| 其他摘要 | With the requirement of weight reduction and improvement of mechanical property for the materials used in automobile and aerospace equipments, the application of lightweight structural materials becomes very imperative. Due to lower density, higher specific strength, higher conductivity, innocuousness, no pollution to environment, easy recycling and etc, Mg alloys have been widely used in the fields of automobile, electronic products and aerospace. Compared with other Mg alloys, Mg-Zn-Y-Zr alloys have attracted great interest because they have higher mechanical properties at both room and elevated temperatures. In this dissertation, we have systematically investigated the influence of heat treatments, texture, main alloy phases and Zn/Y ratios on the mechanical properties of Mg-Zn-Y-Zr alloys, and then deeply indicated the relationship between microstructure variation and strengthening mechanisms. It can provide important guidance and reference for the application and development of new Mg alloys with superior mechanical properties.
Through investigating the influences of main phases (I-phase and MgZn precipitates) on the tensile properties and fatigue behavior of different heat-treated forged Mg-Zn-Y-Zr alloy at room temperature, it firmly disclosed that T5 tempering (24 hours artificial aging at 180℃) was the superior heat treatment. The combination of small zonal distributed I-phase particles and fine lamellar β1΄ precipitates made the alloy having the superior mechanical properties. In the high-cycle and super-long fatigue life regime (1E6-1E9 cycles), we have investigated and compared the fatigue behaviors of T5 tempering forged Mg-Zn-Y-Zr and as-extruded ZK60 alloys through using supersonic fatigue experimental method. It revealed that T5 tempering forged Mg-Zn-Y-Zr alloy had a definite fatigue limit and the value was 85 ± 5MPa. As for the as-extruded ZK60 Mg alloy, there existed a plateau between 5×1E6 and 1E8 cyc in the S-N curve, whereas the fatigue strength gradually decreased in the regime between 1E8 and 1E9 cyc. Therefore, the alloy had no definite fatigue limit and its fatigue strength corresponding to 1E9 cyc was 90±5MPa.
Through investigating the mechanical properties of the as-extruded Mg-5.65wt%Zn
-x%Y-0.8wt%Zr alloys with different Y contents, the strengthening mechanism of Mg-Zn-Y-Zr alloys has been disclosed. XRD analysis indicated that when Zn/Y ratio was higher than 4.38, the main phases of the alloy were I-phase and a-Mg matrix. When Zn/Y ratio was between 1.10 and 4.38, the main phases of the alloy were I-phase, W-phase and a-Mg matrix. When Zn/Y ratio was lower than 1.10, the main phase of the alloy were W-phase and a-Mg matrix. Tensile results indicated that when Y content was between 0 and 1.72wt%, the strength of the alloys increased with the quantity of I-phase increasing. When Y content was between 1.17 and 1.72wt%, I-phase volume fraction reached the maximum, then the alloy would have the highest strength. However, when Y content was between 1.72 and 3.69wt%, the strength of the alloy would gradually decrease with the increase of W-phase volume fraction. Therefore, the strengthening mechanism of Mg-Zn-Y-Zr alloys mainly relied on I-phase particle volume fraction.
The influence of extrusion processing on the phase distribution and mechanical properties of Mg-5.5wt%Zn-xY-0.8wt%Zr (Y contents are 0, 1.08wt%, 1.97wt% and 3.08wt%) at room temperature has been investigated. Texture analysis revealed that Y content in the alloys basically had no influence on {0002} basal texture. However, the alloys exhibited obvious mechanical anisotropy. In addition, with Y content increasing, the anisotropy of ultimate tensile strength between extrusion direction (ED) and transverse direction (TD) became more and more remarkable.
Based on the researches of microstructure, tensile properties and fatigue behavior of Mg-Zn-Y-Zr alloys with I-phase, the influence of higher and lower Zn/Y ratios (10 and 5) on microstructure and mechanical properties has been disclosed.
Through using the strengthening mechanism of I-phase on Mg-Zn-Y-Zr alloys, new kinds of lower density, higher strength and better plasticity Mg-Li-Zn-Y alloys have been developed, which effectively solved the strengthening problem of Mg-Li alloy. These kinds of Mg alloys containing Li belonged to two-phase Mg-Li alloys and its chemical composition and elements content were: Li 5.5~11.5wt%, Zn 0.5~15wt%, Y 0.1~8wt% and balanced Mg. The new developed Mg-Li materials had the ultimate strength = 200~300MPa, yield strength = 200~300MPa, elongation ratio = 17~65% and density = 1.34~1.83 g/cm3. |
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