MD simulations have been employed to investigate damage processes during keV bombardment of metal targets. For self-ion irradiations of Au, Cu, and Pt in the range of 5-20 keV, we have found that both the amount and the character of the damage created in the surface depends sensitively on the details of the energy deposition along individual ion trajectories. In all of these cases, significantly more damage is produced and more atomic mixing takes place relative to corresponding recoil events in the crystal interior. In some cases, enormous craters are formed in an explosive event, while in others a convective flow of atoms to the surface leaves dislocations behind. The results of these simulations will be summarized and their significance for damage studies of ion irradiated materials, discussed.

MD simulations of displacement cascades in a variety of metals of different crystal structure and an ordered alloy are discussed. Frenkel-pair production at the end of the cascade process is well below the NRT theoretical value in all cases and a new empirical relationship between Frenkel-pair number and damage energy is assessed. In contrast with this, antisite production efficiency in ordered alloys increases with increasing energy, as does the atomic mixing of the different atomic species. The degree of clustering of interstitials in cascades is materialdependent. These results are discussed in relation to the highly disordered zone formed in the thermal spike.

A series of high-energy, up to 20 keV, displacement cascades in iron have been investigated for times up to 200 ps at 100 K using the method of molecular dynamics simulation. Thesimulations were carried out using the MOLDY code and a modified version of the many-bodyinteratomic potential developed by Finnis and Sinclair. The paper focuses on those results obtained at the highest energies, 10 and 20 keV. The results indicate that the fraction of the Frenkel pairs surviving in-cascade recombination remains fairly high in iron and that the fraction of the surviving point defects that cluster is lower than in materials such as copper. In particular, vacancy clustering appears to be inhibited in iron. Some of the interstitial clusters were observed to exhibit an unexpectedly complex, three-dimensional morphology. The observations are discussed in terms of their relevance to microstructural evolution and mechanical property changes in irradiated iron-based alloys.

An atomistic model of phase formation in ion bombarded thin binary metallic films is discussed. Compositional changes occur at the interface between collision cascades and crystal matrix. Relaxation arises via elementary charge transfer reactions, with formation of dimers of an effective compound. Comparing surface and thermochemical properties of initial and effective alloys, a set of conditions specific to vitrification and respectively tocrystal formation under ion bombardment is obtained.

Molecular dynamics calculations have been used to study the ion-etching process by a focused-ion beam. Ion-induced atomic displacement in a Ag surface initiates collision cascades which lead to particle sputtering from the surface. Tracing collision cascades reveals that the sputtering is mainly caused by a surface atom being sideswiped by its neighboring atom traveling in a surface channel oriented a certain angle away from the direction of ejection. Depending on the angle of detection and on the kinetic energy of sputtering, the ejecting particles may proceed along crystallographic directions parallel to either the close-packed or the non-close-packed rows of atoms in the surface. The preferred direction of ejection is sensitive to variations in the polar angle of detection and in the ejection energy. For those residual atoms initially located within four lattice spacings from the target atom, the most probable distance of displacement is less than one eighth of the equilibrium bond length in the crystal bombarded by Ar† ions of I keV energy. The structural perturbation increases as the ion energy is increased from 0.2 keV to I keV, but decreases as the energy is increased further.

The thermal spike phase of a displacement cascade is thought to be of fundamental importance to a number of issues in radiation effects. For example, the formation of stable vacancy clusters and the efficiency of ion mixing are thought to be directly controlled by the duration of the liquid-like state during the thermal spike. A number of factors have been proposed to be of significance to the lifetime of the thermal spike. For example, materials which have a high melting temperature, or which exhibit strong electron-phonon coupling, which provides an efficient method of energy transport to the surrounding material, are expected to have short lifetimes. We have examined these factors in detail and demonstrated that, in both elemental metals and alloys, electron-phonon coupling provides the most consistent explanation for variations in the thermal spike lifetime and, hence, the efficiency of ion mixing and the number of stable vacancy defect clusters formed.

