C. A. English

Microstructural evolution in reactor pressure vessel steels

Abstract The irradiation-induced changes occurring at the nanometer level in the microstructure of the reactor pressure vessel have been shown to be responsible for a degradation in mechanical properties that occur in large structures with dimensions in the tens of metres. These changes can place severe restrictions on the reactor both at the startup and under continual operation; and in the long term could compromise the safe operation of the plant. The large financial and safety related matters have given the necessary driving force to study the factors controlling the microstructural evolution, resulting in a multi disciplinary approach to the problem. This paper aims to review the current understanding of the subject, giving were possible examples of the approach and techniques used to obtain this. We also highlight areas of current research activity and indicate the type of work still required to provide information on aspects that currently lack a full understanding.

The molecular dynamics simulation of irradiation damage cascades in copper using a many-body potential

Abstract The evolution of irradiation damage cascades in a metal has been simulated by molecular dynamics, using a many-body potential. Over 100 cascades have been produced with random knock-on directions and primary knock-on atom (PKA) energies ranging from 60 to 2 keV. The cascade evolution has been followed for times typically up to about 10ps and in some cases up to about 30 ps. The cascades are characterized by the sudden emission of replacement collision sequences and with shape variations due to local channelling events. At the higher energies the core has been shown to be a liquid-like structure with cavitation. The annealing phase leaves loosely clustered vacancies at the cascade centre but collapse to a vacancy loop is not observed. The onset of this thermal spike occurs for cascades with energy of 100 eV or higher; below this the cascade is made up of a series of closely coupled short replacement collision sequences. A feature of the more energetic cascades is the production by a ballistic mech...

Molecular dynamics calculations of displacement threshold energies and replacement collision sequences in copper using a many-body potential

Abstract The many-body potential of Ackland et al. is currently being used to investigate the production of displacement damage cascades in irradiated copper, using the molecular dynamics computer program MOLDY. The effect of adopting this potential on both the static and the dynamic properties of irradiation-induced defects is evaluated here. It is shown that this potential provides a good representation of the well known properties of point defects. However, for dynamic events involving the close approach of atoms, a short-range ion core repulsive term needs to be incorporated. The modified core term has been optimized to give an excellent fit to the measured threshold displacement energies without significantly affecting the fit to other physical properties.

Microstructural characterization of irradiation-induced Cu-enriched clusters in reactor pressure vessel steels

Abstract The effect of irradiation on microstructure of four irradiated reactor pressure vessel steels (a low copper A533B-1 plate, a low copper A508-3 forging, a high copper Linde 80 flux weld and a high copper Linde 1092 flux weld) was determined by using complementary microstructural techniques such as optical position-sensitive atom probe (OPoSAP), field emission gun scanning transmission electron microscopy (FEGSTEM) and small angle neutron scattering (SANS). In the low copper steels, irradiation resulted in small shifts in transition temperature and small changes in hardness increments. The microstructural analyzes showed that this response was dominated by matrix damage. In contrast, both copper-enriched clusters and matrix damage formed in the high copper welds. This information was then used as input to the Russell–Brown model to predict the change in hardness resulting from copper-enriched clusters. The calculated hardness increments were found to be consistent with the experimental data.

Considerations of recoil effects in microstructural evolution

Abstract The cascade processes are discussed which potentially give rise to a dependence of microstructural evolution on the recoil spectra set up by a flux of fast particles. It is emphasised that the thermal spike phase of cascade development is central to these processes. Recent results from Molecular Dynamics simulations are presented to highlight the potential importance of interstitial clustering in cascades and the impact of cascades on pre-existing features of the microstructure. A qualitative discussion is presented of the likely importance on microstructural evolution at low and high doses of the different cascade processes.

Collapse of displacement cascades in h.c.p. metals

Abstract The production of vacancy loops by the collapse of displacement cascades is of particular importance in radiation damage, for these loops are known to play a major role in subsequent microstructural development, and yet little is known of the cascade processes in them. In this study, attention has been focused on cascade collapse to form vacancy loops in five of the h.c.p. metals, namely Ti, Co, Re, Ru and Mg, chosen for their wide-ranging material properties. Sb+ ions of either 100 of 150 keV have been used to produce cascades, which may collapse to form vacancy dislocation loops. These loops have been studied using transmission electron microscopy, and this has enabled detailed analysis to be undertaken of loop geometry, size and number density. For the foil orientation used, all the loops observed lay on the prism planes, and from the size distributions of faulted and perfect loops we have estimated values for the prism plane stacking-fault energy. The efficiency of collapse varies dramaticall...

