Capillarity in isothermal infiltration of alumina fiber preforms with aluminum
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INTRODUCTION
REINFORCEMENT materials used to produce metalmatrix composites are generally poorly wetted by the molten metal matrix, t~] For this reason, when a metalmatrix composite is produced by infiltration, mechanical force is often used to drive the molten matrix into the porous reinforcement preform. Because pores have variable shapes and dimensions within a single preform, the metal does not fill all pores at once; it first flows into the largest pores of the preform, because these present both the smallest capillary and viscous resistance to matrix flow. The narrowest pores, which require higher pressures to be infiltrated, are only filled later--if at all. [2] As a result, preforms are infiltrated over a finite range of pressures, and there always exists a region of unsaturated metal flow during infiltration, in which the liquid metal only fills a portion of the preform pore space. Preforms used to produce metal-matrix composites generally have highly complex internal void geometries: the distribution, and often the shape, of pores left between elements of the reinforcing phase (of fibers, particles, or whiskers) is microscopically very intricate and irregular. This geometrical complexity of the preform pore shapes and, hence, of capillarity during infiltration renders prediction of the metal flow path during infiltration too complicated to be realistically modeled at the microscopic level of individual fibers or particles making the preform. In modeling infiltration, therefore, a more macroscopic continuum approach is often adopted, t~] which uses a volume element large enough to contain meaningful local averages of microscopic parameters (such as volume fraction of metal and fiber) but small enough to be treated as a differential element, as is generally done in the analysis of flow through porous media, t3j V.J. MICHAUD, Postdoctoral Associate, and A. MORTENSEN, Associate Professor, are with the Department of Materials Science, Massachusetts Institute of Technology, Cambridge, MA 02139. L.M. COMPTON, formerly Undergraduate Student, Department of Materials Science, Massachusetts Institute of Technology, Cambridge, MA, is Materials Engineer, Altran Materials Engineering Inc., Boston, MA 02110. Manuscript submitted September 8, 1993.
METALLURGICALAND MATERIALS TRANSACTIONSA
To date, in the analysis of metal-matrix composite infiltration, the gradual nature of pore filling with metal is neglected; instead, it is assumed that a sharp twodimensional (2-D) infiltration front divides the preform into an uninfiltrated region and a fully infiltrated region. tl'41 This simplified approach, called the slug flow approximation, has proven useful in the analysis of various aspects of the process, including infiltration kinetics with or without solid metal present, and macrosegregation effects, t4-~~ In some cases, however, especially when the applied pressure is low, the porous zone present behind the infiltration front may be too large to be ignored. The slug flow approximation is then no longer legitimate, an
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