Microstructural thermal stability of CNT-reinforced composites processed by severe plastic deformation

The microstructural thermal stability and mechanical properties of carbon nanotube (CNT)-reinforced Ni matrix composites processed by high pressure torsion (HPT) are investigated. A structural assessment of the CNTs at the different processing steps was performed by means of Raman spectroscopy, not detecting any significant modification of the CNT structure throughout the whole procedure. After deformation, the composites showed a maximum fivefold increase in hardness, related to their microstructure and resembling a Hall-Petch behavior. In order to study the thermal stability, an annealing was made at a homologous temperature of 0.33, which exceeds the grain growth onset temperature for this type of composites. The presence of CNTs helped stabilize the microstructure by pinning the grain boundaries during HPT. This stabilization translated into a significant increase in hardness and remained, to a certain degree, after the thermal treatment. Our results highlight the feasibility of obtaining CNT-reinforced composites by this means, improving the properties without compromising the structural features of the reinforcement and increasing their distribution within the matrix.