Dendrite growth of aluminum-copper alloy reinforced with continuous alumina fibers

Directional solidification studies were carried out in continuous alumina fiber reinforced Al-4.5 and 15 mass%Cu alloy composites in order to clarify the influences of fibers on crystal growth of matrix alloy in the composites. The specimens were designed to have an inner composite region and an outer unreinforced bulk region in order to characterize the dendrite morphology in both regions under the same conditions. The composite specimen was produced by a pressure infiltration process, subsequently remelted, and then directionally solidified. In the composite region of the specimens, the shape of the dendrite was distorted by the presence of fibers, with the primary dendrite tip position being located about 450-750 μm below that in the bulk region. The difference of undercooling was estimated to be 1.2-1.7 K. The concentration of copper at the dendrite tips was 0.05-0.15 mass% higher than that in the bulk region, and the tip composition increased as the fiber interstice became smaller. The solutal undercooling was estimated to be 0.9-2.7 K, so that solutal diffusion field around dendrite tip could have governed the dendrite tip undercooling. Furthermore, in the composite region, the primary dendrite arm spacing decreased to about 70% of that in the bulk region on an average. A model based on the continuity of liquid phase among fibers reveals how fibers influence both the concentration of copper on the dendrite tips and the size of the lateral solute diffusion field.