Infiltration of fibrous preforms by a pure metal: Part III. capillary phenomena
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I.
INTRODUCTION
IN two
preceding publications, t~'2j a theoretical analysis of the infiltration of fibrous preforms by molten metal was presented and compared with experimental data. It was found that the capillary pressure drop a t t h e infiltration front can be estimated on a plot of L / V t (defined as 0) vs the applied pressure APt, driving the metal into the preform, where L is the metal front position and t is time. An example is given in Figure 6 of Reference 2. Use of the capillary pressure drop to estimate wettability is to be contrasted with usual techniques which measure a wetting angle using the sessile drop technique or a variety of dipping experiments. These and, in particular, the sessile drop technique are well established and easy to interpret. However, there can be major drawbacks in using their results to model infiltration, primarily because of the requirement for a smooth substrate and the static nature of these experiments. In this work, these drawbacks are overcome by using the results of dynamic infiltration experiments, in which a capillary pressure is estimated directly on fiber preforms of engineering interest. The resulting estimate can be converted to an apparent wetting angle which is characteristic of the highly transient type of wetting encountered in casting metal matrix composites. In the following, data for the infiltration of alumina fiber preforms by liquid aluminum are presented with a wide range in the variation of the fiber volume fraction (10 to 25 pct) and initial temperature (313 to 558 K). These data are compared with the theory presented in Reference 1 and are also used to estimate the effective wetting angle of aluminum on these preforms during infiltration.
A. MORTENSEN, ALCOA Assistant Professor, is with the Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139. T. WONG, formerly Undergraduate Student with the Department of Materials Science and Engineering, Massachusetts Institute of Technology, is a Consultant with Oliver, Wyman and Company, New York, NY 10036. Manuscript submitted November 27, 1989. METALLURGICAL TRANSACTIONS A
II.
EXPERIMENTAL PROCEDURE
Materials and procedures used in this study were similar to those reported in Reference 2. In brief, molten 99.99 pct pure aluminum was forced under applied pressure APt = Po - Pg into preforms of SAFFIL* alumina *SAFFIL is a trademark of ICI Americas, Inc., Wilmington, DE.
fibers, where P0 is the pressure at the preform entrance applied via pressurized nitrogen onto the metal. The term Pg is pressure in the gas phase initially present in the preform, equal to 1 atmosphere in this work. The fibers were oriented randomly within a plane perpendicular to the infiltration direction, and cohesion of the preform was obtained with a silica binder. Figure 1 is a scanning electron micrograph of the initial preform structure, where the binder is visible as a rugged phase covering part of the fiber surface. The preforms were obtained from various sources, and the
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