SAM INVESTIGATIONS OF THE SEGREGATION OF SULFUR IN POLYCRYSTALLINE ALPHA-IRON

Scanning Auger microscopy (SAM) and AES investigations of the segregation of non-metal impurity atoms (sulphur, nitrogen, carbon and phosphorus) in polycrystalline high-purity (< 99.99%) alpha-iron samples were performed in the temperature range 500-800-degrees-C. At the beginning of the heating, nitrogen segregates homogeneously and quickly but in the equilibrium state the whole surface is covered with nearly half a monolayer of sulphur. During the first measurements, the segregation of sulphur was ruled by a combined bulk and grain boundary diffusion mechanism with a t1/2 law, resulting in a net-shaped structure of the surface concentration of sulphur. After repeated heating and sputtering, the contribution of grain boundary diffusion becomes smaller and the segregation rate of sulphur decreases as a consequence of sulphur depletion of the bulk. The segregation kinetics of the individual grains differ strongly owing to the differences between the extent of sulphur depletion of the grains and can be described very well by a t3/2 law in the initial stage of the segregation process. In a later phase, surface diffusion of sulphur from the grains, which have almost reached the equilibrium sulphur coverage, leads to an almost homogeneous sulphur coverage of the surface with only small and diffuse lateral variations.