Edge dislocations in fcc metals: Microscopic calculations of core structure and positron states in Al and Cu.

Abstract

Structures of edge dislocations (dislocation line direction [11\ifmmode\bar\else\textasciimacron\fi{}2], Burgers vector (a/2)[110]) in aluminum and copper are studied with the molecular-dynamics simulation method which incorporates the effective-medium theory extended to include atomic interactions beyond the nearest neighbors. The observed equilibrium distance between the Shockley partial dislocations in both metals agrees well with the estimate evaluated from elasticity theory by using known values for the stacking-fault energy and the bulk modulus. Effective-medium theory leads to a dislocation core which is narrower than the previous results of the pair-potential calculations for simple metals. The vacancy formation energy in copper is higher at the center of the stacking-fault region than in the bulk, which suggests that pipe diffusion can take place only in the narrow partial dislocation core. In the second part of the work, the role of the dislocation line and of the associated point defects as positron traps are studied in both metals. The pure dislocation line forms only a shallow trap but it can be a precursor state for deeper traps, like vacancies and single jogs on the dislocation line. The calculated lifetimes for these defects are in good agreement with the experimentally observed lifetime components coming from dislocations.