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composites with metallic contents up to 30 vol% have been fabricated via reaction processing of Al 2 O 3 , Fe, and Al mixtures. Low Al contents (<∼10 vol%) within the starting mixture lead to composites consisting of Fe embedded in an Al 2 O 3 matrix, whereas aluminide-containing Al 2 O 3 composites result from powder mixtures with higher Al contents. In both cases, densification up to 98% TD can be achieved by pressureless sintering in inert atmosphere at moderate temperatures (1450°-1500°C). The proposed reaction sintering mechanism includes the reduction of native oxide layers on the surface of the Fe particles by Al and, in the case of mixtures with high Al contents, aluminide formation followed by sintering of the composites. Density and bending strengths of the reaction-sintered composites depend strongly on the Al content of the starting mixture. In the case of samples containing elemental Fe, crack path observations indicate the potential for an increase of fracture toughness, even at room temperature, by crack bridging of the ductile Fe inclusions.

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A reactive sintering process has been used to produce almost fully dense composites with interpenetrating networks of NbAl 3 and Al 2 O 3 . The process involves the reaction synthesis of niobium aluminides and Al 2 O 3 from compacts of intensively milled aluminum and Nb 2 O 5 powder mixtures. During carefully controlled heating under an inert atmosphere, the oxide reduction by aluminum to form niobium aluminides and Al 2 O 3 proceeds at temperatures below the melting point of aluminum. At temperatures of >1000°C, the reaction-formed niobium aluminides and Al 2 O 3 sinter. The present paper discusses processing parameters, such as attrition milling, the heating cycle, and the metal:ceramic ratio in the starting mixture, that control microstructure development and mechanical properties.

Niobium-alumina (Nb-Al 2 O 3 ) composites have been fabricated via the pressureless sintering of compacts of intensively milled Nb-Al 2 O 3 mixtures. Strength and fracture toughness of the composites increase as the niobium content increases. For a composite that contains 50 vol% niobium, strengths of up to 680 MPa, with a corresponding fracture toughness of 6.3 ± 0.3 MPaؒm 1/2 and hardness of 6.4 GPa, have been obtained.