Thermal Mismatch Dislocations in Macroscopic Graphite Particle-Reinforced Metal Matrix Composites Studied by Internal Friction

Abstract

Thermal stresses in metal matrix composites (MMCs) can induce plastic deformation in the region close to the reinforcements. In this paper, commercially pure aluminium (Al)-based MMCs reinforced with 68 and 88% macroscopic graphite (Gr) particles were characterized by internal friction (IF) measurements. The macroscopic Gr particle size was of the order of a millimetre (0.8–1.2 mm). The MMC specimens were prepared by an air-pressure infiltration process. The IF, as well as the relative dynamic modulus, was measured using a multifunction internal friction apparatus at low frequencies of 0.5, 1.0, and 3.0 Hz over a temperature range of 20–400 °C with continuous heating. An IF peak is observed at about 260 °C in the IF–temperature spectrum; this phenomenon is absent in the corresponding unreinforced, commercially pure Al specimens. The position of the IF peak is essentially frequency independent and shifts towards higher temperature with increasing heating rate. The presence of dislocations in the MMCs is highlighted on the basis of transmission electron microscopy studies. In the light of IF measurements and microstructural studies, the IF peak is attributed to the generation and motion of dislocations produced by the difference in the coefficient of thermal expansion between the Al matrix and Gr reinforcement at matrix–particle interfaces. The objectives of this paper are to provide an overview of the sources of dislocation generation in the MMCs, and to provide an insight into the effects that dislocations have on the damping response of the MMCs.