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一个广泛研究的主题。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)是从粗晶单相材料中获得超细晶甚至纳米晶微结构的一种非常有效的方法。不幸的是，由于其大的晶界区域储存了大量的能量，处理后的结构在热上是不稳定的，并且微结构的粗化(即使在室温下)也显著改变了材料的性能。因此，纳米晶和超细晶材料的热稳定性是未来潜在应用的根本问题。本项目的目标是开发新型HPT工艺复合材料，克服结构不稳定性这一主要问题。分散在镍基上的碳纳米管(CNTs)可以稳定微结构，防止晶粒长大。此外，由于碳纳米管具有优异的固有性能，因此有望改善其力学性能和摩擦学性能。为了实现这一目标，将优化HPT工艺以获得碳纳米管在基质中的均匀分散，并将深入研究高能HPT处理对碳纳米管的影响。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。综合微观结构表征，结合碳纳米管对SPD法制备的碳纳米管增强金属基复合材料的力学性能、摩擦学性能和热稳定性的影响，寻找能够描述最优材料性能的主导结构/性能关系。这种结构/性质关系将为优化具有明确性质的碳纳米管复合材料提供工具。因此，建议的项目可以作为该领域进一步研究其他基质材料和其他类型的加固阶段的起点。此外，这些结果将有助于从科学角度评估这些复合材料是否适合用于机械应用。

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.