Metastable liquid phase separation in undercooled molten Pd 40.5 Ni 40.5 P 19
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Metastable liquid phase separation in undercooled molten Pd40.5 Ni40.5 P19 C. W. Yuen, K. L. Lee, and H. W. Kui Department of Physics, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong (Received 11 March 1996; accepted 30 October 1996)
It was demonstrated that molten Pd40.5 Ni40.5 P19 undergoes liquid state phase separation in the undercooling regime DT T1 2 T where T1 is the liquidus of Pd40.5 Ni40.5 P19 and T is the kinetic crystallization temperature. Liquid state phase separation by nucleation and growth takes place for DT < 60 K while that by spinodal decomposition occurs for DT > 100 K. Microstructural analysis of the undercooled specimen obtained in the undercooling regime of 60 < DT < 100 K indicates that it is the transition regime. Finally, it was found that when undercooled molten Pd40.5 Ni40.5 P19 undergoes liquid state spinodal decomposition, it first decomposes into two liquid networks, which is finally replaced by a system of three liquid networks.
Recently, Lee and Kui1 demonstrated that molten Pd80 Si20 undergoes metastable liquid state phase separation (MLPS) in the undercooling regime defined as DT T1 2 T where T1 is the liquidus of Pd80 Si20 and T is the kinetic crystallization temperature. There are two mechanisms for MLPS which are metastable liquid state phase separation by nucleation and growth (MLNG), and by spinodal decomposition (MLSD). Physically, MLPS is possible in undercooled molten Pd80 Si20 for two reasons. First, it is a eutectic alloy and there is apparent partial miscibility in the liquid state deep in the undercooling regime. Second, it can be undercooled to a temperature regime where the partial miscibility is revealed. Chen,2 through thermal analysis by a differential scanning calorimeter (DSC), found that glassy Pd40.5 Ni40.5 P19 specimens undergo partial phase separation as they are heated up from below the glass transition temperature Tg to just slightly above Tg . Further heating would lead to crystallization. Since the phase separation is far from completion, it is difficult to draw meaningful conclusions. In this study, MLPS was investigated by reversing the process: Molten specimen, after purification, was undercooled to way below its liquidus to allow the phase-separation reaction to take place. Recently, Chen3 had predicted the occurrence of two intermediate spinodal networks before the appearance of the final three spinodal networks for a ternary spinodal system. The ternary alloy Pd40.5 Ni40.5 P19 , due to its high resistance to crystallization,4 is an ideal system in confirming the prediction. In this paper, we reported the MLPS in undercooled molten Pd40.5 Ni40.5 P19 . Pd40.5 Ni40.5 P19 specimens were prepared by first melting Ni2 P powder (99.9% pure) in a clean fused silica tube under a vacuum of ,10–3 Torr. The resulting ingots were then put into another clean fused silica tube 314
http://journals.cambridge.org
J. Mater. Res., Vol. 12, No. 2, Feb 1997
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FIG. 1. Thermal histo
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