The infiltration of aluminum into silicon carbide compacts
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INTRODUCTION
D I S C O N T I N U O U S L Y reinforced metal matrix composites have been fabricated by various techniques of which the most successful have been the solid-state processes. These include powder metallurgy processing and lamination of reinforcement and metal by diffusion bonding, u] Liquid-metal processes have had less success, although liquid processing of particulate-reinforced composites is potentially more economical. Moderate success has been experienced with both the addition of the ceramics to the liquid metal prior to casting and the direct infiltration of ceramic preforms. [2] A primary obstacle to the liquid-metal techniques is the nonwetting nature of the ceramics. Flocculation of the ceramic occurs when mixed into the melt prior to casting, t3] voids form at interfaces, and incomplete infiltration results. Degradation of the silicon carbide by reaction with aluminum is also a concern, [4] and various techniques have been employed to overcome these problems. [5] Liquid-metal processes which have had limited success include compocasting and infiltration, u] In compocasting, t6j silicon carbide powder is stirred into the melt at a temperature between that of the liquidus and the solidus. Since this partially solidified melt behaves as a slurry, the silicon carbide powder can then be mechanically entrapped, allowing the mixture to be cast into a metal matrix composite. Composites produced by this process have been characterized by voids and often by poor bonding at the ceramic/metal interfaces. Vacuum coating and pressure (squeeze) casting r71have been applied to the liquid-metal infiltration of silicon carbide fibers. Liquid-metal infiltration processes offer a considerable advantage in the production of intricately
P.B. MAXWELL, Regional Metallurgist, is with the Carpenter Steel Division, Carpenter Technology Corporation, Los Angeles, CA 900580880. G.P. MARTINS, Professor, D.L. OLSON, Professor, and G.R. EDWARDS, Professor and Director, are with the Center for Welding and Joining Research, Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, CO 80401. Manuscript submitted October 31, 1988. METALLURGICAL TRANSACTIONS B
shaped parts such as tubes, where the full benefits of continuous fibers may be realized. The advantage of producing silicon carbide/aluminum matrix composites by liquid-metal infiltration techniques cannot be fully realized without improvements in the infiltration behavior and fiber matrix bonding.t8] Previous investigations have focused on the effects of processing parameters such as pressure, temperature, and alloying. [9,1~ This paper reports on an infiltration model which considers the physical properties of the liquid and preform. These properties include viscosity, density, surface tension, wettability, and pore size. The model has been assessed in terms of its ability to predict infiltration behavior from known physical properties of the material utilized in controlled laboratory experiments. Infiltration problems can be solved
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