The preferential segregation of sulphur to the free surface of nickel via grain boundary diffusion

The segregation of sulfur to the free surface of Ni was investigated with particular regard to the influence that grain boundaries and carbon co-segregation may have on the kinetics of the segregation. High purity polycrystalline Ni was used. Specimens were ion wiped with 3.5 keV Ar exp + ions until the surface was free from contamination and examined in a scanning Auger microprobe. It was shown that grain boundary diffusion has a major effect on the segregation kinetics and distribution of S on the free surface at intermediate temperatures (T = 673-723K). In the temperature range of 673-723K, C rapidly segregated to the free surface via bulk diffusion, whereas the segregation of S to the free surface was dominated by preferential grain boundary diffusion. The C formed a graphitic structured overlayer on the surface of the Ni as monolayer coverage was approached and subsequently saturated at a coverage of approximately two monolayers. Initially the segregated S was highly localized around the grain boundary. Following extended aging, the S appeared to attain an equilibrium concentration and formed S-rich zones on either side of the boundary which broadened with time. The S-rich zones extended up to 10-20 mu m from the grain boundary. The rate of the broadening of the zones was thought to be dominated by the grain boundary mobility of S in Ni. Nickel outgrowths were also observed to form at the intersection of the grain boundaries with the free surface.