Characterization of heavy-ion damage in ruthenium. I: Vacancy-loop crystallography

Abstract Displacement-cascade collapse to form vacancy loops is of fundamental importance to the development of damage structures in irradiated metals. In the present experiments, displacement cascades have been generated in ruthenium, a high melting-point hexagonal metal with a c/a ratio of 1.58, by irradiation with heavy ions having a range of masses and energies. A detailed transmission electron microscopy analysis of the vacancy loops formed by cascade collapse has shown that four distinct sets of loop geometries are present. In a majority of cases the cascades collapse initially on to the {1010} prism planes to form loops with Burgers vector b=½〈1010〉 and it is shown that these can subsequently unfault to form perfect ⅓ 〈1120〉-type loops. When the incident-ion-beam direction lies in the basal plane, a minority of the cascades collapse on to the basal planes to form faulted ½[0001] loops and these can transform to a lower-energy faulted loop with b of the type ⅙〈2023〉. From determination of the loop s...

Characterization of heavy-ion damage in ruthenium. II. Cascade collapse

Abstract Vacancy loop formation in cascades formed during heavy-ion irradiation of the h.c.p. metal ruthenium at room temperature has been investigated by transmission electron microscopy. The ion energy and the ion mass were varied between 10 and 100 keV and 84 (Kr+) to 184 (W+) respectively to produce a wide variety of cascade morphologies. Information about the cascade collapse process has been obtained from the measurement of defect yield and cascade efficiency values. The former defines the probability that an incident ion forms a visible loop, and the latter the proportion of vacancies per cascade that remain in the form of a dislocation loop. The defect yield increases with increasing ion energy and has a complex dependence on ion mass, first increasing and then decreasing with increasing mass at low and high energies respectively. Cascade efficiency is not as complex and increases with increasing ion mass and decreasing ion energy. These effects have been interpreted in terms of current cascade co...

Molecular dynamics simulation of irradiation damage cascades in copper using a many-body potential

Abstract The evolution of irradiation damage cascades in copper has been simulated by molecular dynamics, using a many-body potential. Nearly 200 cascades have been produced with random knock-on directions and primary knock-on atom (PKA) energies in the range from 60 eV to 10 keV. The starting temperature for these simulations was 100 K and 600 K, this report will confine itself to the data obtained at 100 K. The cascade evolution has been followed for times typically up to ∼10 psec and in some cases up to ∼30 psec. The cascades are characterised by the sudden emission of replacement collision sequences and with shape variations due to local channelling events. At the higher energies the core has been shown to be liquid-like structure with cavitation. The annealing phase leaves loosely clustered vacancies at the cascade centre but collapse to a vacancy loop is not generally observed. A feature of the more energetic cascades is the production by a ballistic mechanism of interstitial atom clusters at the pe...

The characterisation of displacement-cascade collapse in Ni-Cr-Fe alloys

Abstract Displacement cascades produced by energetic lattice recoils are the primary damage state in neutron-irradiated metals, among which austenitic stainless steels are an important class of technological alloys. The vacancy constituent of this damage that survives in the form of collapsed vacancy loops is a major component of this damage state, and has been investigated in the present work as a function of alloy composition and the temperature. This has been done by irradiating single crystal foils of a range of high-purity model ternary Fe - xNi -15% Cr alloys ( x = 15–70%) with heavy ions, and then analysing the resulting damage by transmission electron microscopy. A full analysis has been achieved by measuring the areal density of loops and their size distribution for each irradiation condition, and hence obtaining the important parameters defect yield and collapse efficiency. By appropriate choice of ion energy, ion mass, ion dose and specimen temperature, we have been able to vary the factors such as cascade size, cascade energy density, cascade defect density and cascade overlap, and thus study their influence on cascade collapse. The results have been assessed in terms of current models of cascade processes in pure metals and alloys.