Planar to equiaxed transition in the presence of an external wetting surface
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I.
INTRODUCTION
THE morphologies of the solidified microstructure which are normally observed in a bulk directionally solidified nonfaceted material are planar, cellular, and dendritic, tz-wl Often, microcellular structures may also be noted during directional rapid solidification, t~2,~31 Additionally, a new set of morphologies has been reported when restraints like fibers, particles, channels, or wetring surfaces are present during solidification, t2~ When the restraint is nonwetting in nature, half dendrites or half cells may be observed, t22-251 Wetting surfaces can aid the growth of noncrystallographic dendrites t26~ and be the cause for localized convection during solidification. t27j During normal ingot solidification of alloys, the columnar to equiaxed transformation is almost always noted and is related to the loss of the temperature gradient and the presence of favorable sites for heterogeneous nucleation ahead of the growing columnar solidification front. The formation of the equiaxed structure is promoted by low superheat and higher alloy contents.12s~ The nuclei for equiaxed solidification may come from several sources including the detachment of dendrite arms from the growing columnar zone. In this article, it will be shown that a wetting surface may change the solid-liquid morphology from columnar to equiaxed when solidification is induced to occur in the presence of such restraints. First time evidence is presented to show that for the growth of in situ composite materials, fully equiaxed structures may be seen for conditions where columnar structures are expected from the heat flow and composition conditions. The orientation and type of solidification structure influence the physical and mechanical properties of a material. The new morphologies and new dendritic directions observed, arising from the presence of restraints, may influence properties because of a change in the scale of the solidification structure, I22,23,241or a change in the orientation, t25,261 or, as shown in this article, a change in the type of solidification morphology. L.M. FABIETTI, Research Associate, Department of Materials Science and Engineering, and J.A. SEKHAR, Associate Professor, Department of Materials Science and Engineering, and Director, International Center for Micropyretics, are with the University of Cincinnati, Cincinnati, OH 45201. Manuscript submitted May 20, 1992. METALLURGICAL TRANSACTIONS A
II.
EXPERIMENTS
Unidirectional solidification experiments in succinonitrile (SCN)-acetone alloys growing on a TEFLON* *TEFLON is a trademark of E.I. Du Pont de Nemours & Co., Inc., Wilmington, DE.
(TFE-type) bed have been carried out. The technique t25,291 and experimental apparatus have been described elsewhere. [3~ The type of TEFLON is also referred to as the wetting type for the SCN alloy.t25'261 The cells employed in our experiments are shown in Figure 1. The inside bottom of the solidification cell was partially made up of the TEFLON strip 12.5 • 50 • 0.10 mm installed in the center of the glass plates, tzs
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