DEPLETION EFFECTS IN SURFACE SEGREGATION OF SULFUR FROM POLYCRYSTALLINE HIGH-PURITY ALPHA-IRON

Recently, scanning Auger microscopical investigations of repeated surface segregation of non-metallic impurities like sulfur, nitrogen and carbon from high-purity polycrystalline alpha-iron have revealed significant differences in the segregation kinetics of sulfur at the surfaces of the individual grains. This is a consequence of the different extent of the sulfur depletion in a subsurface zone which extends to several 10 mum into the bulk. These differences diminish when surface diffusion of sulfur from grains which have nearly reached the equilibrium S coverage leads to an almost homogeneous equilibrium distribution of sulfur. A computer model is presented which combines the advantages of the layer-by-layer approach and the solutions of the continuous diffusion equations and allows the segregation kinetics to be calculated of highly dilute multi-component alloys. Model calculations performed to simulate the segregation kinetics of sulfur from iron are in good agreement with experimental results.