| P-Mg17Al12 phase in the magnesium alloys can strengthen the Mg matrix, but when the Al content is more than 10%, net-workβ-Mg17Al12 phase will severely dissever from the matrix, and on the other hand, (3-Mg17Al12 phase has low melting point and is easy to soften at high temperatures and thus can not suppress the grain boundary sliding, which has greatly decreased the mechanical properties of the high-aluminum-magnesium alloy at room temperature and high temperatures. Mg3Sb2 phase can enhance the nucleation rate of a-Mg during solidification and plays a role of grain refinement. Moreover, it is a high temperature stable phase due to its high melting point which distributes at the grain boundaries and can hinder the grain boundary sliding, and thus improve the mechanical properties of the Mg alloys with high content of Al at room temperature and high temperatures. However, needle-like Mg3Sb2 also dissevers the Mg matrix, so the strengthening effect of Mg3Sb2 phase is very limited.The microstructures and mechanical properties of a Mg1OAl alloy containing different Sb content at room temperature and 150℃were investigated. The results show that with an appropriate addition of Sb, lot of acicular Mg3Sb2 phase was formed and dispersively distributed in the Mg1OAl alloy. Meanwhile, the network eutectic structure was suppressed with resulting in a refined microstructure with refined primary a-Mg grains and dispersively distributedβ-phase. The best mechanical properties including tensile strength, yield strength and elongation both at room and elevated temperatures are attained in the Mg1OAl alloy containing 0.5 wt.% Sb. The tensile strength and the elongation of Mg10A10.5Sb alloy at 150℃are 180 MPa and 19%, which increase by 30% and 90% compared to those of the Mg1OAl alloy, respectively. Furthermore, the Sb-modified Mg1OAl alloy maintains a higher strength at 150℃, equivalent to that at room temperature, while the strength of the unmodified Mg1OAl alloy decreased obviously.Mg10A10.5 Sb alloy was processed by equal channel angular pressing (ECAP) for one pass at 230℃,250℃,270℃and 280℃with the pressing speed of 1.5 mm/min and 2 mm/min, respectively, to determine the optimal pressing parameters. Microstructure and mechanical property of the processed alloy were investigated. The results show that the cracks on the surface of the processed samples decrease and the mechanical properties increase firstly and then decrease with the increasing of the pressing temperature owing to the occurring of recovery and recrystallization at the same pressing speed. At the same pressing temperature, when the pressing speed is 1.5 mm/min, the surface of the sample is smoother and the grain size of the alloy is more homogeneous, and the alloy obtains the higher values in the mechanical properties in terms of the tensile strength of 240 MPa and the elongation of 3%, compared to the pressing speed of 2 mm/min. As a result, the optimal pressing parameters for the Mg10A10.5Sb alloy are that the pressing temperature is 280℃and the pressing speed is 1.5 mm/min.The Mg10A10.5Sb alloy was processed by ECAP processing through route Bc for 1,2,4,6 and 8 passes at 280℃, respectively and the pressing speed was 1.5 mm/min. With the increase of ECAP passes, primary a-Mg grains are refined gradually, and meanwhile P-phase is fragmented significantly. Moreover, part of fragmentedβ-Mg17Al12 phase are dissolved in a-Mg grains and precipitate from a-Mg grains again during ECAP processing and form a large amount of dispersedly fine Mg17Al12 particles. After ECAP processing for 6 passes, the grain size of a-Mg decreases from 100μm in as-cast state to 10μm, large amounts ofβ-Mg17Al12 phase are precipitated in a-Mg grains, meanwhile theβ-Mg17Al12 phase at a-Mg grain boundaries is fragmented to a maximum extent. The grain size of the a-Mg keeps unchanged after 8 passes, but (3-Mg17Al12 phase tends to coarsening. The mechanical properties of the alloys at room temperature,150℃and 200℃firstly increase and then decrease with the increase of pressing passes, and the best comprehensive mechanical properties are obtained after 6 passes. The strength of the alloys at high temperature decreases obviously, but the elongation greatly improves compared with those at room temperature. The fracture analysis shows that the softening of theβ-Mg17Al12 phase is the main reason of the fracture of the alloys.Microstructure and mechanical properties at room and high temperature after 6 passes were compared between Mg10A10.5Sb alloy and Mg1OAl alloy. With addition of 0.5% Sb, the size of Mg3Sb2 phase decreases from 30μm to 8μm, the grain size of theα-Mg matrix decreases by 1/3, the networkβ-Mg17Al12 phase is completely fragmented, and more P-Mg17Al12 phase are precipitated in a-Mg grains. Mechanical properties at room temperature and high temperature of the Mg10A10.5Sb alloy are higher than those of the Mg1OAl alloy, and decrease slowly with the increasing temperature. |
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