I will review our experimental TEM data on the production of dislocation loops by low energy ion bombardment to low doses, as simulations of similar collision cascades produced by fast neutron irradiation, in various metals and alloys. The dependence of vacancy dislocation loop formation on recoil energy, sample temperature, and specific metal or alloy will be examined. Special emphasis will be placed on the effects of dilute alloy additions. A model for cascade melting will be employed to understand these effects, and will require an examination of the role of electron-phonon coupling in cascade cooling and recrystallization. The formation of interstitial dislocation loops as cascade defects, and the influence of the nearby surfaces in these experiments will be briefly discussed.

Iron alloys containing copper and nickel were irradiated at 288°C to a fluence of 4.63 × 1019 neutrons/cm2. Neutron irradiation produced defects which were observable by TEM in all of the iron alloys studied. The TEM analysis of the defects showed them to be interstitial dislocation loops with a < 100 > and a/2 < 111 > Burgers vectors. The size, the number density, and the Burgers vector of dislocations were affected by the alloy composition. The addition ofcopper and nickel decreased the dislocation loop size and increased the fraction of a/2 < 111 > loops. No voids or vacancy loops were observed in the irradiated iron alloys. The results are discussed in terms of dislocation loop nucleation and growth.

The damage (disordered zones and vacancy loops) produced in the ordered alloy Ni3AI by irradiation with different ions (Ar+, Kr+, Xe+), ion energies (30 and 50 keV), and at 30 and 300 K has been investigated by using transmission electron microscopy. At room temperature and at constant ion energy, the average size of the disordered zones increases as the ion mass increases from Ar+ to Kr+and to Xe+ ions. This trend is opposite to that of the collisional volume which decreases with increasing ion mass and highlights the importance of the thermal spike in causing disorder. At room temperature and at constant ion mass, the zone size increases with increasing ion energy. At 30K, for the same ion mass and ion energy, the size of the disordered zones is smaller than at room temperature. At both irradiation temperatures, the dislocation loop yield increases with increasing ion energy and ion mass. The loop yield was lower for irradiations at 30K compared to at 300 K. Comparison of the dislocation loop yield and the size of the disordered zones indicates that the probability for forming a loop increases with the size of the disordered zone.

Microstructural examinations have been performed on a series of binary Fe-Cr alloys irradiated to 200 dpa at 425°C in a fast breeder reactor. The alloy compositions rangedfrom 3% to 18% Cr in 3% Cr increments, and the irradiation temperature corresponded to the peak swelling condition for this alloy class. Density measurements showed swelling levels as high as 7.4%, with the highest swelling found in the Fe-9Cr and Fe-6Cr alloys. Microstructural examinations revealed that the highest swelling conditions contained welldeveloped voids, often as large as 100 nm, and a dislocation network comprised of both a2 <111> and <100> Burgers vectors. Swelling was lower in the other alloys, and the swelling reduction could be correlated with increased precipitation. These results are considered in light of the current theories for low swelling in ferritic alloys, but no theory is available to completely explain the results.

Energetic particle irradiation may induce athermal processes in addition to the thermallyactivated single atom addition or subtraction events considered by classical nucleation theory. Displacement cascades could destroy sub-critical precipitates. Similarly, either displacement cascades or energetic knock-on atoms could remove single atoms or groups of atoms from subcritical clusters.

Cluster destruction becomes significant in nucleation when the probability of destruction is comparable to the probability of growth by single atom capture. The model is applied to the nucleation of embrittling precipitates in pressure vessel steels. The scaling of accelerated embrittlement tests on the basis of displacements per atom is found only sometimes to be applicable. Athermal single atom loss, as in ion mixing, may appreciably reduce the nucleation if the rate of loss approaches or exceeds the rate of solute atom capture.

