CTB), i.e., $\Sigma$3{112} of BCC Fe is investigated using molecular dynamics (MD) simulations. The growth of an atomistically sharp crack with {112}$<$110$>$ orientation has been examined along the two opposite $<$111$>$ directions of CTB under mode-I loading at a constant strain rate. Separate MD simulations were carried out with crack inserted in the left side, right side and middle of the specimen model system. The results indicate that the crack grows differently along the two opposite $<$111$>$ directions. In case of a crack inserted in the left side, the crack grows in ductile manner, while it propagates in semi-brittle manner in the case of crack inserted in the right side. The directional dependence of crack growth along the CTB is also confirmed by the stress-strain behaviour. This anisotropy in crack growth behaviour has been attributed to the twinning-antitwinning asymmetry of 1/6$<$111$>$ partial dislocations on {112} planes.