The evolution of displacement cascades is similar at all energies, with the development of a highly ener­getic, disordered core region during the initial, colli — sional phase of the cascade. Vacancies and interstitials are created in equal numbers, and the number of point defects increases sharply until a peak value is reached. Depending on the cascade energy, this occurs at a time in the range of 0.1-1 ps. This evolu­tion is illustrated in Figure 5 for a range of cascade energies, where the number of vacancies is shown as a function of the cascade time. Many vacancy — interstitial pairs are in quite close proximity at the time of peak disorder. An essentially athermal process of in-cascade recombination of these close pairs takes place as they lose their kinetic energy. This leads to a reduction in the number of defects until a quasi­steady-state value is reached after about 5-10 ps. As interstitials in iron are mobile even at 100 K, further short-term recombination occurs between some vacancy-interstitial pairs that were initially separated by only a few atomic jump distances. Finally, a stage is reached where the remaining point defects are sufficiently well separated that further recombina­tion is unlikely on the time scale (a few hundred picoseconds) accessible by MD. Note that the number of stable Frenkel pair is actually somewhat lower than the value shown in Figure 5 because the values obtained using the effective sphere identification
procedure were not corrected to account for the interstitial structure discussed above.

A mechanism known as RCS may help explain some aspects of cascade structure.24,41 An RCS can be visualized as an extended defect along a close — packed row of atoms. When the first atom is pushed off its site, it dissipates some energy and pushes a second atom into a third, and so on. When the last atom in this chain is unable to displace another, it is left in an interstitial site with the original vacancy several atomic jumps away. Thus, RCSs provide a mechanism of mass transport that can efficiently sep­arate vacancies from interstitials. The explanation is consistent with the observed tendency for the final cascade state to be characterized by a vacancy-rich central region that is surrounded by a region rich in interstitial-type defects. However, although RCSs are observed, particularly in low-energy cascades, they do not appear to be prominent enough to explain the defect separation observed in higher energy cascades.58 Visualization of cascade dynamics indicates that the separation occurs by a more collective motion of multiple atoms, and recent work by Calder and coworkers has identified a shockwave-induced mech­anism that leads to the formation of large interstitial clusters at the cascade periphery.80 This mechanism will be discussed further in Section 1.11.4.3.1. Coherent displacement events involving many atoms have also been reported by Nordlund and coworkers.81

Defect production tends to be dominated by a series of simple binary collisions at low PKA ener­gies, while the more collective, cascade-like behav­ior dominates at higher energies. The structure of typical 1 and 20 keV cascades is shown in Figure 6, where parts (a) and (b) show the peak damage state and (c) and (d) show the final defect configurations. The MD cells contained 54 000 and 432 000 atoms for the 1 and 20 keV simulations, respectively. Only the vacant lattice sites and interstitial atoms identi­fied by the effective sphere approach described above are shown. The separation of vacancies from interstitials can be seen in the final defect config­urations; it is more obvious in the 1 keV cascade because there are fewer defects present. In addition to isolated point defects, small interstitial clusters are also clearly observed in the 20 keV cascade debris in Figure 6(d). In-cascade clustering is dis­cussed further in Section 1.11.4.3.

The morphology of the 20 keV cascade in Figure 6(b) exhibits several lobes which are evidence of a phenomenon known as subcascade formation.82 At low energies, the PKA energy tends to be dis­sipated in a small volume and the cascades appear as compact, sphere-like entities as illustrated by the
1 keV cascade in Figure 6(a). However, at higher energies, some channeling82,83 of recoil atoms may occur. This is a result of the atom being scattered into a relatively open lattice direction, which may permit it to travel some distance while losing rela­tively little energy in low-angle scattering events. The channeling is typically terminated in a high — angle collision in which a significant fraction of the recoil atom’s energy is transmitted to the next generation knock-on atom. When significant sub­cascade formation occurs, the region between high-angle collisions can be relatively defect-free as the cascade develops. This evolution is clearly shown in Figure 7 for a 40 keV cascade, where the branching due to high-angle collisions is observed on a time scale of a few hundreds of femto seconds. One practical implication of subcascade formation is that very high-energy cascades break up into what looks like a group of lower energy cascades. An example of sub­cascade formation in a 100 keV cascade is shown in Figure 8 where the results of 5 and 10 keV cascades have been superimposed into the same block of atoms for comparison. The impact of subcascade formation on stable defect production will be discussed in the next section.

Figure 6 Structure of typical 1 keV (a, c) and 20 keV (b, d) cascades. Peak damage state is shown in (a and b) and the final stable defect configuration is shown in (c and d).

MD cascade simulations in iron at 100 K: peak damage

10keV

100 keV

5 keV

5keV-0.26ps 10keV-0.63ps 100keV-0.70 ps

Figure 8 Energy dependence of subcascade formation.