Effect Of Surface Diffusion On Mechanical Properties Of Nanoporous Metals

Nanoporous metals have excellent surface properties due to their high specific surface area and a large number of interconnected ligaments,which make them possess prominent electrocatalytic properties,strain invertibility and high yield strength.Because of the large number of nanoscale interconnected pores in nanoporous metal,the mechanical properties are very different from those of the compacted metal at the macro-scale,and its huge application potential requires urgent research on this issue.In this paper,molecular dynamics simulation is used to study the microstructure and mechanical behavior of nanoporous metals.The calculated atomic model is obtained by the phase field method,because the amplitude modulated decomposition of binary alloy simulated by the phase field method can obtain a model that is close to the internal ligament structure of the nanoporous metal prepared by experiment.Firstly,the effect of the phase field parameters on the nanoporous model is studied,and the value range of the phase field parameters of the nanoporous structure required in the practical calculation is obtained,which provide abundant and near-real nanoporous structure models for molecular dynamics simulation.An empirical parameter to characterize the degree of connectivity of nanoporous metal ligaments is proposed,which provides a quantitative standard for selecting nanoporous metal models.Secondly,molecular dynamics simulation is used to simulate the tension and compression of the nanoporous copper models with the same porosity and different sizes.The microstructure changes and phase transitions of different size of nanoporous copper are compared.The results show that the smaller size of the nanoporous copper model is more prone to phase transition in the tension process,and the phase transition changes more drastically.The phase transformation amplitude of nanoporous copper under compression loading is much larger than that under tensile loading,which indicates that the tension-compression asymmetry of nanoporous metal materials is not only caused by the change of macroscopic configuration,but also caused by the change of material microstructure.Finally,three groups of nanoporous copper are unloaded by tension and compression,and the characteristics of nanoporous copper in unloading are analyzed,which are different from the compacted copper.Aiming at the ligament healing phenomenon of nanoporous copper in tension-unloading process,the phase transition and energy analysis of each unloading process are carried out.And the mechanical properties of reloading process are analyzed.The results show that ligament healing is driven by the release of phase transition energy and surface energy,and the main factor is the release of surface energy.Surface atoms in nanoporous copper have high activity.The diffusion of surface atoms leads to various forms of surface reconstruction in the loading-unloading process of nanoporous copper,resulting in different internal structures of the material.In the fine ligament model,a new porous structure is formed in the healing of the ligament after unloading when the deformation is large,so that regular rings appear on the stress-strain curve,and the material becomes soft in macroscopic view.However,the large-size ligament model does not form a new porous structure after unloading when the deformation is large,the regular rings are difficult to appear on the stress-strain curve.But the macroscopic mechanical properties of the material can be basically restored to the initial state.In this paper,an empirical parameter to characterize the degree of connectivity of nanoporous metal ligaments is proposed,and based on this parameter,the recommended range of computational parameters for constructing nanoporous metal models using the phase field method is given.The results of molecular dynamics calculations show that the diffusion of surface atoms in the process of tension,compression and unloading of nanoporous copper makes complex changes in the internal configuration of the material,resulting in a high degree of internal structure tenability;especially in one type of model,the ligaments healed during unloading after being pulled fracture.The analysis of energy changes in the healing process of the fractured ligaments revealed that the releasable surface energy is the main factor that promotes the healing of the ligament.