Abstract
Equilibrium segregation of sulphur on thefree surface of an Fe–36Ni (wt-%) Invar alloy was studied using Auger electron spectroscopy. It was found that sulphur segregates strongly in the temperature range 600–1100°C. The equilibrium segregation of sulphur can be described using the Fowler equation and a value of the free energy can be determined: −114Z·87 kJ mol−1. At high temperatures (1100°C), sulphur disappears completely from the surface leaving the phosphorus to form presumably a phosphide layer on which sulphur cannot segregate. At lower temperatures (≤700°C), phosphorus cosegregates temporarily with sulphur, then reappears in the alloy matrix. The compound BN precipitates on the surface only if the material is both prior annealed at a temperature higher than the precipitation temperature and ion bombarded at room temperature. In spite of the strong surface segregation of sulphur, the Fe–36Ni alloy is never brittle at room temperature. It is highly probable that manganese drastically decreases the solubility of sulphur.
MST/1134
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