IRON ALUMINIDE AL2O3 COMPOSITES BY IN-SITU DISPLACEMENT-REACTIONS - PROCESSING AND MECHANICAL-PROPERTIES

Composites of an intermetallic alloy matrix (such as iron aluminide) a nd ceramic particle reinforcement (such as alumina) have potential for high-temperature structural applications requiring excellent oxidatio n resistance. In contrast to conventional processing methods (e.g. mec hanical alloying), in situ processing techniques can be inexpensive an d can also lead to unique microstructures, such as very fine dispersio ns of the reinforcements or interconnecting phases. The feasibility of processing Fe-Al alloy/Al2O3 composites by an in situ displacement re action between Fe-40at.%Al and the iron oxide Fe2O3 was investigated a nd the results are presented in this paper. Specimens were prepared by hot pressing of the powder mixtures at 1300 degrees C. The final volu me fractions of Al2O3 were determined by the initial ratio of Fe-40%Al to Fe2O3 and composites with 7 and 20 vol.% Al2O3 were investigated i n this study. Compression creep tests performed at 700 degrees C showe d that composites with 7 and 20 vol.% Al2O3 had higher compressive str engths than monolithic iron aluminide (Fe-28at.%Al). In particular, th e compressive strength of the composite with 20 vol.% Al2O3 was three times that of the monolithic iron aluminide. Further increases in comp ressive strength, as well as significant improvements in bend and frac ture strength, were obtained when the particle size of the starting Fe -40%Al powders was less than 10 mu m. Improved fracture toughness with the smaller Fe-40%Al powders could be attributed to the ductile failu re of fine iron aluminide ligaments. The effect of Cr and Ti additions of up to 8 at.% on the strength of the interface between the iron alu minide and alumina phases was also investigated. The compressive stren gth of a composite processed with < 10-mu m diameter iron aluminide po wders and 8 at.% Cr was the highest of all the specimens. However, wit hin the range of compositions studied in this investigation, only slig ht improvements in the mechanical properties could be obtained with th ese alloying additions. (C) 1998 Elsevier Science S.A. All rights rese rved.