Investigations On The Size Effect Of Uniaxial Tension And Tension-Tension Fatigue Behaviors Of Cu-Mn Alloys With High Stacking Fault Energies

With the development of miniaturization of materials,the mechanical properties of block materials are not completely suitable for small-sized materials.Therefore,in recent years,investigations of the size effect of mechanical properties of materials have attracted extensive attention,and many related theoretical models have been established to explain the size effect,but the influence of dislocation slip modes on the size effect of the mechanical behavior is rarely reported.In the present work,the Cu-Mn alloys(Mn=5,10,20 at.%)were selected as the target materials,and their thickness-dependent tensile and fatigue behaviors were systematically investigated.The uniaxial tensile tests of Cu-Mn alloys show that,with the reduction of thickness t,the yield stress ?YS almost keeps constant,but the ultimate tensile strength ?UTS and the uniform elongation ? show a tendency of "the smaller the weaker".When the t is reduced from 2.0 to 0.5 mm,the ?UTS and ? of Cu-Mn alloys slightly decrease,but quickly drop as t is less than 0.5 mm.Nevertheless,the influence of slip modes on the size effect of uniaxial tensile deformation behavior of Cu-Mn alloys cannot be neglected.As the Mn content increases(the slip mode changes from wavy slip to planar slip),the tendency of "the smaller the weaker" for ?UTS and ? is inhibited.Microstructural observations after tensile tests demonstrate that,there exists a significant effect of the t on the plastic deformation micromechanism of Cu-Mn alloys.First,for these three Cu-Mn alloys,dislocation density decreases sharply with decreasing t,especially as t is less than 0.5 mm.Second,the dislocation substructure evolution of Cu-Mn alloys is also strongly dependent upon the t.With decreasing t,the dislocation configurations of the Cu-5at.%Mn alloy exhibit typically wavy-slip characteristics,and the spacing of dislocation cell walls increases significantly.The dislocation structures of the Cu-10at.%Mn alloy are transformed from wavy-slip dislocation structures to planar slip bands.As the Mn content is increased to 20 at.%,the dislocation structures are mainly occupied by planar slip bands,and the slip band width decreases obviously.The above observations indicate that,the sharply decreased dislocation density and the unevenly distributed dislocation structure may cause the strain localization,resulting in the size effect of the tensile behavior of Cu-Mn alloys.However,compared with the wavy slip dislocation structure,the planar slip dislocation structure can effectively inhibit the activation of cross slip of dislocations,and improve the strain-hardening capacity.Thus,the Cu-20at.%Mn alloy with a planar slip characteristic can weaken the tendency of "the smaller the weaker".Tension-tension fatigue tests indicate that,the fatigue properties of Cu-Mn alloys show a strong dependence of t.Under the same stress amplitude,the fatigue lives decrease obviously with the decrease of t.The dislocation microstructures of the Cu-5at.%Mn and Cu-10at.%Mn alloys show a similar varying trend.In both alloys,the dislocation structures change from wavy-slip dislocation structures to the coexistence of planar and wavy slip dislocation structures.On the one hand,the planar slip activating in the thinner specimens restricts the dislocation cross slip to some extent,increasing the difficulty in dislocation multiplication and decreasing the dislocation density,and correspondingly,the cyclic hardening capacity of the alloy becomes weakened and the fatigue life eventually decreases.On the other hand,during the process of deformation,the planar-slip dislocation structures caused by short-range ordering(SRO)undertake most of the plastic deformation,which increases the risk of local strain concentration,leading to the initiation of fatigue damage more easily occurring in the small-t specimen.For the Cu-20at.%Mn alloy,the dislocation microstructures of all specimens are featured by planar slip bands.With the decrease of t,the dislocation density decreases,and obvious dislocation-dense and dislocation-poor areas are observed.Therefore,the reduction and inhomogeneous distribution of dislocation density might be the reason for the size effect of fatigue behavior in the Cu-20at.%Mn alloy.