A study was conducted to investigate a novel route to low cost processing of alumina/calcium-hexaluminate (A/CA6) composites. The objectives of this study were to: (a) develop A/CA6 and ß-spodumene modified A/CA6 composites using an in-situ reaction sintering method and functionally-graded A/CA6 using an infiltration technique, and (b) evaluate the effects of CA6 platelets on the ensuing physical and mechanical properties. The study has revealed that the processing procedures played an important role in the microstructural development of A/CA6 composites. The microstructure-property relationships of these materials were found to be strongly influenced by the presence of CA6 phase.The A/CA6 composites were synthesised by in-situ reaction sintering of alumina powder and (0, 5, 15, 30, 50 and 100 wt%) CA6 precursor. The phase relations and development of this system were monitored using quantitative x-ray diffraction (XRD) and neutron diffraction (ND). Rietveld analysis which showed the CA6 content to increase in proportion with the increase of CA6 precursor added. The XRD study revealed that the CA and CA2 phases started to develop at approximately 1000 and 1100°C and transformed to CA6 phase at 1400T. Similarly, the dynamic high temperature ND study showed that the corresponding calcium aluminates phases commenced to develop at 1000°C and 1200°C and then eventually transformed to CA6 at 1400°C.The presence of the plate-like CA6 grains in the system was revealed by the back-scattered SEM imaging and confirmed by the Ca x-ray map. Although the presence of CA6 caused the reduction of hardness, the fracture toughness of A/CA6 composites were improved when compared with alumina. It was found that the presence of CA6 hindered the processes of sintering and densification in alumina matrix.The use of ß -spodumene had been investigated as a liquid-phase-sintering aid for ++ / the densification of A/CA6 composites. XRD, ND, differential thermal analysis (DTA), scanning electron microscopy (SEM) and Vickers indentation were used to characterise the effects of ß -spodumene on the phase relations, densification, microstructure and mechanical properties. The results showed that the presence of ß -spodumene had a profound influence on the phase relations, densification and microstructure of A/CA6 composites. Quantitative XRD and ND Rietveld analysis showed that the addition of > 2.5 wt% ß -spodumene caused the reduction of CA6 content due to the formation of ß -quartz solid solution. The reduction of porosity in the presence of ß -spodumene suggests that it may be used as an effective sintering aid for improving the densification of A/CA6 composites. However improvements in hardness and fracture toughness were not achieved probably due to the presence of large spherical pores as well as the formation of recrystallised ß -spodumene and ß -quartz solid solution.A functionally-graded alumina/calcium-hexaluminate (A/CA6) composite was successfully synthesised through infiltration of porous alumina preform with a solution containing calcium-acetate. The infiltration kinetics of liquid into porous alumina preform had also been investigated. It was found that the infiltration rate equation proposed by Washburn is most suitable for describing the effects of preform sintering temperature, viscosity and multiple infiltrations on the infiltration characteristics. The influence of applied pressure is consistent with the model proposed by Darcy, where the applied pressure enhances the infiltration rate behaviour. Key parameters for the optimum processing conditions of preforms for subsequent infiltration have also been identified.The graded composition character of the functionally-graded A/CA6 composites were characterised by XRD and synchrotron ++ / diffraction (SRD). Depth-profiling of compositions with XRD and SRD Rietveld refinement showed that the concentration of CA6 decreased with depth, while that of A1203 increased with depth. Both XRD and SRD results showed that CA and CA2 phases formed initially at 1000°C and 1300°C, respectively, but remained stable even at 1400°C, before eventually transformed to CA6 at 1650°C. These results are consistent with those of dynamic high temperature ND data.The graded microstructure was revealed by SEM back-scattered imaging whereby the content of CA6 platelets was most abundant near the surface and decreased with increasing depth towards the bulk. The presence of CA6 phase in the composite fire at 1400°C was also confirmed by the transmission electron microscopy (TEM) observation in conjunction with energy dispersive spectroscopy (EDS). The hardness results of the graded material showed that the graded-region was softer than the non-graded region as a result of the presence of softer CA6 phase in the former. However, the fracture toughness in the graded region was found to be higher than the non-graded region which might be attributed to the display of toughening processes such as crack deflection and grain bridging.

