Rate limiting deformation mechanisms of bcc metals in confined volumes

The influence of microstructure on the strength scaling behaviour of ultrafine-grained bcc metals is investigated by scale-bridging experiments spanning four orders of length on tungsten and chromium. By performing macroscopic compression experiments, nanoindentation and in-situ micro-compression tests in a scanning electron microscope, the plastically deformed volume was thoroughly reduced until a transition from bulk behaviour to single crystalline deformation characteristics was achieved. The stress strain behaviour and local sample deformation characteristics were related to apparent deformation mechanisms established for polycrystalline bcc metals. The influence of small dimensions, interfaces and free surfaces on the deformation behaviour is considered with respect to the single crystal situation. The increasing fraction of free surfaces in small volumes explicitly alters the strength scaling behaviour in dependence of the intergranular dislocation accumulation processes. Furthermore, thermally activated deformation was analysed based on rate- and temperature-dependent properties, such as strain-rate sensitivity and activation volume. To mechanistically interpret this data, a dislocation based model predicting the temperature dependent activation volume was developed. We find that thermally activated kinks control the rate dependent properties below the critical temperature for all length scales and microstructure states. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd.