Abstract
Al-Si alloys have been splat cooled under various cooling conditions and their structures have been examined. The measured cooling rate varied from 105°C/sec to 107°C/sec depending mainly on the condition of thermal contact between specimen and substrate. Assuming the Newtonian cooling, the solidification rate was estimated to be about 10 cm/sec for the cooling rate of 106°C/sec. The lattice parameter of aluminum decreased with silicon content up to about 16% indicating the extended solid solubility, but increased again for higher silicon content. The changes in splat temperatures from 600°C to 1000°C did not affect the extended solid solubility limit. When the substrate temperature was raised to 150°C the solid solubility started to decrease and for the substrate temperature higher than 300°C the lattice parameter of aluminum stayed at the equilibrium value. Transmission electron microscope observations showed no silicon phase up to 12% Si, but the electron diffraction spots of silicon were detected in the specimen with 16% silicon. Silicon particles were observed for specimens of lower silicon content when the cooling rates were small. The silicon particles were finely dispersed in aluminum grains and the crystallographic orientation was found, by careful examinations of selected area diffraction patterns, to coincide completely with that of matrix aluminum. The coincidence in the orientation between aluminum and silicon phases remaind undestroyed by aging at elevated temperatures. The aluminum grain size decreased to the order of 100 Å for specimens with more than 20% silicon. The measurements of the recovery in electrical resistance during the isochronal aging revealed that the supersaturated solid solutions are stable at room temperature and start to decompose at temperatures higher than 100°C.
