Study On Mechanical Behavior And Relevant Mechanisms Of Nanocrystalline Cu

Plastic deformation of coarse-grained metals is mediated by nucleation and movement of dislocations generated from intragranular sources.Interactions of dislocations on slip systems result in the formation of permanent dislocation network that hinders the movement of dislocations and thus hardens materials.However,for nanocrystalline metals,such dislocation mechanism becomes impossible because of the limited intragranular space,which hinder the activity of dislocation.Both dislocation activities and grain boundary mediated processes are activated in nanocrystalline metals.The dislocation activities include the nucleation of dislocations and emission of dislocations from grain boundary sources.The dislocations move across the grain and are absorbed by the opposite grain boundary.The grain boundary mediated processes include grain boundary sliding,diffusion and rotation.Molecular dynamics simulations and theoretical analyses have suggested that there is a critical grain size in nanocrystalline metals.Below the critical grain size,the plastic deformation is controls by the grain boundary sliding,while above the critical grain size,dislocation activities and grain boundary sliding both govern plastic deformation.Furthermore,when the grain size is larger than the critical grain size,the contributions of these two mechanisms to plastic deformation depend strongly on strain rate.We hope that nanocrystalline metals have high and stable mechanical property at different strain rate.Studying the deformation mechanism of nanocrystalline metals can help us understand the plastic deformation and provide the theoretical basis for improving the property of nanocrystalline metals.In this work,nanoindentation tests were performed to examine creep behaviors and unloading behaviors of nanocrystalline metals and acquire the strain rate versus stress data at room temperature.A continuous variation of activation volume,density of dislocations and strain rate sensitivity versus applied stress were obtained based on above data and explained the underlying deformation mechanisms in the plastic deformation behaviors at different stress levels.The results of this work are shown as following:(1)Nanoindentation tests were performed to investigate the creep behaviors of nanocrystalline Cu at room temperature.The results showed that the creep behaviors strongly depend on loading rate.At high loading rate,the initial transition regime is mainly mediated by the rapid absorptions of stored dislocations and newly nucleated dislocations.At low loading rate,there are only a few stored dislocations in grain and the creep behavior is mainly mediated by thermally activated grain boundary sliding.Based on a theoretical model for the probability of dislocation absorption by grain boundary and the interactions of dislocation with twins,the density of dislocation at different stress level is obtained.There is a high density of dislocation at high stress level,because more dislocations can be emitted by grain boundary and they are difficultly absorbed by grain boundary.However,at low stress level,the density of dislocation is low,because less dislocations can be emitted by grain boundary and it is easier to absorb dislocations for grain boundary.It suggests that the contribution of dislocations activity to creep behavior decreases with the decreasing stress and the creep behavior is mainly governed by the grain boundary sliding.(2)Initially,a usual Cottrell-Stokes behavior proceeds as the stress decreases to 530 MPa,i.e.,activation volume increases with decreasing stress.This is because that the distance between pinning points for mobile dislocations is controlled by the inter-dislocations spacing.Lower stress would directly lead to the increasing pinning distances and activation volume.Meanwhile,strain rate sensitivity decreases with decreasing stress,which suggests that the high density of dislocations lead to strain rate sensitivity.When the stress is less than 530 MPa,an inverse Cottrell-Stokes behavior is observed which shows that activation volume decreases with decreasing stress.The reason induced that grain boundaries activities can effectively decrease or even remove the pinning distances controlling the length goes progressively from the maximum length to the dislocation-dislocation distance at the grain boundaries.Strain rate sensitivity increases with decreasing stress,which suggests that the grain boundaries activities lead to strain rate sensitivity.(3)Nanoindentation tests were carried out to investigate the unloading behaviors of nanocrystalline Cu at room temperature.The results showed that initially a usual CottrellStokes behavior proceeds as the stress decreases to 530 MPa,i.e.,activation volume increases with decreasing stress.This is because that the deformation mechanism of nanocrystalline Cu is dislocation motion.Meanwhile,strain rate sensitivity decreases with decreasing stress.When the stress is less than 530 MPa,an inverse Cottrell-Stokes behavior is observed which shows that activation volume decreases with decreasing stress.It is because that the deformation mechanism of nanocrystalline Cu is grain boundaries mediated process.Strain rate sensitivity increases with decreasing stress.Furthermore,when the stress is less than 250 MPa,activation volume is almost a constant,because the plastic deformation is mediated by grain boundary sliding completely and there is no dislocation in grain.