Investigation On High Cycle Fatigue Performance Of Ultrafine Grained Cu-Al Alloys

Strength and hardness are the basic mechanical properties of materials,and the fatigue property is the final criterion of whether the material can be used in practical engineering.Compared with the traditional materials,ultrafine grained(UFG)materials show good strength and hardness,however,the ductility is reduced greatly during tensile test in UFG material prepared by traditional severe plastic deformation(SPD)technologies.Furthermore,local deformation is easy to occur during fatigue deformation,resulting in earlier initiation of fatigue cracks and greatly reduced fatigue strength.The unique microstructure can be obtained by the new technology of friction stir processing(FSP),so good strength and ductility can be achieved in FSP UFG materials.The mode of dislocation slip and the fatigue deformation is determined greatly by the stacking fault energy(SFE),which is an important parameter of the face centered cubic metals.Therefore,Cu-Al alloys with different SFEs were selected as the research object in this thesis.The UFG materials were successfully prepared by FSP with additional water cooling,and the hardness,tensile properties and fatigue properties were investigated.The main conclusions are as follows:FSP UFG Cu-Al alloys exhibited typical dynamic recrystallized characteristics with equiaxed grains,low dislocation density,high ratio of high angle grain boundaries(HAGBs)and a large number of twin boundaries(TBs).The grain refinement mechanism of UFG Cu-Al alloys was changed and the grain size was reduced by decreasing the SFE.At the same time,the strength and the uniform elongation of the FSP UFG Cu-Al alloys were simultaneously enhanced as decreasing the SFE,which was resulted from the increasing of the number of HAGBs and TBs.Compared with UFG pure Cu(Cu-0Al)prepared by equal channel angular pressing(ECAP),higher fatigue strength and fatigue ratio were obtained in UFG pure Cu prepared by FSP.The fatigue damage of ECAP UFG Cu showed a large-scale shear band and grain coarsening.However,the large-scale shear band and grain growth could be inhibited by the special uniform microstructure in FSP UFG Cu,and extrusion was mainly observed on the fatigue surfaces.For the UFG Cu-Al alloys,the fatigue strength was improved clearly by decreasing the SFE.Meanwhile,the dislocation cross slip,recrystallization and grain growth could be inhibited in the FSP Cu-Al alloys with low SFEs,so only few small shear bands were produced in the grain interior.Large-scale extrusion phenomenon was not observed on the fatigue surface and the fatigue crack mainly initiated along the grain boundary under low-stress conditon.