Dynamical stability of radiation-induced C15 clusters in iron

Density functional theory predicts clusters in the form of the C15 Laves phase to be the most stable cluster of self-interstitials in iron at small sizes. The C15 clusters can form as a result of irradiation, but their prevalence and survival in harsh irradiation conditions have not been thoroughly studied. Using a new bond-order potential optimised for molecular dynamics simulations of radiation damage, we explore the dynamical stability of the C15 clusters in iron under irradiation conditions. We find that small C15 clusters make up 5–20% of the interstitial clusters formed directly in cascades. In continuous irradiation, C15 clusters are frequently formed, after which they remain highly stable and grow by absorbing nearby single interstitial atoms. Growth of C15 clusters ultimately leads to collapse into dislocation loops, most frequently into 1/2⟨111⟩ loops and only rarely collapsing into ⟨100⟩ loops at low temperatures. The population, size, and collapse of C15 clusters during continuous irradiation correlates well with their formation energies relative to dislocation loops calculated at zero Kelvin.Peer reviewe

Effects of the short-range repulsive potential on cascade damage in iron

Recent work has shown that the repulsive part of the interatomic potential at intermediate atomic separations strongly affects the extent and morphology of the damage produced by collision cascades in molecular dynamics simulations. Here, we modify an existing embedded atom method interatomic potential for iron to more accurately reproduce the threshold displacement energy surface as well as the many-body repulsion at intermediate and short interatomic distances. Using the modified potential, we explore the effects of an improved repulsive potential on the primary damage production and the cumulative damage accumulation in iron. We find that the extent of the damage produced by single cascades, in terms of surviving Frenkel pairs, directly correlates with the change in threshold displacement energies. On the other hand, the damage evolution at higher doses is more dependent on the formation and stability of different defect clusters, defined by the near-equilibrium part of the interatomic potential.Peer reviewe

Effect of random surface orientation on W sputtering yields

In this study, we investigate the sputtering yield of tungsten surfaces by energetic particles, focusing on the effect of surface orientation and the incoming irradiation angle, by means of molecular dynamics. We develop a simulation approach to simulate sputtering from completely random surface orientations. This allows obtaining the sputtering yields averaged over a sufficiently large number of orientations, to obtain statistically significant yields representative of a polycrystalline sample with random grain orientations. We find that the total sputtering yield is dependent on the surface orientation, and that the results for random surfaces are clearly different from that of any of the low-index ones or their average. The different low index surfaces and the random surfaces also showed that the sputtering yield is dependent on the incoming angle of the ion. The outgoing angle of the sputtered tungsten atoms was observed to be very sensitive to the surface orientation. Different features on the tungsten surface were observed to drastically affect the sputtering yield at certain angles.Peer reviewe

Defect accumulation and evolution during prolonged irradiation of Fe and FeCr alloys

The understanding of materials' behaviour during continuous irradiation is of great interest for utilizing materials in environments where harsh radiation is present, like nuclear power plants. Most power plants, both current and future ones, are based, at least partially, on Fe or FeCr alloys. In this study, we investigate the response of BCC Fe and several FeCr alloys to massively overlapping cascades. The effect of the added chromium on the defect accumulation and defect evolution was studied. Both a bulk setup, for observing the evolution deep inside the material far from grain boundaries and surfaces, and a setup with a nearby open surface, to investigate the effect of a permanent defect sink, were studied. We found that the primary defect production is similar in all materials, and also the build-up before serious overlap is comparable. When cascade overlap starts, we found that different sized clusters are formed in the different materials, depending on the setup. The defect cluster evolution was followed and could be related to the dislocation reactions in the materials. We found that the irradiation mixing was specific to the different chromium concentrations, the low chromium-containing alloy showed ordering, whereas the highest chromium-containing sample showed segregation. (C) 2019 The Authors. Published by Elsevier B.V.Peer reviewe

