Role of nucleation in phase competition in binary Ti-Al alloys

The solidification microstructures of slowly cooled ingots and melt spun ribbons of Ti-50 at.-%Al and Ti-55 at.-%Al have been investigated. in the ingots, alpha was the high temperature primary phase in equilibrium with the melt during solidification. In the ribbons, the nucleation of primary gamma phase was favoured by the high cooling rates achieved in melt spinning. It is shown that metastable phase formation during chill block melt spinning is largely dependent upon nucleation. Growth in the solid state can also be important in the formation of metastable phases depending on the difference in Gibbs free energy Delta G(m)(L-->S) of competing phases.