Microstructural evolution in rapidly solidified Al-Cu-Si ternary alloys

Several Al-Cu-Si alloys were melt spun to produce stable, fine scale microstructures suitable for superplastic deformation and consolidation. Scanning electron microscopy of the ribbon cross-sections reveal two distinct alternating microstructural morphologies, suggesting transitions in solidification behavior. One structure consists of intimately interlocked α-Al and θ (Al2Cu) phases with dispersed spheroids of (Si). The other structure consists of equiaxed or cellular-dendritic α-Al with interdendritic θ and (Si). The latter was found in the middle portion of the ribbon cross-section when cast at a low speed, and throughout the ribbon cross-section when cast at high speed. The dendritic structure appears to result from independent nucleation events in the undercooled liquid ahead of the solid-liquid interface. The solidification mechanism for the interlocked structure appears to involve multiple nucleation of the θ phase followed by its cooperative growth with the α-Al phase. This cooperative growth is unlike that which forms a lamellar structure, as it results in a branched, randomly oriented network. We postulate that the (Si) phase is the first phase to form from the undercooled liquid, and it is uniformly dispersed throughout the undercooled melt. The (Si) spheroids provide nucleation sites for the θ phase because of its observed association with the θ phase. The α-Al grain size varies from 1 μm near the wheel side surface of the ribbon to 8 μm with sub-grains near the free surface. The size of the θ and (Si) phases is on the order of a μm and less. The microstructural size scale appears to be small enough for this material to exhibit superplastic behavior when deformed.