It is commonly known that ultrafine grained (ufg) and nanocrystalline materials exhibit outstanding strength due to grain boundary strengthening. For the case of body centred cubic (bcc) metals, in addition to the thermal component of the flow stress, indicated by the critical temperature of the respective material, the vast fraction of interfaces can strongly influence the rate dependent deformation characteristics, in particular at elevated temperature.
In this work, we examine the temperature and rate dependent mechanical properties of ufg bcc Chromium and Tungsten, which both exhibit strong contributions of the thermal stress component to the flow stress. We address the influence of the free surface and interface fraction as well as the loading condition on the deformation mechanisms at ambient and elevated temperatures by combining uniaxial macroscopic compression experiments, in-situ micro-compression tests in a scanning electron microscope and multiaxial surface sensitive nanoindentation techniques. Moreover, to discriminate between microstructure and Peierls contributions, we compare these ufg results with small scale experiments on single crystalline samples. Based on the derived deformation characteristics such as strain rate sensitivity, activation volume and microstructural evolution, we discuss the underlying microstructure controlled deformation mechanisms.