Browsing by Author "Alexander, J"

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    Analysis of compositionally ungraded FGM analogues: Neutron diffraction measurements of residual stress and mechanical testing of pressure sintered Mo-Y2O3 and Mo-Al2O3
    (Karlsruhe Institute of Technology, 2015-05) Saleh, M; Chavara, D; Toppler, K; Alexander, J; Ruys, AJ; Kaveh, K; Luzin, V
    Functionally graded materials (FGMs) are a type of naturally inspired composite materials whose properties (e.g. microstructure, chemical or phase composition) vary over one or more dimensions. The FGMs were first proposed as an advanced engineering material in 1972 and research into application for Biomaterials, Aerospace, Chemical Plants, Mining, and Building material commodities[1, 2] is ever present. Within the nuclear industry FGMs can be engineered to effectively resist corrosion, radiation and are a potential choice for nuclear reactor components e.g. first wall for fusion reactors and fuel pellets. Additionally FGM’s have been proposed as potential plasma facing components (PFC) whereby the PFC would gradually vary from a refractory material (tungsten, plasma face) to a heat sink material (copper, coolant side). In the case of a metal-ceramic FGM, the composite mates the strength and ductility of a metal with the hardness and toughness of a ceramic [3-5]. The authors have sought to elucidate the development of residual stress in FGMs using neutron measurements on the Kowari Strain Scanner, ANSTO for the Mo-Y2O3 and Mo-Al2O3 system. Due to their extreme gradients FGM’s are not optimal for fast neutron measurements due to the high spatial resolution requirements and long measurement times. An alternative approach was employed to examine compositionally ungraded analogues of varying metal-ceramic ratios. All sample were manufactured using constituent powders, mixed and subsequently sintered using a hot press with close monitoring of the sintering curve. Optical microscopy and scanning electron microscopy were used to look at the resultant samples to observe the grain growth and the defect-like cracks attributable to the thermally induced stresses. Relaxation due to micro cracking and micro-fracturing are evaluated in light of the neutron residual stress measurements and mechanical strength measurements of : (a) bending stiffness using a three point bend tests, (b) bulk modulus through GrindoSonic techniques and (c) statistically averaged micro-hardness. Further evaluation of the residual stress is done through comparison between established analytical models, neutron diffraction and preliminary FEA. The major contribution of residual stress are further realised and evaluated in light of the interfacial instabilities present and the appropriate ways to optimise the thermal protection characteristics of a compositional gradient.
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    A northward shift of the southern westerlies during the Antarctic cold reversal: evidence from Tasmania
    (Australasian Quaternary Association, 2018-12-10) Alexander, J; Fletcher, MS; Pedro, JB; Mariani, M; Beck, KK; Blaauw, M; Hodgson, D; Heijnis, H; Gadd, PS; Lisé-Pronovost, A
    The Last Glacial Termination (LGT) was interrupted in the Southern Hemisphere by the Antarctic Cold Reversal (ACR; 14.7 to 13 ka), a millennial-scale cooling event that coincided with the Bølling– Allerød warm phase in the North Atlantic (14.7 to 12.7 ka). This inter-hemispheric asynchrony of climate change through the LGT, the bipolar seesaw, has been theoretically linked to latitudinal shifts in the southern westerly wind belt (SWW) and their proposed influence over the global carbon cycle via wind-driven upwelling of CO2 rich deep waters in the Southern Ocean (SO). However, while climate models and theory predict a northward shift of the SWW during the ACR in response to ocean-atmosphere heat dynamics, proxy-based reconstructions disagree on the behaviour of the SWW through this interval, and the role of the SWW during the LGT remains contested. Here we present terrestrial proxy palaeoclimate data (pollen, μXRF geochemistry, charcoal) from multiple lakes across Tasmania (40-44⁰S), an island located at the northern edge of the SWW. Our data reveal a clear SWW increase over Tasmania during the ACR, synchronous with reduced SWW-driven upwelling in the SO at the southern edge of the SWW. When combined with evidence from Antarctic ice cores and terrestrial records from New Zealand and Patagonia our results suggest a hemisphere-wide migration of the SWW during the LGT, lending support to the hypothesis that changes in wind-driven ventilation of CO2 from the Southern Ocean were a key driver of the global carbon cycle during the LGT. © The Authors.