Research Of High Cycle Fatigue Microstructure Evolution And Fracture Morphology Of AZ31 Magnesium Alloy

In this paper,the commercial hot-rolled AZ31 magnesium alloy was chosen to study the high cycle fatigue microstructure evolution and the relationship between microstructure and fracture morphology by using SEM and TEM.The cyclic stress we chosen was 50MPa?55MPa?60MPa?65MPa?70MPa and 90 MPa.We also did the post fatigue tensile experiment and analyzed the tensile mechanical properties and tensile fracture morphology.By comparing the fracture morphology and microstructure of magnesium alloy,we draw the following conclusions:(1)The fatigue life of metal material decreases with the fatigue cyclic stress increase.When the cyclic stress is low enough,the material does not get fatigue failure.The experiment results show that when the cyclic stress is lower than 50 MPa,the fatigue cycle number will exceed 106 times,that is,the fatigue limit of AZ31 magnesium alloy is 50 MPa.(2)With the fatigue cyclic stress increase,the inner stress will increase and magnesium alloy is more likely to activate twinning.So the number of {1012} twins? {1011} twins and double twins will increase with the fatigue cyclic stress increase.When the cyclic stress is 50 MPa,the twinning is almost not activated during fatigue deformation.Therefore,the post fatigue samples with cyclic stress of 50 MPa did not found twins.(3)The average grain size of magnesium alloy is obviously refined after fatigue deformation.The dislocations slip and pile-up in low angle grain boundaries will lead to the average grain size decrease.The cycle numbers of and dislocation density have an effect on the average grain size.Therefore,the smaller the fatigue cyclic stress,the smaller the average grain size of the post fatigue magnesium samples.(4)The basal texture strength of the original AZ31 magnesium alloy plate reduced after fatigue deformation.The increase of fatigue cycle numbers and fatigue cyclic stress will make the basal texture of magnesium alloy more weaken.As a result of fatigue deformation,the basal texture strength of magnesium alloy firstly decreases and then increases with the increase of fatigue stress,and when the fatigue cyclic stress is 65 MPa,the maximum basal texture density reduces to the minimum.And the basal texture density value is 13.6.(5)The fatigue fracture morphology of magnesium alloy can be divided into crack initiation,crack propagation and final fracture region.With the increase of fatigue cyclic stress,the stress concentration position become more and the crack can initiate at the stress concentration position.The fatigue propagation region has fatigue striation,and the area of fatigue propagation region decreases with the fatigue cyclic stress increase.The surface smoothness of the propagation region becomes rough with the fatigue cyclic stress increase.The morphology of fatigue final fracture region mainly contains dimples.With the cyclic stress increase,the number of micro holes in the propagation region and the final fracture region will increase.(6)The work hardening and softening both work during the fatigue process.The tensile mechanical properties get better when the fatigue cyclic stress is higher.Moreover,with the cyclic stress increase,the internal defects become more and the tensile mechanical properties become worse.But if the cyclic stress is unstable,a sudden stress overload will not cause fatigue failure.The tensile strength after fatigue will not decrease too much.(7)The tensile fracture surface of the original sample is relatively flat,and the fracture surface consists of dimples and a small amount of micro holes.It is difficult to find the crack initiate region on the fracture surface.In the process of fatigue,the micro defects will be produced and become the crack initiate position.However,compared with the fatigue fracture surface,the post fatigue tensile fracture surface contains the crack initiation region and the final fracture region without the crack propagation region.(8)The micro holes on the tensile fracture surface may have a relationship with the twins,which produced in fatigue process.Because the number of micro holes increase with the fatigue cyclic stress increase.However,in the post fatigue,the fatigue cyclic stress is 50 MPa,tensile fracture surface no microscopic holes were found.