Carbide formation in aluminium-carbon fibre-reinforced composites

New results on the carbide formation in aluminium-carbon fibre composites are reviewed, with their implications for technology, including vacuum infiltration of carbon-fibre preforms with liquid aluminium. The microstructure of infiltrated specimens was studied with the aid of transmission electron microscopy. Most lath-like carbide crystals investigated in this work are twins. Twinning is probably connected with the squeezing stresses during matrix cooling due to a high difference in thermal expansion coefficients of carbide and metal. The analytical description of experimental data in the light of the crystal-growth concept allows us to conclude that carbides grow during infiltration, predominantly at the time of fibre contact with molten aluminium, but not during matrix solidification and its subsequent cooling. The growth rate of carbide crystals is limited rather by the interface kinetics than by carbon diffusion in the melt as was assumed previously. This allows some effective methods of process control to be found, for example, growth step retardation by means of an adsorption-active impurity.