Spark plasma sintering of multiwall carbon nanotubes reinforced titanium-aluminium-vanadium based nanocomposites

Abstract: Innovations in materials development has engendered the improvement of the properties of titanium alloys for diverse engineering applications. In this study, titanium alloy (Ti6Al4V) and multiwall carbon nanotubes (MWCNT) were mixed using shift-speed ball milling (SSBM) technique to achieve the uniform dispersion of MWCNT in the Ti6Al4V matrix. The starting and admixed powders were characterized using scanning electron microscopy equipped Energy dispersive X-Ray spectroscopy (SEM-EDS), X-ray diffraction (XRD), Raman spectroscopy and Transmission electron microscopy (TEM). Detailed TEM characterization was carried out to reveal the structural evolutions of the MWCNT during the dispersion process using selected area diffraction and fast Fourier transform pattern. The admixed powders were then consolidated using the spark plasma sintering machine (model HHPD-25, FCT GmbH Germany). Furthermore, various characterization technique such as XRD, SEM-EDS, optical microscopy (OM) was employed to understand the phase evolutions, morphology, microstructural changes and fractography of the fabricated nanocomposites. The mechanical properties of the fabricated materials were further investigated using the Vickers microhardness tester (FALCON 500 series) and the nanoindentation technique (ultra nanoindenter) UNHT. The study resulted in five (5) research articles, each article investigated the following respectively; (1) the previous works that have been conducted in SPS of titanium-based nanocomposites reinforced with MWCNT, (2) the dispersibility, structural evolutions and interfacial bonding of MWCNT in Ti6Al4V powders using the SSBM technique, (3) the evaluation of the influence of varying sintering temperature on the sintering and densification behaviours and microhardness of the fabricated alloy and nanocomposites, (4) the effects of MWCNT addition on the change in microstructures and mechanical properties of the fabricated nanocomposites, and (5) the influence of MWCNT on the nanomechanical properties of the fabricated nanocomposites. During the dispersion process, the Raman and XRD pattern of the admixed powders showed that mechanical stresses were induced on the walls of the nanotubes which does not result in defects on the MWCNT. Optimal dispersion of MWCNT was achieved on the nanocomposite grades comprising of 0.5 and 1.0 wt.% nanotubes. Additionally, the dispersibility decreased with the increase in concentration of the MWCNT in the Ti6Al4V matrix. Meanwhile, the nanocomposite grades with higher fraction of MWCNT experienced higher deformation of the nanotubes during the dispersion process. The optimal dispersion of MWCNT in Ti6Al4V matrix with minimal...D.Ing. (Metallurgy

Microstructural characterization and mechanical properties of carbon nanotube reinforced nickel aluminide composites

Ph.D. (Metallurgy)Abstract: 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

Enhancement of Mechanical and Tribological Properties of Ti-6Al-4V Alloy

Incorporation of TiC particle was adopted as one of the available methods to study the enhancement in tribological and mechanical properties of Ti-6A14V alloy. The mechanical performance of Ti-6A1-4V-10Vol%TiC sample (TMC) was evaluated using tensile and micro-scratch tests. Ball-on-disk tests at various loads revealed that the wear resistance of Ti-6A1-4V was improved via incorporation of TiC provided that the contact pressure remained lower than 0.98 GPa. Thermal oxidation was adopted to improve the high pressure wear resistance of TMCs. The optimum oxidation condition was characterized using XRD, SEM observations, and micro-scratch tests. The fracture toughness of the coatings was measured by micro-indentation tests. Ball-on-disk tests revealed that oxidation at the optimum condition (at 800°C for 20 min) significantly improved the wear resistance of the TMCs compared to uncoated TMCs. The mechanism and the effect of oxidation process were analyzed and the operative wear mechanisms at different loads were discussed

Fabrication and characterization of aluminum-carbon nanotubes (Al-CNT) functionally graded cylinders

Attention to carbon nanotubes (CNT) reinforced aluminum matrix composites has been growing considerably for the past decade owing to the expected improvement in specific properties that this nano-composite could offer. Functionally graded material (FGM), is also a rapidly developing field of materials science offering the possibility to manufacture components with desired properties at selective locations to serve different applications. Nano FGMs are the newest generation of this class of materials that is currently being theoretically researched intensely with more requirement for experimental research to support and verify the theoretical and numerical models. In the current research, an experimental study was conducted to develop a route to fabricate Al-CNT functionally graded cylinders varying materials along radial direction. Larger content of CNT reinforcement was selectively used at the outer layers to provide the highest strength, hardness and wear resistance at the surface subjected to higher stresses and more severe environments. Pure Al was chosen as the core material since it is subjected to lower stresses and to provide overall ductility to the material. The gradient selected is also cost effective as the expensive CNT is placed only in the layers that require most strengthening while maintaining a soft tough core leading to a balanced set of properties. The produced specimens having increased strength and hardness at the surface in addition to overall combination of high strength, ductility and light weight, together with the cylindrical geometry of the produced FGMs could be used in various applications especially mobility related applications such as drive shafts for aerospace and automotive industries. Testing and characterization of the produced samples were carried out to evaluate the performance of the cylindrical FGMs and the interfacial bonding between the different layers. Two sets of functionally graded specimens were produced; first is “FG2%” having the 2%CNT-Al at the surface then followed by layers of 1%CNT-Al, milled Al and reaching pure Al at the core. The second is “FG5%” having the 5%CNT-Al at the surface then followed by layers of 2%CNT-Al, milled Al and reaching pure Al at the core. Two specifically designed molds were manufactured to produce the functionally graded cylinders; mold A and mold B with enhancements implemented in mold B to produce well bonded cylindrical FGMs. Al-CNT composite powders with 1, 2, 5wt.% CNT fraction as well as milled Al were manufactured deploying planetary high energy ball milling technique (HEBM) of powders of CNT and Al. These were loaded in the designated layers of the manufactured molds. Powder metallurgy process was followed; starting with cold compaction and followed by sintering, then hot extrusion to produce compacts of Al-CNT FGMs of high density. Mechanical testing and characterization were carried out on the produced specimens and these included; mechanical compression, tension, optical microscopy, scanning electron microscopy as well as nanoindentation. It was concluded that the developed route and novel manufactured molds were successful to produce well bonded functionally graded cylindrical structures of Al-CNT. Adding CNT to Al reaching 2%wt. CNT at the outer layer with varying composition to reach pure Al at the core resulted in very unique balanced properties. The samples showed high strength while retaining a higher percentage of the material ductility compared to pure Al and homogeneous composite competitor having the same overall CNT content. The compression strength and hardness values also showed significant enhancement. Increase in content of CNT up to 5%wt. showed a remarkable increase in compression strength and hardness with some enhancement in modulus of elasticity. However, its full potential of strength and ductility could not be confirmed in this study due to fracture of the specimens under tension outside the gauge length. This could be due to increased sample notch sensitivity or increased internal stresses between layers as CNT percentage increases