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

纳米晶和超细颗粒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课题。虽然目前已知几种获得这种材料的方法，但最有效的方法之一是严重的塑性变形。例如，高压扭转（HPT）加工是一种非常有效的方法，可以使原始粗晶单相材料产生超细晶甚至纳米晶组织。不幸的是，由于在其大晶界区域中储存了大量的能量，加工后的结构热不稳定，并且微结构的粗化（即使在室温下）大大改变了材料的性能。因此，纳米晶和超细颗粒材料的热稳定性是未来潜在应用的一个基本问题。该项目的目的是开发新型hpt加工复合材料，其中克服了结构不稳定的主要问题。分散在Ni基体中的碳纳米管（CNTs）可以稳定组织，抑制晶粒生长。此外，由于其优异的内在性能，碳纳米管也有望改善机械和摩擦性能。为了实现这一目标，将对HPT工艺进行优化，使CNTs在基体内均匀分散，并深入研究高能HPT工艺对CNTs的影响。此外，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。综合微观结构表征，结合碳纳米管对SPD加工的碳纳米管增强金属基复合材料的机械和摩擦学性能以及热稳定性的影响的研究，将用于找到可以描述最佳材料性能的控制结构/性能关系。这种结构/性能关系将为优化具有良好性能的碳纳米管基复合材料提供工具。因此，建议的项目可以作为进一步研究其他基质材料和其他类型的增强相的起点。此外，这些结果将有助于从科学的角度评估这些复合材料是否适合机械应用。

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.