Aluminum-matrix composites (AMCs) reinforced with submicron-sized ceramic particles of Al2O3, TiB2 and TiC were in-situ synthesized by reactive sintering of Al, TiO2, and B4C powder mixtures and further densified by hot-extrusion process. The reaction mechanisms for formation of the reinforcing particles, extrusion behavior, microstructure, and tensile properties of the AMCs have been investigated. The reactions of TiO2 and B4C with molten Al were a stepwise process, and there were many intermediate phases including oxygen deficient titanium oxides (Ti3O5, Ti2O3, and TiO), Al4C3, AlB2, and Al3Ti, before the expected reinforcing particles of Al2O3, TiB2, and TiC were formed. The results showed that hot-extrusion process was an effective means to densify reactive-sintered porous composites, and dense AMCs can be obtained through hot-extrusion in a temperature range of 480–550°C. The microstructure of the resulting AMCs was characterized by fine reinforcing ceramic phases with an average particle size of 0.24 μm, which were homogeneously distributed in Al matrix. Furthermore, no significant change in particle sizes could be found after extrusion, and ceramic particle content and extrusion temperature have small influences on the average particle sizes of the reinforcing phases. The presence of these sub-micron hybrid ceramic particles resulted in significant enhancements in yield and tensile strength of the AMCs. The yield strength improvement is mostly due to the coefficient of thermal expansion (CTE) mismatch between the ceramic particles and Al matrix, followed by Orowan strengthening, while the relative contributions of grain refinement and load-bearing effects are much smaller.

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Aluminum/graphite composites have been successfully prepared by a hot-extrusion technique. The effects of processing conditions such as graphite particle size, graphite content, and extrusion temperature on extrusion behavior, microstructure, texture, and thermal conductivity have been systematically investigated. During the hot extrusion, the graphite was subjected to deformation and hence distributed along the extrusion direction in the extruded Al/graphite composites. The (00l) basal planes of the graphite were preferentially orientated along the extrusion direction. The preferred orientation of the graphite resulted in an anisotropy of thermal conductivity in the extruded samples. On the other hand, the utilization of bimodal graphite powder consisting of coarse and fine particles is beneficial to the enhancement of both relative density and thermal conductivity. Moreover, when a pressed green compact was rotated 90°and then subjected to the hot extrusion, the resulting composite exhibited higher thermal conductivity due to its higher density, fewer Al/graphite interfaces, and higher orientation degree of the graphite.

Electroless nickel-coated carbon fibers/aluminum composites were prepared by spark plasma sintering, and the effect of nickel coating on microstructure and thermal properties of the composites has been investigated. Nickel coating on carbon fibers resulted in more homogeneous distributions of carbon fibers in aluminum matrix, higher relative density of carbon fibers/aluminum composites, and stronger interfacial bonding between carbon fibers and aluminum. Microstructural observations exhibited that the majority of carbon fibers were randomly distributed on the sections (X-Y direction) perpendicular to spark plasma sintering pressing direction (Z direction), thus leading to an anisotropic behavior in thermal conductivity of the composites. The thermal conductivity values in the X-Y direction of the carbon fibers/aluminum composites were much higher than those in the Z direction. As a result, the nickel-coated carbon fibers/aluminum composites with a nickel-coating thickness of ∼0.2 µm showed higher thermal conductivity and lower coefficient of thermal expansion values in comparison with those of the uncoated carbon fibers/aluminum samples.

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