Microstructural Evolution, Strengthening Mechanisms, and Fracture Behavior of Aluminum Composites Reinforced with Graphene Nanoplatelets and In Situ–Formed Nano-Carbides

The microstructure and mechanical properties of GNP-reinforced aluminum composites obtained by powder metallurgy and hot extrusion (at 400 °C, 500 °C, and annealing at 3 h at 610 °C), were investigated. It was found that: (i) depending on the processing applied, the composites showed an increase in yield strength (YS) and ultimate strength (US) of up to 283%, and 78%, respectively; (ii) depending on the size of the ex situ GNP and in situ Al4C3 reinforcements, two fracture mechanisms are observed: ductile and brittle–ductile; (iii) annealing for 3 h at 610 °C did not improve the mechanical properties; (iv) the plot of YS vs. the volume fraction of the GNP introduced showed a peculiar pattern not been reported so far. Theoretical analysis of the results showed: (1) the major contributor to the YS increase is the Hall–Petch mechanism; (2) the reinforcements contribution to YS, complements that of Hall–Petch; (3) the main contributor to the composite strength is GNP; (4) a critical size of the reinforcement exists, 1.43 nm, at which the YS is maximal, 260 MPa; (5) the increase in the processing temperature and time leads to Ostwald ripening and increase of Al4C3 size and deterioration of mechanical properties.