Phase separation in undercooled molten Pd 80 Si 20 : Part I
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Phase separation in undercooled molten Pd80Si20: Part I K.L. Lee and H.W. Kui Department of Physics, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong (Received 22 December 1997; accepted 10 June 1999)
Three different kinds of morphology are found in undercooled Pd80Si20, and they dominate at different undercooling regimens ⌬T, defined as ⌬T ⳱ T1 − Tk, where T1 is the liquidus of Pd80Si20 and Tk is the kinetic crystallization temperature. In the small undercooling regimen, i.e., for ⌬T ⱕ 190 K, the microstructures are typically dendritic precipitation with a eutecticlike background. In the intermediate undercooling regimen, i.e., for 190 ⱕ ⌬T ⱕ 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 ⌬T ⱖ 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.
I. INTRODUCTION
By definition, the attraction between the like species in a eutectic alloy is stronger than that between the unlike species. If the composition of the system is allowed to vary, in the free-energy diagram the free energy of the equilibrium solids can be described either by a single smooth curve with a hump in between or by a few different curves. The behavior in the former is termed partial miscibility in the solid state. Consider the same eutectic alloy system (with partial miscibility) in the liquid state. If the interaction between the same species remains stronger than that between the unlike species, it is plausible to assume that when the temperature of the melt is low enough, its free-energy curve is also a smooth line with a hump in between, the same as its crystalline counterpart. Indeed, because it is not necessary to meet the long-range order in a liquid, even if there are independent free-energy curves for its corresponding equilibrium solids, the free-energy curve of the liquid system can be described by a single smooth curve. In short, at high temperatures, because of the entropic contribution, a eutectic alloy melt is homogeneous. On the other hand, at low temperatures liquid state phase separation would occur because the like species in the melt prefer to stay together. Equivalently, a liquid miscibility gap is expected for the eutectic alloy system when composition also varies. J. Mater. Res., Vol. 14, No. 9, Sep 1999
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A liquid has larger configurational entropy S b
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