| At the background of resource, energy and environment crisis, magnesium alloys, which have high specific properties, low density and high damping capacity, have been widely used as structural materials for aerospace,3C and other transport applications. However, commercial potential of Mg-Al alloy as structural materials has not been exploited for their relatively low strength and corrosion resistance.In present study investigated were the influence mechanisms of rare earth on the strength and corrosion resistance of Mg-Al alloy in the process of melting, refining and plastic working. The results of this investigation may enrich magnesium strengthening theory, expand the application field, and provide basic experimental data of magnesium alloys.The Mg-9Al-Zn and Mg-6A1-Mn alloy were melted in vacuum Ar atmosphere. By addition of different amount of rare earth La and Nd, specimens for testing were cast out of the melted metals. The wastage rate of metal components in the process of refining was calculated, the optimum rare earth amount and refining temperature were established; the effects of rare earth for refining structure, and improving mechanical properties and corrosion resistance were analyzed; the optimum parameters of solution and aging treatment were established by the research of microstructure, mechanical properties and corrosion resistance of Mg-Al alloys, the diffusion coefficient of atom Al in the aging process was caculated, and the influence of DP-β phase morphology on mechanical properties of Mg-Al alloys was discussed.(1) During the refining of magnesium alloy in vacuum Ar atmosphere, for longer refining time and higher temperature, the wastage rate became larger. When the addition amount of La and Nd was0.2%and1.2%, respectively, and the amount of Al was more than9%and6%, respectively, the wastage rate of rare earth beame stablished at about20%and30%. The wastage rate of rare earth decreased with increasing temperature. And the proper refining temperature was about983K.(2) The addition of rare earth in Mg-Al alloy shows that when the content of La was0.16%, the Mg-9A1-Zn had the best properties. Maximum tensile strength, yield strength and elongation were245MPa,178MPa and12.18%, respectively. In3.5%NaCl solution, the average corrosion rate reached the minimum of10mg·cm-2·d-1. When the content of Nd was0.9%, the Mg-6A1-Mn had the best properties. Maximum tensile strength, yield strength and elongation were230MPa,127MPa and11.9%, respectively. In3.5%NaCl solution, the average corrosion rate reached the minimum of4.5mg·cm-2·d-1.(3) By solution and aging treatment at410℃/16h/AC+170℃/24h/AC, the precipitation phase of Mg-9Al-Zn-xLa alloy was composed of β-Mg,7Al12and Al11,La3. Because of the diffusion of Al atom, a small number of discontinuous precipitation P-Mg17Al12phases were transformed intp continuous precipitation phase. With the addition of La, the volumes and distribution of lath-sharped DP-β phase were changed as the diffusion coefficient of Al was reduced. When the content of La was0.16%, the tensile strength, yield strength and elongation achieved276MPa,208MPa and13.85%, showing the increase by47.62%,41.84%and65.08%, respectivey, with respect to as-cast alloys. But the corrosion rate was increased, with respect to as-cast alloy, because of the diffusion of Al atom and the change of lath-sharped P phase morphology. (4) By solution and aging treatment at410℃/20h/AC+170℃/24h/AC, the discontinous β-phase in Mg-6Al-Mn-xNd changed to nudolar, and some net-like Al11Nd3changed to globular or near-globular too. Because of the dispersion strengthening of nudolar or globular compound, the turning point of best mechanical properties was transformed from0.9%Nd in as-cast to0.3% Nd in solution and aging condition. The tensile strength, yield strength and elongation of Mg-6Al-Mn-0.3Nd in solution and aging condition were233.9MPa,154.3MPa and12.8%, respectively. |
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