The stability of the ordered γʹ precipitates under 300-keV Ni+ irradiation was investigated between room temperature and 623 K. The two competing mechanisms of destabilization by cascade producing irradiation, i. e. disordering and dissolution of the γʹ precipitates in Nimonic PE16 alloy, has been studied separately by electron microscopy and field-ion microscopy with atom probe. At high temperatures, the precipitates are stable. At intermediate temperatures, the precipitates dissolve by ballistic mixing into the matrix, but the interface is restored by the radiation-enhanced atomic jumps. The order in the precipitates remains stable. At low temperatures, the precipitates are dissolved by atomic mixing. The dissolution proceeds in a diffusional manner with a diffusion coefficient normalized by the displacement rate D/K = 0.75 nm2dpa−1. The precipitates become disordered by a fluence of 0.1 dpa, whereas precipitate dissolution needs much higher fluences.

When a compound is maintained in far for equilibrium configurations by nuclear collisionsunder irradiation, the steady-state properties of the system can no longer be predicted from equilibrium thermodynamics. Here we propose Monte Carlo simulations for addressing the question of phase stability under irradiation. They are based on a kinetic model with two dynamics acting in parallel: thermally activated jumps of vacancies and ballistic events induced by nuclear collisions. Two transformations are studied: the A2-B2 order-disorder transition and the precipitation of copper in iron. In the former case a shift from second to first order of the A2-B2 transition, predicted by the model, has been experimentally checked by I MeV electron irradiations of a FeA1 alloy. In the latter case, the precipitation kinetics are determined by Monte Carlo simulations and are found to be in very good agreement with available experimental data.

Defect kinetic parameters for radiation-induced grain boundary segregation in austenitic stainless alloys are evaluated by comparing model predictions to measured responses. Heavy-ions, neutrons, and proton irradiations having substantial statistical bases are examined. The combined modeling and measurement approach is useful for quantifying fundamental defect parameters. The mechanism evaluation indicates that vacancy migration energies were 1.15 eV or less and the vacancy formation energy at grain boundaries was 1.5 eV. Damage efficiencies of heavy ions and light-water reactor neutrons were about 0.03. Inferred proton damage efficiencies were about 0.15. Segregation measured in an advanced gas-cooled reactor component was much greater than predicted from those parameters alone.

Models for calculating radiation-induced segregation (RIS) in concentrated alloys based solely on differences in the magnitude of the vacancy driven flux typically overpredict the amount of grain boundary segregation. A simple sensitivity analysis is used to show that the overprediction may be linked to the choice of input parameters. By varying the Cr-vacancy migration energy from 1.300eV to 1.320 eV and the Fe-vacancy migration energy from1.300eV to 1.305 eV, well within their experimental uncertainty, RIS model calculations are brought into better agreement with AES and STEM-EDS measurements of grain boundary segregation in Fe-Cr-Ni alloys irradiated with protons at 7x 10−6 dpa/s at various temperatures and doses.

This paper describes the effect of post-irradiation annealing (PIA: 573-873K for 10 h) on the microhardness and intergranular P segregation in P doped and Cu-P doped iron alloys containing residual S. Hardening and softening occurred during PIA at lower and higher temperatures, respectively. PIA-induced hardening was more profoundly observed in the Cu-P doped alloy. Intergranular P enrichment was achieved by lower temperature PIA and P desegregation associated with S segregation dominated at higher PIA temperatures in both the alloys. Changes in the microhardness and intergranular P segregation induced by the PIA were found to be correlated with each other although the Cu-P doped alloy showed more scatter. The dynamic interaction between solute and defects in the grain interior and near grain boundaries during the PIA is discussed in order to explain the experimental observation.

An atom probe field ion microscopy characterization of A533B and Russian VVER 440 and 1000 pressure vessel steels has been performed to determine the phosphorus coverage of grain and lath boundaries. Field ion micrographs of grain and lath boundaries have revealed that they are decorated with a semi-continuous film of discrete brightly-imaging precipitates that were identified as molybdenum carbonitride precipitates. In addition, extremely high phosphorus levels were measured at the boundaries. The phosphorus segregation was found to be confined to an extremely narrow region indicative of monolayer-type segregation. The phosphorus coverages determined from the atom probe results of the unirradiated materials were in excellent agreement with predictions based on McLean's equilibrium model of grain boundary segregation. The boundary phosphorus coverage of a neutron-irradiated weld material was significantly higher than that observed in the unirradiated material.

