Effect of Phosphorus Content on the Microstructure Evolution of Highly Undercooled Al-60%Si Alloys

In order to investigate the effect of P content on the growth morphology and growth mode of the Si phase, the Al-60%Si alloys containing 0.5%P and 1.0%P were subjected to deep undercooling using an electromagnetic levitator. The morphology evolution and growth mode of the Si phase were studied by analyzing the dynamic images recorded by HSV and the SEM images of as-solidified samples. The results reveal that the morphology of the Si phase transformed from the large strip shape to coarse bulks and regularly arranged dendrites, then to spheroidal and rod-shaped with increase of undercooling, and the corresponding growth mode changed from faceted growth to mixed growth, then to non-faceted growth. The P refines the size of the Si phase by enhancing the nucleation rate of the Si phase. With the increase of P content, the critical undercoolings of growth mode transition decrease, and the experimental results are in good agreement with the theoretical predicted results.