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

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.

Automated summary

In this study, bimodal microstructures were prepared in CNT/Al-Mg composites with the weight fraction of coarse grains. The sample with 25 wt% coarse grains showed improved plastic deformation behavior and higher strain hardening ability. The uniform micro-strain distribution in the CG25 sample was attributed to the constrained deformation of soft coarse-grain phase by hard CNT-enriched UFG Al-Mg phase.