D. E. García

REACTION SINTERING OF ALUMINA-ALUMINIDE ALLOYS (3A)

A novel pressureless reaction sintering process is presented for the fabrication of Al{sub 2}O{sub 3}{endash}aluminide alloys (3A). Compacts of intensively milled metal oxide{endash}aluminum mixtures are heat-treated in vacuum or inert atmosphere such that the exothermic reactions take place in a controlled manner essentially at temperatures below the melting point of Al. Dense, homogeneous microstructures were obtained with a variety of Al{sub 2}O{sub 3}-matrix systems with interpenetrating networks of aluminides of Ti, Fe, Nb, Mo, Zr, Ni, etc. By adding modifiers in the form of oxides or metals, volume and phase composition as well as properties can be tailored in a wide range. {copyright} {ital 1996 Materials Research Society.}

Reaction synthesized Al2O3-based intermetallic composites

Abstract Intermetallics-reinforced Al 2 O 3 composites have been fabricated by reaction sintering compacts of intensively milled powder mixtures containing either metal oxides and Al or elemental metals, Al and Al 2 O 3 . During carefully controlled heat-treatment in non-oxidizing atmosphere, the metal oxides, for instance TiO 2 , Fe 2 O 3 and Nb 2 O 5 , are reduced by Al to form the respective aluminides (Ti x Al y , Nb x Al y ,). Alternatively, aluminides can also be formed through reaction between Al and elemental metals (e.g. Fe). By adding Al 2 O 3 to the starting mixture, the intermetallic/ceramic ratio of these alumina-aluminide alloys (3A) can be adjusted within a wide range. At intermetallic volume fractions ⪢~20 vol. %, both phases are continuous, exhibiting a microstructure similar to that of products of directed metal oxidation or reactive metal penetration processes. In this paper, systems based on Ti x Al y , FeAl and NbAl 3 are emphasized. The principles of the reaction synthesis process and the influence of the processing parameters on microstructural development and mechanical properties are described.

Interpenetrating Al2O3-TiAl3 alloys produced by reactive infiltration

Dense alumina-TiAl 3 composites with interpenetrating networks have been fabricated by reactive gas-pressure infiltration and squeeze casting of Al into sintered porous preforms containing 30 vol% TiO 2 and 70 vol% Al 2 O 3 . Strength of up to 543±21 M Pa with corresponding fracture toughness of 8.6±0.4 M Pa√m and hardness of H V10 = 565±27 have been obtained. The present paper discusses processing parameters such as particle size of oxide precursor and preform porosity which control microstructural development and mechanical properties of the composites.

Microstructure and mechanical properties of Al-assisted Sintered Fe/Al2O3 Cermets

Abstract Fe/Al2O3 composites with metal contents between 23 and 35 vol% have been fabricated via Al-assisted pressureless reaction sintering. The effect of variation of oxygen partial pressure during sintering on phase development, microstructure and mechanical properties has been investigated. The formation of the spinel phase FeO·Al2O3 is found to occur at elevated temperatures if the oxygen partial pressure during sintering exceeds a critical value. Microstructural observations and image analysis reveal that the composites exhibit a microstructure with the ceramic and the metal phase forming interpenetrating networks. The fracture toughness increases with increasing metal content and strongly depends on the phase content of the sintered specimens. In composites containing a small amount of FeO·Al2O3, the maximum toughness is 7·1 MPa m . A significant toughness enhancement up to 10·2 MPa m was achieved by avoiding the spinel formation to obtain composites consisting only of Fe and Al2O3.

Fast firing of alumina

Abstract Densification behavior and mechanical performance of alumina fast fired at relatively low temperatures were evaluated and compared to those of conventionally sintered alumina. A density of about 99% of the theoretical value and a grain size of 1.2 μm were achieved by firing pure alumina powder compacts at 1350 °C for 20 min. Fast fired alumina exhibits room temperature strengths (520 MPa) and Weibull moduli (10.4) nearly comparable to those of conventionally sintered alumina although very high heating/cooling rates were used. A qualitative densification model is presented based on the beneficial effects of transient mismatch stresses.

Nb and CrAl2O3 composites with interpenetrating networks

Abstract CrAl2O3 and NbAl2O3 composites containing 50 vol% metal have been fabricated by pressureless sintering of compacts of attrition milled powder mixtures. Successful fabrication of high-strength and high toughness composites requires fine and homogeneous powders. Strength and fracture toughness of the composites increase with increasing milling time. Short milling times do not lead to the required particle fineness and powder homogeneity. For a composite containing 50 vol% Nb, strengths of up to 690 MPa with corresponding fracture toughness of 6.6 ± 0.4 M Pa m 1 2 and hardness of 11.2 GPa (Hv20) have been obtained, whereas strengths of 592 MPa, fracture toughness of 6.6 ± 0.3 M Pa m 1 2 and hardness of 9.3 GPa have been obtained for CrAl2O3 composites.

Fast firing of reaction-bonded aluminium oxide RBAO composites

Al2O3/ZrO2 composites have been prepared by fast firing of oxidized Al/Al2O3/ZrO2 precursors produced by the reaction-bonded aluminium oxide (RBAO) technique. This fabrication route results in high-strength ceramics at relatively low densities. For example, after fast firing for 20 min at 1550 °C, RBAO containing 20 vol% ZrO2 shows four-point bending strengths of > 600 MPa at a density of ∼95% theoretical which is comparable to conventionally sintered RBAO.

Effect of heating rate on grain morphology of in situ reinforced reaction bonded aluminium niobate-based composites

Reaction-bonded aluminium niobate-based composites with tailored microstructures were fabricated through controlled nucleation and growth of AlNbO4 needle-like grains. Using heating rates of 0.5 to < 160 °C min−1, the effects of heat-up period on microstructure development, and the volume content, grain-size distribution as well as aspect ratio of AlNbO4 grains has been investigated. The morphology of the final microstructure is principally determined by the grain growth during heat-up. Equiaxed to elongated grains were obtained by using different heating rates and thermal ageing at 1320 °C in air. SEM observations revealed that increasing the heating rate promotes the development of large and homogeneously distributed needle-like grains. The results presented show that tailoring of final AlNbO4 microstructures become possible by the control of heating rate.