| Magnesium is one of the most metallic elements that store in the lithosphere. Magnesium alloy's density is less than 2g/cm3 generally, so it is one of the lightest metallic structural materials at present time. Magnesium alloy's specific strength is higher than aluminum alloy's and steel's, lower than plastic strengthened by fiber and its specific rigidity is equal to aluminum alloy's and steel's, higher than plastic strengthened by fiber. Magnesium alloys have better mechanical machining property and low machining cost because of their low machining energy which is equal to 70 percent of aluminum alloys. Their grind-resistance properties are better than low carbon steel and exceed the die casting A380 aluminum alloy and damping property and magnetism-shielding property better than aluminum alloys and high heat-conducting, electricity-conducting and non-toxicity. So magnesium alloys which have better comprehensive properties can apply to the areas of automobile, computer and communication. AZ91 alloy which is widely used at the present time has many advantages, such as high ambient temperature strength, good casting property and low cost. But it is unsuitable for use at temperatures above 120 degree due to its poor mechanical properties at elevated temperature. Although Mg-Al-Si and Mg-Al-RE alloys were developed for application at elevated temperature, their limited castability or higher cost has restricted their casting applications. Therefore, it is important to develop some new magnesium alloys with low cost and high mechanical properties at room and elevated temperature. The alloy of Mg-Zn-Al system is promising to meet the above requirements. Mg-Zn-Al alloys have some merits: low manufacture costs, better room strength (same to AZ91D alloy) and creep resistance (higher than AZ91D alloy) and good corrosion resistance. But the solidification range of this system alloy is broad which leads to casting defects. In this paper, we have studied the influents of adding various amount of A15TJB or RE and conducting different heat treatment processes on microstructures, mechanical properties, solidification process and precipitated dynamics of Mg-8Zn-4Al-O.SMn based alloy.In this thesis, the influences of different Al5TiB or RE additions and different heat treatment technologies on microstructures and mechanical properties of Mg-8Zn-4Al-0.3Mn based alloy are systemically studied by using measurements of Optical Microstructure(OM), X-ray Diffraction(XRD), Scan Electric Microscopy(SEM), Energy Dispersive Spectrometer (EDS), Differential Scanning Calorimetric(DSC), Transmission Electron Microscope(TEM), Microhardness and Tensile test.The results of inoculation show that: (1) the microstructures of Mg-8Zn-4Al-0.3Mn based alloy and Mg-8Zn-4Al-0.3Mn based alloys with AlSTiB or RE addition are consisted of matrix - a(Mg) supersaturating solid solution, white T(Mg32(Al,Zn)49) and (p(Al2Mg5Zn2) ternary-phases and black-gray eutectic a(Mg) phases. The differences of these alloys are the amount of phases on the matrix. And the amount of eutectic a(Mg) phases increases with the increasing of AlSTiB addition and block-shape and rod-shape Mg3Al4Zn2RE phases increaseAbstractwith the increasing of RE addition. Lower amount eutectic a(Mg) phase and no Mg3Al4Zn RE phase exist in the alloy with the mixing addition of Al5TiB and RE. (2) ti microstructure of Mg-8Zn-4Al-0.3Mn based alloy can be refined obviously by the additic of 0.5wt%A15TiB. Its grain size reduces from 120~130μm which is the grain size of witho modification to 30~40μm. 1.5wt%RE can also obviously refine the grain size of Mg-8Zi 4Al-0.3Mn based alloy and the grain size reduces from 120~130um to 40~50um. The ro of mixing addition of 0.5wt%A15TiB and 0.5wt%RE is same to the above mentioned and the grain size reduces from 120~130um to 50~60μm. (3)the morphology of T(Mg32(Al,Zn) and (p(Al2MgsZn2) ternary-phases on the grain boundaries changes from quasi-continuou net-shape of alloy without AlSTiB addition to discontinuous-shape and the distribution... |
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