Modeling and experimental study of the interface morphology and growth kinetics of fibrous eutectic solidification
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Experimental studies of fibrous eutectic growth in succinonitrile–camphor reveal that models of the stable range of interfibrous spacings must incorporate the dynamic effects and the instability of the growth front of the fibrous phase, which determines the fineness and regularity of the microstructures. An analytical mathematical model for fibrous eutectic growth has been developed and compared with the experimental results. The selected wavelength scales obey ⳱ √d0ls f (ls /lt), where d0, ls, lt are the capillary, diffusion, and thermal lengths, respectively. While only at a relatively high growth rate or steep temperature gradient, the scaling law 2V ⳱ constant is fulfilled. It is found that the selected band of interfibrous spacings is very narrow. This means that the interfibrous spacing is almost unique at a given growth rate and a fixed temperature gradient. The effects of convection on the interfibrous spacing selection and fibrous phase instability of short wavelength perturbations have also been investigated. These studies reveal that the controlled solidification of a fibrous eutectic produces a very fine and regular microstructure.
I. INTRODUCTION
Fibrous eutectics, or rod eutectics, and similar microstructures constitute one type of in situ filamentary composites that can be formed by directional solidification. Generally, controlled fibrous eutectic growth can form with very refined microstructures (i.e., small spacings). Therefore, fibrous eutectics exhibit superior mechanical properties (e.g., eutectic superalloys and two-phase ceramics) and novel physical properties (e.g., semiconductormetal eutectic systems Si–CrSi2, Si–TaSi2, GaAs–metal arsenide, InSb–NiSb and optical materials such as LaxBy based eutectics, and NaBr–NaF). Copolymer crystals are also being developed as nonlinear optical crystals, which are fabricated by the “analogous eutectic reaction” in a directional ordering process.1,2 The physical or mechanical properties of these types of materials strongly depend on the fineness and regularity of the interfibrous spacing. The optimal wavelength selection, which leads to a solid regular array of fibers in the matrix, is a remarkable feature of this type of eutectics. This selection process occurs at any finite growth rate and with no critical threshold. While a perturbation evolves, although the front
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0374 J. Mater. Res., Vol. 21, No. 12, Dec 2006
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of each fibrous phase is cellular, the evolution of the fibrous pattern does not fit predictively with the MullinsSekerka (M-S) instability criterion.3 To satisfy the equilibrium balance of surface tensions at each triple point, a curvature of each phase at the solid/liquid interface is required. However, at higher growth rates than those of interest here, a M-S instability can be observed as a long wavelength modulation of the basic fibrous patterns. The long-wavelength instability a
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