| 其他摘要 | Going with the advance of energy crisis, the demanding of energy conservative and lightweight materials is being favored increasingly. Magnesium and its alloys have gained considerable interests due to their low density high specific strength and good castability. Among various magnesium alloys, rare earth elements containing alloys show much better mechanical properties at room and high temperatures such as excellent strength and creep resistance due to solution hardening and precipitation hardening. It was recently found that the magnesium alloys containing gadolinium and yttrium have much higher strength than conventional commercial magnesium alloys and other Mg-RE alloys. For this reason much effort has been made to investigate Mg-Gd-Y system alloys. It was found this alloy can offer good combination of strength, elongation, and creep resistance, therefore it is promising for Mg-Gd-Y-Zr system to be chief engineering materials used at elevated temperatures. In despite of the microstructure and mechanical properties of Mg-Gd-Y-Zr system are widely reported, there is little research about the high temperature oxidation behavior of Mg-based alloys, saying nothing of Mg-Gd-Y-Zr system. A comprehensive cognition about the oxidation behavior of Mg-Gd-Y-Zr alloy in some representative environments will be the key information for us to decide whether we can use this material in the future. So this work is mainly focused on the oxidation behavior of Mg-Gd-Y-Zr alloy in the three kinds of oxidation environments (O2, O2+water vapor, NaCl deposit and water vapor). Through the comparison with the oxidation behavior of pure magnesium, a deep comprehension of the rare earth effect on the oxidation behavior of magnesium alloy is obtained.
In this work, the samples were tested in a thermo balance for 10 hours to get the oxidation kinetics of the samples at different environments and different temperatures. Combined with the middle temperature theory presented by Fromhold and Cook, the oxidation kinetics of the samples was analyzed. X-ray photoelectron spectroscopy and SEM were used to character the oxidation film formed at different environments and different temperatures. Through the equation presented by Strohmeier, the thickness of the oxidation film was calculated. According to the ratios of high bonding energy O1s intensity to the total O1s intensity, the concentration of point defects of the oxidation film formed at different environments was analyzed qualitatively. The following conclusions were obtained:
The oxidation kinetics curves for the samples oxidized at 230oC、300oC and 400oC in oxygen show Mg-Gd-Y-Zr alloy has a better oxidation resistance than pure magnesium in this condition.
For the oxidation of samples in O2+H2O at 230oC and 300oC, pure magnesium exhibits improved oxidation resistance; however the oxidation degree is accelerated for Mg-Gd-Y-Zr alloy. The observation of SEM shows there are lots of micro-cracks existed in the oxide film of Mg-Gd-Y-Zr alloy, which means severe oxidation. For Mg-Gd-Y-Zr alloy, the inner layer of oxide film is rich in Y2O3 and Gd2O3. In some places rich in rare earth oxides occupied, it is impossible to form Mg(OH)2. This incomplete Mg(OH)2 layer would make the water vapor easily reach the inner MgO layer which has little oxidation resistance to water vapor. When oxidized in O2+H2O at 400oC, both pure magnesium and Mg-Gd-Y-Zr alloy show accelerated oxidation. A higher temperature would prevent the formation of a complete Mg(OH)2 out layer. As a result, water vapor can easily get into the inner MgO layer and contribute to the serious oxidation of both samples.
Opposition to the behavior of other alloys, the co-existence of water vapor and NaCl deposit has no accelerative effect on the oxidation of pure magnesium and Mg-Gd-Y-Zr alloy at middle temperatures. This means the co-existence of water vapor and NaCl deposit dose not change the oxidation mechanism of pure magnesium and Mg-Gd-Y-Zr alloy. However, at relatively lower temperatures, the co-existence of water vapor and NaCl deposit has accelerative effect on the oxidation of pure magnesium and Mg-Gd-Y-Zr alloy. There is a thin water film at the metal surface in this temperature range. Due to the dissolution of NaCl, a serious electrochemical corrosion will happen at the metal surface.
A two-electrode system was developed to carry out impedance measurement on the oxide film of Mg-Gd-Y-Zr alloy. Based on the analysis of impedance spectra, the principle goal is to establish the relationship between the electrical properties of the oxide film and its oxidation resistance capability. However, the thin oxide film formed Mg-Gd-Y-Zr alloy was shown to have a good electrical conductivity, it is hard to carry out impedance measurement on this system at present. Therefore, the two-electrode system was transferred to investigate the atmospheric rust film with low electrical conductivity. The key parameters were found between the electrical properties of the rust film and its resistance to corrosion through careful analysis of impedance data which is in accordance with the traditional evaluation methods. |
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