Grain Growth And Deformation Mechanism In Nanocrystalline Materials

In the past few decades,nanocrystalline materials have attracted much attention because of their superior physico-mechanical properties.It is well known that the properties of a material are closely tied to its microstructure.Until now,a lot of research work has been conducted to investigate the microstructure evolution and the related deformation mechanisms, and significant successes have been achieved.Our knowledge on grain growth and deformation mechanisms of nanostructure,however,is still far from complete.Thus,a better understanding toward grain growth processes and deformation details is needed,which is the main goal of this work and will provide an effective way to tailor and optimize the fabrication of nanostructured materials.The research work is arranged as follows:In chapter 1,some backgrounds of nanocrystalline materials are introduced and the related research work that has been done before is surveyed,which include the experimental observations,theoretical studies and numerical simulations of grain growth and deformation in nanocrytalline materials.In the last section,a brief description of this work is presented.In chapter 2,a combined stochastic diffusion and mean-field model is developed for a systematic study of the grain growth in a pure single-phase polycrystalline material.A corresponding Fokker-Planck continuity equation is formulated,and the interplay/competition of stochastic and curvature-driven mechanisms is investigated.Numerical results show that when the grains are smaller than several tens of nanometres the dominating mechanism is stochastic diffusion-controlled of boundaries.As the grains grow the influence of the deterministic curvature-driven mechanism increases and finally controls the process.The transition ranges between these two mechanisms are about 2-26 and 2-15 nm with boundary energy of 0.01-1 J m^-2 in two- and three-dimensional systems,respectively.The grain size distribution of a three-dimensional system changes dramatically with time,while it changes a little in a two-dimensional system.The grain size distribution from the combined model is in good agreement with experimental observations.In chapter 3,a modified Potts model is proposed to systematically study normal grain growth in a single- or two-phase volume-conserved system.In this model,the driving forces for grain boundary migration are the interfacial energy between two phases and the boundary energy inside each phase.Simulation results show that the grain growth kinetics follows a power law with a temperature-independent exponent and the normalized grain size distribution is lognormal and time-invariant.The optimal volume fraction for the second-phase is found to be 7-11%for a well control of the primary grain growth,which is consistent with experimental data.Also a simple theoretical model is used to predict the potential microstructure in a two-phase system due to competition between interfacial and grain boundary energies.A critical energy ratio(～2.6) of grain boundary and interfacial energies is found for a common two-phase system and is supported by Monte Carlo simulations.In chapter 4,the basic theories of the molecular dynamics method are introduced, including the motion equations,integration scheme,temperature/pressure control algorithm, empirical potential functions.In addition,some practical methods,such as periodic boundary condition,time step selection and neighboring list method,are also addressed.In chapter 5,the deformation of single-crystalline copper nanowires under bending is studied using molecular dynamics simulations.The length and thickness effects on the stability and deformation of wires are also discussed.The results suggest that the plastic deformation is dominated by atomic slip on close-packed planes,and the twinning deformation is a primary mode in copper nanowires with sizes of～10 nm.It is found that an intermediate icosahedral phase is formed to facilitate the transformation from a low dense (010) plane in a face-centered-cubic lattice to a {111} close-packed fashion,forming tri-junctions composed of three deformation twins.These tri-junctions can easily interact with other deformation twins,forming two conjoint fivefold deformation twins.In chapter 6,a combined Monte Carlo and molecular dynamics scheme is established to investigate the deformation mechanisms of polycrystalline nanostructured copper with grain size in the range of 6-18 nm.The results show that grain boundary mediated plastic deformation,such as grain boundary sliding and grain rotation,mainly occurs in small grains; while large grain deformation is dislocation-accommodated through nucleation,propagation and absorption of partial/extended dislocations.It is also found that these two mechanisms operate in the different stages.With the evolution of deformation,stress-assisted grain growth could also occur due to atomic diffusion and grain boundary migration.Finally,the main contributions of this work are summarized and the further work is suggested.This research work is supported by the National Natural Science Foundation of China under Project Nos.10721062,10640420176 and 50679013,the Program for Changjiang Scholars and Innovative Research Teams in Universities of China,and the National Key Basic Research Special Foundation of China(2005CB321704).