| 其他摘要 | Nowadays, magnesium alloys are becoming a research focus in materials field. As the lightest metallurgical structural material, magnesium alloys would play a vital role on saving fuel consumption and reducing the release of polluting gas. In addition, these alloys have a great potential on the application to the aerospace and aviation fields due to their special requirements to the weight of materials. In the meanwhile, the advantage of plentiful resource makes them own promising prospect in the replace of steel and Al alloys and application in more comprehensive fields in the future.
Conventional cast magnesium alloys usually possess poor strength, especially at higher temperatures. Their product types are not also enough to meet the fast increasing demand in more and more fields. Thereby, it is a strategic requirement to exploit and develop new alloys series with high strength, high corrosion resistance and better properties at higher temperatures.
Adding element Y to ZK60 alloy was considered as a powerful mean to improve strength, thermal resistance and corrosion resistance. Thereby, Mg-Zn-Y-Zr alloys, especially produced through thermal-mechanical processing means, have attracted more and more concerns. Their excellent strength at both room and higher temperature make their application have a prosperous future in many fields.
The purposes in this study include the following: ① to study the tensile and high cycle fatigue(HCF) behavior of wrought Mg-Zn-Y-Zr alloys at room temperature produced by forging and hot extrusion. ② to study the influencing mechanisms of element Y on the mechanical properties of wrought Mg-Zn-Y-Zr alloys. ③ to study the effects and influencing mechanism of heat treatment on the mechanical behavior of wrought Mg-Zn-Y-Zr alloys. ④ to study the influence and their influencing mechanism of space environments (thermal cycling, ionizing radiation) in the low earth orbit (LEO) on the microstructure and mechanical properties of wrought Mg-Zn-Y-Zr alloys. In addition, in order to compare their difference, the corresponding study has been conducted on as-extruded ZK60 alloy. According to the results, the conclusions have been achieved shown as the following:
(1) It is obviously different between the forged and hot-extruded Mg-Zn-Y-Zr alloys on the microstructure and mechanical properties. Furthermore, the microstructure of the extruded plates of the alloys is remarkably different from the surface layer to middle layer.
(2) Phase constitute of Mg-Zn-Y-Zr alloys has an important impact on their mechanical properties. The main factor determining phase compositions of the alloys is the ratio of Zn/Y in weight percentage and their total amount in the alloys. Besides, the others like the metallurgy conditions, processing mode and heat treatment et al can all take an effect on the phase formation and distribution of the alloys. The reason is mainly resulted from the different melt point between W-phase (Mg3Zn3Y2) and I-phase (Mg3Zn6Y). In this study, the alloy 2 (Zn/Y: 2.45 in wt.%) containing 2.30wt.%Y owns the optium mechanical properties among four kinds of alloys with different Y content.
(3) Conventional heat treatments such as T4, T5 and T6 have effectively exerted influence on the microstructure and mechanical properties of the alloys. The mechanisms are mainly to modify the grain size and volume fraction and distribution morphology of the second phases in the matrix. In addition, the loading spectrum also takes important influence on the HCF properties of the alloys. When the stress ratio is -1, the alloys have the highest fatigue strength under T6 condition and the lowest fatigue life under T4 condition. As the stress ratio is 0.1, the fatigue strength of the alloys under T5 and T6 conditions are both higher at one order of magnitude than those under the others conditions.
(4) The thermal cycling test simulated the temperature change rule in low-earth orbit (LEO) environment has strong influence on the mechanical properties of the alloys, especially the HCP properties of the alloys. The operated mechanisms possibly comprises of four kinds of aspects: The first is the softening effect initiated at the interface between the matrix and the second phases. The second is strengthening effect resulting from the dislocation accumulation occurred at the locations of grains boundary and the particles and precipitates of the second phases (I-phase and W-phase). The third is thermal compressing residual stress generated in the surface layer of the samples during the thermal cycling process. The last one is the locations of dislocation accumulation acted as the initiated nuclei of recrystallization and inducing microstructure refinement of the alloys again.
(5) Ionizing radiation simulated the high energy charged particles in LEO just exerts slight effect on the mechanical properties of the alloys. In the meanwhile, after irradiation, their irradiated surface of the samples presents a kind of “dying color” effect. During the irradiation process, the increase of temperature at the local region of the radiated samples has induced the grain growth, the melting of part of second phases and their transformation. In general, the mechanical behavior of the radiated alloys presents a kind of rigidification effect. |
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