Microstructure and mechanical properties of the carbon nanotubes reinforced AZ91D magnesium matrix composites processed by cyclic extrusion and compression

Cyclic extrusion and compression (CEC) was taken to process the carbon nanotubes (CNTs) reinforced AZ91D composites. Effects of CEC passes and CNTs on the microstructure evolution, hardness and tensile properties of the post-processed composites were investigated. Results show that the matrix grain of the 0.5 wt% CNTs/AZ91D composites is greatly refined from ~112 μm to ~126.6 nm after 8 passes of CEC, with Mg17Al12 uniformly distributed along grain boundaries. The addition of CNTs leads to a reduced matrix grain, but exerts a limited effect on the Mg17Al12 precipitates. The initial highly-agglomerated CNTs are gradually dispersed with the progress of CEC, though degrading of CNTs occurs. The implement of CEC markedly enhances the hardness, YS, UTS and elongation to fracture of both the monolithic AZ91D alloy and the CNTs/AZ91D composites, all of which are mainly attributed to the greatly refined matrix grain and the uniformly distributed Mg17Al12 precipitates. Incorporation of CNTs increases the hardness, YS and UTS of the base alloy, but decreases the elongation to fracture, which is closely related with the gradually dispersed but degraded CNTs.