Atom transport under irradiation is determined by the concentration of freely migrating defects, which depends on the dynamical equilibrium between production and annihilation rates. In order to determine effective values of both of these quantities for the case of ion irradiation, spatially resolved self-diffusion measurements were performed on single crystals of nickel which contained several thin tracer layers at different depths.

For fixed depth the radiation-enhanced diffusion coefficient (DK) was determined as function of displacement rate (K0) and fluence (Φ). The DK essentially representing the ratio of the rates of production and annihilation was found to be proportional to K0 for 800 K irradiation temperature and to K00.4for Ni and K00.4for Kr irradiation at 950 K. It is independent of Φ for 800 K and decreases with increasing Φ for 950 K.

As part of an on-going research program, findings are presented from a comparison of the microstructures and associated tensile properties of fcc and bcc materials after high energy proton irradiation, to fluences between 10−4 and 1 dpa, at 300-320 K. Results for this comparison between Cu, Pd, 304 and 316 stainless steel on one side and Fe and the F82H ferritic-martensitic low activation steel on the other are discussed, showing a strong difference in defect accumulation behaviour between the differing crystal structures. The overall deformation behaviour is similar, with an initial localised deformation taking place in all cases, even though the actual deformation mode itself might be different. Furthermore, a comparison is made with some of the materials that have also been irradiated with fission neutrons, showing no influence of the PKA spectra for these irradiation conditions.

Pure copper and Fe-15Cr-2ONi alloy were irradiated at 500°C by 5 MeV Ni ions (single beam), Ni ions plus gas atoms (H and He) (dual beam irradiation) or triple beam of Ni, He and H ions (triple beam irradiation) simultaneously. Irradiation was carried out with accelerators TIARA at Takasaki-establishment of JAERI. Thin specimens were prepared with the combined technique of FIB (Focus Ion Beam) and electro-polishing. Damage structure versus the depth from surface was examined by TEM. In Fe-15Cr-20Ni alloy, voids were formed in the range of Ni ion irradiated specimens. In Fe-Cr-Ni alloy which is irradiated by a dual beam of Ni ion and He ion, the number density of voids increased significantly at the stopping range of He atoms. Remarkable results are void formation in specimens irradiated with dual beam of Ni and H ions, only voids of very small size were observed through the range of specimen. In irradiated specimen with triple beam, the size of voids was suppressed to be smaller than that of irradiated specimens with Ni and He ions. In pure copper which is irradiated with dual beam of helium and Ni ions, number density of voids were increased significantly. While in copper irradiated with hydrogen and Ni ions, number density of voids were not so large. Experimental results show that helium atoms have strong effect for void formation, while hydrogen atoms suppress void formation because the remarkable degradation of movement of small vacancy clusters can be induced by trapping hydrogen atoms.

Single crystalline specimens of pure Pd have been irradiated at ambient temperature with 590 MeV protons to doses ranging between 10−4 and 10−1 dpa. Tensile deformation experiments revealed that irradiation induces hardening and embrittlement, while scanning (SEM) and transmission electron microscopy (TEM) observations showed that plastic deformation of specimens irradiated to a dose ≥ 10−2 dpa is strongly localized and yields the creation of slip bands at the macroscopic scale and of defect-free channels at the microscopic level.

The Spallation Neutron Source (SNS), currently under construction at Oak Ridge National Laboratory, is expected to employ a Hg target encased in a stainless steel. Little is known about the metallurgical behavior of this materials engineering system, which will occur in a service environment involving elevated temperatures and intense radiation. Under normal equilibrium conditions, however, Hg is known to be insoluble in and non-reactive with solid Fe and Cr but to form one or more intermetallics with Ni. Hg has been implanted into alloy 304L. After implantations at 400 and 500 °C to a fluence of 3×1016 cm−2 sub-micron sized precipitates of Hg are formed, as judged, for example, from their solidification behavior on cooling during TEM observation. The formation of such a system of microtargets and possible studies employing them as in situ TEM specimens are discussed, which can provide useful empirical information in conjunction with SNS target development.

Effect of electron irradiation on substitutional ordering was studied in Fe-36.7 wt.% Ni invartype alloy and Fe-25.2 wt.% Cr-20.2 wt.% Ni austenitic stainless steel. It was shown that the fine structure of the hydrogen Snoek-type peak in invar changes after irradiation to a fluence of 1019 e−.cm−2 with electrons with an energy of 3 MeV as compared to the solution treated condition. Irradiation affects the temperature behavior of the shear modulus of invar. No significant effect of irradiation on hydrogen Snoek-type peaks was detected in stainless steel.

Fracture toughness was found to decrease rapidly with increasing temperature in dispersionstrengthened GlidCop®Al-25 copper alloy both in the as-supplied condition and neutron irradiated to a dose of 0.3 dpa. Internal friction study revealed two-component peak. Grain-boundary sliding was recognized to be responsible for the low-temperature component of the peak, which disappears after irradiation and restores after the heating above 900 K. This points out that the changes in the particle — grain boundary interaction, apparently, due to the defects at the interfaces produced by irradiation are responsible for the drop of fracture toughness in A125 alloy.

