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A method is proposed to prepare Al 2 O 3 -AlN-Ni composites. The composites are prepared by sintering Al 2 O 3 /NiAl powder mixtures at 1600°C in a mixture of nitrogen and carbon monoxide. The presence of NiAl particles raises the green density of Al 2 O 3 /NiAl powder compacts. During sintering, NiAl reacts with nitrogen to form AlN and Ni inclusions. A volume expansion accompanies the reaction. Because of the high green density and the reaction, the volume shrinkage of the Al 2 O 3 -AlN-Ni composite decreases with the increase of added NiAl content.

of using Al 2 O 3 -NiAl composites at high temperature is therefore high. In the present study, Al 2 O 3 -NiAl composites Alumina-NiAl composites were prepared by powder were prepared using a straightforward powder metallurgy metallurgy processing for the evaluation of mechanical process and the effect of milling time on the mechanical properties as a function of composite composition. Fine properties investigated. alumina powder and coarse angular NiAl particles were attrition milled for diVerent time periods, using zirconia grinding media, resulting in the NiAl particles EXPERIMENTAL PROCEDURESbeing squeezed into elongated flakes. Dense composites Alumina powder (TM-DR, mean size=0•2 mm, Taimei were prepared by hot pressing in graphite dies at Chemical Co., Tokyo, Japan) and various amounts of nickel 1450°C and samples prepared for microstructure and aluminide powder (NiAl, mean size=5•9 mm, Xform Inc., properties characterisation. Increase in milling time New York, USA) were milled together in ethyl alcohol in was found to increase the aspect ratio of the NiAl an attritor (model 01-HD, Union Process Inc., USA) for 1 flakes which aligned perpendicular to the hot pressing or 12 h using zirconia balls (5 mm dia.) as grinding media direction during fabrication. T he strength and toughand a shaft rotation speed of 300 rpm. ness of the composites were found to be higher thanTo observe the morphology of the NiAl particles in the predicted by the rule of mixtures and this is ascribed Al 2 O 3 -NiAl powder mixtures after milling, the mixtures both to the bridging eVect of NiAl flakes between crack were first ultrasonically treated in water and the suspension surfaces, which is enhanced by the increased aspect of the slurry discarded to remove the fine Al 2 O 3 particles. ratio from increased milling time, and microstructural Most of the Al 2 O 3 particles could be removed after several refinement f rom grain growth inhibition.BCT /395 successive washings and the NiAl particles were collected for observation by scanning electron microscopy (SEM).

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