Size Distribution Of Defect Clusters Research Articles

Characterization of in-situ ion irradiated Fe-21Cr-32Ni austenitic model alloy and alloy 800H at low doses

The microstructural evolution of ternary Fe-21Cr-32Ni (21Cr32Ni) model alloy and of alloy 800H was investigated with a series of in-situ ion irradiation experiments performed using the Intermediate Voltage Electron Microscope (IVEM)-Tandem Facility at Argonne National Laboratory (ANL). Samples were irradiated in-situ with 1 MeV Kr++ ions in the temperature range of 50K to 713K to doses up to 2 dpa. The size distribution of defect clusters, average defect cluster diameter, and defect cluster density were measured and compared. Results showed that the evolution of defects (i.e. the average defect cluster size and number density) in 21Cr32Ni model alloy and alloy 800H with dose were similar at irradiation temperatures up to 300K where they initially increased with dose up to 0.1 dpa after which no significant changes in defect size and density were observed with further irradiation. In addition, both alloys exhibited ordered defect structures along the <100> direction at relatively low temperatures, up to 300K, which remained stable throughout post-irradiation in-situ thermal annealing up to a temperature of 773K. During irradiation at 713K, small defect clusters were observed at low doses (<0.1 dpa) in both alloys. However, at this irradiation temperature, the clusters in 21Cr32Ni grew with a faster rate than those formed in alloy 800H, causing the microstructure in the former to be dominated by numerous large dislocation loops having both {111}- and {110}-type habit planes, and in the latter to be dominated by small defect clusters and small {111}-type dislocation loops. This may indicate that defect trapping by the solute atoms in alloy 800H at 713K can slow point defect migration to defect clusters and limit their growth.

Effects of applied strain on defect production and clustering in FCC Ni

The structural materials of nuclear reactors are subjected to an environment of complex mechanical stresses, which has been reported to significantly affect the primary radiation damage suffered by the structure. In this study, we perform classical molecular dynamics simulations to study the effects of mechanical strains on the defect morphology due to primary radiation damage for a collision energy of 5 keV in face-centered cubic Ni. We consider the effects of four representative types of mechanical strains: hydrostatic, tetragonal shear, monoclinic shear, and uniaxial strains, and we discuss three aspects of the defect morphology: 1) time evolution of generated Frenkel pairs, 2) size distribution of defect clusters, and 3) fraction of clustered defects. We find that volumetric and anisotropic strains differently and significantly influence the primary radiation damage to the structure via affecting the intrinsic physical properties associated with the generation, formation, migration, or binding of Frenkel pairs; this result is generally consistent with those of many simulations. We believe our current study can aid in providing a fundamental mechanistic understanding of primary radiation damage in Ni-based alloys (for e.g., high-entropy alloys) as well as the subsequent long-term microstructural evolution.

Atomistic simulations for the effects of stacking fault energy on defect formations by displacement cascades in FCC metals under Poisson’s deformation

We performed molecular dynamics simulations of displacement cascades in FCC metals under Poisson’s deformation using interatomic potentials differing in stacking fault energy (SFE), in order to investigate the effect of tensile strain on the SFE dependence of defect formation processes. There was no clear SFE dependence of the number of residual defects and the size distribution of defect clusters under both no strain and the applied strain, while the strain enhanced the defect formation to a certain extent. We also observed that the strain affected the formations of self-interstitial atom (SIA) clusters depending on their size and the Burgers vector. These results were consistent with the analysis based on the defect formation energies. Meanwhile, the number of SIA perfect loops was higher at lower SFE under both no strain and the applied strain, leading to an increase in the ratio of glissile SIA clusters with a decrease in SFE. Further, the absolute number of SIA perfect loops was increased by the applied strain, while the SFE dependence of the number of SIA perfect loops was not affected. These findings were associated with the difference in formation energy between an SIA perfect loop and an SIA Frank loop. The insights extracted from this study significantly contribute to the modeling of microstructural evolution in nuclear materials under irradiation, especially for low SFE metals such as austenitic stainless steels.

Primary radiation damage near grain boundary in bcc tungsten by molecular dynamics simulations

Molecular dynamics (MD) cascade simulations of single crystal and five bi-crystal structures in bcc tungsten (W) are conducted to investigate the role of grain boundaries (GBs) on defect production and the size distribution of defect clusters. The cascades in W with cascade energies of 10keV and 20keV are simulated at different distances from the GB plane at 4.2K, 300K and 900K, respectively. The results indicate that the defect production is sensitive to the specific distance between the PKA and the nearby GB. The size of vacancy clusters becomes larger when the overlap region between the cascade and GB is small. Meanwhile, the mean size of interstitial clusters becomes smaller. The number of interstitials decreases with increasing temperature, whereas the number of vacancies is independent of the temperature.

