Influence of alloying additions on the impurity induced grain boundary embrittlement

We have investigated the influence of Mo as a substitutional alloying addit ion on the grain boundary embrittlement induced by interstitial impurity P by using the first-principles full-potential linearized augmented plane wav e total energy/atomic force method within the generalized gradient approxim ation. With Mo segregated to a clean Fe Sigma3(111) grain boundary, the emb rittling potency of P increases from +0.19 to +0.88 eV, suggesting a strong detrimental ternary effect in P-Mo couple. Since Mo has a direct strengthe ning effect of -0.90 eV, the combined effect of a P-Mo couple is -0.02 eV, which confirms the experimental evidence that Mo overcompensates the P embr ittlement. Since Mo has a larger atomic size than Fe, it reduces the volume available for P at the grain boundary and hence an increased elastic energ y. On the other hand, the strong chemical bonding between Mo and the Fe sur face makes the top Fe layers more saturated and hence a weakened vertical P -Fe bonding. Together, they induce a strong embrittling ternary effect in a P-Mo couple. This understanding can also explain the embrittling ternary e ffect of Mn on P embrittlement and is expected to be applicable to more gen eral cases and instructive in quantum design of ultrahigh-strength alloys. (C) 2001 Elsevier Science Ltd. All rights reserved.