Analysis of grain boundary embrittlement by Cu and Sn in paramagnetic γ−Fe by first-principles computational tensile test

Ironmaking processes using steel scrap are becoming increasingly important in the effort to realize a decarbonized society, but their utilization is limited by the surface hot shortness related to cracking at paramagnetic \ensuremath{\gamma}-Fe grain boundaries (GBs) due to tramp elements such as Cu and Sn. To understand its electronic origin, GB embrittlement due to Cu and Sn was investigated by first-principles computational tensile tests (FPCTTs) on paramagnetic \ensuremath{\gamma}-Fe GBs. The paramagnetic \ensuremath{\gamma}-Fe GB was modeled as the $\mathrm{\ensuremath{\Sigma}}5(310)\phantom{\rule{0.28em}{0ex}}\mathrm{GB}$ in the antiferromagnetic double-layer (AFMD) configuration. Both Cu and Sn reduce the fracture stress, fracture energy, and fracture strain of the \ensuremath{\gamma}-Fe GB. The effect of Sn was more pronounced than that of Cu, which agrees well with experimental results on surface hot shortness. Crystal orbital Hamiltonian population (COHP) analysis of the electronic structure during the tensile processes revealed that the atomic orbitals of Cu are more localized than those of Fe in \ensuremath{\gamma}-Fe, resulting in the breaking of Cu--Fe bonds at a lower strain and stress, which leads to GB embrittlement. In addition to this effect, Sn increases the bond lengths between adjacent Fe atoms and weakens the Fe--Fe bonds, causing significant GB embrittlement. The obtained knowledge contributes to the elucidation of the mechanism of the surface hot shortness. Furthermore, the FPCTT using the \ensuremath{\gamma}-Fe GBs in the AFMD configuration is useful to investigate the effects of various solute elements on paramagnetic \ensuremath{\gamma}-Fe GBs, and is expected to be utilized to improve material properties governed by GB fracture.