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The influence of capillarity on the near-plane front solidification of metal matrix composites is examined by analysis of the one-sided solidification of a binary alloy in a planar interstice of constant width in the limit of low Peclet number. We assume that in this limit, solute isoconcentrates in the liquid are everywhere orthogonal to the growth direction. Capillarity causes the alloy to solidify in a cellular mode, even in the absence of constitutional supercooling. Two solution branches are derived for this solidification mode, one for shallow symmetric cells, the other for asymmetric cells. Restricting attention to the former solution branch, as the growth velocity increases, or the temperature gradient decreases, the cell amplitude increases gradually, to reach a critical point which depends strongly on the contact angle along ...
Metal matrix composites composed of high-purity aluminum and Du Pont PRD-166 continuous zirconia-stabilized polycrystalline alumina fibers are fabricated by liquid metal infiltration using three different casting procedures. The microstructure of the composites is analyzed using optical and electron microscopy, including analytical electron microscopy. It is found that discrete faceted particles of ZrAl3 form at the interface and grow into the matrix of samples processed above the melting point of the matrix for 13 minutes or more. The formation of this compound is in agreement with thermodynamic stability calculations. It is also found that there is a reaction between solid aluminum and the fibers at 913 K, yielding a reaction product which has the same morphology as that observed with molten aluminum. When the fibers are infiltrated ...
Pressure infiltration of liquid metal is one of the most important processing routes for the production of aluminum-matrix composites having a self-supporting reinforcement phase. This article briefly examines the physical phenomena governing infiltration processes, to present practical guidelines derived from their analysis for optimization of the process and the materials produced. Engineering aspects that are pertinent to infiltration techniques, including preform preparation, process configurations, flow control, and innovative processes, are summarized.
Models derived in petroleum engineering and soil science for flow of two immiscible fluids in a porous medium are extended to the infiltration of ceramic preforms by a liquid metal. SAFFIL alumina fiber preforms are infiltrated with an aluminum matrix in a series of interrupted unidirectional and isothermal experiments at various low applied pressures, to measure profiles of the volume fraction of metal along the length of the reforms. Comparison of experimental data with theory reveals the existence of a pressure-dependent incubation time for wetting of the alumina preforms by molten aluminum at 973 K. If this incubation time is taken into account, experimental curves of metal distribution are well predicted by theory, confirming the validity of the models after initiation of flow.
Infiltration by a pure matrix in the presence of preform deformation and partial matrix solidification is analyzed using a bounding approach for the preform rheology where solid metal is present. It is found, using parameters for the infiltration of short alumina fiber preforms by aluminum, that the two bounds are close in comparison to other factors of uncertainty in the prediction of infiltration rate. Using this approach, preform compression is shown to exert a significant influence on the infiltration rate for the system explored; in particular, the analysis shows the existence of an optimal value of applied pressure. Simplifications in the analysis are also presented, which yield fairly accurate results while easing their computation significantly.
A replication process for producing fine open-celled ceramic foam from preceramic polymers is presented and analysed. In this process, a porous sodium chloride compact formed by sintering was first infiltrated with polycarbosilane. After dissolution of the salt, the resulting polymer foam was cured by oxidation in air and pyrolysed to form a silicon carbide foam. The curing stage is the most critical step, and was investigated using a series of controlled curing experiments and finite-difference modelling of heat transfer during curing. Good agreement has been found between theory and experiment. The model was then used to investigate the limits of the process and to provide stategies for successful curing of the foams without melting or cracking.
The thermal expansion of three isotropic metal-matrix composites, reinforced with SiC particles or microcellular foam, is measured between 25 degrees C and 325 degrees C. All three composites show initial coefficient of thermal expansion (CTE) values in agreement with the Turner model predictions, and near Schapery's lower-elastic bound for CTE. At higher temperatures, the CTE of foam-reinforced Al decreases, while that of the two particle-reinforced composites increases. These observations are interpreted las resulting from the presence of a very small fraction of microscopic voids within the infiltrated composites. This interpretation is confirmed With finite-element simulations of the influence of voids, cracks, and reinforcement convexity in two-dimensional (2-D) composites featuring: an interconnected reinforcement of SiC surround...
We report the first measurements of a self-healing polymer that combines a microencapsulated liquid healing agent and shape memory alloy (SMA) wires. When a propagating crack ruptures the embedded microcapsules, the liquid healing agent is automatically released into the crack where it contacts a solid catalyst embedded in the matrix. The SMA wires are then activated to close the crack during the healing period. We show that dramatically improved healing performance is obtained by the activation of embedded SMA wires. We conclude that improved healing is due to a reduction of crack volume as a result of pulling the crack faces closed, and more complete polymerization of the healing agent due to the heat produced by the activated SMA wires. (C) 2009 Elsevier Ltd. All rights reserved.
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