Two-liquid Separation in Undercooled Co-Cu Alloys
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CC3.17.1
Two-liquid Separation in Undercooled Co-Cu Alloys
X. Y. Lu1,2, M. Kolbe1, D. M. Herlach1 1
Institute of Space Simulation, German Aerospace Center (DLR), D-51170 Cologne, Germany Department of Applied Physics, Northwestern Polytechnical University, P O Box 624, Xian 710072, P. R. China 2
ABSTRACT Electromagnetic levitation and drop tube processing were used to investigate the solidification behaviour of Co50Cu50 alloys. Deep undercooling was achieved down to 241 K, the alloy melts separated into Co-rich L1-phase and Cu-rich L2-phase before solidification. In drop tube processing, the Cu-rich phases disperse more homogeneously than in EML. This is due to the reduced gravity and high cooling rate during free fall and solidification.
INTRODUCTION Two-liquid phase separation can occur within either a stable or metastable miscibility gap. The former can be found in the monotectic systems [1] as an equilibrium state, while the latter lies beneath the liquidus curve. Cobalt-Copper is a model system for the latter situation. It has nearly flat liquidus line and retrograde solidus line and hence displays a thermodynamic tendency towards the formation of a miscibility gap in undercooled liquid state [2]. In general, the alloy melt is miscible above the liquidus line. But the alloy melt will separate into Co-rich L1-phase and Cu-rich L2-phase when it is undercooled below the binodal temperature of the metastable miscibility gap. The undercooling behaviour of Co-Cu alloys has been studied intensively, and the existence of a miscibility gap in the undercooled region was reported as early as 1958 by Nakagawa, who measured the change of magnetic susceptibility during successive cooling [2]. Later on, it was studied by Elder and coworkers [3]. The solidification path and the resulting microstructure as a function of concentration and undercooling were also studied by Munitz and Abbaschian [4-6]. Yamauchi et al. [7] investigated the undercooling and its effect on solidification structure by glass fluxing methods. Sun et al. [8] studied the supercooling and liquid phase separation by using isothermal soaking at different supercooling rates. And more recently, the location of the metastable miscibility gap was reinvestigated and determined with better accuracy [9]. In this work, in order to investigate the effect of undercooling and cooling rate on liquid phase separation, methods such as electromagnetic levitation and drop tube are used to study the microstructure evolution of Co50Cu50 alloys in the metastable miscibility gap.
CC3.17.2
EXPERIMENTAL DETAILS In the present work, microstructure evolution of Co50Cu50 alloys under different solidification conditions was investigated. The alloys weighing about 1 g were prepared from 99.999% pure Cu and 99.998% pure Co by arc-melting under Ar atmosphere. The alloys were processed by electromagnetic levitation (EML) facility and in an 8-m drop tube (DT). The details of the EML facility and drop tube are based on the facilities described elsewhere [10,11]. After experiments, t
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