Overview of Geometric Effects on Coarsening of Mushy Zones
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INTRODUCTION
DENDRITIC structures invariably form during the solidification of alloys under small temperature gradients. The two-phase region consisting of the dendritic primary solid phase embedded within a liquid phase, or melt, is commonly called the mushy zone. The mushy zone separates the totally frozen solid from the totally melted liquid regions and represents the region of a casting in which the microstructure forms and evolves. The dendritic structures that form in mushy zones are often highly ramified and tend to be reasonably uniform in overall appearance, resulting from the growth of multiple primary dendrites and the subsequent development of secondary, tertiary, and higher-order side branches. The most visually prominent features in dendritic structures are the secondary side branches. It is not surprising that the secondary side branch spacing is often used to characterize the size scale of cast microstructures and their associated chemical microsegregation. In general, the side branch spacing increases with distance behind the tip of each dendrite. The measurement of dendrite arm spacings remains somewhat arbitrary, inasmuch as it requires some consistent, albeit ad hoc, method of averaging individual spacings as measured from various regions of a metallographic section. The solid-phase microstructure within the mushy zone continues to evolve during solidification. This evolution process is known as phase coarsening, or Ostwald ripening. The free energy associated with the interfaces drives a spontaneous process in which the total interphase area decays through dissolution (melting) and reprecipitation (freezing). This capillary-driven diffusional process results in an increase in the characteristic size scales of the microstructure with time.
S.P. MARSH, Research Metallurgist, is with the Physical Metallurgy Branch, Naval Research Laboratory, Washington, DC 20375-5343. M.E. GLICKSMAN, Professor, is with the Materials Science and Engineering Department, Rensselaer Polytechnic Institute, Troy, NY 12180-3590. This article is based on a presentation made at the "Analysis and Modeling of Solidification" symposium as part of the 1994 Fall meeting of TMS in Rosemont, Illinois, October 2-6, 1994, under the auspices of the TMS Solidification Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A
Coarsening in a mushy zone is clearly illustrated in Figure 1.I~lThis series of micrographs was taken from a set of Sn-40 pct Bi alloys in which an initially fine dendritic structure was formed by nucleation and growth within a rapidly cooled alloy melt. The as-cast hypereutectoid samples were then reheated above the Sn-Bi eutectic temperature (179 ~ and held isothermally in the solid-plus-liquid state for various times, and then rapidly quenched to preserve the microstructure. The micrographs in Figure 1 show that the size scales increase dramatically over time and that the initially ramified dendritic structure also undergoes morphological changes; the primary Sn-rich phase becomes distinctly more spheroidal
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