Research On The Low-cycle Fatigue Behavior Of Wrought Magnesium Alloy

The fatigue behaviors of wrought magnesium alloys are affected by grain size, the precipitates, the alloying elements and so on. As structural parts in the practical application,magnesium alloys mostly failed in the form of low cycle fatigue. The strain controlled low cycle fatigue experiments can reveal the law of the fatigue behavior and failure mechanism better, and the fatigue life curve of the actual situation is more instructive.In this paper, the conventional extrusion AZ31magnesium alloy, AZ80magnesium alloy and Mg-10Gd-2Y-0.5Zr alloy are processed by ECAP, annealing, solid solution and aging treatment. To investigate the influence of texture, grain size and the precipitates on the low cycle behavior on these materials, the static mechanical experiments and low cycle fatigue experiments are conducted, microstructures are examined with an optical microscope and a Quanta-200scanning electron microscope following the conclusions:1.The ultrafine-grained AZ31magnesium alloy processed by ECAP exhibited the cyclic softening. Pronounced grain coarsening was observed compared with the conventional extruded AZ31alloy, the ECAP processed AZ31alloy has lower hysteresis strain energy and leads to enhanced fatigue lives.Both the yield stress and low cycle fatigue live of the AZ31alloy processed by ECAP decrease after annealing due to the growth of grains and the change of the texture.The change of the grain size and the texture is not favorable for the start of the silp system,thus induce the stress concentration and the shorter fatigue life.2.The presence of precipitates in the aging treatment samples significantly reduces tension-compression yield asymmetry, compared with solution treated material. This decreased asymmetry significantly reduces tensile mean stress during low-cycle fatigue process. The area of crack propagation zone of the aged samples is smaller than both extruded sample and solid solution one.This decrease in the aged sample leads to the significantly decreased low cycle fatigue lifetime. Due to the enlarged reverse plastic zone size, the aged sample showed microscopically rough faceted fracture surfaces in the fatigue crack propagation zone.3. The rare-earth element contained in Mg-10Gd-2Y-0.5Zr Mg alloy can be dissolved in the matrix after solid solution. The randomized texture through the addition of rare-earth elements helps Mg-10Gd-2Y-0.5Zr Mg alloy eliminate the commonly observed tension-compression yield asymmetry in conventional wrought AZ31alloys. The suppression of deformation twins in solid solution Mg-10Gd-2Y-0.5Zr Mg alloy substantially alleviate the severe strain localization during cyclic loading and benefit the material’s fatigue performance. As the reverse plastic zone size of the solid solution Mg-10Gd-2Y-0.5Zr alloy is smaller,it shows microscopically rough faceted fracture surfaces in the fatigue crack,the rough faceted fracture surfaces can reduce the crack growth rate and results in the longer fatigue life.