Multiscale Atomistic Simulations Of Microstructure Evolution And Interaction Mechanisms Of Defects In Metals

The mechanical properties and radiation damage resistance of crystalline metal could be interpreted by the evolutions of microstructures and the interactions between defects.Atomistic details of such evolutions and interactions are essential to propagate the discussion to meso-sacle or macro-scale.But due to the thermally activated mechanisms involved in the deformation process,bridging the huge gap on the time-scale between Molecular Dynamics(MD)simulation and experiment observation is almost impossible.In nanomaterial where inherent microstructural length scales is quite small,competition between deformation mechanisms becomes unique and different from it is on the macro-scale.The complexity of slip system in hexagonal closed-packed(HCP)metals such as Mg and Zr enhances this so-called "size effect".Meanwhile,new insights on the interactions between dislocations and twin boundaries(TB)in nano-twinned metal are revealed,such as the strong rate sensitivity correlated with the slip transfers induced by TB.In nuclear industry,the high radiation resistance of materials is also contributed by the nanolayered structure,which apparently introduces extremely high grain-boundary density.The point defects and their derivatives in irradiated materials affect the mechanical properties by interacting with dislocations.Based on the discussions above,four subprojects are investigated in this thesis:plasticity on the nanoscale,dynamic response of grain-boundary strengthening,self-healing mechanisms of irradiated materials,and the thermally activated mechanisms in plasticity at low strain rates/high temperatures.Separately the projects are investigated by Molecular Dynamics,Molecular Statics,and a multiscale simulation technique named Autonomous Basin Climbing(ABC)method.We focus on conventional defects such as dislocations,TB and Self-Interstitial Atoms(SIAs)in HCP Mg and Zr,body-centered cubic(BCC)Fe.The main researches and results are listed as follow:1.The MD simulations of nanoscale plasticity in HCP Mg are presented here to investigate the dissociation mechanisms of pyramidal dislocations on two commonly-seen slip planes:{1011} and {1122}.An idea of forcing the atomic system to find the best slip path on restricted slip planes is applied.This idea is achieved by applying Periodic Boundary Conditions(PBC)in certain directions.We found that the dissociation on {1011} plane is more reliable than the one on{1122} plane.The dislocation plasticity dominates the whole deformation process,and the "loss of twinning" phenomenon is also observed in our simulations.2.Molecular Statics(MS)simulations of the interactions between dislocations and TBs in HCP Zr are carried out to reveal the energy accumulation in strain-stress relationship and the details in atomic configurations.We uncovered the contribution of Twinning Dislocation(TD)in the TB-dislocation interaction by emphasizing the relative movement between the interface and the line defect.{1011} TB in Zr plays a different role from the same TB in Mg or Ti does on converting the screw dislocation.By comparing the dynamic responses of four simulation models containing different TBs and dislocations,useful information for 3D simulation or experiments are extracted.3.The diffusion barrier and atomic configurations of transition states in SIA emission process in BCC Fe is investigated by ABC method.By analyzing two previously reported similar self-healing mechanisms from geometric and energetic views,we found the intrinsic relationship between them.The unique character that E3(110){111} grain boundary shows in the emission mechanism is interpreted by its stress and energy distribution of the structural units on the interface.Moreover,the grain boundary influence range is revised on long-time scale by comparing the energy barriers of vacancy hop and SIA emission.4.The dynamic response of the interaction between SIA cluster and edge dislocation in BCC Fe at low strain rates and high temperatures are modeled by a derivative method from ABC,named ABC-T.Dislocation climb is successfully simulated and a "quasi-pinning" mechanism is found to be the reason that Critical Resolved Shear Stress increases at high strain rates and low temperatures.This newly reported mechanism is introduced by SIA cluster which is attached to dislocation line but not absorbed into jog pair.The defect attached has an activation volume which affects its diffusion barrier uniquely.Similar phenomenon has been observed in TEM experiment previously.