Microstructural characterization of crept Mg-Gd(-Zn-Y)-Zr alloys

The development of Mg alloys with attractive creep strength has been one of the Mg alloy research focuses, and Mg-Gd binary alloys and Mg-Gd(-Y)-Zn based alloys were reported to exhibit desirable creep resistance. This thesis presents a systematic investigation of effects of the Gd, Y and Zn elements on the creep properties of Mg-Gd alloys and the evolution of precipitates during creep tests. Four alloy compositions were designed, namely 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.%). The creep tests were performed at 250 °C under 80, 100, 120 MPa and 300 °C under 40, 60, 80 MPa. The microstructures of the samples before and after the creep tests were also investigated using scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and computer simulation. The results of the creep tests show that the Mg-2.5Gd-1.0Y-1.0Zn-0.1Zr alloy exhibits the best creep resistance, for instance, the minimum creep rates obtained at 250 °C under 80, 100 and 120 MPa are 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.It is found that the addition of 1.0 at.% Zn decreases the creep resistance due to a reduction in the number density of the β' precipitates, the substitution of 1.0 at.% Gd with Y improves the creep resistance due to the low diffusion rate of Y in Mg matrix, increasing the Gd concentration can improve the creep resistance due to the increase in the number density of the β' precipitates, the 1.0 at.% addition of Y leads to much better creep performance because of a higher density of the β' precipitates. The distribution of the β' precipitates in the Mg-2.5Gd-0.1Zr alloy during creep tests at 250 ºC was studied. Before the creep tests, the separated lenticular shaped β' precipitates are uniformly distributed. After the creep tests at 250 °C, it is found that the distribution of the β' precipitates is linear precipitate chains extending along the direction that is approximately normal to the direction of the applied stress. The linear precipitate chains consist of alternate β' and β_F' particles. There are four types of transitional zones between the adjoining β_F' and β' particles: continuous zigzag zones, consisting of a Gd atomic zigzag array with a fixed interplanar distance of ~1.11 nm to the nearest β_F' and β' particles; discontinuous hexagonal zones, consisting of a hexagonal array with a wider gap on one side; continuous hexagonal zones, consisting of a hexagonal array with a fixed interplanar distance on both sides; discontinuous zigzag zones, consisting of zigzag arrays with a wider gap on one side, and the width of the wide gap is about 1.66 nm. The denuded zones were observed in the Mg-2.5Gd-0.1Zr alloy after creep tests at 250 and 300 °C. The denuded zones are found in matrix regions adjacent to grain boundaries lying approximately perpendicular to the applied stress direction, and mainly widen in the tertiary creep stage. The majority of the denuded zones are characterized by the distribution of an array of grain boundary particles on one side and a misorientation between the denuded zone and the grain interior. The misorientation is caused by the formation of a dislocation wall between the denuded zone and the grain interior, and it leads to a loss of coherent matching between the otherwise coherent precipitates and the surrounding matrix in this region, which in turn results in the dissolution of the precipitates in this region and hence the formation of a denuded zone.