Eutectic nucleation and growth in hypoeutectic Al-SI alloys at different strontium levels

The effects of different levels of strontium on nucleation and growth of th e eutectic in a commercial hypoeutectic Al-Si foundry alloy have been inves tigated by optical microscopy and electron backscattering diffraction (EBSD ) mapping by scanning electron microscopy (SEM). The microstructural evolut ion of each specimen during solidification was studied by a quenching techn ique at different temperatures and Sr contents. By comparing the orientatio n of the aluminum in the eutectic to that of the surrounding primary alumin um dendrites by EBSD, the eutectic formation mechanism could be determined. The results of these studies show that the eutectic nucleation mode, and s ubsequent growth mode, is strongly dependent on Sr level. Three distinctly different eutectic growth modes were found, in isolation or sometimes toget her, but different for each Sr content. At very low Sr contents, the eutect ic nucleated and grew from the primary phase. Increasing the Sr level to be tween 70 and 110 ppm resulted in nucleation of independent eutectic grains with no relation to the primary dendrites. At a Sr level of 500 ppm, the eu tectic again nucleated on and grew from the primary phase while a well-modi fied eutectic structure was still present. A slight dependency of eutectic growth radially from the mold wall opposite the thermal gradient was observ ed in all specimens in the early stages of eutectic solidification.