Simulation Study Of Microstructure Optimization On Gradient Nanocrystalline Copper With Strength–Ductility Synergy By Crystal Plasticity Finite Element Modeling

With the wide application of nanostructured materials in fields such as aerospace,micro-nano electronic devices,and smart medical devices,the requirements for their comprehensive mechanical properties have been continuously increasing,where strength and toughness are two important mechanical performance indicators for evaluating their engineering serviceability.Compared with traditional coarse-grained materials,the strength of nanocrystalline metals can be significantly improved.However,the strength and toughness exhibit an inverse relationship that is difficult to reconcile due to the lack of tensile plasticity and strain hardening ability,which seriously restricts the application and development of nanocrystalline metals.Constructing gradient grain structures in nanocrystalline structures can achieve coordinated performance of strength and plasticity,which is an effective approach to improve the properties of nanomaterials and a significant research direction for nanocrystalline metals.In view of the difficulties in preparing gradient nanomaterials and the difficulty in controlling the optimal gradient ratio in experiments,computer simulation is used to construct gradient nanocrystalline copper structures with different grain sizes.We explore the gradient structure characteristics with the best strengthductility match by the crystal plasticity finite element method,and study the micromechanism of the high-strength and toughness gradient nanocrystalline structure in this paper.The main conclusions are as follows:(1)To address the difficulty in obtaining plastic parameters in the constitutive relationship,the influence of plastic constitutive parameters on mechanical behavior is studied through crystal plasticity finite element simulation.A plastic parameter calibration method for finite element analysis is provided.Furthermore,by calculating the tensile responses of different dual-phase gradient-structured coppers,it is found that the gradient structure in dual-phase gradient copper has significant advantages in mechanical properties such as strength and plasticity compared to uniform structure.(2)The mechanical behavior,strain,and stress fields of two-dimensional gradient nanocrystalline copper with different grain size gradient structures are analyzed computationally.It is found that when the grain size gradient distribution satisfies a linear relationship(i.e.,the gradient rate is equal to 1),the gradient nanocrystalline copper has optimal strength-plasticity matching.During tensile deformation,coarse grains bear more strain while fine grains bear more stress.When the gradient rate is 1,the gradient nanocrystalline copper exhibits the maximum strain and stress gradient in plastic deformation,and the system reaches a stable strain and stress gradient later.(3)To make the simulated structure closer to the actual situation,three-dimensional gradient nanocrystalline copper is analyzed computationally.The results shows that the optimal strength-plasticity matching,strain and stress variation laws are consistent with the two-dimensional experimental results.When the gradient rate is 1,the deformation process exhibits a multi-axial stress state,and more slip systems are activated in the crystal,resulting in an increase in strain hardening rate and enhancement of plastic flow.The surface roughness can be effectively reduced through mutual constraints between coarse and fine grains,and the strain localization of coarse and fine grains can be effectively suppressed simultaneously.In addition,molecular dynamics simulations are used to analyze different gradient nanocrystalline structures,and it is found that the gradient nanocrystalline structure has the best strength-plasticity matching when the gradient rate is 1,and the stress and strain gradients are the largest.The results of molecular dynamics simulations at the microscopic scale are consistent with those of crystal plasticity finite element calculations at the macroscopic scale,and the system has the highest dislocation density at the optimal strengthplasticity matching,which is consistent with the gradient plasticity theory.This study explores the design of gradient nanostructured materials and investigates indepth the plasticity micro-mechanisms of gradient nanostructured metal,which provides theoretical guidance and a solid theoretical basis for the preparation of high-performance materials.