| A strong energy exchange occurs in the interaction between energetic particles and materials.In the process of ion energy loss,a series of physical and chemical interactions occur between high-energy ions and materials at the atomic scale,thereby affecting the structure and properties of materials,which have important applications in the field of nanomaterials processing.Intense particle collisions inevitably introduce defects in materials,which often lead to deterioration of material properties.With the miniaturization of device size,the problem of material damage is becoming more and more serious,so it is very important to deeply understand the interaction between energy-carrying particles and materials.Irradiation damage has become one of the important drawbacks hindering the application of energy-carrying particles.Due to the characteristics of short time and violent impact between energy-carrying particles and material atoms,it is difficult to capture the impact process in experiments.As an important auxiliary method,theoretical simulation can provide details of system evolution at the atomic scale,which is beneficial to understanding the changing laws of microstructure and properties.In this thesis,molecular dynamics method is used to study the three-dimensional bending of materials,deterioration of mechanical properties,surface morphology evolution and structural damage caused by ion irradiation.This paper mainly achieves the following innovative results:1.By comparing the structural changes of gold films with different crystal structures under various irradiation methods,it is found that the films are prone to bending to the front surface under irradiation.The mechanism is that the material particles migrate to the front surface and cause local modulus softening.The irradiation bending process of single crystal and polycrystalline gold films subjected to local and global irradiation was simulated by molecular dynamics method.The single crystal film is irradiated to produce vacancy defects on the front surface,the local modulus is softened,and the initial bending is formed with the internal stress.With local irradiation,defects rapidly accumulate at the irradiation position and extend to the back of the film to form grain boundaries.The material migration rate at the grain boundaries is much higher than the formation rate of vacancies in the crystal,which accelerates the bending of the film.With global irradiation,defects generated in the crystal can only form small clusters,which cannot be effectively aggregated.These defect clusters act as nucleation sites to form stacking faults with stress.The stacking fault slip makes the whole film tilt toward the ion source.The bending rate of polycrystalline films is related to the relative position of the grain boundary and the irradiation point.When the irradiation site deviates from grain boundaries,it cannot cause significant material migration,and the film bends slowly.Local irradiation will induce the surrounding grain boundaries to migrate to the irradiation center until the intersection of the grain boundary and the surface coincides with the irradiation center,and then the grain boundaries become the main material transport channels to accelerate the bending of the film.With the global irradiation,polycrystalline film exhibits uneven bending due to the different of material migration rate,and the bending is more obvious at the grain boundaries.High-energy irradiated particles can cause instability of the film structure,and the bending directions and angles become random.2.Through the irradiation and stretching simulation of polycrystalline gold nanofilm,it is found that the reason for the deterioration of polycrystalline gold film under large strain is that the grain boundaries converge to the irradiation area.We proposed a force field function of Au-Ga with the combination of the firstprinciples calculation and Force-matching method.The process of polycrystalline gold nanofilms irradiated by Ga ion was simulated by molecular dynamics,and the mechanical properties of the films before and after irradiation were tested.It was found that the tensile strength and ductility of the films after irradiation were lower than those before irradiation.By comparing before and after irradiation,it is found that the structure changes of the films induced by irradiation mainly include two aspects.One is the thinning of the film caused by surface atomic sputtering.The other is grain boundary migration due to local melting and recrystallization in the irradiated region.We introduce a reference model to separate the variables,confirming that the grain boundary migration induced by irradiation is the main reason for the decrease of film strength.Through the analysis of the recrystallized region,it is found that the newly formed grain boundaries have obvious preferred orientation,the degree of interlacing of grain boundaries is reduced,which leads to the reduction of tensile strength,and the shear bands are easily formed at the grain boundaries in the irradiation area during the stretching process to accelerate the film fracture.The ion irradiation of porous films can promote the grain growth and improve the density of the film,which alleviates the strength drop under large strain to a certain extent.3.By simulating the growth process of helium bubbles,we clarified the microscopic mechanism of the formation of nanostructures on the surface of metal tungsten under helium irradiation.The motion of helium atoms in the tungsten lattice at different temperatures is simulated,it is found that the helium atomic group has the largest migration ability in the temperature range of 800 K-1800 K,which is conducive to the nucleation and growth of bubbles.The formation depth of bubbles is directly related to the incident energy of helium.In the simulation of the growth process of helium bubbles at different depths,we found that shallow bubbles lead to the formation of irregular dents on the surface of tungsten,while deep bubbles can lead to the appearance of various nanostructures on the surface.The bubble pressure causes dislocation loops to be continuously generated at the helium/tungsten interface.We analyzed the dislocation loop migration and the interaction between the bubbles and found that the surface nanostructures tend to grow along the <111> direction and connect with each other along the <100> direction,so the surface morphology is closely related to the grain orientation.According to the relationship between the slip direction and the crystallographic orientation of the surface,the growth process of the surface nanostructures with four typical orientations was simulated,and the formation mechanism of the characteristic morphology of the irradiated surface in the experiment was successfully explained.4.The interaction between the tungsten wall of the thermonuclear reactor and the liquid metal lithium protective layer was studied,and it was found that the generation of defects could be effectively suppressed under the protection of liquid lithium.The motion of liquid metal lithium and the tungsten surface is simulated.It is found that lithium can be well adsorbed and wet the tungsten surface and form a layered structure.Lithium has a maximum diffusion rate on the(110)surface of tungsten,while the(100)surface restricts the migration of lithium atoms.On the(110)surface,lithium atoms can form precursor films even at room temperature(below the melting point of lithium).The diffusion rate was found to vary significantly with coverage.At 500 K,the diffusion rate of lithium in the first layer is about two orders of magnitude higher than that in the second layer.The maximum value of the diffusivity of monolayer Li on the(110)surface is related to the phase transition of Li on the surface.Through the simulation of radiation damage on the tungsten surface under the protection of liquid lithium,it is found that when the lithium atomic energy is higher than 5 keV,the total defect concentration in the 10 nm region of the tungsten substrate surface do not vary with the energy of the incident particles,and only increases with the irradiation dose.The concentration of surface vacancy defects generated by high-energy incident particles is greater than that of interstitial atoms.The tungsten atom sputtering maximum occurs at incident particle energies ranging from 100 keV to 300 keV. |
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