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In this work, the influence of Al-solutes on the mechanical behavior of Cu–AlX solid solutions has been studied using indentation strain rate jump tests for single crystalline and ultrafine-grained (UFG) microstructures from high pressure torsion (HPT) processing. Al-solutes in Cu classically lead to a solid solution strengthening, coupled with a decrease in stacking fault energy, which influences also the grain size after HPT processing. For all alloys, a higher hardness is found at lower indentation depths, which can be nicely described by a modified Nix/Gao model down to 100 nm indentation depth. Among the single crystals, the largest size effects are found for the higher solute contents, indicating a stronger work hardening at small length scales for the solid solutions. The dilute UFG solid solutions showed a strong softening after a strain rate reduction test, with a pronounced transient region. Cu–Al15 is, however, quite stable, showing abrupt changes in hardness without strong transients. This indicates that solute solution strengthening does not only influence the indentation size effect and structure formation during HPT processing but also stabilizes the grain structure during subsequent deformation.
A CrMnFeCoNi high-entropy alloy was investigated by nanoindentation from room temperature to 400 °C in the nanocrystalline state and cast plus homogenized coarse-grained state. In the latter case a 〈100〉-orientated grain was selected by electron back scatter diffraction for nanoindentation. It was found that hardness decreases more strongly with increasing temperature than Young’s modulus, especially for the coarse-grained state. The modulus of the nanocrystalline state was slightly higher than that of the coarse-grained one. For the coarse-grained sample a strong thermally activated deformation behavior was found up to 100–150 °C, followed by a diminishing thermally activated contribution at higher testing temperatures. For the nanocrystalline state, different temperature dependent deformation mechanisms are proposed. At low temperatures, the governing processes appear to be similar to those in the coarse-grained sample, but with increasing temperature, dislocation-grain boundary interactions likely become more dominant. Finally, at 400 °C, decomposition of the nanocrystalline alloy causes a further reduction in thermal activation. This is rationalized by a reduction of the deformation controlling internal length scale by precipitate formation in conjunction with a diffusional contribution.
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