Enhanced grain boundary embrittlement of an Fe grain boundary segregated by hydrogen (H)
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First-principles fully relaxed tensile and shear test simulations were performed on R3ð111Þ=½110 tilt Fe grain-boundaries (GBs) with and without hydrogen (H) segregation, to investigate the mechanisms of GB embrittlement enhanced by H segregation. Premature fracture was found in the H-segregated GB, compared with the clean GB, in the tensile test simulations. The Fe–H bond showed covalent-like and ion-like characteristics. The covalent-like characteristics reinforced the Fe–Fe bonds, but the ion-like characteristics weakened the Fe–Fe bonds as a result of charge transfer. The effect of the latter increased with increasing strain, and prevailed over the former, resulting in GB embrittlement. In the shear test simulations, variation in the GB energy for the H-segregated GB was almost the same as that for the clean GB. This is because bond-breaking and rebonding occur concurrently in GB shearing and the variations in charge transfer during shear straining are less than those during tensile straining. I. INTRODUCTION
Grain-boundary (GB) embrittlement in metals is often enhanced by impurity segregation at the GBs. Hydrogen (H)-induced GB embrittlement is one of the most critical problems in GB embrittlement, and it is still under debate.1,2 McMahon showed that there are two types of H-induced GB embrittlement of steels: one is related to a combination of H and other impurities segregated at the GBs and the other results from the presence of H atoms only.3 H atoms dissolve in the GB and presence of only the H atoms causes GB embrittlement because iron hydrides are not formed except under the limited conditions of high pressure and temperature.4 In such a situation, the effects of impurities on GB embrittlement can be estimated using the Rice–Wang thermodynamics model,5 in which the GB embrittlement is enhanced by impurities when the segregation energy at the free surface is lower than that at the GB. H segregation reduces the surface energy more than it reduces the GB energy,6,7 which agrees with the observed enhancement of GB embrittlement by H segregation. Recently, Yamaguchi et al.8 investigated the “mobile” effect of H on GB embrittlement, based on the Rice–Wang thermodynamics model. Also, Farkas et al.9 showed by dynamic simulations of GB fracture of Fe that a crack proceeds without deflection to the inside of the grains in H-segregated Fe GBs. Mechanisms of the enhanced embrittlement of Fe GBs by H segregation have been investigated from the viewpoint of electronic structure, and it a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.111 J. Mater. Res., Vol. 27, No. 12, Jun 28, 2012
has been demonstrated that a charge transfer mechanism is responsible for the H-induced GB embrittlement of Fe.6,10,11 The electron populations of Fe–Fe bonds in the GB plane can be decreased by 60% by charge transfer,11 where the 4s and 4p electrons play a dominant role.10 The H–Fe bonding is ion-like,6 indicating that the charge transfer is due to ion-like characteristics. Gesari
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