Generation of ultrafine grained Ni-CNT composites by severe plastic deformation

Nanocrystalline and ultrafine grained materials have been a subject of extensive research over the past couple of decades due to their unique mechanical and functional properties. Although several routes to obtain such materials are currently known, one of the most efficient processes is severe plastic deformation. High pressure torsion (HPT) processing, for example, is a very effective way to generate ultrafine grained or even nanocrystalline microstructures from initial coarse grained single phase materials. Unfortunately, the processed structures are thermally unstable due to the high amount of stored energy in their large grain boundary area, and coarsening of the microstructures (even at room temperature) alters material properties considerably. As a consequence, the thermal stability of nanocrystalline and ultrafine grained materials is a fundamental issue for future potential applications.

The aim of this project is the development of novel HPT-processed composite materials, in which the major issue of structural instability is overcome. Carbon nanotubes (CNTs) dispersed throughout a Ni matrix can stabilize the microstructure against grain growth. Additionally, due to their outstanding intrinsic properties, CNTs are expected to improve the mechanical and tribological properties as well. To achieve this goal, the HPT process will be optimized to obtain a homogeneous dispersion of the CNTs within the matrix, and the effect of the highly energetic HPT processing on the CNTs will be thoroughly studied. Additionally, an advanced characterization of the microstructural state through the different processing steps (as-sintered, as-deformed and annealed) will be performed. The comprehensive microstructural characterization in combination with an examination of the CNT influence on the mechanical and tribological properties as well as on thermal stability of SPD processed CNT-reinforced metal matrix composites will be used to find the governing structure/property relationships which can describe the optimal material performance. Such structure/property relationships will provide a tool for the optimization of CNT-based composites with well-defined properties. Therefore, the proposed project can serve as a starting point regarding further research in this area with other matrix materials and other types of reinforcement phases. Additionally, these results will be useful for the evaluation of these composites from a scientific perspective as suitable candidates for mechanical applications.