Investigation Of High Temperature Oxidation And Improved Surface Protection Of Magnesium Alloys

Owing to their low density and be friendly to the environments, magnesium alloys have been increasingly used in the automobile industry, aerospace components, communications and computer parts. Unfortunately, their poor oxidation resistance at high temperatures prevents their further applications in many fields. Therefore, it is very important to improve the oxidation resistance of magnesium alloys at high temperatures especially the first high temperature oxidation resistance, and investigate the corresponding mechanism of the oxidation. It would also be paid more attention in the future.In this study, alloying elements and ion implantation were used to add several kinds of reactive elements in the pure magnesium and AZ31 magnesium alloys. Thermogravimetric analyzer (TGA), X-ray diffraction (XRD), Field emission scanning electron microscope (FESEM), Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) were applied to study the oxidation behaviors of these systems, and the corresponding thermal and kinetic models were also formed. In addition, the replacement of Y, Ta ion implantation in Mg and the initial oxidation of Mg3Nd surface were investigated using the density functional theory (DFT). The main results include:1 Oxidation behaviors of magnesium alloys with rare earths at high temperatures The oxidation temperature greatly influenced the oxidation kinetics of MgY, MgNd and Mg-10Gd-3Y alloys, which showed that the higher the oxidation temperature, the greater the oxidation rate was. The flux of O2 had little effect on the oxidation kinetics. After isothermal oxidation, an oxide film with a triplex structure had been formed on the surface of the magnesium alloys. However, the oxidation mechanism of the three kinds of magnesium alloys showed some differences:(1) MgY alloys showed two stages of the oxidation: a fast initial oxidation and a followed oxide growth. At the initial stage, Y2O3 preferentially formed. At the followed oxide growth, oxygen diffused through the Y2O3 layer, reacted with Mg, and finally formed the dense Y2O3/MgO film. So, the oxidation resistance of the alloys was improved. When the oxidation temperature was less than 600℃, there had a critical value (1.09 wt.%Yc, the oxidation kinetics accorded with the parabolic law. On the contrary, for Y nN'd,ΔG <0. At the early stage, Nd2O3 formed prior to MgO, but at the followed stage, Mg became rich and formed MgO, and finally formed the Nd2O3/MgO film. On the contrary, when n Nd < nN'd,ΔG >0. At the initial stage, discontinuous MgO formed prior to Nd2O3, but at the followed stage, Nd became rich and formed Nd2O3, and finally formed the MgO/Nd2O3 film. The corresponding oxidation resistance was improved.(3) For Mg-10Gd-3Y alloys, Y2O3 preferentially formed at the initial stage, while Gd2O3 preferentially formed at the followed stage. Therefore, a dense Y2O3/ Gd2O3 formed on the surface after the oxidation process. The oxidation resistance of the alloys was improved.2 Improvement of ion implantation on the high temperature oxidation resistance of magnesium alloysAfter Y, Ce and Ta ion implantation with different doses, a pre-oxidation film with two sub-layers formed. Moreover, the thickness of the pre-oxidation film increased with the increase of the implantation dose. The formation of the pre-oxidation film would reduce the rate of the oxygen inward diffusion, and thus the oxidation resistance of magnesium alloys was increased. Under the same conditions, the efficiency of the improved oxidation resistance was: Y≈Ta>Ce. There also had some differences for the three ion implantations:(1) After Y ion implantation, the outer layer of the pre-oxidation film was composed of MgO, while the inner layer was made up of MgO and Y2O3. The oxidation kinetics agreed with the parabolic-linear law. The specimen implanted with 5×1017 ions/cm2 showed better oxidation resistance.(2) After Ce ion implantation, the outer layer of the pre-oxidation film was also composed of MgO, while the inner layer was made up of Ce2O3, CeO2 and MgO. The oxidation kinetics agreed with the linear law. The specimen implanted with 1×1017 ions/cm2 had better oxidation resistance.(3) After Ta ion implantation, Ta2Al and little Ta2O5 were formed in the inner layer of the pre-oxidation film. The oxidation kinetics agreed with the parabolic-linear law. The specimen implanted with 1×1017 ions/cm2 exhibited better oxidation resistance. 3 Studying the Y, Ta ion implantation replacement and the initial oxidation of Mg3Nd by the first principlesThe Y, Ta ion implantation replacement in Mg and the oxygen adsorption on the surface of Mg3Nd(001) were carried out within the framework of density functional theory (DFT) by using the Vienna ab initio simulation package (VASP). The main results included:(1) Large crystal lattice aberrance was induced after the replacement of both Y and Ta, and the system was at the nonequilibrium state with high energy. Moreover, the more the number of the replaced atoms, the larger the crystal lattice aberrance was. This showed that larger implantation dose would make the altered film in higher energy states. After structural optimization, the nonequilibrium state changed towards the equilibrium state. According to the electronic structures, the orbits of Y 4p, 4d and Ta 5p, 5d interacted with the orbits of Mg 2p, respectively, and formed the slight metal bonds.(2) Oxygen atom adsorption on Mg3Nd(001) surface was studied. First, the adsorption site between Mg and Nd on the surface layer was found to be preferred, and the chemisorption was characterized mainly by ionic bonds. Second, the stability of Mg3Nd(001) surface as a function of oxygen chemical potential indicated that the clean surface (σ= 2.96 mJ/m2) could be energetically stable only when the oxygen chemical potential was very low. For higher oxygen chemical potential the clean surface became unstable, the oxygen atoms began to adsorb on it, and soon came up to high coverage.