First-principles thermodynamic calculations of diffusion characteristics of impurities in -iron

鉄中の不純物の拡散特性に関する第一原理熱力学解析

都留 智仁   ; 加治 芳行  

Tsuru, Tomohito; Kaji, Yoshiyuki

照射下にあるステンレス鋼では空孔濃度が上昇し拡散が促進されることから偏析が生じやすく、照射脆化等の大きな要因となる。本研究では、第一原理計算と遷移状態理論を基礎とした熱力学解析を応用することにより、偏析機構の素過程となる粒内拡散について検討を行った。空孔や不純物の形成の自由エネルギーのエントロピーの寄与をフォノン状態密度から評価し、拡散の移動エンタルピーをNEB法を用いて評価した結果、-Fe中では自己拡散や他の不純物に比べて半径の大きなMoの拡散が早くなることを示した。

Since the solute impurities have an effect on the embrittlement through segregation under irradiation, the solute stability of impurity elements is of considerable importance. In the present study, the diffusion characteristics of large-radius impurities in -Fe are investigated by first principles calculations. According to the classical diffusion and transition state theories, diffusion coefficient were evaluated nonempirically. The vacancy formation energy and binding energy between impurity and vacancy were obtained by first principles density functional theory (DFT) calculations. And the migration energy was calculated by nudged elastic band method collaborated with DFT. Consequently, it is confirmed that the binding energy between large-radius impurity and vacancy is larger than that with equivalent size of the solvent element, and that the migration enthalpies of these impurities are quite small compared with self diffusion.