Studying The Interaction Between Multidimensional Defects In Materials At High Temperature And Hydrogen Environments By Simulation And Experiment

High temperature and hydrogen environment are two typical extreme conditions for the service of materials and structures.The study on mechanical behavior of materials under such extreme conditions is a very important scientific endeavor,attracting increasing attention from both academic and industrial fields.It is well known that the macroscopic mechanical behavior of materials originates from the underlying deformation mechanisms,which might change with service conditions.At the meso-scale,the plastic deformation and damage of materials are closely related to the movements,evolutions and interactions of multidimensional defects(including the point defects,line defects,plane defects and volume defects,etc.).To reveal and understand the mechanical behavior of materials,it is necessary to quantitatively describe the interactions and evolutions of multidimensional defects at the micro-scale.In this doctoral dissertation,the interactions between multidimensional defects have been investigated under both high temperature and hydrogen environments,and the influences of these interactions on the mechanical behavior of materials have been thoroughly analyzed.The main research works and results are as follows:(1)Based on discrete dislocation dynamics and vacancy diffusion dynamics,a multidimensional defect coupling dynamics framework is developed,which consists of a discrete dislocation dynamics module,a vacancy diffusion module and a finite element computation module.The coupling between dislocation slip and vacancy diffusion is achieved through physical quantity transfer between these three modules.Based on this framework,the compressive creep behaviors of single crystalline and bicrystalline aluminum micro-pillars at high temperature are simulated,and the dislocation-vacancy interaction mechanism is revealed.The simulation results show that different creep stresses result in different steady-state creep rates at high temperature,due to the change of corresponding creep mechanisms;under high stress level,the creep strain rate increases with the increase of micro-pillar diameter,showing a strong size effect.(2)Through nano-indentation tests,the mechanical response at the vicinity of grain boundaries of polycrystalline nickel is studied with both hydrogen free and hydrogen charged samples,and the evolution of indentation hardness versus “indenter-grain boundary distance” is revealed.The experimental results show that the indentation hardness increases with the decrease of indenter-grain boundary distance for both the hydrogen free and hydrogen charged conditions;for the hydrogen charged sample,the pop-in load significantly decreases while the indentation hardness near grain boundaries markedly increases,compared to the hydrogen free counterpart.Based on these experimental results,a dislocation-grain boundary interaction model is proposed through theoretical analysis of geometrically necessary dislocations within the indentation area.The theoretical model shows that the energy barrier of dislocation slip transfer at grain boundary is significantly enhanced due to hydrogen segregation at grain boundary,therefore increasing the indentation hardness near grain boundary in the presence of hydrogen,in line with the nanoindentation results.(3)The effect of hydrogen on mechanical response of single crystalline iron is investigated by nano-indentation tests.The experimental results show that hydrogen leads to a significant decrease in both the pop-in load and hardness while an increase in the recoverable elastic depth.A three-dimensional discrete dislocation dynamics(3D-DDD)algorithm incorporating hydrogen effect is developed and used to investigate,on the one hand the nucleation and slip of a single dislocation loop as well as the interactions between dislocations,on the other hand the nano-indentation process of single crystalline iron.It is shown that hydrogen can promote dislocation nucleation and slip and enhance the shortrange interactions between dislocations,the two mechanisms presenting a competition.Through the comparison between the nano-indentation results and the 3D-DDD simulations,it is found that the hydrogen enhanced dislocation nucleation and slip mechanism is dominant in single crystalline iron.