A nanoindentation strain-rate jump technique has been developed for determining the local strainrate sensitivity (SRS) of nanocrystalline and ultrafine-grained (UFG) materials. The results of the new method are compared to conventional constant strain-rate nanoindentation experiments, macroscopic compression tests, and finite element modeling (FEM) simulations. The FEM simulations showed that nanoindentation tests should yield a similar SRS as uniaxial testing and generally a good agreement is found between nanoindentation strain-rate jump experiments and compression tests. However, a higher SRS is found in constant indentation strain-rate tests, which could be caused by the long indentation times required for tests at low indentation strain rates. The nanoindentation strain-rate jump technique thus offers the possibility to use single indentations for determining the SRS at low strain rates with strongly reduced testing times. For UFG-Al, extremely fine-grained regions around a bond layer exhibit a substantial higher SRS than bulk material.

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Ultrafine-grained (UFG) metals produced by severe plastic deformation (SPD), i.e. equal channel angular pressing (ECAP) or accumulative roll bonding (ARB), exhibit outstanding mechanical properties. They show a high strength often paired with good ductility under monotonic loading and a strain rate sensitive behavior. [1] This is not only true for pure metals like aluminium of technical purity but also for other materials. [2][3][4][5][6] The enhanced strain rate sensitivity (SRS) is widely accepted to be a key issue for the improved ductility of UFG metals.Both processes are in principle suitable for industrial application. However, the ARB process is advantageous due to the possibility of continuous processing of large sheets. It is of interest if the mechanical behavior and especially the strain rate sensitivity SRS depend on the process of SPD. Such comparisons have already been performed for ECAP and high pressure torsion (HPT) by Alkorta et al. [7] on bcc metals. For fcc metals, the correlation of the microstructures and mechanical properties terms of strain rate sensitivity is up to now not completely understood.In this context UFG aluminium of technical purity (Al99.5) was produced with ECAP and ARB up to almost identical effective plastic strains of e ARB ¼ 6.4 and e ECAP ¼ 6.3. The mechanical properties as well as the strain rate sensitivity were investigated. Significant differences were observed and are discussed in terms of microstructural differences in this paper.

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