Effects of edge dislocations on interstitial helium and helium cluster behavior in α-Fe

The properties of interstitial He in the vicinity of an edge dislocation were studied using molecular dynamics (MD) simulation. The distribution of the binding energy of a single interstitial He to the dislocation with and without a jog is calculated. The results show that the distribution of the binding energy is governed by the elastic interaction between the interstitial He and the dislocation. The interstitial He is strongly attracted to the dislocation in the tensile region of the dislocation. The jog acts as a stronger sink to absorb interstitial He. The binding energy to the jog is even larger than that of the dislocation. A small He cluster (composed of three interstitial He atoms) was trapped by the dislocation core in the form of a chain along the dislocation line. The dislocation changes the migration behavior of the He cluster, and provides a pipe for the small cluster to exhibit one-dimensional motion. The diffusion of the He cluster in the dislocation is faster than in the defect-free iron, where the He cluster migrates three-dimensionally (3D). If the dislocation is decorated by a jog, the small cluster sinks deep into the jog. The jog prevents the He cluster from escaping.     

Atomistic simulations of nanometric dislocation loops in bcc tungsten

Small dislocation loops formed from self-interstitial atoms (SIAs) are commonly found in irradiated metals. These defects significantly influence the mechanical properties of the materials. Atomistic simulations are used to describe nanometric circular dislocation loops with Burger’s vectors , and in bcc tungsten. Particular attention is paid to the habit plane of the loop. Two different embedded atom model (EAM) potentials are used. The energetics and geometry of the loops are studied as a function of their size.     

Modeling dislocation structure development and creep-swelling coupling in neutron irradiated stainless steel

Anisotropic nucleation and growth of multi-classes of dislocation loops under the combined actions of fast-neutrons and an external applied stress are considered in modeling dislocation structure development in metals and alloys. The stochastic nature of the nucleation kinetics is formulated via the Fokker-Planck equation. The strain derived from the climb of the anisotropic dislocation structure is separable into volumetric and deviatoric components, corresponding respectively to swelling and creep. The creep contribution resulting from the development of the stress-induced dislocation anisotropy is found to be very significant and exhibits a strong correlation with swelling. For stainless steel, our model explains very well the complex deformation behavior observed in a wide variety of in-reactor experiments.     

Breather mechanism of the void ordering in crystals under irradiation

The void ordering has been observed in very different radiation environments ranging from metals to ionic crystals. In the present paper the ordering phenomenon is considered as a consequence of the energy transfer along the close packed directions provided by self-focusing discrete breathers. The self-focusing breathers are energetic, mobile and highly localized lattice excitations that propagate great distances in atomic-chain directions in crystals. This points to the possibility of atoms being ejected from the void surface by the breather-induced mechanism, which is similar to the focuson-induced mechanism of vacancy emission from voids proposed in our previous paper. The main difference between focusons and breathers is that the latter are stable against thermal motion. There is evidence that breathers can occur in various crystals, with path lengths ranging from 10⁴ to 10⁷ unit cells. Since the breather propagating range can be larger than the void spacing, the voids can shield each other from breather fluxes along the close packed directions, which provides a driving force for the void ordering. Namely, the vacancy emission rate for “locally ordered” voids (which have more immediate neighbors along the close packed directions) is smaller than that for the “interstitial” ones, and so they have some advantage in growth. If the void number density is sufficiently high, the competition between them makes the “interstitial” voids shrink away resulting in the void lattice formation. The void ordering is intrinsically connected with a saturation of the void swelling, which is shown to be another important consequence of the breather-induced vacancy emission from voids.     

Pressure of stable He-vacancy complex in bcc iron: Molecular dynamics simulations

Molecular dynamic simulation was employed to study the stable state of He-vacancy (He-V) complex in bcc iron. The pressure of He-V complex was calculated using the concept of atomic-level stress. In the case of no initial vacancies introduced in the simulation box, self-interstitial atoms (SIAs) are emitted by the small He cluster. As the number of the He cluster is above a critical value, interstitial-type dislocation loops (I-loop) will be generated. After the interstitial-type defects (SIA or I-loop) were created, it is found that the ratio of He atoms to athermal vacancies keeps nearly constant in the He-V complex.     