Friction stir processing (FSP) was applied to produce aluminum based in situ composites from powder mixtures of Al-Fe, Al-Mo, and Al-Fe2O3. Billet of powder mixtures was prepared by the use of conventional pressing and sintering route. The sintered billet was then subjected to multiple passages of FSP. During FSP, the material has experienced both high temperature and very large plastic strain. The basic idea for fabricating the composites is to combine the hot working nature of friction stir processing (FSP) and the exothermic reaction between aluminum and transition metals (Al-Fe, Al-Mo) or metal oxides (Al-Fe2O3). In the Al-Fe alloy, in situ Al¡VFe reaction can be induced during FSP and form Al-Al13Fe4 composite. The size of reinforcing particles formed by the in-situ reaction is ~100 nm. In Al-Mo alloys, fine Al-Mo intermetallic particles with an average size of ~200 nm were formed and uniformly dispersed in the aluminum matrix by FSP. The Al-Mo intermetallic particles were identified mainly as Al12Mo with minor amount of Al5Mo. The exothermic reaction could result in local melting of Al at the Al/TM interface, and the liquid Al may accelerate the reaction. In addition, it is suggested that the critical mechanism responsible for the rapid reaction and the formation of nanometer sized particles in FSP is the effective removal of the Al-TM intermetallic phase from the Al-TM interface, maintaining an intimate contact between TM and Al. In the Al-Fe2O3 system, the reactions taking place during FSP includes the thermite reaction (2Al +Fe2O3 ¡÷ Al2O3 + 2Fe), and the reaction between the reduced Fe and Al to form Al13Fe4. In the FSPed Al-Fe2O3 specimens, there are two types of second phase particles, Al13Fe4 and Al2O3. The Al2O3 particles (about 10 nm in size) usually appear as a cluster of 100-200 nm in diameter. There are two types of Al2O3 phases existed in the Al matrix after FSP passes, depending on the content of Fe2O3. One is £^-Al2O3 in Al-2Fe2O3 specimens, and the other is £\-Al2O3 in Al-4Fe2O3 specimens. It is suggested that the formation of different type of Al2O3 particles in the Al-Fe2O3 composites may be attributed to different heat release in each system. The lower heat release in Al-2Fe2O3 sample favors the formation of the while the higher heat release in Al-4Fe2O3 sample results in the £\-Al2O3. The Al-Al13Fe4/Al2O3 composite produced by FSP exhibits both high strength and good tensile ductility. The higher strength in Al-Fe2O3 specimen may be due to the presence of fine Al2O3 particles. The flow stress of the Al-4Fe2O3 composite can maintain at 100 MPa even at 773 K. The good thermal stability and high temperature strength of Al-Al13Fe4/Al2O3 composites could be attributed to the fine dispersion of second phase particles in the aluminum matrix, especially the nanometric Al2O3 particles. These Al2O3 particles are very stable at elevated temperatures, even after long time exposure at 873 K. The temperature excursion in FSP is determined by both the FSP parameters and the exothermic reaction involved. The peak temperature in Al-Fe or Al-Fe2O3 system during FSP was calculated as a function of the fraction of Fe or Fe2O3 reacted. Based on calculated results, it is noted that with the in situ reaction, the value of can easily reach the melting point of Al, especially for the Al-Fe2O3 system. The reaction mechanism and microstructure evolution during FSP are discussed.

