Microstructure variation and growth mechanism of hypoeutectic Al-Si alloy solidified under high pressure

The microstructure of hypoeutectic Al-9.21wt.%Si alloy solidified under 5.5 GPa was studied. The results show that the solidification microstructure is refined. The primary alpha phase is the extended solid solution. The solid solubility of Si in alpha phase is up to 8.26wt.%. The growth mode of the alpha phase is cellular, and this cellular growth mechanism is interpreted in terms of the decrease of the diffusivity and the extended solid solution pressure. By calculation, it can be known that the the diffusivity of solute in the liquid under normal pressure is high as two hundred times that under high pressure. The microhardness of the hypoeutectic Al-Si alloy solidified under high pressure is higher than that of solidified under normal pressure. After annealing, Si precipitates from the solid solution, the microhardness of the alloy decrease, but, still higher than that of solidified under normal pressure.