A quantitative understanding on the mechanical behaviors of carbon nanotube reinforced nano/ultrafine-grained composites

• Mechanical behavior of atomically disordered regions near CNTs has been discussed.
• CNT-free regions are surrounded by grain boundary affected zone (GBAZ).
• GNDs induced by thermal mismatch and strain gradient effect are limited in GBAZ.
• The roles of three kinds of strengthening mechanisms have been highlighted.

The mechanical responses of carbon nanotube (CNT) reinforced nano/ultrafine-grained metal matrix composites (MMCs) with the reinforcement located at grain boundaries and atomically disordered regions near the reinforcement have not been discussed yet. Here a theoretical model based on dislocation-controlling constitutive relations of metal matrix and strain gradient effect is adopted to quantitatively understand the contributions of strengthening mechanisms to the mechanical response of these composites. CNT-reinforced nano/ultrafine-grained MMCs will be treated as a two-phase composite with CNT-free regions surrounded by CNT-containing domains, whose mechanical behaviors are different from that of the matrix-only regions. The predictions of this model are in excellent agreement with available experimental results. During the process of deformation, geometrically necessary dislocations induced by strain gradient and thermal mismatch are limited within CNT-containing domains, and the thicknesses of these regions are obtained to be linearly proportional to matrix grain size by fitting with the experimental results. When the size of the reinforcement is large, the effect of thermal mismatch strengthening mechanism on the strength of MMCs is slight and can be ignored compared with the roles of Hall–Petch strengthening or load transfer effect.