Influence of alloying additions on the impurity induced grain boundary embrittlement

We have investigated the influence of Mo as a substitutional alloying addition on the grain boundary embrittlement induced by interstitial impurity P by using the first-principles full-potential linearized augmented plane wave total energy/atomic force method within the generalized gradient approximation. With Mo segregated to a clean Fe Σ3(111) grain boundary, the embrittling 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 strenthening 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 embrittlement. 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 energy. On the other hand, the strong chemical bonding between Mo and the Fe surface 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 effect of Mn on P embrittlement and is expected to be applicable to more general cases and instructive in quantum design of ultrahigh-strength alloys.