Radiation-induced segregation (RIS) to grain boundaries in Fe-Ni-Cr-Si stainless alloys has been measured as a function of irradiation temperature and dose. Heavyion irradiation was used to produce damage levels from 1 to 20 displacements per atom (dpa) at temperatures from 175 to 550°C. Measured Fe, Ni, and Cr segregation increased sharply with irradiation dose (from 0 to 5 dpa) and temperature (from 175 to about 350°C). However, grain boundary concentrations did not change significantly as dose or temperatures were further increased. Although interfacial compositions were similar, the width of radiation-induced enrichment or depletion profiles increased consistently with increasing dose or temperature. Impurity segregation (Si and P) was also measured, but only Si enrichment appeared to be radiation-induced. Grain boundary Si peaked at levels approaching 8 at% after irradiation doses to 10 dpa at an intermediate temperature of 325°C. No evidence of grain boundary silicide precipitation was detected after irradiation at any temperature. Equilibrium segregation of P was measured in the high-P alloys, but interfacial concentration did not increase with irradiation exposure. Comparisons to reported RIS in neutronirradiated stainless steels revealed similar grain boundary compositional changes for both major alloying and impurity elements.

Radiation induced segregation (RIS) has been implicated as a mechanism for irradiationassisted stress corrosion cracking (IASCC) in reactor core components. Proton irradiation has been shown to be useful in creating grain boundary chemistries similar to those found in neutron and charged particle irradiated materials for accelerated testing of IASCC susceptibility. This work quantifies grain boundary RIS as a function of proton irradiation dose (0.1-3.0 dpa), temperature (200°−600°C), and alloy composition (20Cr-9Ni, 24Cr-19Ni, and xCr-24Ni, x=16, 20,24). Auger electron spectroscopy revealed Cr depletion and Ni enrichment under all irradiation conditions. As a function of dose, the degree of segregation increased rapidly to near saturation prior to 1 dpa, with a boundary composition of 12.1 at.% Cr and 36.0 at.% Ni at 1 dpa. Segregation peaked at approximately 500°C with 13.0 at.% Cr and 38.6 at.% Ni at the grain boundary at 0.5 dpa; very little segregation was observed at or below 300°C or at 600°C. The trends in segregation as a function of dose agreed well with the Perks' model predictions with the exception of the measurement at 600°C, which showed the sharp decrease in segregation predicted for a higher temperature (700°C-800°C). For alloys containing constant bulk Cr but varying Ni, the Perks' model agreed well with the observed segregation trend; however, for alloys containing constant bulk Ni and varying Cr, agreement was achieved only through the use of composition dependent diffusion parameters.

The evolution of the fine scale microstructural features leading to irradiation embrittlement of reactor pressure vessel steels is described. Copper rich phases undergo accelerated precipitation from supersaturated solution due to radiation enhanced diffusion. In steels with significant trace quantities of copper the precipitates, characterized by high concentrations and small sizes, are the dominant embrittling feature. Precipitate concentrations, sizes, volume fractions and compositions are consistent with thermodynamic and kinetic models that rationalize the effects of a number of irradiation and metallurgical variables. Phosphide and carbonitride phases may also develop along with new manganese nickel rich precipitates, promoted by high nickel contents. These features may lead to severe embrittlement at high fluence even in low copper steels. While their detailed identity and characteristics are not known, defect cluster-solute complexes with a range of thermal stability are important both directly and indirectly; for example, in mediating flux and temperature effects. In conjunction with the application of state-of-the-art characterization methods, development of advanced modeling tools will be needed to address a number of outstanding issues.