Microstructural evolution and strengthening mechanisms in Al7075/ graphene nano-plates/ carbon nano-tubes composite processed through accumulative roll bonding

In the present research, Al7075 matrix composites containing carbon nano-tubes (CNTs) and graphene nano-plates (GNPs) processed through accumulative roll bonding (ARB). Accumulative roll bonding carried out up to 3 passes at 400 °C with 50% thickness reduction per pass. The effect of CNTs and GNPs on the microstructural evolution and strengthening mechanisms were investigated. Microstructural evolutions in processed composites were characterized by using field emission scanning electron microscopy (FE-SEM), energy dispersive spectroscopy (EDS), and electron backscatter diffraction (EBSD). The grain orientation spread (GOS) analysis shows that the volume fraction of recrystallization in deformed grains depends on the level of particle stimulated nucleation (PSN) in the interface between matrix and reinforcement. The occurrence of PSN is affected by the type of reinforcement and the number of ARB passes. The level of PSN depends directly on the extent of the plastic deformation zone (PDZ) in the vicinity of the matrix/reinforcements. In the hybrid composite, the PSN mechanism was restrained due to Zener pinning of grain boundaries. Texture evolution is mainly carried out through β fiber for the composites subjected to the ARB process. In all ARBed samples, the main texture components include one or two of S, Brass, and Copper. The presence of CNTs and GNPs in the composite led to a change in both intensity and type of the main texture components formed during the ARB. In the composite containing GNPs, with increasing of ARB passes the volume fraction of PSN decreased remarkably due to strain hardening and consequently recrystallization was limited. It was found that the most significant strengthening mechanism which has the key role in the increase yield strength is the load transfer mechanism. The role of coefficient of thermal expansion mismatch and Orowan mechanisms in the increase yield strength and hardness of processed composites is less and the effect of the Hall–Petch mechanism is negligible.