Effects Of Solidifying Condition On Microstructure And Nucleation Dynamics Of Mg-Zn-Y Alloys

As the lightest metal structure material, magnesium alloys are found using widely in automotive, electronic and aeronautical industries because of a number of desirable features, including high strength/weight, high hardness/weight, damping characteristic and recycled easily. The microstructures, secondary phase of as-cast alloys are coarse and easily oxidized in high temperature, the room temperature strength and elevated temperature strength is undesirable, which is difficult to meet the needs of high performance structure materials. The rapid solidification appears under this condition, the materials made by rapid solidification have many advantages, such as high strength,high toughness,wonderful corrosion resistance because of fine grains,large solid solubility, new metastable phase. New magnesium alloys with high performance can be developed by rapid solidification. Mg-Zn based alloy is typical high strength magnesium alloy. But its strength is difficult to apply in higher condition. Its strength and heat resistance can be improved by the addition of Y.The rapidly solidified samples of Mg-Zn-Y alloys were prepared using single-roller equipment. In this thesis, the microstructures, phase composition were systemically investigated by using measurements of OM, XRD, SEM, EDS, TEM. The phase transformation was studied by DSC. An expression adapts to the heterogeneous nucleation rate was induced, utilizing time depended nucleation theory.Microstructures of as-cast Mg-Zn-Y alloys are dendritic morphology. As the addition of Y and Zr, the grain size is refined. The grain boundary appears two kinds of morphology: one is "fish bone", which nucleates in triangle grain; the other is continuous graticule, which enclose the whole grain. Microstructures of the cross section of rapidly solidified Mg-Zn-Y alloy ribbons are composed of three regions: fine equiaxed region near the copper roll side, the inner columnar grain region and the outer equiaxed region near the free surface. The microstructure of the region near the copper roll side is fine equiaxed morphology, as the increasing of the rotation speed, the thickness of the outer equiaxed region near the free surface becomes thinner, even disappears in some region.