Phase separation in undercooled molten Pd80Si20:: Part I

Three different kinds of morphology are found in undercooled Pd80Si20, and they dominate at different undercooling regimens Delta T, defined as Delta T = T-1 - T-k, where T-1 is the liquidus of Pd80Si20 and T-k is the kinetic crystallization temperature. In the small undercooling regimen, i.e., for Delta T less than or equal to 190 K, the microstructures are typically dendritic precipitation with a eutecticlike background. In the intermediate undercooling regimen, i.e., for 190 less than or equal to Delta T less than or equal to 220 K, spherical morphologies, which arise from nucleation and growth, are identified. In addition, Pd particles are found throughout an entire undercooled specimen. In the large undercooling regimen, i.e., for Delta T greater than or equal to 220 K, a connected structure composed of two subnetworks is found. A sharp decrease in the dimension of the microstructures occurs from the intermediate to the large undercooling regimen. Although the crystalline phases in the intermediate and the large undercooling regimens are the same, the crystal growth rate is too slow to bring about the occurrence of grain refinement. Combining the morphologies observed in the three undercooling regimens and their crystallization behaviors, we conclude that phase separation takes place in undercooled molten Pd80Si20.