Deformation kinetics in ultrafine-grained (ufg) materials are commonly described by the strain rate sensitivity (SRS) of the flow stress and the related activation volume, which is important for the classification in different dominating deformation mechanisms.
For body centered cubic (bcc) metals such as chromium, the deformation behaviour is significantly influenced by a materials specific critical temperature Tc, but also by the present microstructure. Moreover, it is known that there are significant differences in deformation mechanisms between polycrystals and single crystals but up to know not well understood, in particular for small scale samples.
To contribute to this discussion, scale bridging compression tests on single crystalline and polycrystalline chromium with sample sizes ranging from the macroscopic to microscopic scale were performed. All ufg samples were produced using high pressure torsion (HPT), a severe plastic deformation process which yields to high representative strains.

At the macro scale, ufg compression samples with a specimen size of 2x2x3 mm³ were deformed at different temperatures in a vacuum chamber with varying strain rates between 10-4 and 10-2 s-1 to measure the stress-strain behaviour. From this data, values for SRS and activation volume were quantitatively calculated.
At the micron scale, ufg and single crystalline compression samples with a specimen size range between 0.5 µm and 4 µm were produced by FIB cutting. Compression tests were performed in-situ in a scanning electron microscope at room temperature and varying strain rates between 10-4 and 10-2 s-1 were applied. Occurring slip steps were compared with corresponding stress-strain curves and the initial shape of the samples to identify differences in slip system and global deformation behaviour.

The stress-strain behaviour of macro and micro ufg samples were compared and related to single crystalline chromium, where the well-known sample size effect was investigated. Finally, details of the stress-strain data were used to relate to the occurred differences in deformation mechanisms.