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

纳米晶和超细晶材料由于其独特的力学和功能特性，在过去的几十年里一直是广泛研究的主题。尽管目前已知获得这种材料的几种途径，但最有效的方法之一是剧烈塑性变形。例如，高压扭转（HPT）处理是从初始粗晶单相材料产生超细晶或甚至纳米晶微结构的非常有效的方式。不幸的是，加工的结构是热不稳定的，由于大量的存储能量在其大的晶界区域，和粗化的微观结构（即使在室温下）改变材料的性能显着。因此，纳米晶和超细晶材料的热稳定性是未来潜在应用的基本问题。本项目的目的是开发新型HPT处理复合材料，其中结构不稳定的主要问题得到克服。碳纳米管（CNTs）分散在整个镍基体中可以稳定微观结构，防止晶粒生长。此外，由于其优异的固有性能，碳纳米管有望改善机械和摩擦学性能以及。为了实现这一目标，将优化HPT工艺以获得CNT在基体内的均匀分散，并将彻底研究高能HPT处理对CNT的影响。此外，还将通过不同的加工步骤（烧结、变形和退火）对微观结构状态进行高级表征。结合CNT对机械和摩擦学性能以及SPD处理的CNT增强金属基复合材料的热稳定性的影响的检查的综合微观结构表征将用于找到可以描述最佳材料性能的管理结构/性能关系。这种结构/性能关系将为优化具有明确性能的碳纳米管基复合材料提供工具。因此，拟议的项目可以作为一个起点，在这一领域的进一步研究与其他基质材料和其他类型的增强相。此外，这些结果将是有用的，从科学的角度来评估这些复合材料作为合适的候选人的机械应用。

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.