The interstitial clusters produced by cascades in metals have very high mobility and exhibit thermally activated, one-dimensional glide along <111> directions. Only small interstitial clusters (<4) were observed to change their glide direction during the period of molecular dynamics simulations (10ns), but the directional change for larger clusters was inaccessible to molecular dynamics due to the limited time-scale. In order to overcome the 'time barrier' in molecular dynamics simulations, the dimer method was employed to search for possible transition states of interstitials and small interstitial clusters in α-Fe. The method uses only the first derivatives of the potential energy to find saddle points without knowledge of the final state of the transition. The possible transition states were studied as a function of interstitial cluster size, and the lowest energy barriers correspond to defect migration along <111> directions, as seen in molecular dynamics simulations. Small clusters change their direction by a <110> fragment mechanism involving rotation of each crowdion into and out of the <110> dumb-bell configuration, whereas the directional change for larger clusters was a two-step process consisting of translation along a <100> direction and rotation into an equivalent <111> direction. The mechanism of changing direction for a tri-interstitial cluster was also investigated using molecular dynamics simulations.

Migration and Directional Change of Interstitial Clusters in α-Fe - Searching for Transition States by the Dimer Method. F.Gao, H.Heinisch, R.J.Kurtz, Y.N.Osetsky, R.G.Hoagland: Philosophical Magazine, 2005, 85[4-7], 619-27