Multiscale Modeling of the Deformation of Advanced Ferritic Steels for Generation IV Nuclear Energy

The objective of this project is to use the multi-scale modeling of materials (MMM) approach to develop an improved understanding of the effects of neutron irradiation on the mechanical properties of high-temperature structural materials that are being developed or proposed for Gen IV applications. In particular, the research focuses on advanced ferritic/ martensitic steels to enable operation up to 650-700°C, compared to the current 550°C limit on high-temperature steels

Accuracy & convergence of parametric dislocation dynamics

Abstract In the parametric dislocation dynamics (PDD), closed dislocation loops are described as an assembly of segments, each represented by a parametric space curve. Their equations of motion are derived from an energy variational principle, thus allowing large-scale computer simulations of plastic deformation. We investigate here the limits of temporal and spatial resolution of strong dislocation interactions. The method is demonstrated to be highly accurate, with unconditional spatial convergence that is limited to distances of the order of interatomic dimensions. It is shown that stability of dislocation line shape evolution requires very short time steps for explicit integration schemes, or can be unconditionally stable for implicit time integration schemes. Limitations of the method in resolving strong dislocation interactions are established for the following mechanisms: dislocation generation, annihilation, dipole and junction formation, pileup evolution

Curved parametric segments for the stress field of 3-D dislocation loops

Introduction Numerical simulations of plastic deformation with dislocation distributions are computationally very challenging, especially for engineering levels of strains, strain rates, and volumes. This particular aspect has been recognized in most Dislocation Dynamics (DD) simulations, either in 2-D (e.g., Ghoniem and Amodeo, 1988; Wang and LaSar, 1995), or in 3-D (e.g., DeVincre and Kubin 1994; Dislocations in real crystals are generally curved because of their strong mutual interactions, externally applied stress fields, as well as thermodynamic forces resulting from gradients or changes in local chemical potentials. Moreover, extensive experimental evidence indicates that dislocation lines are generally curved, especially under the action of an externally applied stress, and at temperatures exceeding 0.2-0.3 of the material's melting point. In some special cases, however, long straight dislocation segments are experimentally observed. This is particularly true in materials with high Peierel's potential barriers normal to specific crystallographic orientations (e.g., Si), or large mobility differences between screw and edge components (e.g., some BCC crystals at low temperature). It is apparent that very large curvature variations are expected, especially for strongly interacting dislocation loops. The accuracy of computing the dynamic shape of dislocation loops is thus dependent on how dislocation lines are discretized for field and force calculations. In DD simulations of plastic deformation, the computational effort per time step is proportional to the square of the number of interacting segments, because of the long-range stress field associated with dislocation lines. It is therefore advantageous to reduce the number of interacting segments during such calculations. Recent 3-D calculations of dislocation interactions using straight segments are based on analytical solutions of the elastic field of either mixed segments, e.g.

Perspectives 8 THE ROLE OF THEORY AND MODELING IN THE DEVELOPMENT OF MATERIALS FOR FUSION ENERGY

The environmental and operational conditions of First Wall/ Blanket (FW/B) structural materials in fusion energy systems are undoubtedly amongst the harshest in any technological application. These materials must operate reliably for extended periods of times without maintenance or repair. They must withstand the assaults of high particle and heat fluxes, as well as significant thermal and mechanical forces. Rival conditions have not been experienced in other technologies, with possible exceptions in aerospace and defense applications. Moreover, the most significant dilemma here is that the actual operational environment cannot be experimentally established today, with all of the synergistic considerations of neutron spectrum, radiation dose, heat and particle flux, and gigantic FW/B module sizes. Because of these considerations, we may rely on a purely empirical and incremental boot-strapping approach (as in most human developments so far), or an approach based on data generation from non prototypical setups (e.g., small samples, fission spectra, ion irradiation, etc.), or a theoretical/computational methodology. The first approach would have been the most direct had it not been for the unacceptable risks in the construction of successively larger and more powerful fusion machines, learning from one how to do it better for the next. The last approach (theory and modeling alone) is not a very viable option, because we are not now in a position to predict materials behavior in all its aspects from purely theoretical grounds. The empirical, extrapolative approach has also proved itself to be very costly, because we cannot practically cover all types of material compositions, sizes, neutron spectra, temperatures, irradiation times, fluxes, etc. Major efforts had to be scrapped because of our inability to encompass all of these variations simultaneously. While all three approaches must be considered for th

SURFACE ROUGHENING MECHANISMS FOR TUNGSTEN EXPOSED TO LASER, ION, AND X-RAY PULSES

Tungsten is a candidate material for a variety of applications in Magnetic an