Radiation-induced reduction in the void swelling

Vacancy voids have been produced in Ni by 1.2 MeV Cr ion irradiation at 873 K up to the ion fluence of 10²¹ m⁻². Subsequent irradiation of specimens containing voids at 798 and 723 K has resulted in the reduction of the void size and number density. Accordingly, the void swelling has decreased by a factor of ∼5. The experimental results are explained in the framework of an original model taking into account the interaction of voids with radiation-induced excitations of atomic structure such as focusing collisions and long-propagating self-focusing breathers.     

Steady-state size distribution of voids in metals under cascade irradiation

The theory of radiation damage in metallic materials predicts that under cascade-irradiation conditions the voids should approach a steady state, which is characterised by a maximum mean void size. It is shown in this paper that the steady-state concentrations of voids of different size are described by the Gaussian distribution with the maximum size mentioned above to be the most probable value. The evolution of voids towards the steady state is analysed.     

Electronic effects in radiation damage simulations

A methodology for including electronic effects in classical radiation damage simulations is presented. The method is used to calculate the number of residual defects for low energy (10 keV) cascades in Fe, as a function of the electron-phonon coupling strength. It was found that strong electron-phonon coupling reduced the number of residual defects by rapidly removing energy from the cascade and reducing the thermal spike. Intermediate coupling increased the number of defects by quenching the thermal spike and reducing defect recombination. Thermostatting the cascade with the local, time dependent electronic temperature, rather than the ambient temperature, reduced the number of residual defects by enhancing defect recombination. Swift heavy ion irradiation in tungsten was modeled using the same methodology. In this case we found that the number of residual defects created by a given electronic stopping power was strongly dependent on the temperature variation of the electronic heat capacity. In contrast to cascade simulations, the interstitials were located closer to the core of the ion track than the vacancies.     

Monitoring the embrittlement of reactor pressure vessel steels by using the Seebeck coefficient

The degree of embrittlement of the reactor pressure vessel (RPV) limits the lifetime of nuclear power plants. Therefore, neutron irradiation-induced embrittlement of RPV steels demands accurate monitoring. Current federal legislation requires a surveillance program in which specimens are placed inside the RPV for several years before their fracture toughness is determined by destructive Charpy impact testing. Measuring the changes in the thermoelectric properties of the material due to irradiation, is an alternative and non-destructive method for the diagnostics of material embrittlement. In this paper, the measurement of the Seebeck coefficient () of several Charpy specimens, made from two different grades of 22 NiMoCr 37 low-alloy steels, irradiated by neutrons with energies greater than 1 MeV, and fluencies ranging from 0 up to 4.5 × 10¹⁹ neutrons per cm², are presented. Within this range, it was observed that increased by ≈500 nV/°C and a linear dependency was noted between and the temperature shift ΔT_41 J of the Charpy energy vs. temperature curve, which is a measure for the embrittlement. We conclude that the change of the Seebeck coefficient has the potential for non-destructive monitoring of the neutron embrittlement of RPV steels if very precise measurements of the Seebeck coefficient are possible.     

Modification of a shell model for the study of the radiation effects in BaTiO3

In order to study the radiation effects in BaTiO₃ ferroelectric crystal, a previously developed shell model is modified. The modifications include adding the ZBL universal potentials at short distances and distance-dependent spring constants for core-shell interactions. The phase transition sequences in BaTiO₃ were correctly reproduced using molecular dynamics simulations with this modified shell model. Also, the calculated Frenkel pair formation energies agree well with results obtained by first principles calculations, which suggests that this model is suitable for the simulation of the radiation effects in BaTiO₃. The dependence of polarization on the number of oxygen vacancies was also studied.     

Analyzing the effect of displacement rate on radiation-induced segregation in 304 and 316 stainless steels by examining irradiated EBR-II components and samples irradiated with protons

Recent studies have indicated that, at temperatures relevant to fast reactors and light water reactors, void swelling in austenitic alloys progresses more rapidly when the radiation dose rate is lower. A similar dependency between radiation-induced segregation (RIS) and dose rate is theoretically predicted for pure materials and might also be true in complex engineering alloys. Radiation-induced segregation was measured on 304 and 316 stainless steel, irradiated in the EBR-II reactor at temperatures near 375 °C, to determine if the segregation is a strong function of damage rate. The data taken from samples irradiated in EBR-II is also compared to RIS data generated using proton radiation. Although the operational histories of the reactor irradiated samples are complex, making definitive conclusions difficult, the preponderance of the evidence indicates that radiation-induced segregation in 304 and 316 stainless steels is greater at lower displacement rate.     