High-energy collision cascades in tungsten: Dislocation loops structure and clustering scaling laws

Recent experiments on in situ high-energy self-ion irradiation of tungsten (W) show the occurrence of unusual cascade damage effects resulting from single-ion impacts, shedding light on the nature of radiation damage expected in the tungsten components of a fusion reactor. In this paper, we investigate the dynamics of defect production in 150 keV collision cascades in W at atomic resolution, using molecular-dynamics simulations and comparing predictions with experimental observations. We show that cascades in W exhibit no subcascade break-up even at high energies, producing a massive, unbroken molten area, which facilitates the formation of large defect clusters. Simulations show evidence of the formation of both and interstitial-type dislocation loops, as well as the occurrence of cascade collapse resulting in vacancy-type dislocation loops, in excellent agreement with experimental observations. The fractal nature of the cascades gives rise to a scale-less power-law–type size distribution of defect clusters.

MD simulations to evaluate the influence of applied normal stress or deformation on defect production rate and size distribution of clusters in cascade process for pure Cu

The stress influence on the defect production rate and size distribution of defect clusters in cascades was evaluated by molecular dynamics method. Stress was applied by exerting uni-axial, hydrostatic and isometric strain into the cell. For the uni-axial case, strain was varied between −1% and 1%. Defect production rate in cascade increased significantly under uni-axial tensile stress, and even under uni-axial compressive one. The largest increase of defect production rate was observed under isometric strain, and not so much under hydrostatic one. These results indicate that deformation anisotropy is a key factor to increase defect production rate. It was also found that larger defect clusters were formed under the strain condition in which defect production rate is higher.

Simulation of collision cascades and thermal spikes in ceramics

Classical molecular dynamics simulations have been employed to examine defect production by energetic recoils in UO 2, Gd 2Ti 2O 7, Gd 2Zr 2O 7 and ZrSiO 4. These atomistic simulations provide details of the nature and size distribution of defect clusters produced in collision cascades. The accommodation of recoil damage by lower energy cation exchange and greater occupation of anion structural vacancies is a contributing factor for the greater radiation tolerance of Gd 2Zr 2O 7 relative to Gd 2Ti 2O 7. In addition, electronic energy loss processes in UO 2 has been modeled in the form of a thermal spike to study the details of track formation and track structure. For thermal spikes with energy deposition of 4 keV/nm in UO 2, a track was not formed and mainly isolated Frenkel pairs were produced.

The Size Distribution of Defect Clusters in n-Si (FZ, Cz), Irradiated by Fast Neutrons

The dependence of concentration of defects on doping level for average cluster in n-Si was calculated. It was shown that in the framework of the Gossick's model the concentration of defects for the average cluster is in inverse proportion to the square of a cluster radius. One obtains the size distribution of defect clusters created by fast neutrons of WWR-M reactor, by the transformation of energy spectrum of the primary knock-on atom in n-Si (FZ, Cz). Threshold energy of defect clusters formation 4.7 keV by comparing n-Si crystals irradiated by deuterons and fast-pile neutrons was calculated.

Interactions between vacancy and glissile interstitial clusters in iron and copper

Reactions between defect clusters and point defects are the main mechanism of clusters growth and shrinkage under different conditions and therefore they are necessary input of any model for prediction of microstructure evolution. Traditionally such reactions are treated as the evolution of size distribution of defect clusters where the change in size is considered to be a simple reaction independent of the atomic structure. For example, the interaction between a vacancy and a self-interstitial cluster is treated as an annihilation reaction. However, recent atomistic studies have demonstrated that such reactions could be more complicated. In this paper, we present the results of atomic level modelling of the interaction between clusters of self-interstitial atoms (SIAs) and a vacancy in fcc Cu and bcc Fe. It is shown that vacancies annihilate only with interstitials at the cluster edge. Vacancies inside the cluster glide prism do not recombine but prevent cluster mobility. There are significant differences in the interaction related to the differences in the atomic structure of clusters in particular metals.

Density and size distribution of defect clusters in neutron-irradiated copper

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