Optical micrographs of as-cast Mg-S

Optical micrographs of as-cast Mg-Sn-Zn-Y alloy with and without Y added alloys are shown in Fig. 1. Evidently, the matrix of as-cast alloy is dominated by dendrite and irregular precipitates, the morphology of Mg-5Sn-3Zn(Fig. 1a) exhibits as rose-shaped which is typically the dendritic structure and the dendritic spacing is relatively large, valued about 15 mm. With the addition of Y, the Mg-5Sn-3Zn-xY alloys have changed the dendritic structure into the equiaxed grains because the growth of secondary dendritic arm is restricted, and the grain size is getting finer, The average grain sizes showing in Fig. 1b,c,d are 72.7 mm, 56.2 mm and 66.5 mm, respectively. It implies that 0.2e0.8 wt% addition of Y refines the dendritic structure of as-cast alloys effectively. Lots of works have been done to illustrate the mechanism of grain refinement by rare-earth elements in magnesium alloys. As for Y, which is considered as the surface active element, it would reduce the surface energy of metal liquid, in this way the critical nucleation energy can be decreased to stimulate more nucleation. Y would also refines the grains by segregating in the solid-liquid interface during the solidification to change the growth kinetics, particularly alter the growth restricting factor(GRF) to control the growth process of dendrite. The higher the GRF value is, the more likely it is to hinder the growth of grains, and the high content of Y increases the value of GRF in the liquid phase which would hinder the dendrite growth, thus result in the remarkable refinement of grains.