Activity of aluminum in molten Ti–Al alloys

The activity of aluminum in molten Ti–Al alloy was measured by `free evaporation methodʹ. The specimen was melted and held at constant temperature in an electron beam vacuum furnace. The weight change was measured and converted to the rate of evaporation of constant composition. The Hertz–Knudsen equation was then applied to calculate the vapor pressure of the element, and the activity was obtained by comparing that with the pressure of the pure substance. The activity coefficient at mole fractions of aluminum less than 0.1 is about 0.03 and that for over 0.2 was about 0.1. The activity of titanium was also calculated by integration of the Gibbs–Duhem equation for a binary mixture based on the activity of aluminum.