The dumbbell configuration of a self-interstitial gives it a certain orientation, namely the dumbbell axis, and upon migration this axis orientation may change. This is indeed the case for self-interstitials in fcc metals, as illustrated in Figure 15.

Suppose that the initial location of the self­interstitial is as shown on the left, and its axis is along [001]. A migration jump occurs by one atom of the dumbbell (here the purple one) pairing up with one nearest neighbor, while its former partner

(the blue atom) occupies the available lattice site. Computer simulations of this migration process have shown25 that the orientation of the self-interstitial has rotated to a [010] orientation, and that this combined migration and rotation requires the least amount of thermal activation.

Similar analysis for the migration of self­interstitials in bcc metals has revealed that a rotation may or may not accompany the migration, and these two diffusion mechanisms are depicted in Figure 16. Which of these two possesses the lower activation energy depends on the metal, or on the interatomic potential employed for determining it.

In general, however, the activation energies for self-interstitial migration are very low compared to the vacancy migration energy, and they can rarely be measured with any accuracy. Instead, in most cases only the Stage I annealing temperatures have been measured. In the associated experiments, specimens for a given metal are irradiated at such low tempera­tures that the Frenkel pairs are retained. Their con­centration is correlated with the increase of the electrical resistivity. Subsequent annealing in stages then reveals when the resistivity declines again upon reaching a certain annealing temperature. The first annealing, Stage I, occurs when self-interstitials become mobile and in the process recombine with
vacancies, form clusters of self-interstitials, or are trapped at impurities. Table 8 lists the Stage I temperature,7 Tjm, for pure metals as well as two alloys that represent ferritic and austenitic steels. For a few cases, an associated activation energy Em is known, and in even fewer cases, a preexponential factor, D0, has been estimated.