Ion beam mixing of Cu/Mo multilayer samples and demixing of Cu-Mo alloy samples were examined as functions of temperature and ion mass. Even at liquid nitrogen temperature, the mixing of one component in other was very small under 1.0 MeV Kr irradiation. The maximum solubility was about 10%. Irradiation of homogeneous Cu-Mo alloys under the same conditions, moreover, led to phase separation. These conclusions were deduced from bulk X-ray diffraction measurements and corroborated by EXAFS examination. The maximum solubility obtained by irradiating multilayer samples occurred at about 600 K. At lower or higher temperature, the solubility of Cu in Mo-rich phase was lower. A model based on the ballistic difflusion explains these results.

Helium effect on loss of uniform elongation after low-temperature neutron irradiation of the V-Ti-Cr-Si-Al-Y alloy was studied. Helium implantation to about 30 atomic ppm was carried out before neutron irradiation to 50 dpa at 406°C. The yield stress of the irradiated specimen with helium pre-implantation was slightly smaller than that of irradiated specimens without helium. The uniform elongation and the increase in yield stress of the irradiated specimens were not affected by helium pre-implantation at ambient temperature. It might be possible that the helium effect appears after lower temperature irradiation such as 300°C or lower, which corresponds to the temperature range where the loss of uniform elongation of the alloy is particularly pronounced.

The microstructure of the low activation F82H ferritic/martensitic steel, which is a candidate for future fusion reactor first wall, has been studied in order to determine the differences between 590 MeV proton irradiations and fission neutron irradiations. A range of doses and temperatures are investigated. The present work focuses on the microstructure of irradiation induced defect clusters for cases of proton irradiations to doses up to 1.7 dpa, and for neutron irradiation to a single dose of 2.5 dpa. The irradiation temperatures were 40°C and 250°C in the case of the proton irradiation and 250°C in the case of the neutron irradiation. In the case of the proton irradiation no defects are observed in the transmission electron microscope in the case of the 0.45 dpa dose, while they start to be clearly visible for doses higher than about 1.0 dpa. In the case of the neutron irradiation a high density of defects and small loops is evidenced. These preliminary results are presented here.

Single variable experiments are being conducted to study effects of H/He/dpa on properties based on isotopically tailored alloys. 54Fe has been used to prepare an isotopically tailored duplicate of the commercial steel F82H, and a small number of TEM disks have been irradiated in order to study radiation embrittlement. From single disk specimens, mechanical properties were obtained using a shear punch technique that produces a 1 mm blank from the 3 mm disk, and microstructural information was obtained from the 1 mm blanks thinned to electron transparency.

Scanning electron photomicrographs of U–10 wt.% Mo irradiated at low temperature in the Advanced Test Reactor (ATR) to about 40 at.% burnup show the presence of cavities. We have used a rate-theory-based model to investigate the nucleation and growth of cavities during low-temperature irradiation of uranium-molybdenum alloys in the presence of irradiation-induced interstitial-loop formation and growth. Our calculations indicate that the swelling mechanism in the U–10 wt.% Mo alloy at low irradiation temperatures is fission-gas driven. The calculations also indicate that the observed bubbles must be associated with a subgrain structure. Calculated bubble-size-distributions are compared with irradiation data.

Recent research using molecular dynamics to simulate radiation damage due to displacement cascades in metals is reviewed. It includes results dealing with the effect on defect formation of primary knock-on atom energy and irradiation temperature. Clear dependencies and trends have emerged in these areas. In terms of the development of models to describe the evolution of radiation damage microstructure, the important parameters are not only the total number of Frenkel defects but also the distribution of their population in clusters and the form and mobility of these clusters. Results on these aspects are reviewed and it is shown that computer simulation is providing detailed information that paves the way for successful development of models of the evolution of damage beyond the stage of the cascade process.

Radiation damage formation in iron has been investigated using the method of molecular dynamics simulation. The MD simulations have been used to determine primary defect production parameters for cascade energies up to 50 keV at temperatures from 100 to 900K. The energy dependence of these parameters has been used to determine appropriate neutron-energy- spectrum averaged damage production cross sections for various irradiation environments. Two applications of these effective cross sections are discussed. The first is an evaluation of neutron energy spectrum effects in commercial fission reactor pressure vessels. The second example deals with the use of these cross sections in the source term of a kinetic model used to predict void swelling and microstructural evolution. The simulation of the primary damage event by MD involves times less than 100 ps and a size scale of a few tens of nm, while the kinetic simulation encompasses several years and macroscopic sizes. This use of the MD results to develop an improved source term for rate theory modeling provides a simple example of multiscale modeling.

