Structural Parameters And Mechanical Behaviors Of Laminated Nanotwinned Cu

Heterogeneous nanostructured metals,including laminated nanostructured metals,gradient nanostructured metals,etc.,typically consist of components with dramatic mechanical discrepancy,which are arranged in a certain spatial configuration.They generally exhibit superior comprehensive mechanical properties,such as high strength,good ductility,and enhanced work hardening capability,offering a new approach to breaking through the development bottleneck caused by the paradox between strength and ductility in traditional homogeneous metallic materials.As a typical heterogeneous nanostructured material,the structural parameters of which are complex,including not only the component types but also the spatial distribution,layer thickness,and transition layers between components;the strengthening and toughening mechanisms are also various,such as dislocation pile-up at the interface,mutual constraint between components,and gradient plastic deformation.Therefore,achieving controllable tuning of structural parameters,elucidating the constitutive relationship between structural parameters and mechanical properties,and unraveling the intrinsic strengthening and toughening mechanisms are tremendously challenging.The nanotwinned(NT)structure fabricated by direct current electrodeposition has the characteristics of high controllability and clear deformation mechanisms,which makes it ideal to serve as structural unit to construct laminated nanostructures.In this thesis,the structural parameters are adjusted by changing the microstructural size and spatial distribution of the NT structures,and a series of different laminated nanotwinned(LNT)Cu were consequently designed and fabricated.The effects of different structural parameters on the mechanical properties and plastic deformation behaviors of LNT Cu were systematically investigated,and the intrinsic strengthening and toughening mechanisms are unraveled.The effect of mechanical properties discrepancy between components on the mechanical behaviors of LNT Cu was unraveled.Three LNT Cu with hard component fixed at the surface and soft component as B,C,or D with decreasing strength and increasing ductility and work hardening capacity settled in the core were designed and fabricated.The principle that the extra strengthening and work hardening are simultaneously improved as the mechanical properties discrepancy between components increased was elucidated.By characterizing the strain distribution and geometrically necessary dislocation(GNDs)density distribution after deformation,it is found that enlarging the mechanical properties discrepancy is more effective to suppress strain localization,prolong elastic-plastic transition,and induce larger strain gradient with higher density of GNDs accumulated at the interface,which contributes to a better synergy of strength and ductility.The effect of component proportion on the mechanical behaviors of LNT Cu was investigated.The hard component A and soft component D with the largest mechanical properties discrepancy were selected,and three LNT Cu with A settled at the surface layer and D placed in the core layer were designed and fabricated,in which the soft component proportions are 10%,50%and 90%,respectively.It is found that with the increase in the proportion of soft component,the yield strength of LNT Cu decreased gradually,but the uniform elongation increased significantly.Yield strengths of all three LNT Cu are higher than that estimated by the rule of mixture,indicating an extra strengthening effect from LNT structure.The LNT Cu containing 50%soft component(LNT-50%)demonstrated the higher extra strengthening,resulting from the better suppression of strain localization and the stronger mutual constraint between the soft and hard components.The effect of layer thickness on the mechanical behaviors of LNT Cu was explored.Based on the hard component A and soft component D,LNT Cu samples with equal component proportion and gradually decreased layer thicknesses were designed and fabricated.The results of the tensile tests showed that the strength and work hardening at small strains of all LNT Cu are higher than the average value of the two freestanding homogeneous components,indicating significant extra strengthening and work hardening.LNT Cu with smaller layer thickness presents higher strength and work hardening that even exceeds the hard component.It notes that the extra strengthening of LNT Cu is higher than that of laminated metals without nanotwinned structure.By employing full-field strain measurement and microstructure characterization,the intrinsic mechanism and the origin of the extra strengthening were unraveled.After tensile deformation,gradient plastic deformation occurred from component D to A,forming the strain gradient across the interface and the GND density peak at the interface.The strain gradient and GND density gradually decreased as the distance from interface increased,forming an interface-affected zone covering almost the whole component,the width of which is significantly higher than other laminated metals.The accumulation of GNDs produced strong back stress,which is higher than that calculated by the rule of mixture,indicating extra back stress;the extra back stress keeps increasing continuously as layer thickness decreases,which contributes to higher extra strengthening and work hardening.The effect of transition layer proportion on the mechanical behaviors of LNT Cu was investigated.LNT Cu samples consisted of different numbers of transition layers with super large structural gradients inserted in soft component were designed and fabricated.The results of the study showed that as the proportion of the transition layer increased,LNT Cu presented higher strength with little loss in uniform elongation,and higher elongation to failure compared with the freestanding soft component.In the stage of uniform deformation,gradient plastic deformation in the transition layer induced the strain gradient and the accumulation of GNDs,both of which kept increasing as the tensile strain increased,leading to the improvement of strength and work hardening capacity,and the enhanced work hardening contributed to the excellent uniform elongation.In the stage after necking,the special severe detwinning contributed to the superior tensile plasticity of LNT Cu.In summary,the quantitative design and controllable preparation of laminated nanostructures were realized by using nanotwinned structures as the basic components.The effects of different structural parameters on the mechanical behaviors of laminated nanotwinned Cu were systematically investigated.It is found that by increasing the mechanical properties discrepancy between soft and hard components,regulating the equivalent proportion of components,reducing layer thickness,and increasing the proportion of transition layer,superior mechanical properties of laminated nanostructured metals could be achieved,which is closed related to the gradient plastic deformation and mutual constraint between components.The above finding provides theoretical guidance for the development of high-performance metallic materials.