The Influence of Gravity on the Solidification of Monotectic and Near Monotectic Cu-Pb Alloys
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THE INFLUENCE OF GRAVITY ON THE SOLIDIFICATION OF MONOTECTIC AND NEAR MONOTECTIC CU-PB ALLOYS
A. BERGMAN,
T. CARLBERG, Institute of Technology,
H. FREDRIKSSON AND J. Stockholm, Sweden
STJERNDAHL,
The Royal
ABSTRACT Unidirectional solidification experiments with monotectic and near monotectic Cu-Pb alloys have been performed with the growth direction both parallel and antiparallel to the gravity vector. It was found that a more regular composite structure was possible to achieve in the samples solidified parallel to the gravity vector. It was also found that the amount of lead rich phase, regularily incorporated in the structure, was less than one would expect theoretically. It is proposed that monotectic alloys can solidify to two different kinds of composite structures. One, which can be described by the theory of coupled growth of a rod eutecticum, and the other similar to the growth of primary rods.
INTRODUCTION Since it became possible to make experiments in microgravity an increasing interest has been directed to alloy systems containing miscibility gaps. A number of experiments have already been made in space to study the decomposition within a miscibility gap (1-4). However, also the structure in alloys with a composition at the monotectic point is strongly influenced by the gravity forces. In (5) and (6) investigations were made of Cu-Pb alloys growing antiparallel to the gravity vector. At low growth rates sedimentation of lead occurred and no regular composite structure,reminding of a rodlike eutectic structure appeared. At high growth rates the structure was more regular, but the authors (5,6) Concluded that the interrod spacing was not of the magnitude expected from the theory of rod eutectic growth. Potard (7) studied solidification of Al-In alloys both parallel and antiparallel to the gravity vector. Those experiments showed that a more regular structure appeared in the parallel experiments, although even there segregation phenomena due to gravity was interfering. The aim of this investigation has been to study the possibilities of achieving regular composite structures in monotectic alloys. The Cu-Pb system was chosen due to the large fraction of liquid phase in the monotectic making this system especially sensitive to different gravity conditions. Experiments with growth directions both parallel and anti-parallel to the gravity vector were performed and the composition was varied around the monotectic point. EXPERIMENTAL Solidification with the growth direction anti-parallel to the gravity vector The alloys investigated were Cu-Pb alloys containing 10, 20, 3Q, 36 and 42% Pb. The samples were prepared by melting the alloys in a low frequence (15 k~lz) induction furnace, and sucked up into alumina tubes with bores of 2 mm and outer diameters of 3 mm. The samples were then inserted into a resistantheated furnace, through a water-cooled brasstube from below. The sample tubes
580 were partly positioned above the end of the brasstube, resulting in a remelting of the top of the samples to a d
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