The configuration, thermal stability and mobility of vacancy, and SIA clusters are of importance for the kinetics of damage accumulation and are different in the fcc and bcc metals. In the fcc metals, vacancy clusters are in the form of either dislocation loops or SFTs, depending on the stacking-fault energy, and the fraction of clustered vacancies, ev, is close to that for the SIAs, ei. In the bcc metals, nascent vacancy clusters usually form loosely correlated 3D configurations, and ev is much smaller than ei. Gen­erally, vacancy clusters are considered to be immo­bile and thermally unstable above the temperature corresponding to the recovery stage V.

In contrast to vacancy clusters, the SIA clusters are mainly in the form of a 2D bundle of crowdions or small dislocation loops. They are thermally stable and highly mobile, migrating 1D in the close-packed crystallographic directions.45 The ability of SIA clus­ters to move 1D before being trapped or absorbed by a dislocation, void, etc. leads to entirely different reac­tion kinetics as compared with that for 3D migrating defects, and hence may result in a qualitatively differ­ent damage accumulation than that in the framework of the FP3DM (see Section 1.13.6).

It should be noted that MD simulations provide maximum evidence for the high mobility of small SIA clusters. Numerous experimental data, which also sup­port this statement, are discussed in this chapter, how­ever, indirectly. One such fact is that most of the loops formed during ion irradiations of a thin metallic foil have Burgers vectors lying in the plane of the foil.54 It should also be noted that recent in situ experiments55-58 provide interesting information on the behavior of interstitial loops (>1 nm diameter, that is, large enough to be observable by transmission electron microscope, TEM). The loops exhibit relatively low mobility, which is strongly influenced by the purity of materials. This is not in contradiction with the simulation data. The observed loops have a large cross-section for interaction with impurity atoms, other crystal imperfections and other loops: all such interactions would slow down or even immobilize interstitial loops. Small SIA clusters produced in cascades consist typically of approximately ten SIAs and have, thus, much smaller cross-sections and consequently a longer mean-free path (MFP). The influence of impurities may, however, be strong on both the mobility of SIA clusters and, consequently, void swelling is yet to be included in the theory.