Thermodynamic modeling of Al-Ce-Mg phase equilibria coupled with key experiments

The ternary Al-Ce-Mg phase diagram was calculated using the Calphad method and investigated with selected key experiments. Arc melted alloys were annealed at 400 C for 500 h and the phases were analyzed using quantitative X-ray powder diffraction (XRD). Differential thermal analysis (DTA) was also performed on an alloy with a composition near the ternary phase Al13CeMg6 (tau). Temperatures above 1000 degreesC could be attained due to a special sealing of the sample under argon by welding in a tantalum crucible to avoid evaporation and oxidation. Only with this procedure could reproducible and reliable DTA signals be obtained. The present experimental investigation and the consistent thermodynamic calculation show that the ternary phase Ce(Mg,Al)(2), seemingly isolated in the ternary at 400 degreesC, can be rationalized as a single solid solution phase between the binary end members if a larger temperature range and a solid state miscibility gap is considered. It is demonstrated that previously reported low values of ternary liquidus temperatures must be related to other phase equilibria. The actually found ternary liquidus temperatures are much higher and widely governed by the high melting compound Ce(Al,Mg)(2) and also by Al11Ce3 with primary solidification fields stretching far into the ternary system. (C) 2002 Elsevier Science Ltd. All rights reserved.