Fabrication and properties of graphite fiber reinforced magnesium
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BECAUSEof their
low densities (~0.063 lb/in.3), the specific stiffness (the ratio of elastic modulus to density) of magnesium and its alloys is comparable to those of much stiffer engineering materials such as steel, titanium, or aluminum. This characteristic, together with their excellent castability make magnesium alloys very attractive for applications where light weight and high section modulus are required. By r e inforcing magnesium with advanced graphite fibers, the properties of this very useful engineering metal could be greatly improved. Fig. 1 shows that if all the c r i teria for reinforcement are met, this system would be superior in specific strength and modulus to all c u r rently available resin and metal matrix systems. Thus, the high specific strength and stiffness of the graphite fibers together with the low density and high specific stiffness of the magnesium would yield a new composite material whose improved p r o p e r t i e s would make it even more attractive for light weight, high strength and high stiffness applications. The purpose of this investigation was to fabricate experimental g r a p h i t e - m a g n e sium composites and to determine their room t e m p e r ature mechanical properties with the ultimate objective of utilizing this material in Army equipment. MATERIALS Because of its ready availability, pure magnesium was initially used in this study. The absence of alloying elements during this stage of the study also r e duced the number of variables that must be considered such as diffusion, interfacial reactions, and s e g r e g a tion of alloying elements. The graphite fibers considered in this work are listed in Table I together with some of their pertinent properties. After preliminary wetting and bonding experiments with these fibers, Type B was selected for final composite fabrication because of its high elastic modulus (60 • 106 psi), and because its round c r o s s -
section facilitated its coating with various metals. Fabrication with the other fibers was found to be m o r e difficult because of their irregular c r o s s - s e c t i o n s or twists in their yarns. EXPERIMENTAL PROCEDURE a) Wetting and Bonding Studies To determine whether pure molten magnesium would wet graphite fibers, the apparatus shown in Fig. 2 was designed and built. In operation, the magnesium in the graphite crucible was melted by the induction coil. One end of a small diameter (0.065 in. ID) Vycor tube containing aligned graphite fibers held in place by small magnesium metal plugs was dipped into the molten magnesium. On contact the magnesium plug melted and the hydrostatic p r e s s u r e in the crucible forced liquid m a g nesium around the graphite fibers thereby forming a primitive cylindrical composite. An argon atmosphere minimized oxidation of the magnesium during the infiltration operation. Optical examination of the infiltrated fiber bundle indicated that none of the fibers studied, Table I, was wet by pure molten magnesium. Since direct wetting of these graphite fibers was not possib
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