Deformation behavior of graphite and its effect on microstructure and thermal properties of aluminum/graphite composites

Al/graphite composites were prepared by spark plasma sintering (SPS) and hot extrusion (HE). Two kinds of extrusion routes (with and without 90° rotation of the extrusion billets) were used to understand the deformation behavior of graphite during extrusion process. Microstructure, thermal conductivity (TC), and coefficient of thermal expansion (CTE) of the SPSed and HEed samples were investigated. Microstructural observations showed that HE can lead to deformation, exfoliation, and breakage of graphite. The shear deformation caused preferential distributions of graphite along the extrusion direction. The exfoliation degree of graphite in the HEed sample with rotation was much higher than that in the HEed sample without rotation, thus resulting in more Al/graphite interfaces and smaller Al grains, which inhibited heat transfer in Al/graphite composites. Both graphite deformation and strong interfacial bonding between graphite and Al matrix resulted in decreases in average interlayer spacing d₀₀₂ of graphite. The Al/graphite interfaces in the HEed samples were relatively intricate, and amorphous-like carbon, more defects, and cleavage facets were observed in the severely deformed graphite in the HEed sample without rotation. The HEed sample without rotation showed the higher TC and lower CTE. The higher TC is attributed to the decrease in average interlayer spacing d₀₀₂ of graphite, while lower CTE is associated with the intricate Al/graphite interfaces. These findings can provide new insights for preparing metal matrix/graphite composites with high TC and low CTE.