Energetic Displacement Cascades Research Articles

Quantifying defect production in solids at finite temperatures: Thermally-activated correlated defect recombination corrections to DPA (CRC-DPA)

It has long been recognized from low-temperature defect production experiments and molecular dynamics (MD) simulations of displacement events in metals that the surviving defect fraction relative to the calculated Norgett-Robinson Torrens (NRT) displacements per atom (dpa) ranges from ∼1 for near-threshold displacements (representative of electron irradiation) to ∼0.3 for energetic displacement cascades (relevant for fast neutrons or energetic heavy ion irradiations). This correlated point defect recombination that occurs within a few picoseconds following the creation of a primary knock-on atom (PKA) has recently been formalized in the athermal recombination corrected dpa (ARC-DPA) parameter. However, at elevated temperatures additional thermally-stimulated correlated defect recombination processes need to be considered. For example, at irradiation temperatures above defect recovery Stage I (typically ∼40–100 K for most metals), mobile self-interstitial atoms induce additional thermally-activated correlated defect recombination that can amount to 70–80% reduction in the surviving defect fraction for low PKA energies and ∼20 to 30% reduction for high PKA energies. Since most nuclear material applications involve operation above room temperature, it is generally important to include this additional correlated defect recombination in radiation effects models. Published experimental defect recovery and defect production studies on 26 metals are analyzed to extract the surviving defect fraction following thermally-activated correlated recombination as a function of PKA energy. These experimental estimates of the athermal- plus thermal-activated correlated recombination-corrected DPA (CRC-DPA) are compared with new long-term MD simulation results on Cu and Fe. The analysis indicates the CRC-DPA fraction of surviving defects in most elements for irradiation at temperatures where interstitials are mobile is ∼25% of the NRT DPA over a wide range of PKA energies relevant for electron, ion and fast neutron irradiation conditions. Limited experimental and kinetic Monte Carlo studies suggest a further reduction to ∼10% of the NRT DPA over a wide range of PKA energies when correlated recombination due to thermally activated vacancy migration is considered. This temperature-dependent CRC-DPA should be used as the defect production parameter (rather than NRT DPA or ARC-DPA) for materials irradiated at all temperatures where defects are mobile.

Role of interface on irradiation damage of Cu−diamond composites using classical molecular dynamics simulations

Cu−diamond composites have been proposed as a candidate thermal management material for spacecraft electronics. Nevertheless, irradiation effects on the composites remain poorly understood at present. Here we focus on investigating the influence of Cu−diamond interfaces (CDIs) on energetic displacement cascades using atomistic simulations. Results show that a primary knock-on atom of Cu (PKA-Cu) can induce more significant damage than a PKA-C. Under almost all circumstances, the statistically averaged fraction of surviving interstitials is not only lower than that of vacancies but also no more than 1. Because of the unique nature in the mobility and interactions with CDIs, Cu interstitials exhibit the lowest concentration among all defects in most cases. The high residual rate of displaced defects in diamond makes it relatively difficult to heal. The structural damage is mainly manifested in a short-range disorder of diamond and a long-range disorder of Cu after irradiation. At elevated temperatures, the atomic displacement region may form compact chain-like defects to restrain lattice loosening. Despite the above, CDIs could act as effective sinks to facilitate the recombination and/or annihilation of irradiation-induced defects in all scenarios. This study provides an important insight into the understanding of the microscopic evolution of irradiation defects for the composites.

Atomistic simulation of energetic displacement cascades near an Ni–graphene interface

Ni–graphene nanocomposites have greatly promising applications in supercritical water-cooled reactors due to their outstanding performances. However, many fundamental mechanisms of irradiation behaviors in the composites are still unclear. Here we investigate the displacement cascades near an Ni–graphene interface using atomistic simulations. Different cascade energies and ambient temperatures were introduced to bring out the effect of irradiation. The increasing cascade energy in the interval 0.5–10 keV could significantly promote the formation of defects, while there is slight effect on the number of surviving defects in Ni matrix when adjusting the ambient temperature from 100 K to 900 K. The damaged graphene could be self-healing by the synergistic effects of irradiation annealing and ambient temperature annealing. The interface exhibits high sink efficiency for irradiation defects in all cases. This study provides an important insight into the understanding of the microscopic evolution of defects in cascades for the composites.

