The IAP is a crucial property of a model for it determines all the physical properties of the sim­ulated system. Discussion on modern IAPs is pre­sented in Chapter 1.10, Interatomic Potential Development and so we do not elaborate on this subject here.

Another important property is the spatial scale of the simulated system. The periodic spacing, Lg, in the direction of the dislocation glide has to be large enough to avoid unwanted effects due to interaction between the dislocation and its periodic neighbors in the PAD; 100-200b is usually sufficient.10 Further­more, the model should be large enough to include all direct interactions between the dislocation and obstacle and the major part of elastic energy that may affect the mechanism under study. MD simula­tions have demonstrated that a system with a few million atoms is usually sufficient to satisfy condi­tions for simulating interaction between a dislocation and an obstacle of a few nanometers in size. The biggest obstacles considered to date are 8 nm voids,23 10 nm DLs,25 and 12 nm SFTs26 in crystals containing ^6-8 million mobile atoms. It should be noted that static simulation (T = 0 K) usually requires the largest system because most obstacles are stronger at low T and the dislocation may have to bend strongly and elongate before breaking free.2

Simulation of a dynamic system, that is, T> 0 K, introduces another important and limiting factor for atomic-scale study ofdislocation behavior, namely the simulation time, t, which can be achieved with the computing resource available. Under the action of increasing strain applied to the model, the time to reach a given total strain determines the minimum applied strain rate, є, that can be considered. This parameter defines in turn the dislocation velocity. Consider a typical simulation of dislocation-obstacle interaction in an Fe crystal, for which b = 0.248 nm. For L = 41 nm, a model containing 2 x 106 mobile atoms would have a cross-section area of 5.73 x 10-16m2; that is, a dislocation density pD = 1.75 x 1015m-2. For Lg = 120b = 29.8 nm, the model height per­pendicular to the glide plane would be 19 nm. At є = 5 x 106s-1, the steady state velocity, vd, of a single dislocation estimated from the Orowan relation vd = _/Pob is 11.6 m s-1. The time for the dislocation to travel a distance Lg at this velocity would be 2.6 ns. Thus, even if the dislocation breaks away from the void without traversing the whole of the glide plane, the total simulated time would be ~1 ns.

The lowest strain rate for dislocation-obstacle interaction reported so far27 is 105 s-1 and it resulted in vo = 48 cm s- . This strain rate is about six to ten orders of magnitude higher than that usually applied in laboratory tensile experiments and more than ten orders higher than that for the creep regime. This presents an unresolvable problem for atomic-scale modeling and even massive parallelization gains only three or four orders in є or vo We conclude that the possibilities of modern atomic-scale modeling are lim­ited to dislocation velocity of at least ~0.1 cms — .

Nevertheless, atomic-scale modeling, particularly using MD (T> 0 K), is a powerful, and sometimes the only, tool for investigating processes associated with lattice defect interactions and dynamics. The main advantage of MD is that, if applied properly to a large enough system, it includes all classical phe­nomena such as evolution of the phonon system and therefore free energy, rates of thermally activated defect motion, and elastic interactions. It is, therefore, one of the most accurate techniques for investigating the behavior of large atomic ensembles under differ­ent conditions. We reemphasize that the realism of atomic-scale modeling is limited mainly by the valid­ity of the IAP and restricted simulation time.