Mechanical properties and failure characteristics of FP/aluminum and W/aluminum composites
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I.
INTRODUCTION
THE technology
of metal matrix composite material is being developed very rapidly. Metal matrix composites are superior for their performances at elevated temperatures as well as higher strength and stiffness than those of resin matrix composites over a wide range of temperature. Metal matrices can greatly enhance the ductility of composites and thus reduce the chance of brittle failure. The focus of this paper is on the mechanical behavior of FP* and W fiber reinforced aluminum composites. There is *FP is a tradename of DuPont Company, Wilmington, Delaware for polycrystalline alumina fibers.
a lack of basic understanding of the properties of FP fiber reinforced metal matrix composites. The applications of this relatively new composite must be reinforced by fundamental research aimed at understanding and optimizing properties. The W/aluminum composite is chosen for comparison purposes because of the ductile nature of both the matrix and fiber. Furthermore, the compressive behavior of W/A1 composite is less sensitive to fiber misalignment with the loading axis than FP/A1 composite. As a result, these two types of composites will provide entirely different types of failure modes both in tension and compression because of the relatively large diameter of W fibers. Fiber FP is manufactured by the DuPont Company and prepared in the form of continuous yarns containing 210 filaments. Its basic properties are outlined below. 2 An FP fiber consists of more than 99 pct of c~-alumina and therefore has higher modulus and higher temperature capability than other ceramic fibers containing SiO2. The tensile modulus of 345 to 379 GPa is comparable to boron and high modulus graphite. The filament tensile strength measured at 0.006 m gauge is 1.4 to 1.6 GPa. The strength can be increased to 1.9 to 2.1 GPa by applying a thin coating of
silica to the fiber surface. FP fibers have a density of 3.95 g/cm 3 and 0.4 pct of elongation to break. Because fiber FP is a refractory oxide fiber, it is stable at high temperatures. No significant decrease in properties is observed up to 1273 K. The room temperature tensile strength and modulus of the filaments are unchanged after 300 hours exposure in air at 1273 K. The molten metal infiltration technique has been used to prepare FP/metal composite components. 3 As the first step, fiber FP tape is prepared using a fugitive binder in a manner similar to producing a resin matrix composite prepreg. Fiber FP tapes are then laid up in the desired orientation to form a preform and inserted into a mold of steel, titanium, or other suitable material. The organic binder is burned away and the mold is infiltrated with molten metal. A variety of composite shapes such as plates, rods, beams, and tubes can be prepared by this technique. The FP/metal process permits casting of large parts in a single infiltration step. Since aluminum and commercial Al-alloys do not wet the A1203 fibers, wetting is achieved with AI-Li alloys containing 2 to 4 pct Li. FP/AI composites with high fiber volume f
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