Dynamical evolution of microstructure in finely atomized droplets of Al-Si alloys

The complex time-dependent evolution of microstructure in rapidly solidified droplets of Al-Si eutectic alloys is quantitatively studied. Fine droplets of the alloy were obtained by using a high pressure gas atomization technique. The recalescence effect within each droplet gives rise to non-steady state growth conditions that lead to a systematic variation in microstructure across the droplet. This variation in microstructure is quantitatively characterized first by determining the nucleation undercooling and then by measuring the spatial variation in microstructural scales within the droplet. The results are compared with theoretical models to obtain the interface velocity and interface undercooling as a function of distance from the apparent nucleation site in different sized droplets. Eutectic, dendritic and cellular microstructures have been observed in droplets and the presence of these different morphologies is explained by constructing a microstructure map for the Al-Si system. The volume fractions of these three microstructures, as well as the length scale of each microstructure, are shown to determine the properties of the material. Specifically, microhardness measurements have been carried out for different diameter droplets, and a minimum in the microhardness is observed that is related to the presence of different microstructures. A microstructure map is developed for the Al-Si system to establish regimes of alloy composition and undercooling (as related to droplet diameter) required for the design of materials with optimum mechanical properties.