Verformungsverhalten von ultrafeinkörnigen Metallen untersucht mittels neu entwickelter Nanoindentierungsmethoden

Maier, Verena

PhD Tesis (2013)

The aim of this work was to create a scale-bridging understanding regarding the basic mechanisms which cause the time- and strain-rate-dependent deformation behavior of different materials. To characterize these local mechanical properties and especially the strain-rate sensitivity and the local creep properties, new, more reliable nanoindentation methods were developed. Based on macroscopic strain-rate jump tests, a reliable nanoindentation method for the determination of the local strain-rate sensitivity has been developed by implementing abrupt strain-rate changes within a single indentation process. Experiments on nc-Ni showed that the global and the local deformation behavior are in good agreement. In order to enable a determination of the strain-rate sensitivity at even lower strain-rates, load controlled long-term indentation creep experiments were applied. In order to correct for the known influences, caused by thermal drift, a dynamically corrected indentation method has been improved and applied. Finally, it was found that the strain-rate dependent hardness, investigated by both, nanoindentation strain-rate jump tests and long-term creep tests, are in a good agreement. To determine the strain-rate dependent deformation behavior at elevated temperatures, an initial tip heating segment has been added to the indentation creep method to achieve a constant contact temperature between the heated sample and the not-heated but insulated indentation tip. These new methods were used to determine the local deformation behavior of face-centered cubic Al and Cu, body-centered cubic W and Mo and hexagonal-close-packed Ti and Ti-6Al-4V: On a local scale it has been found that ufg-Al shows significant higher strain-rate sensitivity even at room temperature compared with their cg-counterparts. An increase in the testing temperature led to an even more pronounced strain rate sensitivity, which is generally in very good agreement with literature data from macroscopic compression tests. Within the plastically deformed zone of the resulting impressions a clearly formed, near-surface grain boundary sliding behavior has been observed. This could be directly correlated to the high degree of freedom of the unconstrained surface grains. Thus it might not the dominant contribution to the enhanced strain-rate sensitivity. The interaction between solid-solution hardening and grain refinement has been analyzed investigating single-crystalline and nanocrystalline Cu-Al samples. The results of single crystalline samples were clearly influenced by effects of dynamic strain aging. The indentation size effect was significantly more pronounced with increasing Al concentration and thus lower stacking fault energy. The strain-rate dependent deformation behavior also showed a clear change towards lower strain-rate sensitivities with increasing aluminum content. This could be directly correlated with the stacking fault energy and acting deformation mechanisms. The local deformation behavior of single crystal and polycrystalline bcc metals (Mo and W) has also been investigated. Due to the high Peierls-energy, the single crystalline samples showed an increased strain-rate sensitivity at room temperature. In contrary to fcc-metals, ufg-W showed a reduced strain-rate sensitivity compared to the single-crystalline state. Due to the highly plastic deformation and the fine-grained microstructure the athermal stress component dominates the deformation. The thermally activated component, which is responsible for the strain rate at room temperature, lost its dominance. An increase in the testing temperature for sx-Mo to a level above the critical temperature Tc led to a significant change of the deformation behavior. An initially very weakly pronounced strain rate sensitivity increased with increasing holding time. This behavior could be directly correlated correlated qualitatively as well as quantitatively with dynamic recovery processes and the formation of a subgrain structure within the plastic deformed zone and underneath the indenter. In summary, it has been demonstrated that through the specific development of new nanoindentation methods and improvement of conventional methods, a reliable determination of the local, time-dependent deformation behavior is possible. These new methods allow especially in combination with microstructural investigations, a reliable analysis of the materials properties and they significantly contribute to a better scale-bridging understanding of deformation behavior of materials