Melhorias na eficiência energética motivam o desenvolvimento de isolamentos térmicos cada vez mais eficazes e duráveis. Cerâmicas porosas à base de mulita (Al6Si2O13 ou 3Al2O3.2SiO2) são ideais para essa aplicação devido à alta resistência à corrosão e à densificação. Apesar de rara sua forma mineral, esta é uma das fases mais importantes em cerâmicas tradicionais e avançadas, visto que pode ser obtida a partir de fontes de alumina e sílica, por meio de diversas rotas de processamentos. Sua formação in situ por sinterização reativa tem se destacado dentre os métodos de produção pelos bons resultados mecânicos e eficácia na formação e manutenção de porosidade, entretanto, ainda existem pontos a serem investigados como a influência do tamanho de partícula e porosidade inicial de suas matérias-primas nas propriedades finais das estruturas. Neste trabalho, foram produzidas peças porosas de alumina-mulita in situ a partir de suspensões de sílica coloidal com diferentes concentrações (30, 40 e 50 %) e hidróxidos de alumínio de diferentes granulometrias (fino, HAF e grosso, HAG) pré-calcinados em várias temperaturas (500-1500 °C), pelo processo de moldagem direta para aplicação como isolante térmico em temperaturas acima de 1000 °C. As amostras (verdes e tratadas termicamente - 1500 °C) foram submetidas à ensaios mecânicos (módulo elástico e resistência à ruptura por compressão e flexão), análise microestrutural (MEV e DRX) e de propriedades físicas (porosidade total, densidades e variação térmica dimensional). As aluminas de transição provenientes da calcinação agiram como agentes porogênicos no sistema e juntamente com a sílica coloidal (com funções simultâneas de fluido de mistura, agente ligante, aditivo de secagem e fonte de SiO2 amorfa) formaram estruturas com grande variação de propriedades. Estruturas com HAF apresentaram porosidade próxima a 40% e elevadas propriedades mecânicas, e com HAG foram obtidos níveis de porosidade acima de 50 % e baixas resistências. As composições mistas, contendo tanto HAF como HAG, obtiveram bons resultados mecânicos e porosidades acima de 50 %, mostrando-se bons candidatos para uso como isolamento térmico. Todos os sistemas contiveram a fase de mulita em maior quantidade coexistindo com alfa alumina (Coríndon). / Improvements in energy efficiency motivate the development of more effective and durable thermal insulation. Porous ceramics based on mullite (Al6Si2O13 or 3Al2O3.2SiO2) have great potential for this application due its high resistance to corrosion and densification. Although rare in its mineral form, this is one of the most important phases in traditional and advanced ceramics because it can be obtained from alumina and silica sources, through various processing routes. Among them, the solid-state in situ reactions by reactive sintering stands out for its good mechanical properties and efficiency in the formation and maintenance of pores. However, how particle size and initial porosity of its raw materials influences the final properties still requires investigation. In this work, porous ceramics of alumina-mullite were produced in situ from aqueous suspensions of colloidal silica with different concentrations (30, 40 and 50 %) and aluminum hydroxides of different grain sizes (fine, HAF and coarse, HAG) pre-calcined in several temperatures (500-1500 °C) by direct casting process for application as a thermal insulation at temperatures above 1000 °C. The samples (green and thermally treated - 1500 °C) were submitted to mechanical tests (elastic modulus and resistance to rupture by compression and flexural), microstructural analysis (SEM and XRD) and physical properties characterization (total porosity, densities and dimensional thermal variation). The transition aluminas acted as porogenic agents in the system and with the colloidal silica (with simultaneous functions of mixing fluid, binding agent, drying additive and source of amorphous SiO2), formed structures with great properties\' variation. Samples with HAF had porosity close to 40% and high mechanical properties, and with HAG, porosity levels above 50 % and low resistances were obtained. The mixed compositions, containing both HAF and HAG, obtained good mechanical results and porosities above 50 %, showing great potential to thermal insulation. All systems contained most of mullite coexisting with alpha alumina phase (Corundum).

