Roger T. Pepper

Mechanical Properties of Aluminum-Graphite Composites Prepared by Liquid Phase Hot Pressing

A continuous liquid metal infiltration process has recently been developed for making aluminum-graphite composite wire from commercially available multifiber graphite yarns. Composite specimens have been successfully fabricated by hot pressing the composite wire. The longitudinal properties of both the composite wire and hot pressed specimens approximated rule of mixtures behavior, but the transverse tensile and compressive strengths of the specimens were lower than expected. Examination of the microstructure indicates that better transverse and compressive strengths may be achieved through refinement of the matrix grain size, elimination of continuous networks of intermetallic compounds in the matrix, and homogenization of the matrix alloy constituents.

The tensile properties of a graphite-fiber-reinforced Al-Si alloy

Room temperature uniaxial tensile tests have shown that composites of an Al-13 wt pct Si alloy and Thornel 50 graphite fibers have strengths greater than a theoretical value that was calculated on the basis of the law of mixtures. At 28 vol pct fibers, the average uniaxial tensile strength was found to be 106,000 psi, and several values between 130,000 and 144,000 psi were obtained. The modes of deformation and failure in the composites have been studied by the microexamination of polished surfaces and fractures of tested specimens. The reasons for the high strengths and the unusual modes of fracture that were observed cannot be explained on the basis of the presented data. Specimens of the composite have been thermally cycled between −193° and +20°C twenty times and others between −193° and +500°C twenty times. Tensile tests and microexamination of these thermally cycled specimens show that thermal cycling does not degrade the tensile properties of the composites or change their microstructure.

Directional Solidification of an Off-Eutectic Aluminum-Beryllium Alloy

Aligned fibers of beryllium in aluminum were obtained by zone melting of an aluminum-20 wt% beryllium alloy. Unfortunately, the fiber length was limited by banding caused by freezing rate fluctuations. Thus, the ultimate strength was only about 11 × 10 3 psi and elastic modulus 15 x 106 psi. Analytical expressions were found for the concentration perturbation caused by changes in freezing rate, both for progressive freezing with no convection and for zone melting with much liquid mixing