Metastable phase formation during solidification of undercooled melt

Undercooling behaviour of micron-sized droplets entrained in the matrix of the primary phase of Ag–Ge, Al–Ge and Cu–Ag alloys was investigated. Thermal cycle consisted of equilibration of hypoeutectic alloys above the eutectic temperature in two phase liquid–solid region followed by their slow cooling or quenching from the mushy state. Repeated thermal cycling generated a wide size range of droplets. During slow cooling droplets exhibited a large undercooling below the eutectic temperature of alloys. Solidification structure of the undercooled droplets revealed presence of metastable phases in Ag–Ge and Al–Ge alloys and a non-equilibrium solid solution phase in Cu–Ag alloy. Solidification behaviour of undercooled melt and formation of metastable microstructure are discussed.