The embrittling and strengthening effects of hydrogen, boron, and phosphorus on a $\Sigma 5\left(210\right)\mathrm{}$ [100] nickel grain boundary are investigated by means of the full-potential linearized augmented plane-wave method with the generalized-gradient approximation formula. Optimized geometries for both the free surface and grain-boundary systems are obtained by atomic-force calculations. The results obtained show that hydrogen and phosphorus are embrittlers and that boron acts as a cohesion enhancer. An analysis of the atomic, electronic, and magnetic structures indicates that atomic size and the bonding behavior of the impurity with the surrounding nickel atoms play important roles in determining its relative embrittling or cohesion enhancing behavior.

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 $\Sigma 3$ (111) and the Ni $\Sigma 5$ (210) grain boundaries. Rigorous FLAPW calculations on the effect of Co, Ru, W, and Re on the Fe $\Sigma 3$ (111) grain boundary and Ca on the Ni $\Sigma 5$ (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.

Simple structures of $\mathrm{Mn}X$ binary compounds, namely hexagonal NiAs and zincblende, are studied as a function of the anion $(X=\mathrm{Sb},$ As, P) by means of the full-potential linearized augmented plane wave (FLAPW) method within the local spin density and generalized gradient approximations. An accurate analysis of the structural, electronic, and magnetic properties reveals that the cubic structure greatly favors the magnetic alignment in these compounds leading to high magnetic moments and nearly half-metallic behavior for MnSb and MnAs. The effect of the anion chemical species is related to both its size and the possible hybridization with the Mn d states; both contributions are seen to hinder the magnitude of the magnetic moment for small and light anions. Our results are in very good agreement with experiment, where available, and show that the generalized gradient approximation is essential to correctly recover both the equilibrium volume and magnetic moment.