Microstructures of Conventionally As-Cast Mg-Zn-Y Alloys

Two series ingots of Mg69+xZn30-xY1 and Mg68+xZn30-xY2 (x = 0, 3 and 6, atomic percent) alloys with the diameter of 100 mm were obtained by ordinary gravity casting, respectively. The phase compositions and microstructures were investigated by means of optical microscope (OM), scanning electron microscopy (SEM) and X-ray diffraction (XRD). The law that the number, morphology, size and distribution of the quasicrystal phase varied with the content of Zn and Y was discussed. The results showed that the microstructures of the two series alloys were composed of MgZn matrix, Mg30Zn60Y10 quasicrystal phase (I) and alpha-Mg phase. The I-phase directly nucleated and grew during the cooling process of Mg-Zn-Y melt. The morphology of the two series alloys was different obviously. The I-phase exhibited small pentagonal or five-petal shape and six-petal shape in the microstructure of the Mg69+xZn30-xY1 alloy, while it was not regular polygon shape in the microstructure of the Mg68+xZn30-xY2 alloy. At the same time, with decreasing Zn content, the granularity and homogenization degree of the quasicrystals enhanced gradually. The quasicrystals increased and distributed more evenly.