Simulation of Ion Irradiation of Nuclear Materials and Comparison with Experiment

Radiation defects generated in various nuclear materials such as Mo and CeO2, used as a surrogate material for UO2, formed by sub-MeV Xe and Kr ion implantations were studied via TRIM and MD codes. Calculated results were compared with defect distributions in CeO2 crystals obtained from experiments by implantation of these ions at the doses of 11017 ions/cm2 at several temperatures. A combination of in situ TEM (Transmission Electron Microscopy) and ex situ TEM experiments on Mo were used to study the evolution of defect clusters during implantation of Xe and Kr ions at energies of 150-700 keV, depending on the experimental conditions. The simulation and irradiation were performed on thin film single crystal materials. The formation of defects, dislocations, and solid-state precipitates were studied by simulation and compared to experiment. Void and bubble formation rates are estimated based on a new mesoscale approach that combines experiment with the kinetic models validated by atomistic and Ab-initio simulations. Various sets of quantitative experimental results were obtained to characterize the dose and temperature effects of irradiation. These experimental results include size distributions of dislocation loops, voids and gas bubble structures created by irradiation

Helium bubble formation in ultrafine and nanocrystalline tungsten under different extreme conditions

We have investigated the effects of helium ion irradiation energy and sample temperature on the performance of grain boundaries as helium sinks in ultrafine grained and nanocrystalline tungsten. Irradiations were performed at displacement and non-displacement energies and at temperatures above and below that required for vacancy migration. Microstructural investigations were performed using Transmission Electron Microscopy (TEM) combined with either in-situ or ex-situ ion irradiation. Under helium irradiation at an energy which does not cause atomic displacements in tungsten (70 eV), regardless of temperature and thus vacancy migration conditions, bubbles were uniformly distributed with no preferential bubble formation on grain boundaries. At energies that can cause displacements, bubbles were observed to be preferentially formed on the grain boundaries only at high temperatures where vacancy migration occurs. Under these conditions, the decoration of grain boundaries with large facetted bubbles occurred on nanocrystalline grains with dimensions less than 60 nm. We discuss the importance of vacancy supply and the formation and migration of radiation-induced defects on the performance of grain boundaries as helium sinks and the resulting irradiation tolerance of ultrafine grained and nanocrystalline tungsten to bubble formatio

Research on the physics of solid materials Semiannual status report, 1 Nov. 1966 - 30 Apr. 1967

Crystal structure, solids-electromagnetic interactions, and physical properties of single crystals, thin films, and bulk solid

Engineering self-organising helium bubble lattices in tungsten

The self-organisation of void and gas bubbles in solids into a superlattices is an intriguing nanoscale phenomenon. Despite the discovery of these lattices 30 years ago, the atomistics behind the ordering mechanisms responsible for the formation of these nanostructures are yet to be fully elucidated. Here we report on the direct observation via transmission electron microscopy of the formation of bubble lattices under He+ ion bombardment. By careful control of the irradiation conditions, it has been possible to engineer the bubble size and spacing of the superlattice leading to important conclusions about the significance of vacancy supply in determining the physical characteristics of the system. Furthermore, no bubble lattice alignment was observed in the directions pointing to a key driving mechanism for the formation of these ordered nanostructures being the two-dimensional diffusion of self-interstitial atoms

Molecular dynamics simulation of nucleation and growth of defects in the alloy fe-cr in the conditions of high-energy loading

Features of primary radiation damages in the near-surface layers of the Fe-Cr crystallite were investigated. The calculations were based on the molecular dynamics method. It was found that the number of surviving defects at the generation of atomic displacement cascades near the free surfaces is almost twice their number than in case of cascade generation far away from the various interfaces. Besides it the cascades can knock out some atoms from the free surfaces and form some specific structural defects: craters, adatom islands, dislocation loops of vacancy type. The crystallographic orientation of the irradiated surfaces has a significant influence on the features of the material damage. Craters are much more frequently formed at the irradiation of the (111) surface. There is a correlation between the size of the vacancy loops and the number of adatoms on the free surface. The size of the vacancy loops formed by the irradiation of the (111) surface is slightly larger than the number of adatoms. The inverse relationship was found at the irradiation of the (110) surface of Fe-Cr crystallite

Mechanisms of Cavity Growth During Ion Implantation

This paper will review two separate areas of cavity growth under ion implantation, firstly bias-driven void growth occurring in metals over the range 0.3 to 0.5 of the melting point at high displacement doses, and secondly, bubble growth during inert gas implantation. Interesting phenomena take place in both areas, e.g. void swelling and void lattice formation in the first and blistering, bubble lattices and the precipitation of heavier inert gases in the solid phase in the second. These phenomena will be described together with the extension of mechanisms to other implant species such as carbon where the possibility of precipitation in the diamond phase might be of interest