The Microstructure And Mechanical Properties Of Mg-Ni-Zn-Y Amorphous Matrix Composites

The high specific strength and good plasticity for Mg-based amorphous matrix composites (AMC) make them have many potential applications in such as micro electro mechanical system. Therefore the search for Mg-based AMC with higher plastic deformation has become a highlight in the development of bulk metallic glasses. In this paper, Mg-Ni(Cu)-Zn-Y amorphous matrix composites were prepared by copper mould casting. The microstructure, the fracture surfaces and the phase constituent of those alloys were analyzed by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The mechanical properties of the AMC were tested by universal electronic material testing machine. The results are given as following:Comparative investigations were made for Mg₇₇Cu₁₂Zn₅Y₆ and Mg₇₇Ni₁₂Zn₅Y₆ as-cast alloys with 3mm in diameter. The fracture strength and plastic strain are 532MPa and 2.4% for the former and 667MPa, 7% for the latter. The different crystalline phases in the amorphous matrix result in the different mechanical properties for the two alloys. Furthermore, the plastic deformation mechanism is suggested by analyzing the stress-strain curves.The microstructure and mechanical properties for Mg_77+xNi_12-xZn₅Y₆(x=4, 6, 8) alloys were investigated by considering both Mg and Ni variation. The homogenous distribution of needle-like Mg phase was only observed for x=4 alloy and it decreased with the decrease of Ni element. At the same time, the intermetallic Mg₂Ni was increased and Mg₁₂YZn phase was increased. It is interesting to find that the plastic strain for the alloy with x=4 is as high as 21%, which is the largest plastic strain found up to now. The fracture strengths and plastic strains are 640MPa and 15.8% for x=6 alloys, respectively, 600MPa and 17% for x=8 alloys, respectively.The microstructure and mechanical properties for Mg_73+xNi₁₂Zn₅Y_10-x(x=0, 2, 6, 8) alloys were investigated by decreasing Y and increasing Mg content. The microstructure evolution from flower-like for x=0 to needle-like for x=6 and then to thick dendritic for x=8 alloys respectively. While a combination microstructure containing both flower and needle Mg solid solution was observed for x=2 alloy, which presents the best mechanical properties among the alloys mentioned above. The fracture strength and plastic strain for it are 692MPa and 4% respectively. When we further decreased Ni content and re-designed Mg_87-xNi₈Zn₅Y_x(x=2, 4, 8) alloys, however, the mechanical properties of the new alloys were not improved. The effect of cooling rate on the mechanical properties was also investigated. The fracture strength and plastic strain were becoming worse when as-cast sample size was increased for both Mg₇₅Ni₁₂Zn₅Y₈ and Mg₇₇Ni₁₂Zn₃Y₈ alloys.