It is well established that ultrafine grained (ufg) and nanocrystalline materials exhibit outstanding strength, which is related to grain boundary strengthening. In addition to the thermal component of the flow stress, which is indicated by the critical temperature of the respective material, the high number of interfaces can strongly influence the deformation behaviour, in particular at elevated temperature.
In this work, we concentrate on the temperature and rate dependent mechanical properties of ufg body centred cubic chromium, which exhibits a strong contribution of the thermal stress component to the flow stress. We investigated the influence of the free surfaces on the deformation mechanisms at ambient as well as elevated temperatures above the critical temperature by using macroscopic compression experiments, in-situ SEM microcompression tests and nanoindentation techniques. Moreover, we compared these ufg results with single crystalline chromium to eliminate the grain boundary contribution. The results in terms of strain rate sensitivity, activation volume and microstructural evolution are discussed based on tested volume, surface fraction and stress state.