Distributions of structures and solute in directionally solidified Al - 7 wt % Si

Abstract : Bridgman experiments for various withdrawal rates were conducted in an Al - 7 wt % Si alloy. The dendrite arm spacing and the grain size of the primary dendritic aluminium-rich phase were measured, together with the spatial distribution of the interdendritic eutectic micro structure. A fine grain structure is observed for large pulling rates at constant temperature gradient, corresponding to a uniform distribution of the eutectic structure very close to the Gulliver-Scheil approximation. When decreasing the cooling rate, the average fraction of the eutectic structures decreases as it is expected based on back-diffusion. However, the distribution of the eutectic structure progressively looses uniformity and the grain size increases. Detailed analyses of these observations are conducted using a cellular automaton - finite element model. It allows the simulation of the local solidification path for an open system coupled with a solution of the heat and solute mass balances in the presence of fluid flow for the entire Bridgman sample. Evolutions of the size and density of structures are retrieved when increasing the cooling rates. This demonstrates the key role played by the thermosolutal buoyancy forces and their interaction with the solidifying dendritic grain structure to explain the distributions of structures and solute segregation.