Computational Simulation Study For Structural Defects And Mechanical Properties Of Iron-based And Carbon Materials

Structural defects in materials and the concomitant degradation of its mechanical properties have been confirmed to seriously influence its service performance.In this dissertation,the research is mainly focus on the evolution of structural defects and its influences on mechanical properties in iron-based materials,graphite and composites based on carbon nanotubes,using computational simulation methods.Dislocation and dislocation loops have great influence on the mechanical properties of iron-based materials.In this work,the mechanical behavior of and effect on materials mechanical properties from the 1/2<111>interstitial loops in BCC-Fe are studied by using the Molecular dynamics simulation method.Results show that:(1)the anti-stretching ability of BCC-Fe crystal decreases significantly when the radius of dislocation loop exceeds a certain critical value.Small shear loops around the initial prismatic loop are formed as an intermediated state before the final dislocation network is formed.(2)Under shear stress,the habit plane of the loop changes from the initial {111} to {100} and then {112},which makes the loop changes its property from a prismatic loop to a shear loop,and the prismatic loop will reform from the shear loop if removed the shear stress.(3)When a torsion is applied to a cylindrical computational box of BCC-Fe,the dislocation loops can delay the phase transition of Fe atoms,which results in the formation of a micro-cracks on the free surface of the cylinder.These results provide an important reference for further understanding the properties of interstitial dislocation loops and their effects on radiation damage of iron-based materials.Helium atoms in reactor iron-based materials which induced by the neutron irradiation can combine with vacancies or pores to form helium bubbles.In this work,the stress state and the obstacle effect on the dislocation motion of helium bubbles in BCC-Fe are investigated by using the Molecular dynamics simulation method.Results show that with the increase of He/V ratio.the helium bubble will undergo the states of low-pressured,equilibrium,high-pressured.extreme high-pressured and over-pressured successively.The dislocation line can absorb vacancies and interstitials when interacts with the low-pressure or high-pressure helium bubble,respectively.The extreme high-pressured bubbles could result in the significant deformation of the closed dislocation.The surface of the over-pressured helium bubble will be distorted,and the bubble will release the stress by punching out dislocation loop or segment.The obstacle strength of the helium bubble on dislocation motion is mainly determined by three factors:(1)the deformation of dislocation line caused by the bubble;(2)the area swept by the dislocation line across the helium bubble on the slip plane;(3)the different of interactions between the bubble and the different sides of the dislocation line.These results are conducive to understand the interaction of helium bubbles and edge dislocations in iron-based materials.The formation and evolution mechanisms of the structural defects in graphite at the atomic scale still need further research.In this work,the displacement cascades in graphite induced by helium atom have been simulated.Results show that the structures of defects induced in a single implantation are mainly single-vacancy,double-vacancy,interstitial and small in-plane defect clusters,which are determined by the energy transferred from the bombarded atom to the target atom during the collision and the direction of the initial velocity of the displaced target atom.Large defect clusters including unstable in-plane defect clusters and interlayer long carbon atomic chains will be introduced by continuous implantations.In the few-layered graphite,the density and the distribution along the penetration direction of defects are directly related to the incident energy.In the annealing simulation,the Frenkel-pairs are directly observed to recover or evolve into Stone-Thrower-Wales defects,and single vacancies are observed to migrate on the basal plane and aggregate to form the in-plane defect clusters.The large cross-layer defect clusters will transfer to a more stable state,which composed of stable in-plane defect structures or clusters and interlayer interstitial atoms or six-membered rings perpendicular to the basal plane,which is the key factor for the change of the bending properties of graphite.while the inplane defect without connection with other carbon layers has little impact on it.The above results will be helpful to understand the radiation effect of graphite materials and provide a reference for the application of graphite materials in nuclear reactors.The preparation technology of thin film thermoelectric materials with good flexibility and thermoelectric properties is important for the development of modern electronic technology.First principles.Molecular dynamics and ab initio Molecular dynamics methods have been used to study the growth of thermoelectric nanocrystals on the substrate of single-walled carbon nanotube bundles.Results show that there is no chemical bonding between the deposited nanocrystals and the surface of nanotubes bundles.Thus,the excessive stress and subsequent crack or deformation of the nanocrystal caused by the lattice mismatch are avoid.Therefore,the new approach can be used for the growth of various of nanocrystals with different lattice structures.The spatial distribution characteristic of van der Waals forces between the nanotubes and deposited nanocrystals result in a specific crystallographic orientation of the deposited nanocrystals.The bending properties of nanocrystals with low-tilt-angle grain boundaries of specific orientation are closed to those of single crystals,while the grain boundary with largetilt-angle decreases the flexibility of the nanocrystals significantly.The above results provide a theoretical explanation and mechanism analysis for the experimental method.The results provide a powerful theoretical explanation and mechanism analysis for the experimental method and theoretical support for the fabrication of micro flexible thermoelectric devices.