Investigation On The Fatigue Behavior And Failure Mechanisms Of Nanostructured Magnesium Alloy

Magnesium(Mg)alloys have been increasingly applied as structural materials in automotive,aerospace and electronics industries due to their extremely low density,high specific strength,and abundant resources.Since the structural components in these industries would undergo cyclic loading in air or corrosion environment during service,it is crucial to understand the fatigue behavior of Mg alloys.However,Mg alloys usually possess poor wear and low corrosion resistance,which seriously limits their application in industries.On the other hand,fatigue fracture and corrosion usually initiate on the surfaces of materials.Hence,many techniques have been developed to improve the mechanical properties of Mg alloys through surface nano-crystallization(NC).In the present study,an NC surface layer was generated on AZ31 b Mg alloy by means of surface mechanical attrition treatment(SMAT)with 3 mm balls(3 mm-SMAT)and with 2 mm balls(2 mm-SMAT).Fatigue tests in air under different strain rates and corrosion fatigue tests in 3% Na Cl solution were conducted.It is found that the yield strength and ultimate tensile strength of the Mg samples increase after SMAT process,while the elongation of the samples decrease.The fatigue life of the SMATed samples is significantly enhanced compared with the coarse grained(CG)samples,and the fatigue life of 3 mm-SMATed samples is longer than that of 2 mm-SMATed samples.The enhanced fatigue life of the SMATed samples is mainly attributed to the combined influences of compressive residual stress and NC surface layer suppressing the initiation of cracks.Meanwhile,for both the SMATed and the CG samples,the fatigue life increases with increasing strain rate,owing to the fact that high strain rate favors twinning activity and restricts dislocation slip,thus enhancesing the fatigue crack resistance and prolonging the fatigue life.In the hydrogen evolution experiment and electrochemical experiment,the corrosion resistance of the samples decreases after SMAT process,mainly due to their high surface roughness,and the crystalline defects induced by SMAT favor the corrosion rate of the Mg samples.The fatigue life of the SMATed samples is significantly enhanced in the stress controlled fatigue test in air,and the 3 mm-SMATed samples show better fatigue behavior than the 2 mm-SMATed samples.3% Na Cl solution environment markedly reduces the fatigue life of all kinds of Mg samples,however,due to the compressive residual stress underneath the surface hinders the crack initiation and propagation of the SMATed samples,their corrosion fatigue behaviors are enhanced,and the corrosion fatigue behavior of the 3 mm-SMATed samples is better than that of the 2 mm-SMATed samples when the stress amplitude is higher than 80 MPa.