PNNL

Abstract

It is well known that grain boundaries in metals can be sinks for migrating defects such as mobile interstitial atoms, but less is known about the effects of grain boundaries on defect production and defect-grain boundary interactions due to displacement cascades in the vicinity of grain boundaries. Molecular dynamics simulations were performed for displacement cascades in the vicinity of both a symmetric S3{112} and a symmetric S11{323} grain boundary (GB) in a-Fe to investigate cascade-GB interactions and defect creation near GBs. Both self-interstitial atoms and vacancies are created within the S11 GB as well as within the S3 GB, although fewer defects are trapped in the S3 GB than in the S11. See Figures 1-3 and typical cascades in Figure 4. The relative numbers of surviving vacancies and interstitials per cascade residing within the GB vary as a function of the distance of the primary knock-on atom from the GB, with more interstitials than vacancies arriving at the GB from distant cascades. For both S3 and S11 GBs the total number of surviving defects per cascade increases somewhat with decreasing distance of the cascade from the GB, indicating that having some fraction of the defects trapped in the GB promotes the initial survival of more cascade defects overall relative to cascades in the perfect crystal. Molecular statics simulations of defect formation energies within the GBs (Figure 5) confirm that both vacancies and self-interstitials have lower defect formation energies in both the S3 and S11 GBs relative to their formation energies in the perfect crystal.