Microstructure and mechanical properties of AZ91 alloy reinforced by carbon nanotubes coated with MgO

Carbon nanotubes (CNTs) are novel reinforcement of Mg-based composites because of their attractive mechanical properties. However, in general, nanosize CNTs did not completely impart their exceptional properties to the Mg matrix because of the poor interfacial bonding between CNTs and the matrix and the aggregation of CNTs, which largely limit their applications in Mg-based composites. In this study, a novel method was developed to increase the interfacial bonding strength by coating magnesium oxide (MgO) nanoparticles on the surface of CNTs. The detailed crystallographic analysis by transmission electron microscopy (TEM) confirmed that two types of bonds were formed at the CNTs/MgO interface: nanoscale-contact bonds and diffused interfacial bonds. Moreover, for the first time, the orientation relationships of (11¯1)MgO//(011¯1)α−Mg and [011]MgO//[21¯1¯0]α−Mg with the semicoherent interface characteristics were observed between MgO and the matrix of α-Mg. The interatomic spacing misfit at the MgO/α-Mg interface is 6.5%, which indicates a good lattice space matching between MgO and the matrix of α-Mg. The yield strength and elongation of AZ91-3.0MgO@CNT composite were enhanced by 69.0% and 22.9%, respectively, when compared with the AZ91 alloy. The mechanism of strengthening of AZ91 alloy composites reinforced by MgO@CNTs was discussed in detail.