High-temperature diffusion processes in ultrafine-grained aluminum and its composites containing multi-walled carbon nanotubes

The present study investigates the effect of multi-walled carbon nanotubes (MWCNTs) on atomic diffusion processes in the metal matrix, by comparing growth kinetics of interfacial phases in Al-Cu and Al/MWCNT-Cu diffusion couples. Multiple intermetallic layers such as Al₂Cu, AlCu, Al₃Cu₄, Al₂Cu₃, and Al₄Cu₉ are formed at the interface between Al and Cu during heat treatment of an Al/Cu diffusion couple at 530°C. For the diffusion couple of ultrafine-grained Al and cast Cu, the growth rate of intermetallic layers is comparable with theoretical expectations based on Fick's second law. On the other hand, MWCNTs significantly restrict the diffusion of Al atoms in the composite because the atoms tend to detour around the long tube for diffusion, particularly when MWCNTs are oriented perpendicular to the atomic diffusion path. This accelerates the growth of voids at the contact interface of the diffusion couple during heat treatment.