Low energy sputtering of Mo surfaces

Surfaces of materials subject to irradiation will be affected by sputtering, which can be a beneficial effect, like in the coating industry where a material is sputtered and redeposited on to another material to coat it. However, in most cases sputtering is an unwanted side-effect, for instance in nuclear fusion reactors, where the wall material will be degraded. This effect needs to be understood in order to be able to predict its consequences. To understand the sputtering, on an atomistic level, we have thoroughly investigated molybdenum surface sputtering by computational means. Molybdenum was chosen as detailed experimental studies have been carried out on it and it is one candidate material for the diagnostic mirrors in ITER, facing the plasma. In this study, we thoroughly investigate the molybdenum samples of different surface orientations, and their response to low energy argon plasma irradiation, by molecular dynamics simulations. We find both a surface orientation and ion energy specific sputtering yield of the samples, and a very good agreement with the experiments available in the literature. A few different setups were investigated to observe differences as well as to understand the key features affecting the sputtering events. The different simulation setups revealed the optimal one to represent the experimental conditions as well as the mechanisms behind the observed discrepancies between different modelling setups. (C) 2020 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).Peer reviewe

Radiation damage buildup and dislocation evolution in Ni and equiatomic multicomponent Ni-based alloys

Single-phase multicomponent alloys of equal atomic concentrations ("equiatomic") have proven to exhibit promising mechanical and corrosion resistance properties, that are sought after in materials intended for use in hazardous environments like next-generation nuclear reactors. In this article, we investigate the damage production and dislocation mobility by simulating irradiation of elemental Ni and the alloys NiCo, NiCoCr, NiCoFe and NiFe, to assess the effect of elemental composition. We compare the defect production and the evolution of dislocation networks in the simulation cells of two different sizes, for all five studied materials. We find that the trends in defect evolution are in good agreement between the different cell sizes. The damage is generally reduced with increased alloy complexity, and the dislocation evolution is specific to each material, depending on its complexity. We show that increasing complexity of the alloys does not always lead to decreased susceptibility to damage accumulation under irradiation. We show that, for instance, the NiCo alloy behaves very similarly to Ni, while presence of Fe or Cr in the alloy even as a third component reduces the saturated level of damage substantially. Moreover, we linked the defect evolution with the dislocation transformations in the alloys. Sudden drops in defect number and large defect fluctuations from the continuous irradiation can be explained from the dislocation activity. (C) 2017 Elsevier B.V. All rights reserved.Peer reviewe

CASCADE DEBRIS OVERLAP MECHANISM OF <100> DISLOCATION LOOP FORMATION IN Fe AND FeCr

Two types of dislocation loops are observed in irradiated alpha-Fe, the 1/2 loop and the loop. Atomistic simulations consistently predict that only the energetically more favourable 1/2 loops are formed directly in cascades, leaving the formation mechanism of loops an unsolved question. We show how loops can be formed when cascades overlap with random pre-existing primary radiation damage in Fe and FeCr. This indicates that there are no specific constraints involved in the formation of loops, and can explain their common occurrence. Copyright (C) EPLA, 2017Peer reviewe

Memory Effect, Rejuvenation and Chaos Effect in the Multi-layer Random Energy Model

We introduce magnetization to the Multi-layer Random Energy Model which has a hierarchical structure, and perform Monte Carlo simulation to observe the behavior of ac-susceptibility. We find that this model is able to reproduce three prominent features of spin glasses, i.e., memory effect, rejuvenation and chaos effect, which were found recently by various experiments on aging phenomena with temperature variations.Comment: 10 pages, 14 figures, to be submitted to J. Phys. Soc. Jp

Effect of cascade overlap and C15 clusters on the damage evolution in Fe : An OKMC study

In order to investigate the long-term evolution of radiation-induced defects in the fission- and fusion-relevant material iron, we introduce cascade overlap effects into Object Kinetic Monte Carlo simulations. In addition to cascade overlap, we study the effect of introducing discrete C15 Laves phase clusters into the simulations. By applying either, none, or both of these effects we identify how they influence the evolution of the system. We find that both cascade overlap and C15 clusters affect the evolution of the radiation damage in different ways and on different time scales. Cascade overlap is found to reduce the number of Frenkel pairs. On the other hand, the explicit consideration of C15 Laves phase clusters increases the accumulation of defects at low dose. The results are compared to Molecular Dynamics simulation results under similar conditions.Peer reviewe