STRUCTURE FORMATION DURING PROCESSING SHORT CARBON-FIBER-REINFORCED ALUMINUM-ALLOY MATRIX COMPOSITES

Nickel- and copper-coated, as well as uncoated, short carbon fibers we re dispersed in melts of aluminum or aluminum alloys by stirring follo wed by solidification of composite melts. Microstructural examination of cast composites indicated extensive damage to the surface of the ca rbon fibers when uncoated carbon fibers were introduced into the melt under the conditions of the present investigation. When nickel- or cop per-coated carbon fibers were used to make composites under similar co nditions, the fibers generally did not exhibit observable amounts of f iber surface degradation at the interface, except for small islands of an Al4C3 phase. When nickel-coated carbon fibers were used to make co mposites, the coating reacted with the melt, and NiAl3 intermetallic p hase particles were observed in the matrix away from the fibers, indic ating a preference for nucleation of NiAl3 away from the fiber surface s. Under a transmission electron microscope (TEM), the NiAl3 phase was not observed on the surface of carbon fibers, except in some regions where the NiAl3 phase engulfed the carbon fibers during growth. When c opper-coated carbon fibers were used to make composites, the coating r eacted with the melt, and particles of CuAl2 intermetallic compound we re 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 microstructures are discussed in terms of nucl eation of primary alpha aluminum and NiAl3 or CuAl2 eve and the intera ction between short carbon fibers and these phases during growth while the composite was solidifying. Additionally, the role of the reaction between nickel or copper coatings and the melt on structure formation is discussed; some of the differences between the nickel and copper c oatings are attributed to the fact that nickel dissolves with an exoth ermic reaction. The differences between solidification of short fiber composites and particle or fiber composite are also discussed.