Microstructure, tensile properties and deformation behaviors of aluminium metal matrix composites co-reinforced by ex-situ carbon nanotubes and in-situ alumina nanoparticles

In this study, the origin of the high tensile ductility (>10% strain to fracture) of a new class of high-strength (yield strength >500 MPa) aluminium (Al) metal matrix composites (Al MMCs) co-reinforced by ex-situ carbon nanotubes (CNTs) and in-situ γ−Al₂O₃ nanoparticles was investigated. The microstructures of these unique Al MMCs prior to and post tensile testing were characterized in detail using high resolution transmission electron microscopy, in order to understand their response to tensile deformation. The pathways to the in-situ formation of γ−Al₂O₃ nanoparticles were identified and attributed to the high energy ball milling process, and the subsequent spark plasma sintering, which converted the amorphous Al-oxide into γ−Al₂O₃ nanoparticles. The in-situ γ−Al₂O₃ nanoparticles and Al matrix exhibited a semi-coherent interface, which led to a markedly increased dislocation density in the matrix around the nanoparticles during tensile deformation. These dislocations were precursors to microvoid formation and consequent dimples on further deformation. The co-operation of CNTs and γ−Al₂O₃ nanoparticles resulted in a long strain-softening stage and therefore excellent tensile ductility. The results suggested that the exploitation of hybrid reinforcement by ex-situ CNTs and in-situ nanoparticles can enable the fabrication of high-performance metal matrix composites.