The Creep Properties And Microstructures Of Mg-Gd(-Y-Zn)-Zr Alloys

Mg alloys are promising in applications and research for their low densities, high specific strength, high ability of resistant to electric-magnetic interference et al. For example, they can be used in the automative industry to replay many components made of steel, resulting in decreasing the vehicle weight and reducing the energy consumption. Now the application of commercial Mg alloys contains the shells of electronic products, car dashboard, seat support, steering wheel et al. In future, Mg alloys will be used in the engine part and power transmission system such as gearbox, engine block and piston, which work at high temperatures. These potential applications require heat-resistant Mg alloys, while normal Mg alloys show poor strength at elevated temperatures. These requirements drive the development of heat-resistant Mg alloys. The commercial Mg alloys used at elevated temperatures are WE series alloys(Mg-Y-RE) with precipitation strengthening, and the applied temperature of WE series alloys is below 200°C. However, the engine part requires the material working at 300°C. Therefore it is urgent to develop a new heat-resistant Mg alloy.Recently, Mg-Gd based alloys have been the focuses of developing heat-resistant Mg alloys, because they were reported to have superior strength(tensile strength and creep resistance) than WE series alloys at elevated temperatures. Moreover, it has been reported that the addition of Y or/and Zn element(forming Mg-Gd-Y, Mg-Gd-Zn, Mg-Gd-Y-Zn) will further improve the creep resistance. However, the effects of the additions of Gd, Y, Zn elements on the creep properties and microstructure evolution during creep of Mg-Gd(-Y-Zn)-Zr alloys are not clear. So this project will investigate the creep properties obtained at 250°C and 300°C, and the microstructures before and after creep tests of four alloys with a fixed total concentration of Gd and Y elements: Mg-2.5Gd-0.1Zr, Mg-2.5Gd-1.0Zn-0.1Zr, Mg-1.5Gd-1.0Y-1.0Zn-0.1Zr and Mg-2.5Gd-1.0Y-1.0Zn-0.1Zr(at.%) using back scattered electron scanning microscopy(BSE)、electron back scattered diffraction(EBSD) 、 transmission electron microscopy(TEM) 、 high-angle annular dark-field scanning transmission electron microscopy(HAADF-STEM) and phase filed simulation, and reveal the effects of Zn, Y, Gd addition in creep properties, which can provide the theoretical guidance for the future development of heat-resistant Mg alloys.Chapter III focuses on the creep properties obtained at 250°C and 300°C, and the microstructures before and after creep tests of the four alloys. It is revealed that the Mg-2.5Gd-1.0Y-1.0Zn-0.1Zr alloy has the best creep resistance: the minimum creep rate obtained at 250°C under 80, 100 and 120 MPa is 1.7×10-9, 4.5×10-9, 1.3×10-8 s-1, respectively. In contrast, the Mg-2.5Gd-1.0Zn-0.1Zr alloy shows the worst creep resistance. Comparing the Mg-2.5Gd-0.1Zr and Mg-2.5Gd-1.0Zn-0.1Zr alloys, it is found that the addition of 1.0 at.% Zn decreases the creep resistance due to the reduce in the density of β′ precipitate. Comparing Mg-2.5Gd-1.0Zn-0.1Zr and Mg-1.5Gd-1.0Y-1.0Zn-0.1Zr alloys, it is found that the substitution of 1.0 at.% Gd with Y improves the creep resistance due to the low diffusion rate of Y in Mg matrix. Comparing Mg-1.5Gd-1.0Y-1.0Zn-0.1Zr and Mg-2.5Gd-1.0Y-1.0Zn-0.1Zr alloys, it is found that increasing the Gd concentration can improve the creep resistance due to the increasement in the density of β′ precipitate.Chapter IV focuses on the crystal structure, morphology and distribution of precipitates in the Mg-2.5Gd-0.1Zr alloy during creep tests, and the effects of different creep temperatures, stresses and time are studied. It is found that the β′ phase has three variants before creep tests, while only one variant is observed after creep tests and these β′ precipitates are lines, which is approximately normal to the applied stress direction. The precipitate lines contain alternated β′ phase and βF′ phase, which has not been reported in Mg-Gd alloys. The reason for this phenomenon is discussed at the end of this chapter.Chapter V focuses on the denuded zones formed in the Mg-2.5Gd-0.1Zr alloy during creep tests. These denuded zones are found next to the grain boundary, which is approximately normal to the applied stress direction. Two types of wide denuded zones are found: type A is next to one array of grain boundary precipitates and having a misorientation with the interior of the grain, and type B is between two arrays of grain boundary precipitates and the grain boundary shuffles between the two arrays of particles. The process of the formation of denuded zones can be described as dislocation flow, which is from the region adjacent to the grain boundary that is parallel to the applied stress direction to the region next to the grain boundary that is normal to the applied stress direction. The difference in the orientation of the two adjoining grains determines the denuded zone type.