CARBIDE FORMATION IN ALUMINUM-CARBON FIBER-REINFORCED COMPOSITES

New results on the carbide formation in aluminium-carbon fibre composi tes are reviewed, with their implications for technology, including va cuum infiltration of carbon-fibre preforms with liquid aluminium. The microstructure of infiltrated specimens was studied with the aid of tr ansmission electron microscopy. Most lath-like carbide crystals invest igated 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 de scription of experimental data in the light of the crystal-growth conc ept allows us to conclude that carbides grow during infiltration, pred ominantly at the time of fibre contact with molten aluminium, but not during matrix solidification and its subsequent cooling. The growth ra te of carbide crystals is limited rather by the interface kinetics tha n by carbon diffusion in the melt as was assumed previously. This allo ws some effective methods of process control to be found, for example, growth step retardation by means of an adsorption-active impurity.