Carbon nanotubes (CNTs) reinforced Al matrix composites containing high-density stacking faults were successfully fabricated through spark plasma sintering (SPS) and subsequently hot rolling with high strain rate. It is found that the inclusion of CNTs facilitate the formation of stacking fault in Al matrix. With increasing the CNTs from 0.5 vol% to 1.0 vol%, the density of stacking fault increases, which is mainly because of the reduced stacking fault energy of Al matrix in Al/CNTs composites, as revealed by the density functional theory simulation. Combined with the effective exploitation of the strengthening effectiveness of CNTs due to uniform dispersion of CNTs and well bonded Al-CNTs interface, the highest tensile strength (378 ± 8 MPa) together with good ductility (17.1 ± 1.5%) can be achieved for 1 vol% CNTs reinforced Al matrix composite. Moreover, the enhanced strength deriving from each strengthening mechanism relies on the CNTs content. When the CNTs content is 1.0 vol%, Orowan looping and stacking fault strengthening are the dominant strengthening mechanisms. The reported microstructure design and control can be informative for improving the mechanical properties of CNTs reinforced metal matrix composites.

Exploiting the synergic strengthening effects of stacking faults in carbon nanotubes reinforced aluminum matrix composites for enhanced mechanical properties

Carbon nanotubes (CNTs) reinforced Al composites with a unique intragranular dispersion were obtained by using ultra-short CNTs. Nano-scaled CNTs were uniformly distributed inside the ultrafine Al grains. In comparison with typical CNT/Al composites with intergranular CNTs dispersion, this intragranular CNT/Al composites enable stronger dislocation pinning and retention, thereby exhibits simultaneous improvement in strength and ductility. The present intragranular dispersion strategy will provide inspirations for fabricating strong and ductile nanocarbon reinforced metal matrix composites.

Reinforcement with intragranular dispersion of carbon nanotubes in aluminum matrix composites

Superior high-temperature tensile properties of aluminum matrix composites reinforced with carbon nanotubes

The uniform distribution of carbon nanotubes (CNTs) can lead to severe grain refinement to ultrafine-grained (UFG) regime of Al matrix, resulting in low strain hardening ability, thereby low ductility in CNT/Al composites. To evade this dilemma, CNT/Al–Mg composites with bimodal microstructures were prepared by a powder assembly process in this study. The effects of weight/volume fraction of coarse grains on microstructure evolution, plastic deformation behavior and micro-strain distribution were investigated. The elongation improved from 4.2% of the uniform microstructure to 5.2% for the bimodal CNT/Al–Mg composites with 25 wt% (CG25) coarse grains. The compression stress relaxation test revealed the enhanced effective stress and activation of multiple dislocation-mediated mechanisms during plastic deformation in the CG25 sample, which lead to sustainably higher strain hardening ability. The uniform micro-strain distribution was revealed in the CG25 sample by Kernel average misorientation analysis, attributed to the constrained deformation of soft coarse-grain phase by hard CNT-enriched UFG Al–Mg phase.

Enhanced strain hardening by bimodal grain structure in carbon nanotube reinforced Al–Mg composites

For nanocarbon reinforced aluminum matrix composites (AMCs), the architecture design of reinforcements is an effective route to obtain superior mechanical performances. In this work, a novel AMC reinforced by CNTs wrapped with γ-Al2O3 (CNTs@Al2O3) reinforcement with balanced strength and toughness was fabricated by powder metallurgy methods. The wrapping structure of γ-Al2O3 was synthesized through the in-situ reactions of Al matrix and boric acid decorated CNTs, thus improving the interface coupling of the composite. The CNTs@Al2O3/Al composite with the addition of 0.5 wt% CNTs and 10 wt% in-situ Al2O3 exhibited a mean ultimate tensile strength of 404 MPa, which is significantly higher than unreinforced pure Al (183 MPa) and those reinforced by individual CNTs (207 MPa) or Al2O3 (289 MPa). An improvement of ductility was also achieved in CNTs@Al2O3/Al composites compared with the Al2O3/Al composites with the same Al2O3 content. The synergistic strengthening and toughening effects are ascribed to the ingenious in-situ CNTs@Al2O3 wrapped structure, which highlights the importance of architecture design of reinforcements in AMCs, and sheds new light on the development of CNT-reinforced AMCs.

Yiqiang Chen

Papers

Enhancement of strength and ductility by interfacial nano-decoration in carbon nanotube/aluminum matrix composites