Influence of alloying additions on grain boundary cohesion of transition metals: First-principles determination and its phenomenological extension

The toughness and ductility of ultrahigh-strength alloys is often limited by intergranular embrittlement, particularly under conditions of unfavorable environmental interactions such as hydrogen embrittlement and stress corrosion cracking. Here we investigated the mechanism by which the segregated substitutional additions cause intergranular embrittlement. An electronic level phenomenological theory is proposed to predict unambiguously the effect of a substitutional alloying addition on grain boundary cohesion of metallic alloys, based on first-principles full-potential linearized augmented plane-wave method (FLAPW) calculations on the strengthening and embrittling effects of the metals Mo and Pd on the Fe grain boundary cohesion. With the bulk properties of substitutional alloying addition A and the matrix element M as inputs, the strengthening or embrittling effect of A at the grain boundary of M can be predicted without carrying out first-principles calculations once the atomic structure of the corresponding clean grain boundary is determined. Predictions of the embrittlement potency of a large number of metals, including the 3d, 4d, and 5d transition metals, are presented for the Fe Σ3 (111) and the Ni Σ5 (210) grain boundaries. Rigorous FLAPW calculations on the effect of Co, Ru, W, and Re on the Fe Σ3 (111) grain boundary and Ca on the Ni Σ5 (210) grain boundary cohesion confirm the predictions of our model. This model is expected to be applicable to other high-angle boundaries in general and instructive in the quantum design of ultrahigh-strength alloys with resistance to intergranular fracture.