Low-cycle fatigue behaviors of hot-rolled AZ91 magnesium alloy under asymmetrical stress-controlled cyclic loadings

摘要

The low-cycle fatigue behaviors of hot-rolled AZ91 magnesium alloy are investigated by the uniaxial asymmetric stress-controlled cyclic experiments at room temperature. The fatigue failure mechanisms are analyzed by the fracture morphology characteristics. The effects of stress ratio and peak stress on the low-cycle fatigue life are discussed. Considering the coupled effects of ratcheting damage and fatigue damage on the material failure, a phenomenological fatigue life model is developed to evaluate the low-cycle fatigue life of the studied magnesium alloy. Results show that: (1) The fatigue cracks initiate from the surface of specimens. The cleavage-like facet features appear in the crack propagation region, while the residual ductile dimples and tearing edges appear in the fracture region. (2) The fatigue life increases with the increase of stress ratio and decrease of peak stress, and the stress intensity factor range and twinning deformation mechanism are sensitive to the stress ratio and peak stress. (3) The amount of residual twins on the fracture surface of specimens are greatly affected by the stress ratio and peak stress. Increasing the stress ratio and decreasing the peak stress can obviously decrease the amount of residual twins. The residual twins kink with each other, which deteriorate the grain boundary. (4) Comparisons between the measured and predicted fatigue life confirm that the developed phenomenological fatigue life model can give an accurate estimate of the low-cycle fatigue life of hot-rolled AZ91 magnesium alloy.