Precipitation kinetics of dilute Cu-W alloys during low-temperature ion irradiation

The kinetics of W precipitation in dilute Cu-W alloys during room temperature irradiation is investigated using in situ electrical resistivity measurements and transmission electron microscopy. For a series of alloys with W concentrations varying from ∼1 at.% to 6 at.%, resistivity measurements show that high dose irradiation leads to steady-state solubility values which are concentration dependent, while electron microscopy shows that the precipitate structures are stabilized at high doses at a size of about 2 nm. These steady states are independent of the initial alloy microstructure: whether it is a solid solution or it contains large W precipitates within the Cu matrix. The effective tracer impurity diffusion coefficient of W in Cu in energetic displacement cascades is determined by in situ electrical resistivity measurements on multilayer structures of alternating Cu/W layers, yielding a value of 2.1 nm2/dpa. These multilayer structures are observed to undergo significant interfacial roughening during irradiation, showing signs of transforming from a 2-dimensional to 3-dimensional structure under prolonged irradiation. A model based on a dynamical competition between recoil mixing and thermal spike diffusion is proposed to explain these various results; it is implemented in kinetic Monte Carlo simulations.

Atomic Scale Modelling of the Primary Damage State of Irradiated UO2 Matrix

AbstractLarge scale classical molecular dynamics simulations have been carried out to study the primary damage state due to a-decay self irradiation in UO2 matrix. Simulations of energetic displacement cascades up to the realistic energy of the recoil nucleus at 80 keV provide new informations on defect production, their spatial distribution and their clustering. The discrepancy with the classical linear theory NRT (Norton-Robinson-Torrens) law on the creation of the number of point defects is discussed. Study of cascade overlap sequence shows a saturation of the number of point defects created as the dose increases. Toward the end of the overlap sequence, large stable clusters of vacancies are observed. The values of athermal diffusion coefficients coming from the ballistic collisions and the additional point defects created during the cascades are estimated from these simulations to be, in all the cases, less than 10-26 m2/s. Finally, the influence of a grain boundary of type Sigma5 is analysed. It has been found that the energy of the cascades are dissipated along the interface and that most of the point defects are created at the grain boundary.

Annihilation of freely migrating defects by cascade remnants in Cu-1% Au alloys

The effects of cascade remnants on freely migrating defects (FMD) were studied by measuring radiation-induced segregation in Cu-1%Au at 400°C during simultaneous irradiation with 1.5 MeV He and (400–800) keV heavy ions (Ne, Ar or Cu). Radiation induced segregation during single-ion irradiation with He, Ne, Ar and Cu, and the effects of pre-irradiation with the same ions, were also investigated. The large segregation observed during 1.5 MeV He-only irradiation was dramatically suppressed under simultaneous Cu irradiation. This suppression disappeared immediately after the Cu irradiation ceased, indicating that it was caused by a transient population of cascade remnants, i.e., vacancy and/or interstitial clusters. For simultaneous inert-gas (He and Ne or Ar) irradiation, a similar suppression of the radiation induced segregation was observed. However, in contrast to the Cu-irradiation results, the suppression persisted after the Ne or Ar beam was turned off. The present results demonstrate that the energetic displacement cascades created by heavy ions introduce additional point-defect annihilation sites, which reduce the steady-state concentration of FMD. This finding implies that recombination dominates defect annihilation under the present irradiation conditions, which is indeed what is observed experimentally. As the cascade remnants produced by the Cu ions are thermally unstable at 400°C, the suppression of radiation induced segregation occurs only during simultaneous irradiation. During Ne and Ar irradiation, the inert gas atoms which accumulate in the specimen apparently stabilize the cascade remnants, allowing the suppression to persist during subsequent He-only irradiation.

