Study On Cyclic Deformation And Damage Behavior Of Pre-deformed AZ31B Magnesium Alloy

As magnesium alloys are often subjected to cyclic loading in service,fatigue is a main failure mode of structural components with magnesium alloys.Therefore,a good understanding of cyclic deformation and fatigue behavior in magnesium alloys is critical.After cyclic pre-deformation at low strain amplitude(referred to as "cyclic pre-deformation")and pre-compression,dislocations and twins were introduced into the extruded AZ31 B alloy,respectively.This paper investigated the influence of pre-deformation on monotonic and cyclic deformation behavior as well as fatigue life of AZ31 B alloy.The evolution of cracks in low cycle fatigue was quasi in-situ observed using optical microscope(OM),scanning electron microscope(SEM)and electron backscattered diffraction(EBSD).The mechanism of fatigue damage failure in pre-deformed AZ31 B alloy was analyzed.The results of monotonic and cyclic loading tests of cyclic pre-deformed AZ31 B alloy show that the monotonic tensile yield strength improves by 28 MPa,and the monotonic compression yield stress is increased by 8 MPa after cyclic pre-deformation.The fatigue lives of extruded and cyclic pre-deformed samples at the strain amplitude of 1% are identified to be 400 and 380 cycles,respectively.The cyclic pre-deformation has almost no effect on the fatigue performance during cyclic loading under high strain amplitudes.The fatigue lives of extruded and cyclic pre-deformed samples at the strain amplitude of 0.6% are 2206 and 975 cycles.Induced dislocations can lead to severe dislocation pile-ups and stress concentration,and significantly reduce the fatigue life at low strain amplitudes.The results of initial microstructure and cyclic loading tests of pre-compressed AZ31 B alloy show that approximate 60% volume fraction of tension twins were introduced into the alloy after 6% compression strain.The fatigue lives of extruded and pre-compressed alloys at a strain amplitude of 1% are found to be 400 and 349 cycles,respectively.Pre-compression can bring about the inverted hysteresis-loops due to the prior activation of detwinning in AZ31 B alloy,and has no obvious effect on the fatigue performance at high strain amplitudes.The fatigue lives for extruded and pre-compressed alloys at a strain amplitude of 0.3% are 37400 and 23000 cycles,respectively.Pre-compression substantially reduces the fatigue life at low strain amplitudes dominated by slips.The results of fatigue damage behavior at low strain amplitudes of pre-compressed AZ31 B alloy show that the percentage of fatigue life when surface damage first appears are changed from 30% to 13% after pre-compression.Fatigue cracks are mainly initiated on the prismatic or prismatic-basal slip bands in the parent grains,and these microcracks hardly propagate.The volume reduction of the parent grains after pre-compression reduces the crack initiation life,thereby resulting in a shorter fatigue life.In addition,the fatigue damage inside twins is hardly observed.