Bauschinger and size effects in thin-film plasticity

We present an experimental investigation of the effects of surface passivation, film thickness and grain size on the plastic behavior of freestanding Cu thin films. The stress-strain curves of the films are measured using the plane-strain bulge test. Films with a passivation layer on one or both surfaces have an offset yield stress that increases significantly with decreasing film thickness; the yield stress of unpassivated films, by contrast, is relatively independent of film thickness and increases mainly as a result of grain-size strengthening. The stress-strain curves of passivated films show an unusual Bauschinger effect on unloading. This effect is not observed for unpassivated films. Our experimental results suggest that passivation layers prevent dislocations from exiting the films and that they block slip bands at the film-passivation interface. The back stresses associated with these blocked slip bands increase the resistance to forward plastic flow on loading and cause reverse plastic flow on unloading. The effect of the back stresses increases with decreasing film thickness and leads to the observed strengthening of the passivated films. The constraint of a passivating layer on dislocation motion and hence on plastic flow cannot be described by classical plasticity theories, but can be modeled with some strain-gradient plasticity theories. We evaluate the suitability of the strain-gradient plasticity theory developed by Fleck and Hutchinson to describe our experimental results in a continuum framework. Comparison between experimental results and calculations yields very good agreement for the effect of film thickness, but the strain-gradient plasticity model fails to describe the Bauschinger effect observed in passivated films. (c) 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.