Radiation-induced Defect Production Research Articles

Unveiling the radiation-induced defect production and damage evolution in tungsten using multi-energy Rutherford backscattering spectroscopy in channeling configuration

Radiation-induced defect production in tungsten was studied by a combination of experimental and simulation methods. The analysis of structural defects was performed using multi-energy Rutherford backscattering spectroscopy in channeling configuration (multi-energy C-RBS). To create different microstructures, (111) tungsten (W) single crystals were irradiated with W ions at two different doses (0.02 and 0.2 dpa) at 290 K. Detailed transmission electron microscopy (TEM) analysis of the samples revealed the presence of dislocation lines and loops of different sizes. The RBSADEC code was used to simulate the measured C-RBS spectra, recorded with four different He beam energies along the 〈111〉 direction. For the first time for tungsten, molecular dynamics (MD) simulations of overlapping cascades were used as input. The well-known method of randomly displaced atoms (RDA) was applied for comparison. RDA does not provide a satisfactory understanding of the nature of the induced defect structure. With MD, a very good agreement between the simulated and experimental spectra was obtained for the sample prepared at a lower dose, despite the fact that the absolute defect densities are two orders of magnitude higher than those found with TEM. A discrepancy is observed for the high-dose-irradiated sample, which is ascribed to the presence of extended defects such as dislocation lines, which are clearly observed by TEM, but cannot be formed in finite size MD cells. RBSADEC with MD cells as input can describe correctly the response of the RBS signal with analysing beam energy while RDA as input gives the wrong trend.

A machine-learning interatomic potential to understand primary radiation damage of silicon

Harsh radiation environments cause displacement damages in semiconductor components, resulting in performance degradation. Molecular simulations provide a unique approach to study the dynamic processes of radiation-induced defect production, clustering, and evolution to design and reinforce novel semiconductor components. In this paper, we developed a more efficient machine learning (ML) potential with DFT accuracy to investigate the radiation damage in silicon material. The accuracy of the potential was verified by comparing the static properties, defect formation energy, and threshold displacement energy with experiments and DFT data. By simulating the excitation of a single PKA we found that with the ML potential PKA has an impact over a larger spatial area compared to the empirical potentials. Finally, by simulating multiple PKA excitations in sequence, we found that the first several excited PKAs produce an amorphous region. For the later excited PKAs, the energies are dissipated when they cross through the amorphous region, resulting in a 36% decrease in newly generated defects.

Role of chemical disorder on radiation-induced defect production and damage evolution in NiFeCoCr

Understanding chemical disorder in many concentrated solid solution alloys (CSAs) at the levels of electrons and atoms has attracted increasing attention as a path forward to reveal and identify underlying mechanisms for extraordinary mechanical properties and improved radiation tolerance. Single-phase NiFeCoCr CSA is a common base for many high-entropy alloys (HEAs) that have shown improved mechanical strength and radiation tolerance. In this study, defect production and damage evolution in NiFeCoCr under ion irradiation at room temperature to dose over 20 dpa are determined using ion channeling technique along both <100> and <110> directions utilizing multiple probing beam energies. The results obtained from the multi-axial and multi-energy channeling analysis are compared with those previously obtained for Ni crystals irradiated under similar conditions. The influence of chemical complexity on defect production and clustering at early-stage under room temperature irradiation up to dose of 1 dpa is discussed based on positron annihilation spectroscopy results. Defect structure evaluation in Ni and NiFeCoCr is also discussed based on transmission electron microscopy results over a prolonged irradiation at both room and elevated temperatures. Compared with chemically complex NiFeCoCr, larger dislocation loops thus less lattice strain are expected to form in pure Ni. Moreover, the role of chemical disorder in this CSA is also investigated based on ab initio calculations using large supercells. To understand the impact of chemical complexity effect on defect structure evolution, this integrated research effort attempts to link the relatively large charge redistribution due to difference in valence electron counts resulting from alloying different 3d transition metal elements, moderate lattice distortion arising from similar adaptable atomic size, and notable suppressed or delayed damage evolution in NiFeCoCr.

