A new technique for vapor pressure measurement applied to the Fe−Si−Mg system

Abstract

Since most magnesium additions to nodular cast iron contain a ferrosilicon base, the Fe−Mg−Si system is of particular commercial interest. The purpose of this investigation was to determine the extent of the miscibility between iron and magnesium-rich liquids in the Fe−Mg−Si system and also to determine the magnesium vapor pressure relations within the miscibility envelope. Two-layer melts were equilibrated in specially developed crucibles and sampled under argon pressure up to 15 atm at 2650° and 2750°F. The liquid miscibility gap between the iron-rich and magnesium-rich liquids shows the following relations: at lower silicon levels (5 to 17 pct Si) in the iron-rich layer the magnesium solubility rises slowly from 0.85 to 3 pct. Above 17 pct Si the magnesium solubility increases rapidly, reaching 15.2 pct at 34 pct Si. In the magnesium-rich liquid the iron solubility is little changed (about 3 pct Fe) up to 10 pct Si. Beyond this point the iron solubility rises quickly to reach 21.5 pct at 33 pct Si. Magnesium vapor pressure measurements were made using a boiling technique specifically developed for this investigation. The validity of the method was checked by determining the vapor pressure of pure magnesium over a range from 3 atm to 15 atm. From the data for the two liquid melts, the activity coefficients are calculated. Additional work involving fourth components and temperature effects is described.