Influence of laser melting treatment on surface composition of Fe–17Cr–13Ni single crystal alloy Part 2 Surface segregation of nitrogen and sulphur

Abstract

An Fe–17Cr–13Ni (wt-%) single crystal alloy was melted using a continuous wave (CW) CO2 laser. The surface segregation of sulphur and nitrogen at high temperature in vacuum is investigated on the various surface zones (base alloy, heat affected zone, and melted zone) using Auger electron spectroscopy (AES). Below ~700°C, cosegregation of sulphur and nitrogen is observed, while above 700°C, only sulphur segregates on the surface on all three zones. The segregation of sulphur or the cosegregation of sulphur and nitrogen induce, on the three zones, the segregation of chromium in the first atomic plane. The enthalpy for the surface segregation of sulphur ∆HsegS is found to be −164±10 kJ mol−1 for the base alloy. The values of ∆HsegS are about 40% lower for the treated zones. The result for the base alloy is in agreement with the segregation enthalpy calculated from the reported data on adsorption and dissolution of sulphur, i.e. ∆HsegS ≍−157 kJ mol−1. The pseudodiffusion parameters for sulphur and nitrogen are determined. The activation energy for nitrogen diffusion is the same for all three zones and is in agreement with literature data (~−180 kJ mol−1). For sulphur, the energies for diffusion are highly dependent on the laser treatment. The activation energy for diffusion is maximum in the melted zone: ~ 146 kJ mol−1; the values in the untreated and heat affected zones are 75 and 85 kJ mol−1, respectively.MST/1175