A MOLECULAR-DYNAMICS STUDY OF DISPLACEMENT CASCADES IN ALPHA-IRON

The mechanisms of defect production in displacement cascades in alpha-iron have been investigated by computer simulation. Cascades of up to 5 keV in energy have been simulated by molecular dynamics in crystals with atomic interactions described by a many-body potential. The effects of lattice temperature have been studied by using block temperatures of either 100 or 600 K. 80 cascades have been modelled overall. The morphology of cascades during the collisional phase changes at about 1-2 keV, due to the collective nature of atomic displacements at higher energy. This transition is reflected in the relaxation time during the subsequent recombination phase, and it also decreases the efficiency factor for defect production. This factor is similar in size to that obtained from recent modelling of copper, an fee metal. Although the cascade zone contains a large number of displaced atoms, true melting was not observed in alpha-Fe, and vacancy clustering did not occur in the thermal spike phase. Interstitial clustering has been analysed, and found to be less pronounced than in copper. One large cluster was observed to grow by interstitial movement during the thermal spike, and visual analysis has shown that it formed a perfect dislocation loop: it was not nucleated by the Eyre-Bullough mechanism, however. Statistics on the cascade parameters are presented, and comparisons with work on other crystal structures are drawn where possible.