| The most significant advantages of magnesium alloys is their particularly low dendity. Magnesium alloys are the lightest metal materials in engineering application and have great potential application in the car and aerospace fields. However, most of magnesium alloys belong to hexagonal close-packed (HCP) crystal structure with limited slip systems, lead to their poor plastic and strength, especially their bad high temperature performance, which greatly limits the wide application of magnesium alloys.Intermetallic compound Mg2Si is a kind of ideal reinforcing phase for magnesium matrix composite, because it has lots of advantages, such as high melting point, high hardness and high elastic modulus and low density, etc. However, Mg2Si phase, which often appears in the shape of course Chinese characters in as-cast alloy, can separate the matrix easily, reducing the mechanical properties of alloys. Equal channel angular pressing (ECAP) is a severe plastic deformation (SPD) technology, can refine matrix effectively, broke coarse reinforcing phase fully and make it tend to be redistributed dispersively, thereby improving the mechanical properties of alloys. Accordingly, the Si element is added to the ZK60magnesium alloy to introduce the Mg2Si phase in this paper, then we change its shape and size and distribution through ECAP extrusion. The microstructure and mechanical properties of cast and ECAP processed alloys were investigated.In the experiment,2%and4%of Si element were added to ZK60magnesium alloy, respectively. The influence of Si content on the microstructure and properties of as-cast alloys is researched. The results indicate that, the components of as-cast ZK60alloy are mainly a-Mg matrix, Mg7Zn3and MgZn phases. After the addition of Si element, the components of ZK60+2Si alloy are mainly α-Mg matrix, Mg2Si and MgZn phases. The Mg2Si phase mainly appears in the shape of course Chinese characters with sharp edges and corners, and distribute unevenly. With the increase of Si content, the mechanical properties of ZK60, ZK60+2Si, ZK60+4Si three kinds of alloys reduce gradually. The room temperature tensile strength and elongation rate decreases from170MPa and7%, to122.IMP and2.3%, respectively. It is mainly because that the coarse Chinese-script Mg2Si phaes dissevere the matrix seriouslyThe ZK60+2Si alloy was processed for2,4and6passes by ECAP processing through Bc route at300℃, and the pressing speed is1.6mm/min. The results show that, as the increase of ECAP passes, the matrix is been refined significantly and the average grain size decreases from100pm to15um. Meanwhile, the original coarse Chinese-script Mg2Si phases are significantly broken into particles which tend to dispersively redistribute, and its average size decreases from50um to5um. After4passes of EACP, the brinell hardness of alloy is improved from56.5to62.6, and the tensile strength and elongation of alloy at room temperature increases from154.8MPa and4.5%to270MPa and17.5%, respectively. After6passes of EACP, elongation further increases to21%, but tensile strength drops to261MPa. After ECAP, the improvement of mechanical properities is mainly because the role of matrix fine-grain strengthening and Mg2Si phase dispersion strengthening.The effects of EACP passes on creep resistance of ZK60+2Si alloy in the conditions of200℃and70MPa constant load. It turned out that, after6passes of EACP, the creep life of alloy extends as much as10times, and the steady creep rate decreases by about an order of magnitude. It is mainly because the tiny Mg2Si particles prevent grain boundary sliding effectively. |
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