Segregation of niobium solute in nickel toward grain boundaries and free surfaces

The spatial redistribution of niobium atoms near the (100) and (111) free s urfaces and selected grain boundaries (GBs) of pure nickel has been conside red in the low niobium concentration limit. There is one key difference bet ween the situation for placing a niobium atom at a free surface and at a GB , At the free surface, an energetic comp remise is required between having space for the large niobium atom and being able to place that atom at a pos ition of high electron density. For a GB, no such compromise is required. A n extremely interesting feature is the presence of a region around the thir d layer of (111) and the fourth layer of (100) free surfaces where the subs titutional internal energy reverses its sign, The authors' simulations show a significant depletion in concentration of niobium immediately at free su rfaces. However, under the first two or three layers of pure nickel, there exists a niobium-enriched region with a strongly temperature-dependent conc entration. This predicted nonmonotonic distribution of niobium in the surfa ce region may be important for many applications and calls for experimental confirmation. In contrast, at the grain boundaries, the concentration of n iobium, which is pertinent to GB oxidation embrittlement, is predicted to b e much higher than in the bulk. It monotonically decreases with the distanc e from the GB until reaching the bulk value. The calculation of the free en ergy uses atomistic potentials based on ab initio quantum mechanical calcul ations, includes lattice relaxation around niobium atoms by using molecular dynamics (with 1440 or 2880 atoms in the modeling cell), and includes vibr ational entropy phenomenologically within the local harmonic approximation. The entire approach is ab initio based and does not require any empirical information.