| As the lightest metal engineering structural material,magnesium alloy plays an important role in solving the lightweight problems in China's automobile industry,aerospace and other fields.Fatigue is one of the main factors of component failure.It is of great theoretical and application value to study the fatigue behavior of magnesium alloys.In this paper,the effects of different deformation processes on the microstructure and properties of Mg-Gd-Y-Zn-Mn alloys are first compared.Then,the Mg-Gd-Y-Zn-Mn alloy with the best performance technology was selected as the fatigue material to study the high and low cycle fatigue properties and fatigue fracture behavior of different fatigue conditions and different types of samples.The microstructures of Mg-Gd-Y-Zn-Mn alloys obtained by different deformation processes were composed of?-Mg,eutectic phase and LPSO phase,showing bimodal microstructure characteristics.The degree of recrystallization after the double extrusion was greater,and the recrystallized grain size obtained was smaller,which showed more excellent comprehensive mechanical properties.The tensile strength reached 392 MPa,the yield strength is 310 MPa,and the elongation is 14.2%.During the low-cycle fatigue experiment,the finely recrystallized grains,a high amount of RE(Gd,Y)elements and the special second phase-layered LPSO in double extrusion Mg-Gd-Y-Zn-Mn alloy hindered the twinning activity of during cyclic loading,which leaded to the symmetrical hysteresis loops under different strain amplitudes.Although slight cyclic hardening occurred at low strain amplitude,cyclic softening was the main feature at most strain amplitudes.The alloy had excellent low-cycle fatigue properties,and fatigue life of the alloy could be well described by Coffin-Manson law and Basquin's equation,which could be used for the anti-fatigue design of alloy components.Fatigue cracks initiated from the surface and near-surface of the samples.cracks propagated through a mixed intergranular and transgranular mode.Crack propagation area was characterized by layered structure with quasi-cleavage features,and secondary cracks that originated within the LPSO.The present alloy had extremely excellent high-cycle fatigue properties.The tension-tension fatigue properties of the smooth specimens(K_t=1)was 211.7 MPa(10~7),and the rotating bending fatigue properties of the smooth specimens was 188.3 MPa(10~7).Which were higher than other deformed magnesium alloys,and was equivalent to some deformed high-strength aluminum alloys.However,the alloy had a higher notch sensitivity.The tensile high-cycle fatigue performance of notched specimens(K_t=3)was reduced by about 60.2%,and the rotational bending high-cycle fatigue strength was reduced by about 43.6%.During the tensile-tension axial loading and rotating bending loading of smooth alloy specimens,fatigue cracks tended to initiate at surface processing defects and inclusions near the surface,and showed single-source cracking characteristics,and the crack propagation zone presented the characteristics of quasi-cleavage.During the tensile-pulling axial and rotational bending loading process of notched specimens,it was easier to generate multiple fatigue crack sources around the notch,and then the cracks expanded from the periphery to the center,which also exhibited the characteristics of quasi-cleavage.The crack growth area of notched specimen was rougher than that of smooth specimen. |
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