Molecular Dynamics Simulations of displacement cascades in metallic systems

We use Molecular Dynamics Computer Simulations to investigate defect production induced by energetic displacement cascades up to 10 keV in pure metals (Cu, Ni) and in ordered intermetallic alloys NiAl, Ni 3 Al. Various model potentials were employed to describe the many-body nature of the interactions: the RGL (Rosato-Guillope-Legrand) model was used in pure Cu and Ni simulations; the modified version of the Vitek, Ackland and Cserti potentials (due to Gao, Bacon and Ackland) in Ni 3 Al and the EAM potentials of Foiles and Daw modified by Rubini and Ballone in NiAl, Ni 3 Al were used in alloy simulations. Atomic mixing and disordering were studied into details owing to imaging techniques and determined at different phases of the cascades. Some mixing mechanisms were identified. Our results were compared with existing data and those obtained by similar Molecular Dynamics Simulations available in the literature.

Molecular dynamics simulations of displacement cascades in metallic systems

We use Molecular Dynamics Computer Simulations to investigate defect production induced by energetic displacement cascades up to 10 keV in pure metals (Cu, Ni) and in ordered intermetallic alloys NiAl, Ni 3Al. Various model potentials were employed to describe the many-body nature of the interactions: the RGL (Rosato-Guillope-Legrand) model was used in pure Cu and Ni simulations; the modified version of the Vitek, Ackland and Cserti potentials (due to Gao, Bacon and Ackland) in Ni 3Al and the EAM potentials of Foiles and Daw modified by Rubini and Ballone in NiAl, Ni 3Al were used in alloy simulations. Atomic mixing and disordering were studied into details owing to imaging techniques and determined at different phases of the cascades. Some mixing mechanisms were identified. Our results were compared with existing data and those obtained by similar Molecular Dynamics Simulations available in the literature.

Intercascade annihilation of freely migrating defects

To characterize putative interactions between freely migrating defects (FMD) and remnants of energetic displacement cascades, radiation-induced segregation (RIS) in Cu-1 at. % Au was measured by Rutherford backscattering during separate and simultaneous irradiation at 400 °C with 1.5 MeV He and 800 keV Cu ions. The strong RIS observed during only He irradiation was greatly reduced under simultaneous Cu irradiation at approximately the same displacements per atom rate; increasing the Cu flux by a factor of 5 suppressed the RIS from the He beam almost completely. The suppression of RIS at 400 °C disappeared quickly when the Cu irradiation ceased. These results demonstrate that a transient population of interstitial and/or vacancy clusters from the Cu irradiation greatly reduces the survival rate of FMD produced by the He.

In-Situ Hvem Studies of Radiation-Induced Segregation In Ni-Al Alloys During Simultaneous Irradiation with Electrons and Ions

AbstractThe effects of 75-keV Ne+ and 300-keV Ni+ bombardment on electron radiation-induced segregation (RIS) in a Ni-9at%Al alloy were investigated in-situ using the HVEM (high voltage electron microscope)/Tandem accelerator facility at Argonne National Laboratory. The radial component of defect fluxes generated by a highly-focused 900-keV electron beam was used to induce segregation of Al atoms towards the center of the electron irradiated area via the inverse-Kirkendall effect. The radial segregation rate was monitored by measuring the increase in the diameter of the Al enriched zone within which γ'-νi3αl precipitates form during irradiation. Both dual electron-ion and pre-implanted ion-electron irradiations were performed in an attempt to separate the contributions of energetic displacement cascades and implanted ions acting as defect trapping sites to RIS suppression. It was found that 75-keV Ne implantation has a retarding effect on RIS.

