| Research works of this thesis aim at high performance wrought magnesium alloys and Mainly focus on the effects of ECAP on Magnesium-Aluminum-RE Alloys with High Level Aluminum. Some Mg-Al magnesium alloys, such as Mg-15Al-xNd alloys are investigated systematically for the relations of ECAP and their chemical composition, microstructures, ambient and elevated mechanical properties.All of the Mg-15Al-xNd alloys were pressed using route BC for 4 passes, at 250°C. And the effects of the deformation behavior of ECAP 4 passes on the microstructures,hardness and ambient and elevated mechanical properties before and after ECAP are discussed. At the same time , influences of rare earth on the microstructures and mechanical properties of Mg-15Al alloy , especially for the elevated properties are studied.The mechanical properties and microstructures of Magnesium-high level Aluminum Alloys added rare earth are investigated for the first time and the results show that the strengtheningβphase plays fully the role and the tensile strength of Mg-Al alloy is improved greatly, on the basis of ensuring adequate toughness and plasticity. The mechanism of grain refine and properties improvement of moderate rare earth on Mg-15Al alloy is investigated. The grain size of Mg-15Al alloy changes from about 62.4μm to 27.9μm and the morphology ofα+βeutectic structure in the as-cast Mg-15Al alloy changes from massive network to small and discontinuous one. The mechanism of grain refine of rare earth on Mg-15Al alloy is the increasing of undercooling degree between the solid/liquid interface during the solidification and resulting in more nucleus. And the hardness,tensile strength and elongation of the Mg-15Al alloy are improved remarkably. The ultimate tensile strength changes from 141.3 MPa to 189.6MPa, and the elongation is increasing from 0.23% up to 1.1%.The mechanical properties and microstructures of Mg-15Al-xNd Alloys after ECAP 4 passes are discussed and the results show that the size of Mg-15Al-xNd is refined remarkably andβ,Al-Nd phases were obviously broken into small particles after ECAP. The grain size of theα-Mg is decreased to 2~3μm,and in some areas, it's less than 1μm. The mechanism of grain refine after ECAP 4 passes is continuous dynamic recovery and recrystallization. Since the average grain size is refined greatly after ECAP deformation, the effect of fine-grain strengthening is significant, So the mechanical properties are improved significantly. The yield strength, tensile strength and elongation are drastically increased to 224.6MPa, 310.9MPa and 7.5%.The mechanical properties at elevated temperature of the Mg-15Al-xNd magnesium alloy after ECAP are discussed. The results show that the mechanical properties present different forms at different temperatures and strain rates. At elevated temperatures, grain boundary sliding is a key factor influencing material deformation behavior,microstructures and properties. As the Al-Nd phase having a pinning effect on the grain boundary, grain boundary and dislocation sliding are effectively prevented and grain boundary sliding shear resistance is increased, thereby, the high temperature strength of Mg-15Al alloy increases greatly. All in all, the Mg-Al-Nd alloy has a higher heat resistance than Mg-Al alloy. So it has enriched the theory of rare earth improving heat resistance of magnesium alloy.This study shows that it's feasible to obtain high-strength magnesium alloy by appropriate ECAP processing. It not only has good mechanical properties, and also has good heat resistance at high temperature. Therefore, it makes a valid exploration for the development of magnesium alloys in China. |
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