The growth of tubular or vermicular structures in organic monotectic systems
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I. I N T R O D U C T I O N A L L O Y S of monotectic composition, Figure 1, partially solidify according to the phase reaction Liquid, L1 ~ Solid, S + Liquid, L2; the L2 phase is the minor product phase and there is typically negligible solid solubility in the solid product, S. It has been shown by Gmgel et al. ~1-51 in a number of metallic and organic systems, that at low growth rates ( 4 1 to 5 / x m s -1) there are two types of solidification behavior, one yielding an aligned fibrous or tubular distribution of the minor L2 phase, when grown unidirectionally, and the other a coarser, irregular or globular distribution, poorly aligned in the growth direction. The first type appears to be characteristic of systems having a high upper consolute temperature, Tc, relative to the monotectic temperature, Tin, and the second, of systems having lower miscibility gaps, i.e., larger ratios of Tm/Tc. A suggested explanation for this difference has been offered by Cahn r6,7~ in terms of a critical wetting temperature, T~, above which one liquid, L1, will wet a third phase, S, to the exclusion of the other liquid, L2. The above-mentioned distinction between monotectic microstrnctures seems to be compatible with this critical wetting argument, and, if the Tin~To ratios are compared for a series of binary and ternary systems, the transition in growth behavior appears to take place at a ratio of about 0.9 (in K), although we know of no theoretical justification for this value. The present study relates to four binary systems based on succinnonitrile, SCN, for which the temperature ratios are close to 0.9, as in Table I. The distinction between the t y p e s in Table I has already been made elsewhere t3,4,5~ for glycerol and ethanol and will be further illustrated here for the water and benzene systems. At the same time, it has also been shown that at more rapid growth rates all monotectic alloys grow in such a way that the minor L2 phase is incorporated into the solid, S, as a complex arrangement of twisted tubes which subsequently spheroidize. Direct observations of the HUI SONG, Graduate Student, and ANGUS HELLAWELL, Professor, are with the Department of Metallurgical Engineering, Michigan Technological University, Houghton, MI 49931. Manuscript submitted May 27, 1988. METALLURGICAL TRANSACTIONS A
growth fronts which produce such structures have been reported by Grugel et al. [3"4"sl for SCN-glycerol and SCNethanol and by Kaukler et a/. t8'91 for SCN-water and SCNbenzene, the latter drawing particular attention to strange morphologies having worm-like or fungoid appearances. In the present communications we use a series of time lapse pictures to illustrate steady state growth in the SCNwater system and further examples of the remarkable nonsteady state morphologies which can develop in this and the SCN-benzene system.
II. E X P E R I M E N T A L
The materials used in this work were zone ref'med (SCN --99.8 pct purity), distilled water and benzene of stated commercial purity 99.9 pct. All samples examined were of nominal mono
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