Migration and directional change of interstitial clusters in a-Fe: Searching for transition states by the dimer method

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) are observed to change their glide direction during the period of molecular dynamics (MD) simulations (10 ns), but the directional change for larger clusters is inaccessible to MD due to the limited time-scale. In order to overcome the 'time barrier' in MD simulations, the dimer method is employed to search for possible transition states of interstitials and small interstitial clusters in -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 are studied as a function of interstitial cluster size, and the lowest energy barriers correspond to defect migration along 〈111〉 directions, as seen in MD simulations. Small clusters change their direction by a 〈110〉 fragment mechanism involving rotation of each crowdion into and out of the 〈110〉 dumbbell configuration, whereas the directional change for larger clusters is 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 is also investigated using MD simulations.