Discrete Dislocation Dynamics Simulations Of The Relationship Between Microstructure And Mechanical Properties Of Nanotwinned Copper

In the field of materials science and engineering,the research on high-strength and hightoughness materials has been an important issue for a long time.The strength and toughness are equivalent to those of fish and bear’s paw.It is difficult to control the toughness of materials while seeking to improve their strength.The team in Lu Ke,Institute of Metallurgy,Chinese Academy of Sciences,synthesized nano-twinned copper materials through electrodeposition technology,which has ten times the tensile strength of the traditional coarse-grained copper,and still maintains good toughness and conductivity.Through a large number of in-situ characterization experiments,dislocation theories and molecular dynamics(MD)simulations,it is found that the excellent mechanical properties derive from the special dislocation-twin boundary(TB)microstructure.TBs not only have the strengthening characteristics of ordinary grain boundaries and hinder dislocation slip,but also can be used as the sink for dislocation interactions and dislocation nucleations,resulting in a large amount of plastic deformation and maintaining the toughness of materials.This shows that the excellent mechanical properties of twin materials are closely related to their microstructure evolution,while the traditional microstructure characterization experiments are costly and difficult to capture the evolution process of microstructure.In MD simulations,the sample size is small and the applied strain rate is high,so it is difficult to perform the interaction behaviour between collective dislocations and twin boundaries in real twinned materials.Discrete dislocation dynamic(DDD)simulation can study the evolution behaviour and mechanical response of dislocation microstructure at the nanometer to microns scale.However,the basic framework of DDD lacks a model describing the interaction between TB and dislocation,so it cannot be performed in the study of twinned materials.Therefore,in order to further reveal the correlation between twinned microstructural parameters and mechanical properties in twinned copper materials,the following work was carried out in this study DDD simulations and dislocation theoretical models.(1)Based on microstructure characterization experiments,MD simulations and dislocation theories,an anisotropic nonlinear mobility law are introduced into the basic DDD framework,and a dislocation-twin boundary interaction model is established to improve the DDD framework of twinned copper materials.(2)The mechanical behaviours of compression tests of twinned bicrystal pillars were studied by the method of DDD simulations,the twin boundary orientation effects,twinning dislocation source effects and compound effects of orientation and size are systematically analysed.The results are in good agreement with the experiments,which reveal the relationship between microstructure evolution and mechanical behaviour dominated by twin boundary orientation and size.(3)Based on the simulation results in(2),a set of theoretical models are developed to quantitatively describe the evolution of the dislocation-TB microstructure in the micropillars,including the activation model of a single Frank-Read source,the orientation effect and size effect of the collective dislocation sources,collinear reaction model,line tension model of dislocation source-TB reaction,continuous dislocation model of dislocation pile-ups at TB.Based on these theoretical models,the initial source structure is analyzed to predict the orientation effect,compound effect of orientation and size in micropillar compression tests,and further quantitatively analyze the strengthening mechanism of TB.(4)The multilayer twin structure is established in DDD framework,and the effects of loading orientation,twin thickness,TB defects,grain size and gradient on the microstructural evolution and mechanical response of the multilayer twin structure are simulated,and the anisotropy model,confined layer slip models of hard modes and dislocation pile-up model applicable to nanotwinned materials are established to describe the effects of microstructural parameters on mechanical properties.