Sulfur-induced embrittlement of nickel: a first-principles study

We study the embrittlement of Ni due to the presence of S impurities, considering their effect in the bulk and at grain boundaries (GBs). For bulk Ni, we employ Rice's theory based on generalized-stacking-fault energetics and the unstable stacking energy criterion. We use first-principles density-functional-theory calculations to determine the ductility parameter D = γ s / γ us , the ratio of the surface energy γ s to the unstable stacking energy γ us , for bulk Ni with substitutional S impurities. Similar arguments based on Rice's theory for the mechanical properties of GBs are invoked. We study the Σ5(0 1 2) GB with interstitial S impurities, in which case D is defined as the ratio of the work of separation W s and the unstable stacking energy γ us , to model the competition between grain decohesion and shear-induced plastic deformation due to grain boundary sliding (GBS). The presence of S impurities is found to increase the value of D by ∼40% in bulk Ni, but reduces it by over 80% for the GB. These results support earlier suggestions that embrittlement of Ni by S impurities is related to their effect on GBs. We further calculate relevant tensile and shear stresses to study the expected fracture mode and find that intergranular crack propagation accommodated by GBS is inhibited in the system considered here.