Investigation Of Metastable Phase Reinforced Mg-Base Alloys

In this paper, Mg-based amorphous alloys and quasicrystalline alloys and their composites were prepared by copper mould casting. The microstructures and phase constituent were analyzed by optical microscopy(OM),scanning electron microscopy (SEM), X-ray diffraction (XRD) and Transmission Electron Microscope (TEM). The mechanical properties of the alloys were tested by universal electronic material testing machine, and the fracture surfaces were analyzed by SEM.Mg₇₀Zn₂₅Ca₅Cu_x(x=l, 2, 3.5, 5) alloys were investigated. The results show that the glass-forming ability is the best among the alloys when x=l, the critical size for glass formation is 3mm in diameter. At the same time, Mg₆₉Zn₂₅Ca₅Cu₁ alloy exhibits the largest strain up to 1.7% and stress of 690MPa respectively, which is the largest plasticity found in the Mg-based bulk metallic glasses.Mg(70-x)Zn₂₅Ca₅Y_x(x=l, 3) amorphous alloy matrix composite was prepared by replacing Cu by Y in Mg-Zn-Ca-Cu alloys. The results show that crystalline phase appears in the microstructure due to the addition of Y element, which further reduces the fracture stress and strain of the alloys. For the Mg(80-x)Zn(14+x)Ca₄Y₂(x=0, 2, 4) alloys, the shape of the crystalline phases is changed from dendritic to stick and then to petal-like companying by dotted-like crystalline phases. The fracture strength and plastic strain were reduced from 540MPa and 2.4% for x=0 alloy to 430MPa and 0.8% for x=4 alloy.Mg_100-7xZn_6xY_x(x=l, 2, 3, 4) alloys were desigened by Zn/Y ratio of being 6. The microstructures of as-cast alloys with low and high cooling rate were investiaged respectively. The results show that the grains become finer in the alloy with the increasing of Y element and the changing of Zn/Y ratio makes the morphologies of quasicrystals different. The microstructure changes ofα-Mg and the appearance eutectic-like quasicrystal were observed with the increase of cooling rate.Mg_87-xZ₁₁Y₂Ni_x(x=0, 3, 5, 7, 9) alloys were desigened by adding proper Ni element in Mg-Zn-Y alloys. The results show the morphology of quasicrystal was changed by addition of Ni element. The fish-bone morphology in the quasicrystal was observed when x=7, which has never been reported in the litetature.