The effects of Mo and Pd segregation upon the cohesion of Σ = 3 (111) grain boundaries were investigated by using the first-principles full-potential linearized augmented plane-wave total-energy atomic-force method in the generalized gradient approximation. The present total-energy calculations, based upon the Rice-Wang model, showed that Mo had a significant beneficial effect upon grain-boundary cohesion, whereas Pd behaved as a weak embrittler. An analysis of the geometry indicated that Mo had an intermediate atomic size which fitted well into the grain-boundary hole, whereas Pd introduced a larger distortion into the atomic structure near to the grain boundary. The elastic energy which was associated with Mo and Pd segregation was estimated using a rigid-environment approximation. It was found that both Mo and Pd introduced a beneficial volume effect. Studies of the electronic structures showed that its strong-bonding nature made Mo a cohesion-enhancer (-0.90eV) for the above grain boundary. On the other hand, its weak-bonding nature caused Pd to be a weak embrittler (0.08eV). The present first-principles quantum-mechanical results supported the idea that the elemental cohesive energy difference between the substitutional element and the host element played an important role in determining its effect upon grain-boundary cohesion. However, numerical results for Pd, which had a similar cohesive energy to that of Fe, indicated that an important role was played by the volume effect. It was suggested that, in a lower-angle grain boundary, which had a greater grain-boundary volume expansion, Pd might become a cohesion-enhancer.

Effect of Mo and Pd on the Grain-Boundary Cohesion of Fe. W.T.Geng, A.J.Freeman, R.Wu, G.B.Olson: Physical Review B, 2000, 62[10], 6208-14