PRIMARY DAMAGE FORMATION IN BCC IRON

Primary defect formation in bcc iron has been extensively investigated using the methods of molecular dynamics (MD) and Monte Carlo (MC) sim ulation. This research has employed a modified version of the Finnis-S inclair interatomic potential. MD was used in the simulation of displa cement cascades with energies up to 40 keV and to examine the migratio n of the interstitial clusters that were observed to form in the casca de simulations. Interstitial cluster binding energies and the stable c luster configurations were determined by structural relaxation and ene rgy minimization using a MC method with Clusters containing up to 19 i nterstitials were examined. Taken together with the previous work, the se new simulations provide a reasonably complete description of primar y defect formation in iron. The results of the displacement cascade si mulations have been used to characterize the energy and temperature de pendence of primary defect formation in terms of two parameters: (1) t he number of surviving point defects and (2) the fraction of the survi ving defects that are contained in clusters. The number of surviving p oint defects is expressed as a fraction of the atomic displacements ca lculated using the secondary displacement model of Norgett-Robinson-To rrens (NRT). Although the results of the high energy simulations are g enerally consistent with those obtained at lower energies, two notable exceptions were observed. The first is that extensive subcascade form ation at 40 keV leads to a higher defect survival fraction than would be predicted from extrapolation of the results obtained for energies u p to 20 keV. The stable defect fraction obtained from the MD simulatio ns a smoothly decreasing function up to 20 keV. Subcascade formation l eads to a slight increase in this ratio at 40 keV, where the value is about the same as at 10 keV. Secondly, the potential for a significant level of in-cascade vacancy clustering was observed. Previous cascade studies employing this potential have reported extensive interstitial clustering, but little evidence of vacancy clustering. Interstitial c lusters were found to be strongly bound, with binding energies in exce ss of 1 eV. The larger clusters exhibited a complex, 3D structure and were composed of [111] crowdions. These clusters were observed to migr ate by collective [111] translations with an activation energy on the order of 0.1 eV. (C) 1997 Elsevier Science B.V.