Columnar dendritic solidification in a metal-matrix composite

Results are presented of a study on columnar dendritic solidification of the matrix of a fibrous metal-matrix composite, the fibers of which are aligned SiC fibers 140 mum in diameter and the matrix of which is Al-4.5 wt pet Cu. Samples were produced by pressure infiltration of the metal-matrix into a preform of the fibers. The matrix was subsequently remelted and resolidified under controlled thermal gradient and growth rate. Dendrite growth begins in the center of the interstices left between the fibers. The dendrite tip temperature is not significantly influenced by the fibers, but the usual linear dependency of dendrite arm spacing on t(1/3) (where t is time during solidification) is altered significantly in the narrower interstices at long solidification times. The underlying mechanism is dendrite arm coalescence which takes place at a sufficiently rapid rate in the composite that the microstructure gradually becomes nondendritic. The solid/liquid interface then is parallel to the matrix/fiber interface. A model is presented for the kinetics of dendrite arm coalescence and compared with experimental results. The amount of microsegregation that was found in the matrix within interstices is significantly less than that found in the usual cast alloy, especially at long solidification times (low cooling rates). The mechanism responsible for the observed reduction in microsegregation is solid-state diffusion which is enhanced in the composite by the fact that the fibers place an upper limit on the dendrite arm spacing, and hence on the required diffusion distance.