Effects of the density of collision cascades: Separating contributions from dynamic annealing and energy spikes

We present a quantitative model for the efficiency of the molecular effect in damage buildup in semiconductors. Our model takes into account only one mechanism of the dependence of damage buildup efficiency on the density of collision cascades: nonlinear energy spikes. In our three-dimensional analysis, the volume of each individual collision cascade is divided into small cubic cells, and the number of cells that have an average density of displacements above some threshold value is calculated. We assume that such cells experience a catastrophic crystalline-to-amorphous phase transition, while defects in the cells with lower displacement densities have perfect annihilation. For the two limiting cases of heavy (500 keV/atom ²⁰⁹Bi) and light (40 keV/atom ¹⁴N) ion bombardment of Si, theory predictions are in good agreement with experimental data for a threshold displacement density of 4.5 at.%. For intermediate density cascades produced by small 2.1 keV/amu PF_n clusters, we show that dynamic annealing processes entirely dominate cascade density effects for PF₂ ions, while energy spikes begin contributing in the case of PF₄ cluster bombardment.     

Cluster Dynamics modelling of irradiation growth of zirconium single crystals

This paper aims at modelling irradiation growth of zirconium single crystals as a function of neutron fluence. The Cluster Dynamics approach is used, which makes it possible to describe the variation of irradiation microstructure (dislocation loops) with neutron fluence. From the irradiation microstructure, the strain can be calculated along the axes of the lattice structure. The model is applied to the growth of annealed zirconium single crystals at 553 K measured by Carpenter and Rogerson in 1981 and 1987. The model is found to fit the experimentally measured growth of Zr single crystals very nicely, even at large neutron fluence where the ‘breakaway growth’ occurs. This was made possible by considering in the model the growth of vacancy loops in the basal planes. This growth of vacancy loops in the basal planes could be modelled by taking into account that diffusion of self-interstitial atoms (SIA) is anisotropic and that there exist in the basal planes some nucleation sites for vacancy loops (iron clusters), the density of which is considered constant over time.     

Mechanical properties of Eurofer 97 in Pb-16Li and irradiation effect

To be used in a fusion reactor, structural materials, and in particular steels, has to be selected and optimised in their composition to achieve a reduction in the long-term radioactive waste. A reduction in the long-term radioactive inventory could be reached substituting elements like molybdenum, niobium and nickel with other ones like tantalum and tungsten which have the same functions as alloying elements and, if irradiated, do not produce long lived radioisotopes. The martensitic steel belonging to the family of 8-9% Cr Eurofer 97 is considered the reference structural steel for fusion application. However, only few information are available about its mechanical properties in the liquid eutectic alloy Pb-16%Li. Particularly, the problem of liquid metal embrittlement (LME) has not been studied in detail and the effect of neutron irradiation on LME has not been investigated at all so far. This work presents the results obtained irradiating tensile specimens of Eurofer 97 up to 5.9 dpa in lead lithium. Tensile tests of samples have been performed out of pile in the same alloy at the same temperature at which irradiation was carried out.     

Investigation by slow positron beam of defects in CLAM steel induced by helium and hydrogen implantation

Hydrogen and helium ion beams delivering different doses are used in the ion implantation, at room temperature, of China Low Activation Martensitic (CLAM) steel and the induced defects studied by Doppler broadening of gamma-rays generated in positron annihilation. Defect profiles are analysed in terms of conventional S and W parameters, measures of relative contributions of low and high-momentum electrons in the annihilation peak, as functions of incident positron energies E up to 30 keV. The behaviours of the S-E, W-E and S-W plots under different implantation doses indicate clearly that the induced defect size has obvious variation with depth, taking values that interpolate between surface and bulk values, and depend mainly on helium ion fluences. The S-W plot indicates that two types of defects have formed after ion implantation.     

