Atomic-scale Simulation Of Dynamic Damage And Fracture Of Metallic Materials

The dynamic mechanical behaviors of materials under extreme loading conditions are important basis for understanding the evolution of material properties and assessing material reliability in major engineering applications.Among them,the shock response and radiation damage of metallic materials are key scientific issues of concern in the fields of weapons physics and nuclear materials,respectively,all involving the dynamic evolution process of microstructures.Through molecular dynamics simulations,this thesis is intended to investigate the damage and fracture of metallic materials at nanoscale under strong shock loading or high radiation dose,focused on the micromechanical behaviors and underlying physical mechanisms in these non-equilibrium processes.The contents and insights in this thesis are as follows:1.The dynamic fragmentation and transport processes of microjets from grooved tin surface in inert gases are investigated.The mutual interactions between mircojets and inert gases are analyzed.The results show that microjets experience significant deceleration in the presence of drag force and the particle flow ahead of jet tips is suppressed.In turn,gas is heavily compressed by ejecta,resulting in the formation of transmitted shock waves in background gases.At the same time,microjets progressively break up under internal velocity gradient and form mixing zone with ambient gas.By comparing the spatial density distributions in different ambient gases,the change law of the thickness and density in the mixing zone is obtained.The microscopic mechanism on the atomization of ejecta droplets during transport is analyzed,revealing the dominant role of gas stripping and its significant effect on the size distribution of ejecta particles.2.The influence of helium bubbles on the dynamic response of Al under shock loading is studied.Based on the helium parameters in aged plutonium,Al models containing corresponding concentration of randomly distributed helium bubble are constructed.The spall strength of Al with He bubbles is investigated under decaying shock wave of different shock strengths and a gradual decrease with increasing He concentration is found.The analysis of microvoids indicates that microvoids are preferentially nucleated near helium bubbles under tension,and this kind of heterogeneous nucleation stems from the facilitation of shear strain localization from He bubbles.In addition,further study reveal that plastic deformation is promoted by He bubbles during dynamic tension,with bubbles acting as preferential dislocation sources,so dislocation density increases significantly with increasing helium concentration.3.In-depth research has been carried out on the microstructural evolution of helium-containing metallic materials under radiation conditions.The high energy self-radiation process adjacent to He bubbles in ?-phase plutonium-gallium alloy is investigated and compared to that in pristine crystal.It is found that the mass effect of He promotes displacement damage at thermal spike phase and affects the distribution morphology of kinetic energy and point defects.The microscopic evolutions of cascade-induced bubble coalescence,resolution and re-nucleation are revealed.For single crystal Cu containing He bubbles,the coupling of collision cascades with He bubbles is systematically investigated.The results show that the radial interstitial dislocation network tends to form around big He bubbles as the radiation damage is aggravated with increasing bubble size.The microscopic mechanism of how He bubble evolution leads to this dislocation structure is revealed.When He bubble is able to maintain its shape under cascade collision,it is motivated to expand as a whole,thus punching out interstitials and stabilizing vacancies,which promotes the formation of interstitial-type dislocation network.The synergistic evolution of defective structures and He atoms under overlapping cascade collisions is investigated for Cu containing substitutional He.The significant effect of the continuous replacement reactions between substitutional He and interstitials on the defect accumulation process is analyzed.Such reactions generate a large number of Cu-He interstitial complexes,which inhibits their migration and growth,leading to a reduced size of dislocation loops.On the other hand,the transformation of substitutional He to interstitial He promotes the aggregation of He atoms,which is an important mechanism for the nucleation of helium bubbles at room temperature.