Fabrication of ultrahigh-strength aluminum matrix composites reinforced with high-content carbon nanotubes and ex-situ added alloying elements

An ex-situ alloying approach was developed to tackle the uniform dispersion challenge of high-content carbon nanotubes (CNTs) in hard aluminum (Al) alloy powders that are difficult to deform during the high energy ball milling (HEBM) process. The approach includes (1) attaining uniform distribution of high-content CNTs in soft pure Al powders via HEBM, followed by (2) short-duration HEBM with elemental Zn/Mg/Cu powders. Microstructural analysis showed that the high-content CNTs promoted dynamic recrystallization in the Al matrix, leading to a heterogeneous grain structure consisting of fine-grained and coarse-grained zones. The high density of dislocations introduced by CNTs resulted in more and finer precipitates in the fine-grained zones. The synergistic enhancement between CNTs and precipitates in composite yielded an exceptionally high tensile strength of 855 MPa, which registered a new record among the bulk CNTs/Al composites in literature. The strengthening mechanisms were discussed based on thorough microstructure characterizations. This study proposes a paradigm for microstructure tailoring to fabricate Al alloy composites reinforced with high-content CNTs via optimized preparation techniques and precipitation engineering.