Abstract:Excess free energy of the molten binary system and partial mole excess free energy of the components in ternary system molten metal were calculated. A model for calculating the activity and activity coefficient of the molten ternary system alloys was derived based on Miedema’s model to calculate the formation heat of binary alloys, and Toop’s equation to calculate the excess free energy of ternary system solution. The activity coefficient and evaporation rate of the elements in Ti-6Al-4V alloy were also calculated. The results shows that Al is an asymmetrical component, Ti and V are symmetrical ones, the change of the activity coefficients of Ti and V with temperature is small but the change of Al with temperature is very notable, the activity coefficients of Ti and V are close and higher than that of Al in Ti-6Al-4V alloy. When melting Ti alloy in high temperature, Ti is easy to be polluted, and Al is very easy to evaporate and burn.

Key words: ternary system; activity; activity coefficient; evaporation