M. Samaras

Radiation damage near grain boundaries

Large-scale molecular dynamics of cascade production of the primary damage state are performed in nanocrystalline nickel with an average grain diameter of 12 nm and primary knock-on atom kinetic energies ranging from 5 to 30 keV. The role of the grain boundary during the cascade production of irradiated NC Ni is discussed in terms of grain-boundary structure. It is shown that regions of misfit in the grain boundaries can absorb self-interstitials and that stacking-fault tetrahedra are formed in the neighbourhood of the grain boundary.

Multiscale Modelling: the role of helium in iron

The mechanisms and processes of bubble nucleation and growth are still not completely solved and research in this field is ongoing. This is an important issue for materials used in fission and fusion reactors. In such environments, one of the main gaseous by-products is helium, whose presence and further production is known to decrease ductility, fatigue life and weldability, induce creep and stress rupture properties of materials, as well as promote swelling. These effects lead to the drastic modification of the materials mechanical properties. In the past, experiments have been used to offer clues into the structure of the materials; now, modelling offers the possibility to understand the structure of the material and from this information, to elucidate a fundamental understanding of material properties. This review discusses the modelling paradigms used to investigate and obtain an understanding of the mechanisms at play in helium bubble nucleation and growth in ferritic steels.

NUCLEAR ENERGY MATERIALS PREDICTION: APPLICATION OF THE MULTI-SCALE MODELLING PARADIGM

The safe and reliable performance of fusion and fission plants depends on the choice of suitable materials and an assessment of long-term materials degradation. These materials are degraded by their exposure to extreme conditions; it is necessary, therefore, to address the issue of long-term damage evolution of materials under service exposure in advanced plants. The empirical approach to the study of structural materials and fuels is reaching its limit when used to define and extrapolate new materials, new environments, or new operating conditions due to a lack of knowledge of the basic principles and mechanisms present. Materials designed for future Gen IV systems require significant innovation for the new environments that the materials will be exposed to. Thus, it is a challenge to understand the materials more precisely and to go far beyond the current empirical design methodology. Breakthrough technology is being achieved with the incorporation in design codes of a fundamental understanding of the properties of materials. This paper discusses the multi-scale, multicode computations and multi-dimensional modelling undertaken to understand the mechanical properties of these materials. Such an approach is envisaged to probe beyond currently possible approaches to become a predictive tool in estimating the mechanical properties and lifetimes of materials.

Discrete dislocation dynamics simulations of dislocation interactions with Y2O3 particles in PM2000 single crystals

In oxide dispersion strengthened steels the interactions between dispersoids and dislocations determine the materials plasticity. Using three-dimensional Discrete Dislocation Dynamics simulations, the effect of Y2O3 dispersoids on the motion of dislocations in BCC single crystal PM2000, a commercial alloy candidate for gas-cooled reactors, has been studied. The dispersoid distribution used in this model has been derived from experimental TEM observations of PM2000. As screw dislocations are predominant in the studied material, the behaviour of a single screw dislocation under shear loading in a distribution of spherical Y2O3 dispersoids is studied. The critical resolved shear stress, the minimum value of the external stress needed to move the dislocation through the obstacle field, is found to be only slightly lower than the experimentally determined value revealing that dispersoids are the main hardening contributions.

Modelling in nuclear energy environments

Producing energy to supply the demands of our societies is reaching a critical limit. To tackle this issue, there is a slow renaissance of fission reactors and the push to realise fusion reactors. The safe, reliable and optimal performance of fusion and fission plants is dependent on the choice of suitable materials used as components and fuels. As these materials are degraded by their exposure to high temperatures, irradiation and a corrosive environment, it is necessary to address the issue of long term degradation of materials under service exposure in advanced plants. A higher confidence in life-time assessments of these materials requires an understanding of the related physical phenomena on a range of scales from the atomic level of single defect energetics all the way up to macroscopic effects.

