Investigations On The Microstructural Evolution Of Cu-Mn Alloys Under Uniaxial Tensile And Fatigue Loads

As one of the most crucial plastic deformation mode of metallic crystals,dislocation slipping has attracted widespread attention.Stacking fault energy(SFE)has been all along regarded as the important factor determining the slip mode in face-centered cubic(FCC)crystals.Contrary to this recognition,some of recent studies on the plastic deformation behavior of high-SFE Cu-Mn alloys under tension and compression have confirmed that the slip mode depends mainly on the short-range ordering(SRO)structure as the SFE maintains constant in these alloys.However,investigations on the influence of SRO content on the microstructural evolution during the process of monotonic and cyclic deformation were still not reported.In light of this,in the present work,a series of uniaxial tensile tests at different amounts of strains and total-strain-amplitude-controlled push-pull fatigue tests at different cycles have been conducted on the coarse-grained Cu-Mn alloys containing a wide range of Mn contents(5～20 at.%),and the relevant evolution of plastic deformation micromechanisms was systematically explored with the aid of transmission electron microscope(TEM)and scanning electron microscope(SEM)observations and analyses.During the process of uniaxially tensile tests of Cu-Mn alloys,the varied evolution of microstructures occurs,depending upon the Mn content(or SRO degree).In the Cu-5at.%Mn alloy,the characteristic deformation microstructures at different strains are all dislocation cells.In the Cu-7at.%Mn and Cu-l0at.%Mn alloys,the characteristic deformation microstructures shift from planar slip bands to dislocation cells and cell-walls with the increase of strain amount.In the Cu-20at.%Mn alloy,planar slip bands are always the characteristic deformation microstructures at different strains.Apparently,the SRO degree plays a determining role on the microstructural evolution.On the one hand,the increase in SRO degree promotes not only the formation but also the increment of planar slip bands at the very beginning of deformation.On the other hand,a higher SRO degree would hinder the formation of wavy-slip dislocation structures in the further-going stages of deformation.The Cu-Mn alloys containing different Mn contents exhibit a similar cyclic stress response behavior.At the chosen strain amplitude,all of alloys present a cyclic stress saturation phenomenon right after an initial stage of cyclic hardening.The microstructures corresponding to the stress saturation stage are,respectively,related to the formation of persistent slip bands(PSBs)in the Cu-5at.%Mn and Cu-10at.%Mn alloys and the occurrence of persistent Lüder’s bands(PLBs)in the Cu-20at.%Mn alloy.Careful transition electron microscope(TEM)observations demonstrate that,in the Cu-5at.%Mn alloy,the characteristic deformation micro structures transform from dislocation veins generated in the early cyclic stage into dislocation walls,cell-walls and labyrinth structures at the late or end stage of cycling.In the Cu-10at.%Mn alloy,planar slip bands dominate the microstructures along with dislocation veins appearing and developing gradually in the early cyclic stages,while dislocation walls and labyrinth structures become predominated with few planar slip bands being remained at the final cyclic rupture.In the Cu-20at.%Mn alloy,planar slip of dislocations dominates the whole cyclic deformation process.With increasing number of cycles,planar slip bands will be gradually developed into the densely-arranged PLBs.The above micro structural evolution phenomena under cyclic deformation have further provided a convincing evidence for the fact that SRO have a critical impact on the evolution of micromechanisms during the deformation process of fcc metallic materials.