Study On Low Cycle Fatigue Behavior Of AZ80 Magnesium Alloy Gradient Microstructure

Magnesium alloy is the lightest metal structural material.It has the advantages of high specific strength and stiffness,excellent damping performance and easy recovery.It plays an important role in the realization of lightweight equipment,and is widely used in different fields,such as aerospace,national defense and transportation.The defects such as uneven microstructure and poor mechanical properties of magnesium alloy components prepared by casting and traditional plastic deformation often limit their industrial application.However,the gradient texture materials prepared by torsion deformation can effectively refine the grain and weaken the texture,thus improving the mechanical properties of the materials.In this paper,AZ80 magnesium alloy was taken as the research object.The extruded magnesium alloy samples were prepared with grain size gradient microstructure by torsion deformation under continuous cooling at 380 ℃ to 280 ℃ using Gleeble-3500 thermal simulation tester.The low cycle fatigue properties of the samples with gradient microstructure and the microstructure in the process of cyclic deformation are analyzed,and the low cycle fatigue properties of the samples with gradient microstructure after aging treatment are discussed.The results show that the grain refinement effect of the gradient microstructure materials prepared by torsion deformation is obvious.The samples were transformed from the original average grain size of 18.6μm and uniformly distributed equiaxed grains to the average grain size gradient of 14.3μm,10.8μm and 6.9μm in the center,middle and side with the gradual increase of deformation through torsion deformation.In the process of cyclic deformation,twinning plays an important role in the process of original tissue deformation,so that the stress-strain hysteresis curve presents an obvious asymmetric shape.Compared with the samples with gradient microstructure,the asymmetry phenomenon is weaker,and the stress-strain hysteresis curve is basically symmetrical,which is caused by the effect of grain refinement and texture weakening of the samples with gradient microstructure after deformation.It indicates that the twining-detwining phenomenon of the samples with torsion deformation is less obvious than that of other samples with traditional deformation in the process of cyclic deformation.In the process of cyclic deformation,the fatigue life of raw samples without aging treatment under different strain amplitudes is higher than that of gradient samples.After aging treatment,the fatigue life of the original tissue sample is still higher than that of the gradient tissue sample under the control of 0.3%,0.5%,0.7% and 0.9% strain amplitude,but the fatigue life of the gradient tissue sample is higher than that of the original tissue sample under the control of 1.1% large strain amplitude.The fracture study shows that,compared with before aging treatment,more second phase precipitates and more microcracks are formed in the direction of propagation near the fatigue crack source.Fatigue cracks can be found in the fatigue crack growth zone.The average fatigue cracks spacing of the raw texture without aging treatment is narrower than that of the gradient texture at low strain amplitude,and the average fatigue cracks spacing of the raw texture without aging treatment is larger than that of the gradient texture at high strain amplitude.After aging treatment,the average fatigue scratch spacing of the original microstructure at low strain amplitude is still narrower than that of the gradient microstructure.The average fatigue scratch spacing of the original microstructure at high strain amplitude is1.5μm,but no fatigue scratch spacing is observed in the gradient microstructure.On the whole,after aging treatment,the average fatigue striation spacing of different structures under different strain amplitude control becomes smaller.