Influence of surface melting treatment by laser on the segregation of sulphur, nitrogen and chromium on the surface of a single-crystal FeCrNi alloy

After melting the (100) surface of a FeCrNi single crystal by a CWCO2 laser, a quasi‐epitaxial regrowth of the melted zone takes place2 within a misorientation of ∼4°. Three distinct areas are obtained: unaffected, heat affected and melted surfaces. On all areas, the segregation of sulphur alone occurs at high temperature, while there is a cosegregation of sulphur and nitrogen at T < 700°C. The thermodynamic interpretation of the segregation results is reported. For the surface segregation of sulphur, no interactions are noted with other alloy elements, except with chromium on the unaffected surface. The difference between the free enthalpies of segregation of sulphur on the untreated and treated surfaces is associated with the evolution of the defect content. When sulphur and nitrogen cosegregate on the surface of the untreated alloy, a repulsive coefficient of interaction between sulphur and nitrogen (i.e. βSN) of the order of 20 ± 5 kJ mol−1 is calculated. For the laser‐treated surfaces, the chromium content increases on the surface. Then, the regular solid solution model is applied to the CrNS ternary system. The calculated βSN coefficient is now 45 ± 5 kJ mol−1 and agrees with the experimental data obtained from the experimental evolution of ΔG S0 versus YN (with ΔG S0 = free energy of segregation of S and data YN = nitrogen concentration on the surface). More complex interactions are also discussed, especially for nitrogen, on account of the site competition between N and S and the influence of the vacancy content on the treated surfaces.