| Magnesium alloys as the lightest structural metallic materials have large application potential in the industry and become more and more attractive for material researchers. It is becoming the third largest metal structural materials following steels and aluminum alloys, known as the "21st Century Green Engineering Materials". However, the mechanical properties of most commonly used magnesium alloy are very low. But the cost of high strength and high toughness magnesium alloys are high because they always high content of RE. Therefore, research and development low-cost of high strength and high toughness magnesium alloys is very necessary. In this thesis, the effects of Zn, Y, Nd, Sn, Zr and Ca contents on microstructure and mechanical properties of ZW21 alloy (2.0%Zn,1.0%Y,0.5%Nd,0.5%Sn, 0.3%Zr and 0.05%Ca) have been investigated. In addition, the effect of heat treatment on microstructure and mechanical properties of ZW21 alloy have been studied. The results are shown as followings:The additions of the above alloying elements have large effect on the microstructure. The addition of different amounts of Zn, Y and Nd can change the microstructure.morpgologies and phase constitutes. The phases containing Mg, Zn, RE (Y or Nd) are determined by Zn/RE ratio:the formed phase(s) should be H-phase, H-phase+W-phase, W-phase and W-phase+ I-phase When Zn/RE ratio is less than 0.60, and in the ranges of 0.60～0.85,0.85～2.05,2.05～5.00, respectively.The tensile strength, elongation and hardness of ZW21.alloy are 207MPa, 16.9%,52.8HV, respectively. It as-cast microstructure is consisted ofα-Mg+W-phase. After being solutionized at 525°C for 4h, it evolves into a uniqueα-Mg phase. During the subsequently ageing at 250°C, the W-phase graudually precipitates. Ageing treatment (24h at 250°C) can improve the mechanical properties of the ZW21 magnesium. Compared with the as-cast ZW21 alloy, the tensile strength, elongation and hardness of aged alloy are increased by 17.4%,23.1% and 11.2%, respectively.
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