Browsing by Author "Riley, D"

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    Accommodation, accumulation, and migration of defects in Ti3SiC2 and Ti3AlC2 MAX phases
    (John Wiley and Sons, 2013-10-10) Middleburgh, SC; Lumpkin, GR; Riley, D
    We have determined the energetics of defect formation and migration in Mn+1AXn phases with M = Ti, A = Si or Al, X = C, and n = 3 using density functional theory calculations. In the Ti3SiC2 structure, the resulting Frenkel defect formation energies are 6.5 eV for Ti, 2.6 eV for Si, and 2.9 eV for C. All three interstitial species reside within the Si layer of the structure, the C interstitial in particular is coordinated to three Si atoms in a triangular configuration (C–Si = 1.889 Å) and to two apical Ti atoms (C–Ti = 2.057 Å). This carbon–metal bonding is typical of the bonding in the SiC and TiC binary carbides. Antisite defects were also considered, giving formation energies of 4.1 eV for Ti–Si, 17.3 eV for Ti–C, and 6.1 eV for Si–C. Broadly similar behavior was found for Frenkel and antisite defect energies in the Ti3AlC2 structure, with interstitial atoms preferentially lying in the analogous Al layer. Although the population of residual defects in both structures is expected to be dominated by C interstitials, the defect migration and Frenkel recombination mechanism in Ti3AlC2 is different and the energy is lower compared with the Ti3SiC2 structure. This effect, together with the observation of a stable C interstitial defect coordinated by three silicon species and two titanium species in Ti3SiC2, will have important implications for radiation damage response in these materials. © 2013, Commonwealth of Australia.
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    Colloidal processing of zirconium diboride ultra-high temperature ceramics
    (John Wiley and Sons, 2013-05-21) Tallon, C; Chavara, DT; Gillen, AL; Riley, D; Edwards, L; Moricca, SA; Franks, GV
    Colloidal processing of the Ultra-High Temperature Ceramic (UHTC) zirconium diboride (ZrB2) to develop near−net-shaping techniques has been investigated. The use of the colloidal processing technique produces higher particle packing that ultimately enables achieving greater densification at lower temperatures and pressures, even pressureless sintering. ZrB2 suspension formulations have been optimized in terms of rheological behavior. Suspensions were shaped into green bodies (63% relative density) using slip casting. The densification was carried out at 1900°C, 2000°C, and 2100°C, using both hot pressing at 40 MPa and pressureless sintering. The colloidally processed materials were compared with materials prepared by a conventional dry processing route (cold pressed at 50 MPa) and subjected to the same densification procedures. Sintered densities for samples produced by the colloidal route are higher than produced by the dry route (up to 99.5% relative density by hot pressing), even when pressureless sintering is performed (more than 90% relative density). The promising results are considered as a starting point for the fabrication of complex-shaped components that can be densified at lower sintering temperatures without pressure. © 2013, The American Ceramic Society.
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    The effect of extreme temperature in an oxidising atmosphere on dense tantalum carbide (TaC)
    (Springer, 2013-01-01) Lashtabeg, A; Smart, M; Riley, D; Gillen, AL; Drennan, J
    This study describes the microstructure development as dense tantalum carbide (TaC), which is subjected to extreme temperature environments (3,000 °C) in the presence of oxygen. These are conditions that structural materials are expected to experience in hypersonic aero-propulsion applications. The conditions produce molten oxide which may provide a temporary resistance to rapid oxidation and may go some way to repair thermal shock cracks, however, at the same time the liquid is observed to attack the dense ceramic both chemically and mechanically. A reaction mechanism is suggested which involves dissolution of TaC in the oxide melt and a two step oxidation; first the reaction of TaC with oxygen to form Ta(O,C) and TaO x , resulting in dissolved dissociated carbon, followed by the reaction of dissolved carbon with oxygen to produce gas. This microstructural analysis of one of the candidate ultra-high temperature ceramic materials for hypersonic flight provides new insight into the mechanism of TaC oxidation and the role of the liquid oxide layer in acting not only as a protective layer to further oxidation, as is commonly reported, but also as a dynamic component that promotes erosion of the TaC surface and is a source of further oxygenation of the TaC surface. If the formation of the liquid phase can be better controlled and the reaction of the liquid phase with the matrix be slowed and stabilised, then the formation of a liquid phase at the surface of TaC may provide a key to designing materials that can withstand the rigours of hypersonic flight. © 2012, Springer.
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    Friction stir forming to fabricate copper–tungsten composite
    (Elsevier B.V., 2015-03-01) Ahuja, Y; Ibrahim, R; Paradowska, AM; Riley, D
    Tungsten embedded composite of copper (C1100) was fabricated through probeless tool aided friction stir forming (FSF). The heat input conditions and forging were determined to be most effectively controlled by the tool rotation speed. A void-free and continuously bonded Cu–W interface was established at the parameter combination of 1200 rpm tool rotation speed with 100 mm min−1 traverse speed, 0.05 mm plunge and 3° tool tilt angle. The Cu–W interface was characterized via SEM and EDS analysis and was determined to be a purely mechanical interlock due to the absence of new phases. Microstructure of the friction stir formed copper near the interface was examined by optical microscopy. Mechanical properties of the processed copper were investigated by Vickers indentation and shear punch tests, and they showed good correlation with the microstructure. Grain refinement induced work hardening was observed in the copper close to the interface. Cu–W interface remained intact during the shear punch testing and failure occurred in the grain coarsened region of the copper 1 mm away from the interface. The bond strength of the Cu–W mechanical interlock fabricated by FSF was determined to be 130 MPa. © 2014 Elsevier B.V.