Nucleation of β/α phases in undercooled Ti-48(at%)Al peritectic alloy

The Ti-48(at%)Al peritectic alloy was solidified at various bulk undercooling using electromagnetic levitation method. The maximum undercooling of the alloy melt was up to 295K. Detailed thermal histories were acquired during experiments using optical pyrometry. The solidification microstructures were investigated by OM. Solidification and other high-temperature transformation pathways were deduced from the thermal data and microstructural analysis. The dependence of nucleation between the primary phase and the secondary phase on the undercooling was identified by analyzing the recalescence behaviour and microstructure, and investigated systematically by using the classical solidification theory, the negentropic model and the transient nucleation theory. The interfacial critical work of nucleation, steady state nucleation rates, incubation time and time dependent nucleation rate of two phases β(bcc) phase and α(hcp) phase were calculated, and the calculated results were analyzed by comparing with the experimental data. The results are as fellows: β phase is always prior to nucleate from all undercooled(≤295K) melt below the cooling rate ~10K/s.