The evolution of microstructure and its effect on high-cycle fatigue behavior in Mg-9Gd-4Y-2Zn-0.7Zr alloy processed by repetitive upsetting-extrusion

The microstructure evolution and its influence on high-cycle fatigue behavior of Mg-9Gd-4Y-2Zn-0.7Zr alloy processed by repetitive upsetting-extrusion (RUE) are investigated. The microstructure is observed by using optical microscopy (OM), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). The result shows, the fatigue limits of the as-cast and as-homogenized alloys are 75 MPa and 101 MPa, and the fatigue limits of the RUE-extrusion direction (ED) and RUE-radial direction (RD) samples are 140 MPa and 123 MPa, respectively. The fatigue properties of the alloy during homogenization and RUE are greatly improved, and RUE samples exhibit obvious fatigue properties anisotropy. The oxide inclusions have the most significant impact on fatigue crack initiation of as-cast and homogenized alloys, while the fine-grained microstructure, fragmented and dispersed long period stacking ordered (LPSO) phases significantly impede crack propagation of RUE samples. The higher fatigue strength of ED plane is related to the finer grain size, more bimodal microstructure, and more broken bulk LPSO phases. The anisotropy in fatigue strength is mainly due to the combined effects of grain refinement difference, LPSO phase distribution difference, and bimodal microstructure difference in different directions. The weaker basal texture and fatigue anisotropy behavior can be realized during the RUE process, which is related to the strong shear force.