Study On Low-Cycle Fatigue Damage Behavior Of Extruded AZ31B Magnesium Alloy

Fatigue failure is among the most destructive failure mode for structural materials.Therefore,the understanding of fatigue damage development and behavior is important for the further application of magnesium alloys.On the other hand,twining is the important deformation mechanism of magnesium alloys,especially the alloy with strong texture.Although the influence of twinning on cyclic behavior has been reported,twinning's influence on fatigue damage needs to be further studied.The current work firstly acquired pre-compressed AZ31 B alloys with two different amont of compression,and investigated the low cycle fatigue properties of extruded and pre-compressed AZ31 B alloys;then characterized the sample surface morphologies in order to analysis the damage mechanisms of low cycle fatigue using optical micrograph(OM),scanning electron microscope(SEM)and electron backscattered diffraction(EBSD),and finally explain the damage mechanism of extruded AZ31 B alloy.The results show that hysteresis loops of extruded and 6% pre-compressed AZ31 B alloys own a strong asymmetry,while the loop of 2% pre-compressed alloy is symmetric.Extruded AZ31 B alloy reaches the largest tensile stress and 6% pre-compressed alloy reaches the largest compressive stress during the cyclic deformation at 1% strain amplitude,suggesting that initial twin influences the cyclic behavior of AZ31 B alloy and makes the average stress level moving towards the compressive direction.At 0.3% strain amplitude,the largest tensile and compressive stress is reached by extruded alloy,stresses of pre-compressed alloys shrink due to twinning.Fatigue life of extruded,2% pre-compressed and 6% pre-compressed alloys at 1% strain amplitude are 440 cycles,380 cycles and 160 cycles,respectively.And fatigue life at 0.3% strain amplitude are 40000 cycles,22000 cycles and 14000 cycles,respectively.The fatigue life is shortened by the increasing amount of initial twin.Slip crack is the dominant mechanism of fatigue damage of three AZ31 B alloys with different pre-compression quantity at both 1% and 0.3% strain amplitudes,and two types of twin-induced crack are discovered.Twin-induced crack and grain boundary crack also participate the damage process of pre-compressed AZ31 B alloys,and they are more and more destructive due to the inceasing amount of initial twins.As no crack caused by pre-compression process is observed,all the damage is considered as the result of low cycle fatigue.