First-principles computational tensile test of γ-Fe grain boundaries considering the effect of magnetism: Electronic origin of grain boundary embrittlement due to Zn segregation

Abstract

The development of high-strength steels requires detailed understanding of the effect of solute elements on \ensuremath{\gamma}-Fe grain boundaries (GBs). In this study, first-principles computational tensile tests (FPCTT) were conducted on \ensuremath{\gamma}-Fe GBs to elucidate the mechanism of GB embrittlement due to Zn segregation. The paramagnetic \ensuremath{\gamma}-Fe GB was simulated by the $\mathrm{\ensuremath{\Sigma}}5\phantom{\rule{0.28em}{0ex}}(310)$ GB in the antiferromagnetic double-layer (AFMD) configuration. The FPCTTs revealed that the fracture stress and fracture energy of the \ensuremath{\gamma}-Fe GB were reduced by Zn segregation, which is consistent with experimental results. Crystal orbital Hamilton population analysis was also performed to investigate the change in electronic states during the tensile process, and the enhancement of GB fracture by Zn segregation is caused by the breaking of the covalent-like bonds between Fe and Zn at a relatively small strain compared to the Fe--Fe bonds. This behavior is attributed to the localized nature of the $3d$ orbitals of Zn in \ensuremath{\gamma}-Fe. The FPCTTs of \ensuremath{\gamma}-Fe GBs using the AFMD properly consider the effect of magnetism in paramagnetic \ensuremath{\gamma}-Fe under tensile strain and is useful for investigating the effects of various solute elements on GB fracture and for the development of high-strength steels.