40207_13 - METAL MATRIX COMPOSITES V I K ostikov a nd V S...

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METAL MATRIX COMPOSITES 13 V. I. Kostikov and V. S. Kilin 13.1 INTRODUCTION Industrially developed countries are sucess- fully producing materials and shapes based on metal matrix reinforced with carbon fibers (MMC). Metal matrix, ceramic and carbon composites reinforced with both discrete and continuous reinforcements have been described’-6. The present chapter will describe the results of investigations of the properties and preparation of aluminum/car- bon composites MMC production deals with the solution of specific problems which involve the formation of useful structural components, with the attainment of their potential properties. The first problem is to fill the interfiber space of the carbon fibrous framework with matrix metal or alloy. Several molding methods are available. Optimization of the materials and the processes for obtaining usable MMC may fur- ther the use of MMCs in industry. When the MMC is reinforced with high molecular and high strength fibers the composite may be very brittle and can have thermal dynamic incom- patibility with many metals used as matrices. MMC production technology is compli- cated and requires satisfaction of the following conditions, of which the most significant are as follows: 1. maintaining the reinforcing fibers’ strength; 2. ensuring a strong bond of fibers with matri- ces and between the matrix layers; Handbook of Composites. Edited by S.T. Peters. Published in 1998 by Chapman & Hall, London. ISBN 0 412 54020 7 3. providing the correct fiber length, greater than the critical length; 4. even distribution of fibers in the matrix; 5. orientation of fibers in the direction of the applied load; 6. achieving the required shape and dimen- sions of the MMC; 7. obtaining a MMC strength reasonably near to theoretical. Contemporary processes for obtaining an alu- minum/carbon MMC can be divided into three main types: solid stage, liquid stage, and solution sedimentation. In the solid stage, the greatest effort has been spent in obtaining the MMC by hot extrusion. In this case, a foil, used as a matrix, is interspersed with carbon fibers to form a preform. The preform can then be subjected to optimum pressure, time and tem- perature in air, vacuum or inert gas medium. Considering metal bonding in the liquid stage, chemical bonds predominate; rarely are there mechanical bonds and there are no physical bonds7. Strong chemical bonds are possible because the atoms (or surface) of one sub- stance come close enough to the surface of the other (1.5-3 A) to enter the zone of the surface atoms’ field-of-forces effectivity. When MMCs are produced by solid stage methods (diffusion welding, rolling, extru- sion), it is practically impossible to provide full convergence of fiber and matrix surface due to uneven surface contours. Only applica- tion of extreme external forces makes the convergence possibleH. This pressure increase leads to brittle fiber breakage. Also, some
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