Abstract
Ph.D. (Metallurgy)
Toughened nickel aluminides were successfully synthesized by incorporating multi-walled carbon nanotubes (MWCNTs) into nickel aluminide (NiAl) intermetallic matrix. The drive to retain the relative lightweight of NiAl motivated the choice of MWCNTs as the reinforcing agent in this study. Moreover, enhanced mechanical properties were anticipated in the reinforced composites owing to the exceptional properties of the MWCNTs. Elemental powders of nickel and aluminium were blended together with MWCNTs in a novel two stage ball milling for optimum dispersion and preservation of the structural integrity of the MWCNTs. The milled powders were consolidated by Spark Plasma Sintering (model HHPD- 25, FCT GmbH, Germany). The milled powders and sintered samples were characterized using Scanning Electron Microscopy, Transmission Electron Microscopy and X-Ray Diffraction. The nano-structural evolution of the MWCNTs during their dispersion via dry ball milling was further evaluated using Raman Spectroscopy. The mechanical properties and fracture behaviours of the reinforced sintered samples were critically evaluated using nanoindentation techniques. Results show that the integration of MWCNTs into the NiAl matrix led to an enhancement of the fracture toughness. An inverse relationship between the hardness and fracture toughness of the NiAl composites was established. The intergranular fracture morphology of the unreinforced NiAl transited to a dominantly dimpled fracture morphology in the NiAl-1.0 wt% CNTs composites indicating enhanced ductility and fracture toughness. The improvement of the fracture toughness of the reinforced NiAl is attributed to the uniform dispersion of theMWCNTs within the NiAl matrix, the preservation ofMWCNTs aspect ratios and the disordering of the B2 ordered NiAl intermetallic structure.