| As is known, magnesium as the lightest metal, possessing some important advantages such as low density, high specific strength, machined easily, high damping and renewable utility, are increasingly used in aerospace, automotive, electronics and other military and civilian fields. Moreover, since Mg-based alloys have good casting properties, high temperature strength and good resistance to corrosion, the production of the Mg alloys materials become the focus in recent researches in the effort to lower the industrial cost.Rare earth element is one of the components in the Mg-based alloys. According to the observed results up to now, rare earth element can effectively decrease the grain size, reduce the content of the formed phase with lower melting point. Also the second phase produced by the reaction between Mg and rare earth element, will have higher melting points and stability, which can result in the good performance of Mg-based alloys, respectively at room or even the higher temperatures. In this thesis, we take the cost as the purpose, and research the effect of rare earth element Y with small content on the microstructures and mechanical properties of the Mg-Al-Mn alloys which are produced through casting and rolling, separately. Also, the Y element has an important role in the microstructures and mechanical properties for Mg-Mn-Ce alloys. All this can provide theoretical guide for the exploitation and application of new Mg-based alloys in industry. Under the condition that the study on the semi-solid Mg-based alloys is still not mature, the evolution in the formation of the semi-solid alloy Mg-8Gd-2Y-1Nd-0.3Zn-0.6Zr is studied in this paper, offering the references for the similar formation process. Also the idea by combining the mechanical properties with the resistance to oxygen is also brought out in this thesis. From some points such as the oxidation in thermodynamics and kinetics, and the mechanism in restraining combustion, the resistance to oxygen for the elements involved in the alloys. The obtained results provide the theoretical basis for the next work. The main conclusions and the results obtained are listed as following:1. The effect of the rare earth element Y on the mechanical properties of the Mg-Al-Mn alloy which are produced by casting and hot rolling methods, respectively, is studied, and also their microstructures and the mechanical properties under different state. The results show that Al2Y phase with higher melting point and thermal stability can be formed when Y is added. As the content of Y element is increased, the grain size of the alloys drops correspondingly, with the mechanical property enhanced. The alloy reaches to the optimal value in mechanical property at room temperature when the content of Y is 0.9% (wt. %). Mg-6Al-0.3Mn-0.9Y alloy behaves good mechanical property at room temperature after rolled at 400℃with 70% thickness reduction. The corresponding tensile and yield strengths are 303MPa,255MPa and the elongation is 17.1%, respectively. When compared with the as-cast Mg-6Al-0.3Mn alloy, the increases in the extent are respectively 70%, 335% and 50%. In the rolling process, Al2Y can effectively strengthen the dispersion through the movements of dislocation. As a result, the plasticity in alloys can be attributed to grain refinement, hardening, and dispersion strengthening. The optimal parameters for annealed alloys should be temperature of 350°C and time of 30 min. For the annealed Mg-6Al-0.3Mn-0.9Y alloys, tensile and yield strengths, the elongation are 261MPa,149MPa and 32%, respectively, where the increases are 45.8%, 166.1% and 154% compared to the as-cast Mg-6Al-0.3Mn alloy. Also, it should be noted that the elongation increases 87.1% when compared with the rolled alloy.2. Mg-1Mn-0.6Ce-xY (x = 0.5%, 1%, 2%, 4 %) magnesium alloys are prepared by casting and extrusion, and the role of the content of Y element on the microstructure and mechanical properties of the Mg-1Mn-0.6Ce magnesium alloys is discussed. When adding Y elements to Mg- Mn-0.6Ce alloys, the alloy is mainly composed of theα-Mg phase,Mg12Ce phase and Mg24Y5 phase, and moreover the size of Mg12Ce phase particles increases as the content of Y element increases, accompanying the coarsening in Mg24Y5 phase. A large number of small recrystallined grains are distributed in paralle as shown in the microstructure image for the extruded alloys. For the as-cast alloy, the adding of Y element can enhance the mechanical properties, resulting in the best property in Mg-1Mn-0.6Ce-1Y alloys at room or higher temperature, with the values in tensile and yield strengths, the elongation being 152 MPa, 72 MPa, and 13.4% at room temperature. The increases are 23.6%, 63.6% and 38.1% when compared with Mg-1Mn-0.6Ce alloy. Moreover, the elongation in Mg-1Mn-0.6Ce-1Y alloy increases 118.5% without lowing strength at higher temperature. In the case of extruded alloys, the mechanical properties are also enhanced, especially for the yield strength and elongation property when compared with the as-cast ones. It is found that the tensile and yield strengths, the elongation are 254 MPa, 224 MPa and 26% for Mg-1Mn-0.6Ce-1Y alloy, and the corresponding increases are 106.5%, 409.1% and 168% respectively when compared with as-cast Mg-1Mn-0.6Ce alloy. The elongation is highest for the extruded Mg-1Mn-0.6Ce-3Y alloy at room temperature, with its value being 37.5%. The best mechanical properties are obtained for Mg-1Mn-0.6Ce-3Y alloy at higher temperature, where the tensile and yield strengths, the elongation are 178 MPa, 127 MPa, and 69.5%, showing the increase in 49.6%, 39.6% and 88.3% when compared with the extruded Mg-1Mn-0.6Ce alloy by the same treatment. Therefore, the enhancement in the mechanical properties of the alloys obtained can be contributed to the grain refinement and high stability of the Mg12Ce phase, and the hardening induced by the movement of the dislocation and grain boundaries of Mg24Y5 phase.3. The semi-solid slurry of Mg-8Gd-2Y-1Nd-0.3Zn-0.6Zr (wt. %) alloy is prepared using strain-induced activation method (SIMA). The microstructure evolution of the isothermal treatment was also discussed. It is found that the alloy is mainly composed withα-Mg matrix, embedded with Mg5RE, Mg24RE5 and Mg41RE(5RE=Gd,Y,Nd)phases. After extruded, the recrystallization of the alloys occurs and the average grain size in about 17μm. The microstructure is composed of Mg3RE phase andα-Mg phases after isothermal treatment. Taking the extruded Mg-8Gd-2Y-1Nd-0.3Zn-0.6Zr alloy for isothermal treatment, we can found that the fraction in liquid phase increases, where the solid particles becomes more and more spherical and smaller. So the evolution in the formation of the semi-solid alloys is the Ostwald mechanism, that is, the fraction of liquid phase increases with the number of solid particles increasing when the time for heat preservation is elongated. By analyzing the experimental data presented in this study, It is found that the optimal process parameters should be isothermal temperature of 630℃and time of 30 min, where the content of the solid phase is 52%.4. From the points of the thermodynamics, kinetics and the integrity of the formed oxide films, the mechanism of the resistance to oxidation by adding rare earth elements into Mg-based alloys is analyzed. The results show that rare earth elements have larger affinity to oxygen than that of magnesium element, which results in the higher stability of the oxides of the rare elements when compared to that of magnesium under standard state. Compared the Mg and rare earth elements, the ability affinitive with oxygen from larger to smaller is: Mg, La, Nd, Yb, Ce, Pr, Gd, Sm, Dy, Tb, Tm, Lu, Ho, Er, Sc, and Y. In the molten of Mg-based alloys at 750℃, any rare earth element can react with MgO in the way of replacement and combines with O element in MgO. Except Yb and Eu elements, the density of the formed oxidation layer through reaction between rare earth element and oxygen element is a litter higher than 1, which will have good protection for Mg-based alloys.
|
PDF Full Text Request