Molecular dynamics simulations of radiation damage generation and dislocation loop evolution in Ni and binary Ni-based alloys

As a great potential structural-material candidate for nuclear power applications, single-phase concentrated solid-solution alloys containing different elements have outstanding mechanical properties. In order to explore the effects of their elemental compositions on their radiation tolerance properties, in this work, radiation-induced defect production, as well as defect cluster and dislocation loop evolution, are investigated for equiatomic NiCo and NiFe alloys, and pure Ni, using molecular dynamics simulations. We find that the binary alloys are more radiation-resistant than pure Ni, while the NiFe binary alloy has the best radiation tolerance due to a higher recombination of defects and a lower number of surviving Frenkel pairs after cascade, compared to NiCo and pure Ni. Small defect clusters appearing in the three materials exhibit different migration behaviors, which migrate in a simple one-dimensional mode in pure Ni, but randomly change directions in the binary alloys; this behavior in the latter is thought to enhance the recombination of defects. Vacancy-type stacking fault tetrahedra and mixed interstitial dislocation loops with different types of dislocation segments are also observed. Interstitial dislocation loops generated in the NiCo and NiFe alloys show delayed and sluggish nucleation and evolution processes, which may lead to a higher probability of defect recombination during their evolution. Moreover, the formation of mixed dislocation loops is also discussed.

Thermodynamic mixing energy and heterogeneous diffusion uncover the mechanisms of radiation damage reduction in single-phase Ni-Fe alloys

Understanding and predicting radiation damage is of central importance to develop radiation-tolerant structural materials for current and next-generation nuclear systems. Single-phase solid solution alloys constitute attractive choices due to their promising mechanical properties and radiation tolerance. Here, by examining radiation-induced defect production and evolution in single-phase Ni-Fe alloys, we show that radiation damage resistance directly correlates with thermodynamic mixing energy and heterogeneity of defect diffusion. We found that radiation damage in materials decreases linearly with lowering mixing energy, and the relationship holds true for all studied Ni-Fe compositions. The damage reduction with varying composition is further ascribed to the increasing heterogeneity of point defect migration across a complex potential energy landscape that enhances defect recombination. This new insight into the dynamical evolution of radiation defects points to a thermodynamic criterion for designing radiation-tolerant materials.

Surface, structural and tensile properties of proton beam irradiated zirconium

This paper reports the surface, structural and tensile properties of proton beam irradiated pure zirconium (99.8%). The Zr samples were irradiated by 3.5MeV protons using MC-50 cyclotron accelerator at different doses ranging from 1×1013 to 1×1016 protons/cm2. Both un-irradiated and irradiated samples were characterized using Field Emission Scanning Electron Microscope (FESEM), X-ray Diffraction (XRD) and Universal Testing Machine (UTM). The average surface roughness of the specimens was determined by using Nanotech WSxM 5.0 develop 7.0 software. The FESEM results revealed the formation of bubbles, cracks and black spots on the samples’ surface at different doses whereas the XRD results indicated the presence of residual stresses in the irradiated specimens. Williamson–Hall analysis of the diffraction peaks was carried out to investigate changes in crystallite size and lattice strain in the irradiated specimens. The tensile properties such as the yield stress, ultimate tensile stress and percentage elongation exhibited a decreasing trend after irradiation in general, however, an inconsistent behavior was observed in their dependence on proton dose. The changes in tensile properties of Zr were associated with the production of radiation-induced defects including bubbles, cracks, precipitates and simultaneous recovery by the thermal energy generated with the increase of irradiation dose.

