| 其他摘要 | Magnesium and magnesium alloys possess many advantages, such as low density, high specific strength and rigidity, good electromagnetism shield and good damping capacity, etc., which are one of the lightest metallic structural materials and have widely application prospect in the industry. The wrought magnesium alloys have higher strength, better ductility, a wider variety of comprehensively mechanical properties and exhibit more advantages than the as-cast ones in the application of magnesium alloys. However, there are some problems with the present wrought magnesium alloys, such as few alloy systems, poor formability, low room temperature strength and low high temperature performances, which limit their extensive applications.
In order to increase strength and improve heat resistance of the magnesium alloys, one of the important directions is to develop new type wrought magnesium alloys by adding rare earth (RE) elements. The present dissertation systematically investigates the microstructures, mechanical properties and high temperature deformation behaviors of some magnesium-RE alloys and representative Mg-based laminated composites. The age-strengthening and mechanical properties are investigated for Mg-5%Y-4%Nd-0.5%Zr (wt.%, WE54), Mg-10%Gd-3%Y-0.5%Zr (GW103) and Mg-10%Gd-3%Y-1%Zn-0.5%Zr (GWZ1031) alloys under different process conditions, respectively. The three alloys possess obvious age-hardening characteristics, and the results show that the three alloys have higher mechanical strength and lower elongation when the ageing temperature is lower. After peak ageing at 200℃, the as-extruded WE54, GW103 and GWZ1031 alloys exhibit the highest mechanical properties, and their ultimate tensile strength, yield strength and elongation are 351MPa, 247MPa, 7% and 397MPa, 311MPa, 5% as well as 428MPa, 339MPa, 4%, respectively; after the secondary ageing treatment, the highest strengths of as-extruded GW103 and GWZ1031 alloys decrease slightly, but their elongations increase in about 2%. The investigations on high temperature deformation properties of WE54, GW103 and GWZ1031 alloys reveal that the good heat resistant is obtained for the three alloys from room temperature to 200℃; at the temperature above 200℃, there is a rapid degradation in mechanical properties of the alloys, and the cast-T6 specimens have better heat resistance than the as-extruded ageing (T5) specimens. By comparison, it is found that the GW103 and GWZ1031 alloys have higher mechanical properties than the WE54 alloys at both room temperature and elevated temperature, which are attributed to the more alloy elements in the former two alloys, where more heat resistance precipitates are formed.
The deformation behaviors, microstructural evolutions and high temperature deformation mechanisms of WE54 and GW103 alloys are investigated during the high temperature tensile deformations. The mechanical properties exhibit that both as-extruded WE54 and GW103 alloys exhibit high elongations at elevated temperatures, whose maximum elongations are 290% and 390%, respectively; the high strain rate sensitivity exponents (m) are also obtained for the two alloys, where the dynamical recrystallization (DRX) has taken place at elevated temperatures and the grains have been refined. Subsequently, the investigation results show that the microstructures of as-extruded WE54 and GW103 alloys are significantly refined after the equal channel angular extrusion (ECAE), whose grain sizes are below 3μm, and their elongations also further increase at room and elevated temperatures. The ECAE WE54 and GW103 alloys exhibit high elongations, even superplasticity, whose maximum elongations are 600% and 490%, respectively, and their strain rate sensitivity exponents are close to 0.5. The microstructural evolutions reveal that the grain boundary sliding (GBS) operates during the high temperature deformations for the two alloys, whose grains occur coarsening after tensile tests with the increasing temperatures and/or decreasing strain rates. By analyzing the activation energy in the temperature range of 350—450℃, it is suggested that both the deformations of as-extruded WE54 and GW103 alloys are controlled by the lattice diffusion mechanism; further carrying out ECAE for the two as-extruded alloys, the deformation mechanisms are transformed to the combination effect of grain boundary and lattice diffusions.
Except the present explosive welding and roll cladding techniques, a new ECAE conception of fabricating laminated composites is brought forward by means of the extruding and shearing effect during the ECAE procedure, and the Mg-5Y-4Nd/Mg-6Zn-1Y (WE54/ZW61) and AZ31/pure Al laminated composites are successfully fabricated by ECAE technique. The microstructural evolutions and mechanical properties are studied for WE54/ZW61 composites during the high temperature deformation, and the effect of annealing treatment on the microstructure and performance of AZ31/Al composite is also investigated. The WE54/ZW61 laminated composite exhibits the 620% maximum superplastic elongation, and the GBS takes part in the deformation for the component materials during the high temperature tests, where many cavities with different amounts and configurations are formed after high temperature deformations in the composites. It is found that there are more cavities in the ZW61 component, whose coalescence and interlinkage are also more deteriorative, and the necking or rupture is also preferentially originated from the ZW61 component. The grain coarsening with different extents is also noticed for the components with increasing temperature. The effects of ECAE and annealing temperatures on the microstructures and joining boundary properties of AZ31/Al composite are also investigated and compared. The results exhibit that the thicker diffusion layer is formed near the joining interface of AZ31/Al composite with increasing ECAE temperature; there are reaction phases forming when the ECAE and annealing temperatures are high, whose amounts increase with increasing temperature, which are mainly Mg2Al3 and Mg3Al2 intermetallic compound phases. The mechanical properties of laminated composites will be deteriorated if the excessive intermetallic compounds are formed. |
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