Atomic-scale simulation by computer has become a powerful tool for investigation of material properties and processes that cannot be achieved by experimen­tal techniques or other theoretical methods. This is due to the increasing power of computers, the devel­opment of new efficient modeling codes, and the extensive usage of ab initio calculations for probing atomic mechanisms and generating data for design of new IAPs. Moreover, increasing length and time scales attainable by atomistic modeling provides overlap with experimental scales in some cases, thereby allowing direct verification of modeling results.26 In this chapter, we have described only a selection of the results obtained within the last decade by atomic-scale modeling of DD in irradiated bcc and fcc metals. The examples presented and references cited demonstrate how detailed insight into mechanisms can be gained by such modeling. For some obstacles to dislocation motion, for exam­ple, many inclusion-like obstacles, strengthening is controlled by the dislocation line shape at breakaway and can be parameterized using the existing elastic­ity theory models. In other cases, for example, dislocation-like obstacles, reactions and their results, including obstacle strength, depend very much on the material, and dislocation character and core struc­ture; dislocation behavior is also sensitive to condi­tions such as interaction geometry, temperature, and strain rate. The variety of outcomes for dislocation­like obstacles is complicated and wide, and although features of these reactions can be understood within the framework of the continuum theory of disloca­tions, for example, Frank’s rule, a general formaliza­tion of the reactions in terms of obstacle strength and reaction product does not exist. Nevertheless, it has been seen that the insight gained by simulation has allowed the outcome of reactions to be classified in a meaningful way (Table 1). This will allow for validation of higher-level DD modeling of these reac­tions using the continuum approximation. An excel­lent example of the way in which this can be done has been provided by Martinez and coworkers,96,97 who used MD and DD to simulate the same dislocation — SFT interactions. The continuum modeling of these nanometer-scale obstacles was verified by the atomic simulation, and this enabled a large number of inter­action geometries and conditions to be investigated successfully by DD. Unfortunately, successful over­laps in scale of atomistic modeling and experiment or/and continuum modeling are still rare. Efforts in all techniques are necessary to progress understand­ing of mechanisms and their parameterization for predictive modeling tools that can be applied to irradiated materials.

Investigations such as those just discussed bring assurance that atomic-scale modeling is correct at least qualitatively and is invaluable in cases where scale overlap of techniques is not yet achieved. Two main problems exist with regard to the quality of its quantitative outcomes. One is concerned with time scale. As already mentioned, the limit on time scale is the main disadvantage of current atomic-scale mod­eling. The maximum simulation time achieved so far is of the order of a few hundred nanoseconds. For dislocation studies, this allows dislocations with velocity as low as 0.5 ms-1 to be modeled — and it may be that some processes are insensitive to veloc­ity at this level27 — but the overall strain rate (105 s-1) is fast compared with experiment, and the interaction time (~100 ns) with an obstacle is too short for thermally-activated processes to be sampled. Devel­opment of new methods for keeping atomic-level accuracy over at least microsecond to second time scales is necessary to progress to the next step toward predictive modeling for engineering applications. An example of a new generation technique for simulating realistic strain rates whilst retaining atomic-scale detail was published recently.98 The new technique combines atomic-scale modeling for estimating vacancy migration barriers in the vicinity of an edge dislocation and kinetic Monte Carlo (MC) for simulating vacancy kinetics in a crystal with a specified dislocation density. The technique was successfully applied to simulate the process of power-law creep over a macroscopic time scale with microscopic fidelity. The other problem is concerned with accuracy in describing interatomic interactions. Much of the research described in this chapter, based as it is on empirical EAM IAPs, is more related to the behavior of model metals with bcc or fcc crystal structure in general than to the elements Fe or Cu in particular. This difficulty will become more acute with the demand for more sophisticated, radiation — resistant alloys, and future investigations of chemical effects on plasticity will require IAPs that incorpo­rate chemistry in a meaningful way. More on this is presented in Chapter 1.10, Interatomic Potential Development.

We would like to conclude on an optimistic note. It is clear that significant progress has been achieved over the last decade in understanding the details of the atomic-scale mechanisms involved in dislocations dynamics in structural metals in a reactor environ­ment. The small, nanoscale nature of the obstacles created by radiation damage is such that the tech­niques described here provide uniquely valuable

information, despite the limitations they currently experience.