Stacking fault tetrahedra formation in the neighbourhood of grain boundaries

Large scale molecular dynamics computer simulations are performed to study the role of the grain boundary (GB) during the cascade evolution in irradiated nanocrystalline Ni. At all primary knock-on atom (PKA) energies in cascades near GBs, the damage produced after cooling down is vacancy dominated. Truncated stacking fault tetrahedra (TSFTs) are easily formed at 10 keV and higher PKA energies. At the higher energies a complex partial dislocation network forms, consisting of TSFTs. The GB acts as an interstitial sink without undergoing major structural changes.

Abnormal subgrain growth in a dislocation-based model of recovery

Simulation of subgrain growth during recovery is carried out using two-dimensional discrete dislocation dynamics on a hexagonal crystal lattice having three symmetric slip planes. To account for elevated temperature (i) dislocation climb was allowed and (ii) a Langevin type thermal noise was added to the force acting on the dislocations. During the simulation, a random ensemble of dislocations develops into a subgrain structure and power-law type growth kinetics are observed. The growth exponent is found to be independent of the climb mobility, but dependent on the temperature introduced by the thermal noise. The in-depth statistical analysis of the subgrain structure shows that the coarsening is abnormal, i.e. larger cells grow faster than the small ones, while the average misorientation between the adjacent subgrains remains nearly constant. During the coarsening Holts relation is found not to be fulfilled, such that the average subgrain size is not proportional to the average dislocation spacing. These findings are consistent with recent high precision experiments on recovery.

Condition Monitoring of High Temperature Components With Sub-Sized Samples

Advanced nuclear plants are designed for long-term operation in quite demanding environments. Limited operation experience with the materials used in such plants necessitate a reliable assessment of damage and residual life of components. Non-destructive condition monitoring of damage is difficult, if not impossible for many materials. Periodic investigation of small samples taken from well defined locations in the plant could provide an attractive tool for damage assessments. This paper will discuss possibilities of using very small samples taken from plant locations for complementary condition monitoring. Techniques such as micro/nano-indentation, micropillar compression, micro bending, small punch and thin strip testing can be used for the determination of local mechanical properties. Advanced preparation techniques such as focused ion beam (FIB) allow the preparation of samples from these small volumes for micro-structural analyses with transmission electron microscope (TEM) and advanced X-ray synchrotron techniques. Modeling techniques (e.g. dislocation dynamics DD) can provide a quantitative link between microstructure and mechanical properties. Using examples from ferritic oxide dispersion strengthened materials the DD approach is highlighted to understand component life assessments.

Impact of Grain Boundaries on Structural and Mechanical Properties

For some polycrystalline metals with grain sizes in the nano regime, experiments have suggested a deviation away from the Hall-Petch relation relating yield stress to average grain size [1]. The debate continues whether or not such deviations are a result of intrinsically different material properties of nanocrystalline (nc) systems, or due simply to inherent difficulties in the preparation of fully dense nc-samples and in their microstructural characterization. Nevertheless, it suggests that the traditional work hardening mechanism of pile-up of dislocations originating from Frank-Read sources may no longer be valid at the nanometer scale. In-situ deformation testing in the transmission electron microscope (TEM), performed on Cu and Ni3Al nc samples, reveals a limited dislocation activity in grains below 50nm [2,3]. However, due to the presence of large internal stresses which make grain boundaries (GB) in TEM images difficult to observe, and also possible artifacts induced by thin-film geometry such as dislocations emitted from the surface [4], in-situ tensile tests did not until now, bring convincing evidence for abundant dislocation activity. Mechanical testing also revealed the issue of the “GB state” by means of a property dependence on thermal history and internal strains. It is shown that a substantial strengthening can be obtained by a short heat treatment. The cause of the strengthening is possibly associated with a reduction in internal strains and/or dislocation content produced by the annealing [5]. The effect of strengthening has been measured both on nc materials obtained by grain refinement techniques and those obtained by consolidation of clusters.