Neutron hardening and embrittlement of pressure vessel steels is due to a high density of nm scale features, including copper-manganese-nickel rich precipitates and what are generally believed to be defect cluster-solute complexes. It has been postulated that the sub nanometer defect cluster-solute complexes form directly in displacement cascades. Cluster-complexes that are thermally unstable mediate the effect of flux on embrittlement kinetics. Larger cluster-complexes, that are relatively thermally stable for irradiation times up to 1 Gs, cause embrittlement in low copper steels. Robust characterization of these two types of so-called matrix defects has been an elusive goal. In this work, Kinetic Lattice Monte Carlo (KLMC) simulations of the long term evolution of the vacancy-rich cascade core regions were carried out for both pure iron and dilute iron-copper alloys at the nominal irradiation temperature of 563°K up to times when the vacancy clusters completely dissolve. Energetics were based on lattice embedded atom method potentials. Special time scaling and pulse annealing techniques were used to deal with the enormous range of inherent time scales involved, viz., rapid free vacancy jumps to slow emission from large complexes. Three-dimensional clusters rapidly form, containing a wide range of vacancies, as well as copper atoms in alloys. Small complexes are very mobile and growth takes place primarily by coalescence. The vacancy clusters ultimately dissolve at times from less than 0.1 to more than 100 MS. These simulations support the hypotheses that cascade cluster- complexes constitute both thermally stable and unstable matrix defect features.

The steel vessels of pressurized water reactors are embrittled by neutron irradiation. It is well known that copper atoms play an important role in the embrittlement and that different Cu- containing defects such as Cu-rich clusters (sometimes called atmospheres), Cu precipitates and Cu-vacancy complexes have been identified experimentally. It is still difficult to link the formation of these defects to the primary damage resulting from the neutron inducing displacement cascades. Therefore, we investigate the evolution of the primary damage in FeCu alloys using kinetic Monte Carlo simulations based on a vacancy diffusion mechanism. The calculations rely on adapted, phenomenological, n-body potentials that satisfactorily reproduce properties of FeCu. At room temperature, experimentally identified defects such as Cu-vacancy complexes (one Cu atom bound to three or four vacancies) form in the course of our simulations. Furthermore, it appears that complex defects such as a Cu atom linked to two vacancies are very mobile and are responsible for the formation of small Cu clusters.

The structure, stability and thermally-activated motion of interstitial and vacancy clusters in Fe and Cu have been studied using atomic scale computer simulation. All studied interstitial clusters and perfect interstitial loops (PILs) in Fe are mobile whereas their mobility in Cu can be suppressed at large sizes (bigger than 49–61 self-interstitials depending on the temperature) due to dissociation. A comparative study of relaxed configurations has shown that the structure of small perfect dislocation loops of vacancy and self-interstitial nature is very similar. Molecular dynamics simulation has demonstrated that small perfect vacancy loops (PVLs) in Fe consisting of more than 37 vacancies are stable over a wide temperature range and produce atomic displacements by a thermally-activated movement in the direction of the Burgers vector. The mechanism is qualitatively similar to that of SIA clusters studied earlier. Motion of vacancy loops in Cu does not occur because they transform into sessile configurations similar to stacking fault tetrahedra. These results point to the possibly important contribution of vacancy loop mobility to the difference in radiation damage between bcc and fcc metals, and between fcc metals with different stacking fault energy.

Phase stability in alloys under irradiation is studied considering effective thermodynamic potentials. A simple kinetic model of a binary alloy with phase separation is investigated. Time evolution in the alloy results from two competing dynamics: thermal diffusion, and irradiation induced ballistic exchanges. The dynamical (steady state) phase diagram is evaluated exactly performing Kinetic Monte Carlo simulations. The solution is then com- pared to two theoretical frameworks: the effective quasi-interactions model as proposed by Vaks and Kamishenko, and the effective free energy model as proposed by Martin. New developments of these models are proposed to allow for quantitative comparisons. Both theoretical frameworks yield fairly good approximations to the dynamical phase diagram.

Molecular dynamics (MD) simulations are used to obtain detailed information on defect production and disordering produced in the primary cascade state of radiation damage in Ni3AI as a function of irradiation temperature, Tirr Although the number of Frenkel pairs decreases with increasing Tirr, the size of interstitial clusters and the interstitial clustering fraction increase with Tirr. The number of antisites defects per cascade increases by about 20% as Tir increases from 100 to 600K, and then by about 90% from 600 to 900K due to the increase in the intensity and lifetime of the thermal spike. The average size of disordered zones at different Tir, is compared with experimental transmission electron microscopy data and the results are in reasonable agreement. The long-range order parameter in the cascade region is found to be consistent with values obtained experimentally on Ni3A1 irradiated by ions to a similar dose. The chemical short-range order parameter is about 0.6 over the whole range of Tirr, considered, suggesting that the cascade core retains some short-range order at high temperature due to local reordering.