Silvia Schicker

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.

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.

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.