Basic Research About Controlling Of Microstructure And Mechanical Properties For Mg-Zn-Ca Magnesium Alloys

Magnesium alloys are the lightest structural alloys commercially available and have great potential for applications in automotive, aerospace and other industries. However, the elevated temperature properties of the current magnesium alloys are relatively poor. Therefore, it is very necessary to develop new elevated temperature magnesium alloys. The Mg-Zn-Ca based alloys are thought as the potential elevated temperature magnesium alloys, and their research have received much global attention. However, the research about the Mg-Zn-Ca based alloys presently focuses on the microstructure characterization and age hardening behaviour, the investigations about the controlling of the microstructure and mechanical properties by further alloying and/or micro-alloying are very scarce. Therefore, the research about the controlling of the microstructure and mechanical properties by further alloying and/or micro-alloying for the Mg-Zn-Ca based alloys has very important theoretical significance and practicable value in developing the Mg-Zn-Ca alloys with high properties and widening the application of magnesium alloys.Based on the Mg-3.8Zn-2.2Ca (wt.%) ternary new type alloy, the effects of Ce, Sn and Zr additions on the microstructure and mechanical properties of the Mg-3.8Zn-2.2Ca alloy, especially the effects on as-cast microstructure, tensile and creep properties, were investigated by using optical microscope(OM), scanning electron microscope (SEM), X-Ray diffraction(XRD) , DSC and tensile and creep properties testing, and the following main results were obtained:1) The Mg-3.8Zn-2.2Ca magnesium alloy is mainly composed ofα-Mg, Mg₂Ca and Ca₂Mg₆Zn₃ phases, and the Ca₂Mg₆Zn₃ phase mainly exhibits continuous and/or quasi-continuous distribution. The phase transformations during the solidification of the Mg-3.8Zn-2.2Ca alloy mainly consists of the binary eutectic reaction (L→α-Mg+Mg₂Ca) at ⁴30°C and the binary eutectic reaction (L→α-Mg+Ca₂Mg₆Zn₃) at ⁴00°C.2) Adding 0.5-2.0wt.%Ce can effectively refine the grains of the Mg-3.8Zn-2.2Ca alloy, and an increase in Ce amount from 0.5 wt.% to 2.0wt.% causes the grain size to gradually decrease. Furthermore, adding 0.5 wt.% and 1.0wt.%Ce respectively causes the morphology of the Ca₂Mg₆Zn₃ to partly change from initial continuous blocks to fine particles but after adding 2.0wt.%Ce, parts of the Ca₂Mg₆Zn₃ and Mg12Ce phases are mixed, and the morphology of the Ca₂Mg₆Zn₃+Mg12Ce mixed-phases changes to coarse quasi-continuous blocks. In addition, adding 0.5-2.0wt.%Ce can improve the tensile and creep properties of the alloy. Among the Ce-containing alloys, the alloy with 1.0wt.%Ce exhibits the best ultimate tensile strength and elongation while the alloy with 2.0wt.%Ce has the best yield strength and creep properties.3) The CaMgSn phase is formed in the Sn-containing alloys, and with the increase of Sn amount from 0.5 wt.% to 2.0wt.%, the amount of the CaMgSn phase increases, and the morphology of the Ca₂Mg₆Zn₃ phase changes from initial continuous and/or quasi-continuous net to quasi-continuous and/or disconnected shapes. Furthermore, the grains of the Sn-containing alloys are effectively refined, and the grains of the alloy with the addition of 1.0wt.%Sn are relatively finer. In addition, adding 0.5wt.% and 1. 0wt.%Sn can improve the tensile and creep properties while adding 2.0wt.%Sn gives a detrimental effect on the ultimate tensile strength and elongation but is beneficial to the yield strength and creep properties. Among the three Sn-containing alloys with the additions of 0.5wt.%, 1.0 wt.% and 2.0wt.%Sn, the alloy with the addition of 1.0wt.%Sn exhibits the optimal tensile and creep properties.4) Adding 0.5-1.5wt.%Zr to the Mg-3.8Zn-2.2Ca does not cause the formation of any new phases but causes the morphology of the Ca₂Mg₆Zn₃ phase to change from initial continuous and/or quasi-continuous net to quasi-continuous and/or disconnected shapes. Furthermore, the grains of the Zr-containing alloys are effectively refined, and an increase in Zr amount from 0.5wt.% to 1.5wt.% causes the grain size to gradually decrease. In addition, adding 0.5-1.5wt.% Zr to the Mg-3.8Zn-2.2Ca alloy can improve the tensile properties but decreases the creep properties. Among the Zr-containing alloy, the alloy with the addition of 1.5wt.%Zr exhibits the relatively optimal tensile and creep properties.