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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. [ABSTRACT FROM AUTHOR]

Abstract: The grain growth stagnation produced by the addition of multiwalled carbon nanotubes to bulk nickel composites manufactured by hot uniaxial pressing is investigated. A hardness improvement up to 27% compared to pure nickel was achieved, mainly based on the microstructural refinement. However, this enhancement is only observable up to concentrations of 3wt%. Beyond this point, no significant improvement is observed, thus setting an empirical reinforcing limit. The main strengthening mechanism identified was the Hall–Petch effect, which hinders the dislocation movement and is strongly dependent on the final grain size of the composites. Finally, a model for the prediction of the final grain size is proposed based upon the Zener pinning equation for the particular system studied in this work. This model is intended to provide a basic tool for a further tailoring of the mechanical properties, adjusted for a certain application. [Copyright &y& Elsevier]