Structure formation during processing short carbon fiber- reinforced aluminum alloy matrix composites

Abstract

Nickel- and copper-coated, as well as uncoated, short carbon fibers were dispersed in melts of aluminum or aluminum alloys by stirring followed by solidification of composite melts. Microstructural examina-tion of cast composites indicated extensive damage to the surface of the carbon fibers when uncoated carbon fibers were introduced into the melt under the conditions of the present investigation. When nickel- or copper-coated carbon fibers were used to make composites under similar conditions, the fibers generally did not exhibit observable amounts of fiber surface degradation at the interface, except for small islands of an Al₄C₃ phase. When nickel-coated carbon fibers were used to make composites, the coating reacted with the melt, and NiAl₃ intermetallic phase particles were observed in the matrix away from the fibers, indicating a preference for nucleation of NiAl₃ away from the fiber surfaces. Under a transmission electron microscope (TEM), the NiAl₃ phase was not observed on the surface of carbon fi-bers, except in some regions where the NiAl₃ phase engulfed the carbon fibers during growth. When cop-per-coated carbon fibers were used to make composites, the coating reacted with the melt, and particles of CuAl₂ intermetallic compound were generally dispersed in the matrix away from the fibers, except for a few locations where the CuAl₂ phase was found at the interface under TEM observation. These micro-structures are discussed in terms of nucleation of primary α aluminum and NiAl₃ or CuAl₂ phases and the interaction between short carbon fibers and these phases during growth while the composite was so-lidifying. Additionally, the role of the reaction between nickel or copper coatings and the melt on struc-ture formation is discussed; some of the differences between the nickel and copper coatings are attributed to the fact that nickel dissolves with an exothermic reaction. The differences between solidification of short fiber composites and particle or fiber composite are also discussed.