| As the lightest structural material, research and application of magnesium alloy has drawn more and more attention. Despite the fact that performance of magnesium alloy at room temperature is excellent, its creep strength reduces dramatically when the temperature is above120℃.The poor heat-resistant of magnesium-alloy has been one of the primary cause that limits its applications.Early the temperature of heat-resistant magnesium alloys was up to200℃and its main alloy elements were rare elements. But the expensive cost, recycle difficulties and protection hardness restrict the application of Mg-RE heat-resistant magnesium alloys. Therefore the focus of study on the heat-resistant magnesium alloys is to develop lower cost and better new alloy.First, the effect of element Sr on the microstructure and mechanical properties of Mg-8A1alloy was studied, especially at elevated temperature. Experimental results showed that Sr refined the grain of Mg-8A1alloy distinctly and restrained the formation of β-Mg17Al12and the structure component changed with Sr addition. When more Sr was added up to the alloy the new phases appeared. The new phases include the compound of Al4Sr, which is strip-shaped like and Al2Sr, which is irregular block-shaped like. The melt points of Al4Sr and Al2Sr are1040℃and936℃respectively, and both of them are heat resistant and located mainly at grain boundaries, which is helpful to enhance the mechanical properties at elevator temperature. TEM results showed the morphology of each is different and Al4Sr belongs to (bct) body-centred tetragonal and Al2Sr remains fcc(face-centred cubic).The tensile strength of Mg-8Al-3.5Sr alloy took the maximum of186MPa,158MPa,160MPa at25℃,150℃and200℃respectively.Next, the effect of Y on the microstructure and mechanical properties at both room temperature and high temperature of Mg-8Al-3.5Sr alloy was studied. Results showed that Y refined the grain of Mg-8Al-3.5Sr alloy. During the melting process Y captured the Al atoms firstly and Al2Y arose, which brought out that the amount of Al4Sr became less and that of Al2Sr became more.Research results showed Al2Y distributed at grain boundaries and could restrain the grain boundaries sliding at elevated temperature. The tensile strength of Mg-8Al-3.5Sr-0.60Y alloy reached201MPa,191MPa and192MPa and increasing about by8%,21%and20%respectively comparing with that of Mg-8Al-3.5Sr-0Y.HRTEM results showed that the diameter of granulated A12Y at grain boundary was about50nanometers.At last, the effect of a small amount of La on the microstructure and mechanical properties at both room temperature and high temperature of Mg-8Al-3.5Sr-0.60Y alloy was studied. Results showed that that La refined the microstructure much more. Moreover, small acerate high melting point Al4La arised at grain boundary. The tensile strength of Mg-8Al-3.5Sr-0.60Y-0.05La alloy took the maximum at both room temperature and elevator temperature. At200℃/70MPa the creeping test showed that Mg-8Al-3.5Sr-0.60Y-0.05La still kept secondary creep after100hours. According to the creep curves for100hours the total strain of Mg-8Al-3.5Sr-0.60Y-0.05La was0.26.In conclusion, Al4Sr、Al2Sr、A12Y and Al4La were heat-resistant phases and helpful to improve the mechanical properties at high temperature together. |
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