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First-principles study of the grain-boundary embrittlement of metals

第一原理計算による粒界脆化の研究

山口 正剛; 海老原 健一; 板倉 充洋; 鈴土 知明; 蕪木 英雄

Yamaguchi, Masatake; Ebihara, Kenichi; Itakura, Mitsuhiro; Suzudo, Tomoaki; Kaburaki, Hideo

粒界脆化のメカニズムは長年の間謎とされ、溶質元素がどのぐらい粒界偏析するとどのぐらい粒界の凝集エネルギーが低下するのかもわからなかった。われわれは第一原理計算により、鉄粒界の粒界凝集エネルギーがホウ素,炭素,リン,硫黄元素の偏析によってどの程度変化するのかを計算した。その結果、ホウ素,炭素は粒界凝集エネルギーを増大させ、逆にリンと硫黄は粒界凝集エネルギーを減少させることを示した。その変化分は、実験的に観測されている延性脆性遷移温度の変化とよく相関しており、粒界凝集エネルギーが粒界脆化の鍵を握る物理量であることを示している。

It is not known in detail how much solute atoms segregate in grain boundaries of metals and how much the cohesive energy (work of fracture) of grain boundary is decreased by the segregation. 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 with varying the segregation density. We find that these elements can segregate up to a high concentration in the grain boundary. We also find that the segregation energy on the fracture surface is significantly larger than that in the grain boundary for the embrittling elements like P and S. 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.

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