Deformation Mechanism And Fracture Mode Of AZ31 Magnesium Alloy During Low Cycle Torsional Fatigue

Magnesium(Mg)and its alloys have been widely applied as structural components in mechanical engineering due to their low density,specific strength and specific stiffness.Fatigue performance is an important parameter to be referenced when designing structural components,so it is very meaningful to study the cyclic deformation mechanism and fatigue failure behavior of Mg alloys.In this paper,fatigue specimens processed along the normal direction(ND)of commercial hot-rolled and annealed AZ31 Mg alloy sheet are taken as the object.First,the quasi-in-situ electron backscattered diffraction(EBSD)and slip trace analysis methods was used to study the deformation mechanism of the shear strain(?)of 0.5%,1%,2%,and 4%under unidirectional pure torsion loading.Next,0.42%,0.887% and 1.732% strain amplitudes are selected to study its low cycle torsional fatigue properties.Lastly,the quasi-in-situ EBSD and trace analysis methods are used to compare and analyze the deformation mechanism and fracture mode of the alloy at low strain amplitude of 0.42%and high strain amplitude of 1.732%:(1)The deformation mechanisms of AZ31 magnesium alloy during unidirectional pure torsion deformation is basal slip(??1%)(?)basal slip + a little {10(?)2} tension twinning(?=2%)(?) basal slip + {10(?)2} tension twinning(?=4%).(2)AZ31 magnesium alloy has undergone twinning-detwinning under the control of 0.42% and 1.732% strain amplitude.The deformation mechanism is basal slip and a little {10(?)2} tension twinning during torsional fatigue at the 0.42% strain amplitude,the percentage of grains with slip and twinning trace after fatigue failure is72.7% and 7.3%.The deformation mechanism is basal slip + {10(?)2} tension twinning at the 1.732% strain amplitude,the percentage of grains with slip and twinning trace after fatigue failure is 92.4% and 88.7%,respectively.(3)Most of the microcracks along the persistent slip bands(PSBs)and some cracks along the grain boundaries(GBs)after fatigue failure at the 0.42% strain amplitude,and the larger of the Schmidt factor(SF)of PSBs in the grains,the easier of microcracks initiate along the PSBs.The microcracks mainly along the GBs and twin boundaries(TBs)and a few along the PSBs after cyclic failure at the 1.732% strain amplitude.and the larger of the SF of TBs in the grains,the easier of microcracks initiate along the TBs.(4)For GBs type microcracks generated in low cycle torsional fatigue,the misorientation angle(?)between adjacent grains is larger and the geometric coordination factor(m')between slip and(or)twinning of neighboring grains is smaller.