O hexaluminato de cálcio (CaAl12O19 ou CA6) poroso é um material promissor para aplicações de isolamento térmico pois combina baixa condutividade térmica (~0,33 Wm-1K-1 a 1400 °C), resistência mecânica razoável (2 – 8 MPa), inércia química, boa refratariedade (Tf ~1830 °C) e alta resistência ao choque térmico. Existem várias rotas para se obter o CA6 por meio de reações em temperaturas acima de 1300 °C, usando diversas fontes de Al2O3 e CaO, assim como diferentes métodos de processamento. No entanto, embora suas propriedades físicas tenham sido avaliadas, dois pontos principais ainda requerem investigação: o impacto das características das matérias-primas no desenvolvimento da microestrutura de sistemas porosos formados in situ, e a evolução da microestrutura e propriedades de sistemas obtidos a partir de CA6 pré-formado. Neste trabalho, foram produzidas peças de CA6 in situ a partir de diferentes fontes de Al2O3 (alumina calcinada e hidróxido de alumínio) e carbonato de cálcio (CaCO3) de diferentes granulometrias, processados por prensagem uniaxial e moldagem direta de suspensões e submetidas a diferentes tratamentos térmicos. As amostras (verdes e secas e após tratamento térmico) foram submetidas à análise microestrutural (MEV e DRX) e dilatométrica, ensaios para determinação das propriedades físicas (porosidade total, distribuição de tamanho de poros e condutividade térmica) e propriedades mecânicas (resistência à ruptura por compressão e módulo elástico). Estruturas à base de CA6 formado in situ obtidas por prensagem e moldagem direta apresentaram elevada porosidade (até 71 %) e uma resistência à compressão acima de 10 MPa. Verificou-se que o processo de conformação determinou a porosidade à verde inicial da peça, enquanto o tamanho de partícula de alumina induziu a um crescimento de grão assimétrico (partícula grossa) ou à densificação da peça (partícula fina). Dois mecanismos antagonistas acontecem ao mesmo tempo na reação in situ: a reação expansiva da formação de aluminatos intermediários (efeito porogênico) e a densificação das partículas de Al2O3. As partículas de carbonato tiveram uma grande influência no tamanho final dos poros. O efeito porogênico do hidróxido de alumínio foi efetivo até um conteúdo máximo de 50 %vol. / Porous calcium hexaluminate (CaAl12O19 or CA6) is a promising material for thermal insulation applications because it combines low thermal conductivity (~0,33 Wm-1K-1 at 1400° C), reasonable mechanical strength (2 – 8 MPa), chemical inertia, good refractoriness (Tf ~1830 °C) and high resistance to thermal shock. There are several routes to obtain CA6 by reactions at temperatures above 1300 °C, using various sources of Al2O3 and CaO, as well as different processing methods. However, although its physical properties have been studied, two main points still require investigation: the impact of the characteristics of the raw materials on the development of the microstructure of in situ formed porous systems, and the evolution of the microstructure and properties of systems obtained from preformed CA6. In this study, in situ CA6 bodies were produced from different sources of Al2O3 (calcined alumina and aluminum hydroxide) and calcium carbonate (CaCO3) of different granulometries, processed by uniaxial pressing and direct molding of suspensions and thermally treated at different temperatures. The samples (green and heat treated ones) were submitted to microstructural analysis (SEM and XRD) and dilatometry, also tests to determine the physical properties (total porosity, Hg porosimetry and thermal conductivity) and mechanical properties (compression strength and elastic modulus). In situ formed CA6-based structures obtained by pressing and direct molding showed high porosity (up to 71%) and a compressive strength above 10 MPa. It was found that the conformation process determined the initial porosity of the green body, while particle size of alumina may induce asymmetric grain growth (coarse particle) or densification of the ceramic body (fine particle). Two antagonistic mechanisms occur at the same time in the in situ reaction: the expansive reaction of the formation of intermediate aluminates (porogenic effect) and the densification of Al2O3 particles. The carbonate particles had a great influence on the final pore size. The porogenic effect of aluminum hydroxide was effective up to a maximum content of 50% vol.