A polycrystalline modeling of the mechanical behavior of neutron irradiated zirconium alloys

Zirconium alloys used as fuel cladding tubes in the nuclear industry undergo important changes after neutron irradiation in the microstructure as well as in the mechanical properties. However, the effects of the specific post-irradiation deformation mechanisms on the mechanical behavior are not clearly understood and modeled. Based on experimental results it is discussed that the kinematic strain hardening is increased by the plastic strain localization inside the dislocation channels as well as the only basal slip activation observed for specific mechanical tests. From this analysis, the first polycrystalline model is developed for irradiated zirconium alloys, taking into account the irradiation induced hardening, the intra-granular softening as well as the intra-granular kinematic strain hardening due to the plastic strain localization inside the channels. This physically based model reproduces the mechanical behavior in agreement with the slip systems observed. In addition, this model reproduces the Bauschinger effect observed during low cycle fatigue as well as the cyclic strain softening.     

Effects of energetic heavy ion irradiation on the structure and magnetic properties of FeRh thin films

Fe-54at.%Rh thin films were irradiated with 10 MeV iodine ions at room temperature. Before and after the irradiations, the changes in magnetic properties and the lattice structure of the samples were studied by means of a SQUID magnetometer and X-ray diffraction. For the low fluence irradiation, the SQUID measurement at 20 K shows that the anti-ferromagnetic region of the thin film is changed into ferromagnetic region by the irradiation. As the film thickness is much smaller than the ion range, we can discuss the relationship between the density of energy deposited by ions and the change in magnetization quantitatively. For the high fluence irradiation, the magnetization of the film is strongly decreased by the irradiation, which can be explained as due to the change in lattice structure from B2 into A1 structure by the irradiation.     

First-principles study of hydrogen in perfect tungsten crystal

Tungsten-based materials are used as the first wall materials in ITER. Hydrogen impurities were introduced via bombarding with the reaction plasma, which are important for the behavior and stability of the tungsten wall. Using the first-principles density functional theory and planewave pseudopotential technique, we have simulated the behaviors of hydrogen atoms inside the perfect tungsten bcc lattice. The binding energies for different interstitial sites were compared to determine the optimal trapping site for the hydrogen atom inside the tungsten lattice. The diffusion barriers for hydrogen atom between nearby trapping sites and the interaction between two interstitial hydrogen atoms were also calculated. The implication of our theoretical results on the hydrogen diffusion and accumulation behavior was discussed.     

A kinetic Monte Carlo approach for the analysis of trapping effect on the defect accumulation in neutron-irradiated Fe

The trapping effect of self-interstitial atom (SIA) clusters in neuron-irradiated Fe was analyzed in terms of generic traps. The effect of the cut-off size between sessile and glissile SIA clusters was investigated. The accumulation of SIA clusters decreased drastically as the cut-off size increased, which originated from the elimination of the SIA clusters at a grain boundary through its one-dimensional motion. When the immobile generic traps were introduced to the kinetic Monte Carlo simulation model, the effect of trap parameters was assessed. An increase in the binding energy between the trap and SIA-species resulted in a decrease in the number of mono-SIAs that were dissociated from the trap and a corresponding delay in visible SIA clusters. The size-dependent prefactor for the dissociation rate of trapped SIA clusters was necessary for a realistic accumulation behavior of SIA clusters. The trap density affects the density and size of the accumulated SIA cluster density during irradiation. This parameterization of generic traps provided insight into the mechanism of accumulation of SIA and SIA cluster.     

Mechanisms of damage formation in semiconductors

The damage accumulation in ion-implanted semiconductors is analysed using Rutherford backscattering spectrometry (RBS). When energetic ions are implanted in a material, they transfer their energy mainly into atomic collision processes (nuclear energy loss) and in electronic excitations (electronic energy loss). For a given material this primary energy deposition is determined by the mass and energy of the implanted ions and the ion fluence (number of ions per unit area). However, the damage concentration which is measured after implantation does not only depend on the primary energy deposition, but is strongly influenced by secondary effects like defect annealing and defect transformation. For the latter processes the target temperature and the ion flux (number of ions per unit area and time) play an important role. In this presentation the influence of the various parameters mentioned above on the damage accumulation is demonstrated for various materials. Simple empirical models are applied to get information about the processes occurring and to systematize the results for the various semiconductors.