Radiation-induced disordering and defect production in Cu 3Au and Ni 3Al studied by molecular dynamics simulation

We use molecular dynamics computer simulations to investigate the kinetics of disordering induced by energetic displacement cascades in two ordered intermetallics, Cu 3Au and Ni 3Al. We also study the mechanisms of defect production in these systems and compare the results to recent findings in pure metals. We employ embedded atom potentials which reproduce the main features of the phase diagram, including a disordering transition below the melting point ( T m) in Cu 3Au and an ordered compound up to T m in Ni 3Al. We simulate events with 5 keV recoil energy in crystals containing up to 157216 atoms at 10 K. We present results of the influence of the displacement cascade on the order parameters and describe their temporal and spatial evolution. We discuss atomic mixing in the cascade regions in terms of the atomic mobilities in each sublattice. The influence of the ordering energy and the melting temperature in the behavior of the compounds is emphasized.

Kinetics of radiation-induced disordering of A3B intermetallic compounds: A molecular-dynamics-simulation study.

Intermetallic compounds are susceptible to disordering when exposed to irradiation with energetic particles. However, the atomistic mechanisms of this process are not fully understood. We present molecular-dynamics studies of the kinetics of disordering induced by 5-keV energetic displacement cascades in ${\mathrm{Cu}}_{3}$Au and ${\mathrm{Ni}}_{3}$Al. Our results show that the underlying crystalline structure is lost in the local region of the cascade core. However, kinetic constraints imposed by the fast quenching rate of the cascade region prevent a complete loss of chemical short-range order from occurring. After the cascade energy dissipates, the Bragg-Williams long-range-order parameter in the central region is found to have a value near zero. The shape of the interface between the cascade-induced disordered region and the surrounding ordered lattice is shown to depend on the ordering energy of the irradiated compound.

Defect production efficiency in Fe 60Co 40 thin films irradiated with swift heavy ions

The damage production in Fe 60Co 40 thin films has been studied at liquid nitrogen temperature during low energy (15–360 keV He +, Ne +, Ar +, Xe +) and high energy ion irradiations (445 MeV Ag ions, 186 MeV and 40 MeV N ions). The irradiated films are polycrystalline with a very small grain size of 5–8 nm are in an initially chemically disordered state. Elastic collisions occurring during low temperature ion irradiations induce displacement cascades and this Frenkel pair creation results in an electrical resistivity increase of the alloy. In situ electrical resistivity measurements have allowed the experiment determination of the damage production. It is shown that the radiation damage produced by low energy ions and 40–186 MeV N ions are well explained by elastic collisions effects only. Moreover, a reduced defect production efficiency is observed in the case of energetic displacement cascades in agreement with previous experimental results obtained in thin pure metals and also with recent molecular dynamic computer simulations. On the other hand, an important defect production efficiency increase is observed in the case of a 445 MeV Ag ion irradiation where the slowing down of the ions occurs mainly via inelastic collisions. This result is discussed by comparison with previous experimental data demonstrating that an additional contribution to the classical defect production by elastic collisions exists in some metals above a given electronic stopping power threshold.

Molecular dynamics studies of defect production and clustering in energetic displacement cascades in copper

We review recent molecular dynamics studies of displacement cascades in copper at energies from near threshold to 25 keV where nascent subcascades develop. We discuss the structure of the cascade region and present results on the dynamical evolution and lifetime of the cascades. We show that the defect production efficiency in cascades follows a downward trend with energy, in good quantitative agreement with experiments. The cluster size distributions from the simulations show a strong recoil energy dependence. At low energy, loosely correlated vacancy distributions are generally observed. However, as the energy and hence the lifetime of the cascade increase, large vacancy clusters appear. Evidence is presented for the athermal collapse of a 25 keV cascade to a vacancy dislocation loop. Interstitial production mechanisms are identified. At all energies studied replacement collision sequences are observed. Moreover, vacancy-interstitial separation mechanisms based on ballistic clustering and dislocation loop punching from cascades are presented.

