Directional solidification compositions

An important example of an MMC in situ composite is one made by directional solidification of a eutectic alloy. The strength, (, of such an in situ metal-matrix composite is given by a relationship similar to the HaH-Petch relationship used for grain boundary strengthening of metals ... 

In our treatment of directional solidification above, only one diffusion field was treated explicitly, namely the compositional diffusion. If a simple material grows dendritically (thermal diffusion) one may worry about small amounts of impurities. This was reconsidered [132], confirming a qualitative previous result [133] that impurities may increase the dendritic growth rate. Recently some direct simulation results have been obtained with two coupled diffusion fields, one for heat and one for matter, but due to long computing times they are not yet in the state of standard applications [120,134]. 

Connection between Transport Processes and Solid Microstructure. The formation of cellular and dendritic patterns in the microstructure of binary crystals grown by directional solidification results from interactions of the temperature and concentration fields with the shape of the melt-crystal interface. Tiller et al. (21) first described the mechanism for constitutional supercooling or the microscale instability of a planar melt-crystal interface toward the formation of cells and dendrites. They described a simple system with a constant-temperature gradient G (in Kelvins per centimeter) and a melt that moves only to account for the solidification rate Vg. If the bulk composition of solute is c0 and the solidification is at steady state, then the exponential diffusion layer forms in front of the interface. The elevated concentration (assuming k < 1) in this layer corresponds to the melt that solidifies at a lower temperature, which is given by the phase diagram (Figure 5) as... 

It has been established that the structure of the shaped Al203-Zr02(Y203) eutectic composite, as well as the structure of composites obtained by other methods of directional solidification in this system, consists of two phases the matrix, which is alumina, and stabilized zirconia [47]. In this Al203-Zr02 systan, alumina is the... 

Directional solidification to produce fiber reinforced composites with high strength and elastically stiff discontinuous Ti5Si3 fibers aligned with the solidification direction. 

J. Sha, H. Hirai, T. Tabaru, A. Kitahara, H. Ueno and S. Hanada, Effect of carbon on microstructure and high-temperature strength of Nb-Mo-Ti-Si in situ composites prepared by arc-melting and directional solidification , Mat. Sci. Engg., A343 (2003) 282-289. 

Principal advantages of gas-pressure infiltration over other mechanically driven pressure infiltration processes are that infiltration parameters are controlled with precision and that weak molds can be used, which can be used to minimise impurities in the composite. It is thus an attractive method for academic research however, it is also used industrially, to produce electronic substrates for example. After infiltration, the metal is solidified. We use directional solidification, induced by lowering the mold onto a chill (Fig. 1), to avoid shrinkage cavity formation in the composite. The applied... 

The Inorganic discussed in Chapter 7. Eutectic or directed solidification also makes use of heat hh a diffusion limitation to make a composite of anisotropic stacked layers from an eutectic melt, as described in Chapter 8. 

Figure 8.10. Morphologies in directed solidification. From V. S. Stubican, R. C. Bradt, F. L. Kennard, W. J. Minford, and C. C. Sorrel. Ceramic eutectic composites. In R. E. Tressler, G. L. Messing, C. G. Pantano, and R. E. Newnham (eds.). Tailoring Multiphase Composite Ceramics. Plenum, New York (1984). Morphologies from D. Michel, Y. Rouaux, and M. Perez y Yorba. J. Mater. Sci. 15,61 (1960). With kind permission of Kluwer Academic Publishers.

Direct Firing. The firing of pottery or vitreous enamelware in a fuel-fired kiln or furnace without protection of the ware from the products of combustion. Directional Solidification. This is a technique for preparing lamellar or fibrous composites of ceramics and/or metals by growth from a molten eutectic... 

In short, there is little research related to the study of mechanical and electrochemical properties of Zn-Al alloys as well as Zn-Al alloys MMCs containing SiC and AI2Q3 particulations with different grain structures in the matrix. Also there is lack of fundamental study on the performance of Zn-Al alloys and their MMCs in corrosive environments when both solidification microstructure and type of particle distribution are in consideration. In the present research, Zn-Al-SiC and Zn-Al-AI2O3 composites are prepared and solidified by vertical directional solidification method. By means of voltammograms and electrochemical impedance spectroscopy, the corrosion resistances of Zn-Al matrix composite materials with different types of particles are obtained and analyzed and the results are compared. 

Ares, A.E. Gueijman, S.F. Schvezov, C.E. (2010) Experimental Study of the Columnar-to-Equiaxed Transition During Directional Solidification of Zinc-Aluminum Alloys and Composites, /, Crystal Growth, Vol. 312, pp. 2154-2170, ISSN 0022-0248... 

NiAl-based in-situ composites, in particular those fabricated by directional solidification (DS) processing, have been of inter-... 

In the growth of single crystals (or other application) where uniform composition is required, it is also necessary to maintain a near planar solidification front. In the above discussion of directional solidification, it was shown that the segregation coefficient causes a buildup of the rejected phase in front of the advancing solidification front. 

A major method of controlling solidification porosity is directional solidification. Such solidification of eutectics results in unidirectional leunellar or rod "reinforced" composites, wherein the thickness of the lamellae or rod diameters are inversely proportional to the solidification... 

---