Solidification, instability during

S. Coriell, M. Cordes, W. Boettinger, R. Sekerka. Convective and interfacial instabilities during unidirectional solidification of a binary alloy. J Cryst Growth 49 13, 1980. 

O. Richmond, L. G. Hector, and J. M. Fridy, Growth Instability during Nonuniform Directional Solidification of Pure Metals, J. Appl. Mech., 57, pp. 529-536,1990. 

F. Yigit, N.-Y. Li, and J. R. Barber, Effect of Thermal Capacity on Thermoelastic Instability during the Solidification of Pure Metals, J. Therm. Stresses, 16, pp. 285-309,1993. 

Example 6.2 Characteristics of Interfacial Instability DURING Solidification... 

Although equation 33 gives a physical description of the mechanism of the instability that leads to microstructure formation during solidification, it is not rigorous because it does not consider the effects of the rates of heat and species transport on the evolution of the disturbance. Because of this deficiency, equation 33 cannot be used as a basis for further analysis of microstructure formation. This deficiency is shown clearly by the inability of equation 33 to predict the spatial wavelength of the microstructure formed along the interface. 

Light elements like O, C, and N will form inclusions above the solubility limits and is found in feedstock as SiC>2, SiC, and Si3N4. These impurities can create problems in the wafering process and during solidification. The consequence can be structure loss in the CZ process and instability in the MC process. 

So far we have dealt with reactions in which a solid has been consumed. When a solid surface is built up by a reaction, transport of reactants in the fluid phase is destabilizing, just as is the case during solidification (Seshan, 1975). An example of considerable practical importance is electrodeposition. In this case, instability is normally undesirable because it leads to an irregular surface that does not have the preferred bright appearance. It has been found that instability can sometimes be prevented by including small quantities of certain surface active organic additives in the electrodeposition bath (Edwards, 1964). The additives diffuse to the solid-liquid interface and are either incorporated into the developing deposit or consumed by an electrochemical reaction. Since they are surface active, they adsorb at the interface and produce a decrease in the rate of the electrodeposition reaction, possibly because they block some sites where metal ions would otherwise deposit. 

There are, however, some drawbacks to this kind of structure. As a poly Schiff s base, one would expect hydrolytic instability and that is the case polyazomethine fiber, for the most part, under hot-wet conditions, loses physical properties quite rapidly. Secondly, most of these structures have a glass transition temperature (Tg) at about 100°C at or near this temperature, those fibers show a dramatic loss of tensile modulus. A third concern involves the difficulty in controlling polyazomethine molecular weight during polymerization and melt processing. If melt residence time is not carefully monitored, polymer solidification in the apparatus can become a problem. 

Impurities dissolved in the liquid can influence and are influenced by the solidification process [7]. Trapping, dependence of the interface segregation on the velocity during resolidification, instabilities of the planar interface and formation of cell structures represent another area of interest to the meeting and some relevant experiments will be described. At this stage we prefer, in view also of our background to point out mainly, the experimental results and a simple thermal treatment [8]. 

Worster MG (1992) Instabilities of the liquid and mushy regions during solidification of alloys. J Fluid Mech 237 649-669... 

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