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Yamaguchi, Masatake
no journal, ,
From first-principles, we calculated the segregation energy of some solute elements like boron (B), carbon (C), phosphorous (P), and sulfur (S) in bcc Fe Sigma 3 (111) symmetrical tilt grain boundary and on (111) fracture surface with varying the segregation density. We found that the segregation energy on the surface is significantly larger than that in the grain boundary for the embrittling elements like P and S. Therefore, the cohesive energy of the grain boundary, which is the energy difference between fracture surface and grain boundary, can be significantly decreased by P and S segregation. On the contrary, the cohesive energy is increased by B and C segregation. The increase-decrease rate in the calculated cohesive energy by solute segregation is found to be well correlated with experimentally observed shift in ductile-to-brittle transition temperature by solute segregation.
Yamaguchi, Masatake; Ebihara, Kenichi; Itakura, Mitsuhiro; Suzudo, Tomoaki; Kaburaki, Hideo; Kadoyoshi, Tomoko*
no journal, ,
From first-principles, we calculated the trapping energy of hydrogen atoms in bcc Fe Sigma 3 (111) and fcc Al (Cu) Sigma 5 (012) symmetrical tilt grain boundaries and on the fracture surfaces with varying the trapping density of hydrogen. For Fe case, the cohesive energy of the grain boundary is decreased by hydrogen trapping by about 30% at most. Moreover, the cohesive energy can be decreased by about 70% at most if hydrogen in solid solution state moves quickly and then adsorbs onto the newly generated fracture surfaces. For Al case, we find a similar trend as Fe case. For Cu case, on the other hand, the decrease in the cohesive energy of the grain boundary by the hydrogen trapping is much smaller than that for Fe and Al cases. This is because the trapping density of hydrogen atoms is very low in the Cu grain boundary and its fracture surface. The hydrogen trapping may cause brittle fracture in the grain boundaries for Fe and Al.