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I.

INTRODUCTION

M E T A L - m a t r i x composites (MMCs) are now attracting enormous interest. One of the prime reasons for this is that significant advances have been made in recent years on the development of fabrication routes which are economically attractive and generate material of high microstructural quality. In particular, it is possible to produce composites which are relatively free from gross defects and exhibit consistently intimate contact between the metallic matrix and the ceramic reinforcement. The techniques available for MMC fabrication include (a) diffusion bonding of thin sheet interleaved with ceramic fibers, ~2 (b) mechanical processing of ceramic suspensions in molten metal ("compocasting"), 3'4 (c) premixing and consolidation of metallic powder and ceramic whiskers, fibers, or particles, s'6 and (d) melt infiltration into a preform consisting of an assembly of ceramic fibers. 7'8 (There is, in addition, current interest in the potential of spray co-deposition of metal droplets and ceramic particles and in the use of superplastic matrices.) Of the techniques listed, (a) is very cumbersome and expensive (and is applied only when other routes present severe problems, as with titanium), and (b) tends to give rise to severe oxidation difficulties, in addition to being technically awkward and rather uneconomic. Powder routes, including some involving semisolid slurries, 9 offer many attractive features and are being extensively explored, particularly for production of stock material. However, the melt infiltration process is in some respects the most highly developed, being in significant commercial use ~~and offering a combination of technical versatility, attractive process economics, and good microstructural control. The technittue is based on the injection of a fully liquid melt into the interstitial space within an assembly of ceramic fibers, it has been most extensively employed with aluminum alloys and with fibers of carbon, H silicon carbide, t2'13 and alumina (particularly the ICI 'Saffil' 6-alumina staple fiber 7'~4and the DuPont FP a-alumina monofilamentlS). The process has certain features in common with "SqueezeCasting". 16'17 There have been some previous publications

analyzing different features of the squeeze infiltration process, including coverage of the microsegregation characteristics,~S the critical pressures for infiltration, 7 the interfacial structure, 19 and thermal parameters. 12 However, a complete analysis of the sequence of events during processing, and consequent comprehensive recommendations about process optimization, are not yet available, The present paper aims in that direction. The modeling and data presented refer to preforms composed of a nominally planar random array of discontinuous fibers of large aspect ratio, and are oriented toward the fine (3/xm diameter) "Saffil' product. However, the broad implications for other fibers, and for different fiber distributions, are in most cases reasonably clear or can be readily inferred. A nomenclature listing for