Modification of hypoeutectic Al-Si alloys

Modification of the Al-Si eutectic microstructure has been studied by using the entrained droplet technique in a range of hypoeutectic Al-Si alloys doped with different levels of phosphorus and sodium. Differential scanning calorimetry has been used to investigate the kinetics of silicon nucleation during eutectic solidification, and transmission and scanning transmission electron microscopy has been used to investigate the eutectic microstructure and microchemistry. The Al-Si eutectic microstructure can be modified from coarse-faceted Si particles nucleating at low undercooling to clusters of fine-scale Si particles nucleating at high undercooling by either increasing alloy purity, adding Na, or increasing cooling rate during solidification. The unmodified coarse Si particles are heterogeneously nucleated on pre-existing AlP particles. Increasing alloy purity removes P and prevents the formation of AlP; adding Na leads to the formation of Na sub 3 P in preference to AlP; and increasing cooling rate allows insufficient time for the precipitation of AlP from low levels of dissolved P. At a cooling rate of approx10K/min, the transition from an unmodified to a modified Al-Si eutectic microstructure takes place when P levels fall below 0.25-2 ppm, and when Na levels increase to 80-850 ppm.