The results of simulations such as those presented above should not be viewed as being quantitatively accurate. As already mentioned, subtle changes in the fitting of the interatomic potential can alter the cascade simulations both qualitatively and quantita­tively. Even if a sufficiently accurate potential can be identified, the results represent a certain limiting case of what may be observed experimentally. This is because all the simulations mentioned so far were carried out in perfect material — computer-pure material. Nowhere in nature can such perfect metal be found, particularly for iron, which is easily contaminated with minor interstitial impurities such as carbon. In this section, a few examples will be discussed to illustrate how reality may influence cascade damage production relative to the perfect material case. The examples include the influence of preexisting defects, free surfaces, and grain boundaries.

1.11.4.4.1 Influence of preexisting defects

Even if a well-annealed, nearly defect-free, single crystal material is selected for irradiation, radiation — induced defects will rapidly change the state of the material. A simple calculation employing typical elastic scattering cross-sections for fast neutrons and the cascade volumes observed in MD simulations will demonstrate that by the time a dose of ~0.01 dpa is reached, essentially the complete volume will have experienced at least one cascade. There have been relatively few studies on how cascade damage produc­tion may be different in material with defects.95-97 The results of cascade simulations reported in Stoller and Guiriec97 were carried out at 10keV and 100 K to expand the range of previous work carried out using 1 keV simulations in copper95 and 0.40, 2.0, and 5.0 keV simulations in iron.96 A 10 keV cascade energy is high enough to initiate in-cascade clustering, is near the plateau region of the defect survival curve, and involves a limited degree of subcascade formation. For these conditions, the database discussed above (see Figure 11) includes two independent sets of cascades, seven in a 128 k atom cell and eight in a 250 k atom cell that can be used to provide a basis of comparison. A cell size of 250 k atoms was used for the cascade simulations with preexisting damage.

The study in Stoller and Guiriec97 involved three simple configurations of preexisting damage that were all derived from cascade debris. This is perhaps the simplest possible damage structure, a collection of point defects and point defect clusters. The first configuration was simply the as-quenched debris from a 10 keV cascade in perfect crystal. A total of 30 vacancies and interstitials were present, including one di — and one 7-interstitial cluster. The second case was similar, but the point defects were reconfigured so that the 30 vacancies included a 6-vacancy void and a 9-vacancy loop, and the interstitial clusters included four di-, one tri-, and one 8-interstitial cluster. The third configuration contained only a single 30-vacancy void. These configurations are shown in Figure 23. Eight simulations were carried out with different initial PKAs and the same <135> PKA direction. The selected PKAwere 15—20 lattice parameters from the center of the cascade debris and located such that the < 135 > direction pointed them toward the center of the debris field. The same set of PKAs was used for all three defect configurations.

As expected, substantial variation was observed between the different simulations for any given pre­existing defect configuration; in some cases the cas­cade produced more defects than in perfect crystal,

while in others fewer were produced. The most dra­matic visible effects were observed for the 30-vacancy void. In one case, the void was completely intact after the second cascade, while in the others it was destroyed to varying degrees. The impact of preexisting damage on stable defect formation in the 10keV cascades is shown in Figure 24, where results from the three different defect configurations are compared with those obtained in perfect crystal. The variation between two sets of perfect crystal sim­ulations is provided for comparison purposes. The statistical information from analysis of defect survival

and interstitial clustering is summarized in Table 3. On average, a significant reduction in defect forma­tion was observed for the two configurations most typical of random cascade debris. A slight increase (that may not be statistically significant) in defect production was observed when the cell contained only a small void. Only the second defect configuration led to a significant change in interstitial clustering.

Although the approach in Stoller’s investigation of preexisting damage was slightly different, the results are consistent with previous studies by Foreman and coworkers95 and Gao and coworkers.96 They observed substantial reductions in defect production when a cascade was initiated in material containing defects. The reductions in defect production observed by Stoller (Figure 24 and Table 3) are somewhat smaller. This difference may partially be due to the higher cascade energy employed here (10 keV vs. 0.4— 5 keV), but the statistical nature of cascade evolution is also a factor. Gao and coworkers analyzed the results of several simulations as a function of dis­tance between the center of mass (COM) of the new cascade and that of the preexisting damage. A good correlation was found between this spacing and the number of defects produced. In the work of Stoller and Guiriec,97 the distance between PKA location and the preexisting damage was nearly constant. As the morphology of each cascade is quite different, the COM spacings varied. This is certainly part of the reason for the variety of behaviors mentioned above for the case of the small void. The average behavior for a fixed initial separation cannot be
directly compared to any of Gao’s results for the average at a fixed distance. Many more simulations need to be carried out at different energies to develop a more complete picture of cascade damage formation in material with typical defect densities, particularly to assess the clustering behavior. Over­all, the reduced defect survival observed in material containing defects suggests that it may be appropri­ate to employ defect formation values that are some­what lower than the perfect crystal results in the kinetic models used to simulate microstructural evolution over long times.