中文摘要：

利用第一原理理论计算了轻杂质P、N在体心Fe∑3[110]（111）晶界和Fe（111）表面的偏聚能，从能量方面对P、N的韧脆机制进行了探讨．结果表明，P在晶界七的偏聚能比表面七更高，更有利于偏聚在表面而非晶界，而N则相反．因此，P为脆性杂质，N为韧性杂质，此结果与实验观察相符．还分析了造成这一结果的化学因素和力学因素的影响，指出了在计算中对原子结构进行驰豫的必要性．

英文摘要：

By use of the first principles theory, the segregation energies of light impurities P.N on the ∑3[110]（111） grain boundary （GB） and the （111） surface of body-centered cubic Fe were computed, and the brittleductile mechanisms of P.N were discussed energetically. It was found that the segregation energy of P on the GB is higher than on the surface, which benefits its segregation on the surface than on the GB, and N is opposite. Therefore, P is a brittle impuity and N a ductile impurity, in accordance with the experimental investigations. In addition, the influences of relevant chemical and mechanical factors were analyzed, and the necessity of the atomics tructure relax in the computation is emphasized.