Multi-timescale investigation of radiation damage near TiO2 rutile grain boundaries

To understand the interactions between defects and grain boundaries (GBs) in oxides, two atomistic modeling methods were used to examine the role of GBs in a model system, rutile TiO2, in modifying radiation-induced defect production and annealing. Molecular dynamics was used to investigate defect production near a symmetric tilt GB at both 300 K and 1000 K. The damage production is found to be sensitive to the initial distance of the primary knock-on atom from the GB. We find three distinct regimes in which GBs have different effects. Similar to GBs in metals, the GB absorbs more interstitials than vacancies at certain distances while this behavior of biased loading of interstitials diminishes at other distances. Further, we obtain the statistics of both interstitial and vacancy clusters produced in collision cascades in terms of their compositions at two temperatures. Perfectly stoichiometric defect clusters represent a small fraction of the total clusters produced. Moreover, a significant reduction in the number of interstitial clusters at 1000 K compared to 300 K is thought to be a consequence of enhanced migration of interstitials towards the GB. Finally, the kinetic properties of certain defect clusters were investigated with temperature accelerated dynamics, without any a priori assumptions of migration mechanisms. Small interstitial clusters become mobile at high temperatures while small vacancy clusters do not. Multiple migration pathways exist and are typically complex and non-intuitive. We use this kinetic information to explain experimental observations and predict their long-time migration behavior near GBs.

Positron annihilation study and computational modeling of defect production in neutron-irradiated reactor pressure vessel steels

Positron annihilation spectroscopy (PAS) and a computer simulation were used to investigate a defect production in reactor pressure vessel (RPV) steels irradiated by neutrons. The RPV steels were irradiated at 250°C in a high-flux advanced neutron application reactor. The PAS results showed that mainly single vacancies were created to a great extent as a result of a neutron irradiation. Formation of vacancies in the irradiated materials was also confirmed by a coincidence Doppler broadening measurement. For estimating the concentration of the point defects in the RPV steels, we applied computer simulation methods, including molecular dynamics (MD) simulation and point defect kinetics model calculation. MD simulations of displacement cascades in pure Fe were performed with a 4.7keV primary knock-on atom to obtain the parameters related to displacement cascades. Then, we employed the point defect kinetics model to calculate the concentration of the point defects. By combining the positron trapping rate from the PAS measurement and the calculated vacancy concentrations, the trapping coefficient for the vacancies in the RPV steels was determined, which was about 0.97×1015s−1. The application of two techniques, PAS and computer simulation, provided complementary information on radiation-induced defect production.

Radiation Induced Defect Production in Iridium Doped Silicon

Presented are the results of studies of some parameters of the Ir-centers in n-Si and of defect production processes in n-Si under the effect of ^{60}Co \gamma-rays. After irradiation with a does of 10^{7} Gy five energy levels have been observed in the DLTS spectrum of n-i. A conclusion has been drawn that of these the E_{c}-0.20 eV level belongs to the \lq\lq P atom +Ir atom + Vacancy" complex, two levels belong to well- known conventional A- and E- centers and the rest to Ir centers in silicon themselves.

Channeling effects and defect accumulation in ion implantation

In channeling implantation range and damage distributions and the dechanneling probability are strongly influenced by lattice vibrations, by a thin amorphous surface layer, by radiation-induced defect production during ion bombardment and by pre-existing defects. This is demonstrated in the example of〈100〉 channeling implantation of 80 keV boron into silicon using our binary collision code Crystal-TRIM. The comparison with experimental data shows that the simple model of defect accumulation used is sufficient to simulate the dose dependence of the range profiles. However, the simulations do not produce realistic damage distributions for high-dose implantation.

The Effect of Temperature on Defect Production in Copper Doped with Beryllium

AbstractThe effect of temperature on radiation-induced defect production was investigated in Cu single crystal films doped with ∼ 100 ppm Be. Frenkel pair production was determined at 55K and 190K as a function of electron energy and incident beam direction using in situ electrical resistivity measurements in the high voltage electron microscope. The crystallographic anisotropy of defect production was found to be considerably greater for irradiations at 55K than for irradiations below 10K. This difference is related largely to an observed anisotropy in the stage I defect recovery. It was found in isochronal annealing experiments after irradiation nearthreshold that the resistivity remaining after stage I was nearly twice as large for recoils along &lt;110&gt; as compared to &lt;100&gt;.