Quantification of grain boundary equilibrium segregation by NanoSIMS analysis of bulk samples

Abstract

A technique for the quantification of equilibrium grain boundary segregation by high resolution secondary ion mass spectroscopy (NanoSIMS) on simple metallographically polished surfaces has been demonstrated for the model system of sulphur segregation to nickel grain boundaries. Samples of nickel containing 5.4 wt ppm of sulphur were annealed at different temperatures to achieve different equilibrium sulphur grain boundary concentrations, ranging from less than 1% to about 50% of a monolayer. Quantification was carried out from sulphur concentration profiles acquired across about 20 grain boundaries in each sample. An internal standard (nickel containing a known concentration of sulphur in solid solution) was used for calibration. It is found that, depending on the annealing temperature, the average grain boundary sulphur concentration ranges from 0.9 to 25.8 ng cm–2 (or 1.7 1013 to 4.8 1014 atoms cm–2), i.e. ~0.015 to ~0.43 monolayer. Thermodynamic analysis gives a segregation free energy of −97.8 kJ mol–1 and a grain boundary sulphur concentration at saturation of 26.7 ng cm–2 (or 5.0×1014 atoms cm–2), i.e. ~0.44 monolayer, in good agreement with previous measurements on this system. The limit of detection of the technique is shown to be as low as 0.24 ng cm–2 (or 4.5×1012 atoms cm–2), i.e. ~0.004 monolayer, with a counting time of only 10 min. Copyright © 2012 John Wiley & Sons, Ltd.