ON THE CHARACTER OF SELF-INTERSTITIAL LOOPS IN VANADIUM--L. A. Zepeda-Ruiz (Lawrence

Isolated self-interstitial atoms (SIA) and SIA clusters produced during collision cascades are key components of the microstructure observed when metals are irradiated with high-energy particles. The evolution of these defects may cause undesirable changes in the mechanical properties of the material under irradiation. Therefore, knowledge of the properties, formation and diffusion mechanisms of SIA is essential for understanding and predicting the effects of radiation damage. In this study, we used molecular statics and molecular dynamics (MD) simulations based on a new Finnis- Sinclair potential, fit to first-principles calculations of point defect properties, to investigate the energy and structure of SIA dislocation loops in vanadium. We found that SIA dislocation loops with a/2<111> Burger's vector were the lowest energy configuration in vanadium, and migrated rapidly along their <111>-glide cylinder. Initial dislocation loop configurations with Burger's vector of a/2<110> or a<100> rotated into a/2<111> orientations at very low temperatures during the computational relaxation scheme used in this work and indicate that the formation energy of a/2<110> and a<100> loops is much higher than a/2<111> loops. Our results were compared to experimental observations and recent results in ferritic alloys which detail the formation mechanism responsible for the nucleation and growth mechanism of a<100> dislocation loops. Unlike in Fe, where a metastable a<100> loop is very close in energy to the ground-state a/2<111> orientation, constrained a<100> loops in V have considerably higher formation energies than a/2<111> loops, and the energy difference increases with size. Finally, our MD simulations of the interaction between two mobile a/2<111> clusters, according to the reaction proposed for a<100>-loop formation in Fe, revealed the formation of a single resulting a/2<111> loop. The simulations indicated that while a junction reaction occurs (a <100>-junction does form), the junction has a low thermal stability and rotates into a a/2<111> orientation at temperatures of 600-800K. The simulations performed to date provide no indication that the smaller a<100> junctions will propagate across the loop, but instead will dissolve with low thermal stability.