Microstructural design and characterisation of alumina/aluminium titanate composites
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A new but relatively simple processing study was conducted to investigate the microstructure-property relationships of alumina/aluminium titanate (AAT) composites. The objectives of this study were: (a) to develop a process for fabricating AAT and β-spodumene modified AAT composites using a solid-state reaction method and functionally-graded AAT using an infiltration technique, and (b) to evaluate the effects of dispersed aluminium titanate (AT) on the phase relations, microstructure and mechanical properties of alumina-based composites. The study has revealed that the processing procedures played an important rule in the microstructural development of AAT composites. The microstructure and properties of AAT composites have been found to be strongly influenced by the presence of dispersed AT. The phase relations in the AAT system have been characterised by x-ray diffraction (XRD) and neutron diffraction (ND). Rietveld analysis showed that the AT content increased in proportion with the amount of rutile added. The dynamic ND study showed that AT commenced to form at ~1310°C The presence of AT caused a reduction of hardness but an improvement in fracture toughness. In addition, the presence of AT hindered the processes or kinetics of sintering and densification. The use of β-spodumene has been investigated as a liquid-phase-sintering aid for the densification of AAT composites. XRD, ND, differential thermal analysis (DTA), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Vickers indentation were used to characterise the effect of β-spodumene on the phase relations, densification, microstructure and mechanical properties of AAT composites. The presence of β-spodumene was found to have a profound influence on the phase relations, densification, microstructure and properties of AAT composites.The addition of β-spodumene caused a small reduction of AT content and a commensurate increase of alumina phase. Functionally-graded AAT composites have been successfully synthesised through infiltration of porous alumina preform with a solution containing TiCl4. The infiltration kinetics of liquid into porous alumina preform has also been investigated and modelled. It was found that the infiltration rate equation proposed by Washburn was proven to be suitable for describing the kinetics of infiltration in terms of preform sintering temperature, viscosity, and multiple infiltrations. The influence of applied pressure was consistent with the model proposed by Travitzky and Shlayen, where the applied pressure enhanced the rate of infiltration. Pre-sintering of alumina preform at 900, 1000 and 1100°C for 2 h resulted in different rates of infiltration which may be attributed to a varying degree in tortuosity of the pore channels. The graded composition character of functionally-graded AAT composites has been determined by XRD and grazing incidence synchrotron diffraction (GISRD). Graded compositions from Rietveld refinement analysis showed that the concentration of AT decreased with depth. In contrast, the α-A12O3 content increased with depth. Microstructural examination by SEM showed that the content of AT grains was the most abundant near the surface and decreased gradually with an increase in depth. The hardness results showed that FGM had a soft graded-region (AT rich) but hard non-graded alumina region.The lower hardness in the graded region can be attributed to the presence of intrinsically soft AT phase. The presence of graded AT caused a considerable improvement in damage tolerance. The isothermal decomposition of AT at 1100°C both in air and vacuum has been studied. Both ex-situ and in-situ studies have been conducted to examine the effect of environment on the decomposition behaviour of AT. The addition of MgO was effective in enhancing the thermal stability of AT against decomposition both in air and in vacuum.
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