A discrete dislocation dynamics simulation is used to investigate dislocation motion in the confined geometry of a polycrystalline thin film. The repeated activation of a Frank-Read source is simulated. The stress to activate the sources and to initiate plastic flow is significantly higher than predicted by models where the dislocations extend over the entire film thickness. An effective source size, which scales with the grain dimensions, yields flow stresses in reasonable agreement with experiments. The influence of dislocations deposited at interfaces is investigated by comparing calculations for a film sandwiched between a substrate and a capping layer with those for a free standing film.
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