| In the rapid development of the automotive and electronics industries,Mg-Zn-Al alloy is a new low-cost magnesium alloy with good creep resistance, molding properties.However, some properties of the alloy have not yet reached satisfactory levels, in particular, there are still some problems in room and high temperature mechanical properties. Ca is an important alloying element in improving high temperature performance, one of the main measures to improve high temperature performance is alloying with calcium.By using as-cast forming technique and test facilities such as optical microscope,scanning electron microscope(SEM),X-ray diffraction (XRD),electronic universal testing machine, et cetera,studied the effects of Zn addition on the microstructures and mechanical properties of the Mg-Zn-Al alloys, meanwhile, effects of Ca addition on the microstructures and mechanical properties of the Mg-(8%～12%)Zn-4Al alloys have been investigated, and effects of Ca on microstructures refinement mechanism and the forming principle of alloy phase in the solidification process of liquid metal have been in-depth explored. The study shows that:1) The matrix phase of Mg-Zn-Al cast alloys areα-Mg,τ-Mg32(Al,Zn)49 phase andφ-Al2Mg5Zn2 phase,τ-Mg32(Al,Zn)49 phase, who exist as intermittent strip-like, semi-continuous and continuous net-like.φ-Al2Mg5Zn2 phase exist as dispersive island-like. With the addition of Zn content, the morphology is changed from small strip-like,net-like and island-like to semi-continuous and continuous thick net-like. In particular, with the Zn addition up to 12 wt.%, there are numerous continuous thick honeycomb net-like texture in local region. The macro-hardness increases gradually, meanwhile, brinell hardness is rise to the maximum 71.3HB. The tensile strength and impact toughness are straight-in penetration. When Zn addition is 8 wt.%, the tensile strength and impact toughness is 189MPa,8.4J/cm2,When Zn addition is 12 wt.%, the tensile strength and impact toughness is 168MPa,5.5J/cm2,and Zn addition in 8 wt.% to 10wt.%, the tensile strength and impact toughness decline is slow; when it's addition above 10 wt.%, decreases rapidly.2) When the Ca has been added, a small amount of Ca have been dissolve in the alloy, as a result the peak has been shifted. Meanwhile,τ-Mg32(Al,Zn)49 phase peaks significantly increases, whileα-Mg peaks, and theφ-Al2Mg5Zn2 peaks become relatively weaker. It shows that, according to the addition of Ca,φ-Al2Mg5Zn2 phase separation has been inhibited,τ-Mg32(Al,Zn)49 phase separation has been accelerated. Furthermore, there is no new phase which contain Ca in ZA84 alloy. However, in ZA104 and ZA124,with the change of calcium content, the alloy phase have been changed. In ZA104 alloy, with Ca addition up to 0.3 wt.%, there is a new phaseτ1,τ2[(Mg,Ca)32(Al,Zn)49]; in ZA124 alloy,the new phase appears when the the Ca addition up to 0.6 wt.% yet. Meanwhile,Ca have a certain impact on the formation of microstructure and phase of ZA series alloy. The main manifestations show that the organizations and the grain have been refined with addition of small amounts of Ca, the net-like phase also have been refined,the joint of the net-like phase have been fractured, some phase has broken into isolated strengthening phase. Then with addition of Ca content,microstructure become coarse gradually. When Ca content up to 0.9 wt.%,the morphology is semi-continuous and continuous net-like.3) Effects of Ca addition on the microstructures and mechanical properties of the Mg-Zn-Al alloys have been investigated. With the increase of calcium content, the macro-brinell hardness of the alloy increased accordingly, when Ca addition up to 0.3 wt%,brinell hardness of the alloy obvious changes,curve rising more rapidly (especially for ZA104 alloy),then with the calcium content continues to increase, the curve rises relatively slowly, and the highest hardness values of the ZA84, ZA104, ZA124 alloy are 71.2HB, 78.5HB, 73.9HB. With the increase of calcium content,the impact toughness of alloy first increases and then decreases. When Ca content is 0.3 wt.%,the impact ductility of ZA124,ZA104 achieve the top point,they are 6.4J/cm2,8.9 J/cm2, while ZA84 reaches its maximum 10.4J/cm2 with 0.6 wt.% Ca content. With 0.3 wt.% calcium content addition, tensile strength tests were all at the optimal under room temperature, ZA104 alloy which shows the most significant, and it's maximum value is 203MPa. Then with the increase of calcium content, all the alloy show a linear decline in tensile strength. Up to 0.9 wt.% calcium, ZA84, ZA104, ZA124 tensile strength values decrease to 149MPa,157MPa,143MPa. Yield strength and elongation corresponding changes with the tensile strength. Ca can improve the tensile properties of alloys in high temperature, particularly in ZA104 alloy, with Ca addition up to 0.3 wt%., obtains optimum elongation rate 17.3%, then high temperature tensile strength is 190MPa.4) Fracture analysis shows : With the addition of Zn content , the quasi-cleavage fracture is slowly transforming into cleavage fracture,when Zn content achieve 12 wt.% from 8 wt.% in test alloys, the fracture morphology gradually transformes into rock candy-like cleavage plane,from dimples and cleavage quasi-cleavage crack. Ca improves the tensile fracture surface,and when the addition of Ca is up to 0.3 wt.%, the result is the most obvious. The fracture analysis of all the ZA104 alloy in high temperature are quasi-cleavage fracture. Although some of the region show cleavage stage, compares with room temperature tensile fracture,the level of cleavage stage is significantly higher.
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