How to control melt composition is the key for getting high quality alloy m
elt. The paper made the following efforts: (1) The activity coefficients in
Ti-15-3 melt have been calculated. (2) Taking advantage of the activity co
efficients, the evaporation losses of components in Ti-15-3 melt during ISM
process have been studied. The calculated results show that there is a cri
tical vacuum degree (about 1.33 Pa) during melting process.