Molecular dynamics studies of defect production and clustering in energetic displacement cascades in copper

Molecular dynamics studies of defect production and clustering in energetic displacement cascades in copper

Electron-phonon coupling in dense collision cascades

Abstract The rate of the energy exchange between electrons and ions in the liquid-like dense collision cascade is derived. It is shown that only the interaction with the collective modes of ion motion may cause the energy dissipation to the electrons and affect the cooling of the energetic displacement cascades. The interaction of valence electrons with collective modes leads to a similar rate of the energy exchange as is obtained for metals at elevated temperatures. This result is reached without the need to postulate the existence of phonons and electron-phonon coupling in thermal spikes.

Interactions of energetic particles and clusters with solids

Ion beams are being applied for surface modifications of materials in a variety of different ways: ion implantation, ion beam mixing, sputtering, and particle or cluster beam assisted deposition. Fundamental to all of these processes is the deposition of a large amount of energy, generally some keV, in a localized area. This can lead to the production of defects, atomic mixing, disordering and in some cases, amorphization. Recent results of molecular dynamics computer simulations of energetic displacement cascades in Cu and Ni with energies up to 5 keV suggest that thermal spikes play an important role in these processes. Specifically, it will be shown that many aspects of defect production, atomic mixing and cascade collapse can be understood as a consequence of local melting of the cascade core. Included in this discussion will be the possible role of electron-phonon coupling in thermal spike dynamics. The interaction of energetic clusters of atoms with solid surfaces has also been studied by molecular dynamics simulations. This process is of interest because a large amount of energy can be deposited in a small region and possibly without creating point defects in the substrate or implanting cluster atoms. The simulations reveal that the dynamics of the collision process are strongly dependent on cluster size and energy. Different regimes where defect production, local melting and plastic flow dominate will be discussed.

The effect of electronic energy loss on the dynamics of thermal spikes in Cu

We present results of a molecular dynamics simulation study of the effect of electron-ion interactions on the dynamics of the thermal spike in Cu. Interatomic forces are described with a modified embedded atom method potential. We show that the electron-ion interaction acts to reduce the lifetime of the thermal spike and therefore the amount of atomic rearrangement that takes place in energetic displacement cascades in Cu. The results point toward the important effect that inelastic energy losses might have on the dynamics of displacement cascades in the subcascade energy regime where the lifetime of the thermal spike is expected to exceed the electron-phonon coupling time.

Effect of temperature on the dynamics of energetic displacement cascades: A molecular dynamics study.

Molecular dynamics simulations of 3-keV displacement cascades in Cu at temperatures of 0, 300, 500, and 700 K have been performed using the Gibson II potential. Most atomic displacements occur in the quasimolten core region that develops at {ital t}{similar to}0.5 ps and persists for several ps. The size and lifetime of the molten regions increase with ambient temperature, which results in a fourfold increase in atomic mixing between 0 to 700 K and a decrease in defect production. The instantaneous diffusion coefficients in the cascade melt are in close agreement with those in the equilibrium liquid.

Molecular dynamics simulation of displacement cascades in Cu and Ni: Thermal spike behavior

Molecular dynamics simulations of energetic displacement cascades in Cu and Ni were performed with primary-knock-on-atom (PKA) energies up to 5 keV. The interatomic forces were represented by the Gibson II (Cu) and the Johnson-Erginsoy (Ni) potentials. Our results indicate that the primary state of damage produced by displacement cascades is controlled basically by two phenomena: replacement collision sequences during the ballistic phase, and melting and resolidification during the thermal spike. The thermal-spike phase is of longer duration and has a more marked effect in Cu than in Ni. Results for atomic mixing, defect production, and defect clustering are presented and compared with experiment. Simulations of “heat spikes” in these metals suggest a model for “cascade collapse” based on the regrowth kinetics of the molten cascade core.