Microstructural and mechanical analysis of carbon nanotube reinforced magnesium alloy powder composites

Carbon nanotube (CNT) is an effective reinforcement used to improve the mechanical and thermal responses of metal matrix composites. It is, however, obvious that segregation of CNTs due to their strong van der Waals forces will produce material defects, decreasing the material properties. An advanced powder metallurgy process that disperses un-bundled nanotubes has been developed, and it is applied to fabricate a Mg matrix composite reinforced with CNTs in the present study. When approximately 1 vol.% CNTs were added, the extruded pure Mg and AZ31B alloy composites displayed an extremely large increase of the tensile yield stress of 25-40%, compared to Mg materials containing no CNT. HR-TEM verified the presence of MgO thin layers of 2-4 nm thickness, originating in the oxide surface films of the raw Mg and its alloy powders. These layers exist at the interface between α-Mg and the un-bundled nanotubes. The oxide layer showed a homogeneous mixture containing both α-Mg and CNT. This mixture resulted in an effective tensile loading transfer at the interface, which significantly improved the tensile strength (TS) and yield stress (YS) of the Mg composites. However, the elongation was less than 5%, and the composites exhibited a very poor ductility. This was because the MgO layers at the interface between α-Mg and CNT were very ductile.