Transformations in undercooled molten Pd 40.5 Ni 40.5 P 19
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Transformations in undercooled molten Pd40.5 Ni40.5 P19 C. W. Yuen and H. W. Kui Department of Physics, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong (Received 16 July 1996; accepted 28 January 1998)
It was demonstrated that liquid phase separation by nucleation and growth (LNG) occurs in undercooled molten Pd40.5 Ni40.5 P19 for undercoolings DT < 60 K (DT Tl 2 T where Tl is the liquidus and T is the kinetic crystallization temperature), and liquid state spinodal decomposition (LSD) occurs for DT > 100. For 60 < DT < 100 K, it is the transition regime from LNG to LSD. A ternary phase diagram is introduced to summarize the reactions occurred in undercooled molten Pd40.5 Ni40.5 P19 . Finally, it is suggested that LSD has an important impact on glass-forming ability of metallic alloys.
I. INTRODUCTION
In a eutectic alloy, the like species prefer to stay together. When the alloy is in the molten state at an elevated temperature, the entropic contribution in G H 2 TS forces complete mixing of the constituent elements at all compositions. On the other hand, when the temperature of the molten specimen is low enough, even lower than the liquidus Tl of the alloy, and if the alloy can remain as a melt, the TS term may no longer dominate and phase separation can take place in the liquid state. Furthermore, if the free energy curve of the liquid is a continuous function of composition like that of the monotectic system, phase separation both by nucleation and growth, and by spinodal decomposition is expected to happen at different undercooling regimes. Chen and Turnbull1 found that when a glassy Pd– Cu–Si (minute amount of Cu) specimen is heated up from below its glass transition temperature Tg to just slightly above Tg , it undergoes phase separation. Crystallization interrupts on further heating of the glassy specimen. Chou and Turnbull,2 by small angle x-ray scattering method, confirmed that the phase separation taken place in amorphous Pd–Au–Si is consistent with predictions of spinodal decomposition. Later, amorphous phase separation was observed in other systems as well, for example, in Pd40.5 Ni40.5 P19 by Chen.3 Indeed Tanner and Ray4 observed amorphous phase separation in asquenched Ni–Ti–Zr–B glass by transmission electron microscopy (TEM) method. Since then, phase separation in amorphous metallic alloys and undercooled alloys melt has been studied quite intensely by a number of research groups.5–9 However, the most definite experimental confirmation of the phase separation mechanism still remains to be solved. From the above observations, it appears that the difficulty in demonstrating liquid phase separation in undercooled molten eutectic alloy is the interruption of crystallization deep in the undercooling regime. Experience10 indicates that with the removal of heteroge3034
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J. Mater. Res., Vol. 13, No. 11, Nov 1998
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neous impurities from an alloy melt, it can be undercooled substantially belo
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