Iron aluminide–Al2O3 composites by in situ displacement reactions: processing and mechanical properties

Composites of an intermetallic alloy matrix (such as iron aluminide) and ceramic particle reinforcement (such as alumina) have potential for high-temperature structural applications requiring excellent oxidation resistance. In contrast to conventional processing methods (e.g. mechanical alloying), in situ processing techniques can be inexpensive and can also lead to unique microstructures, such as very fine dispersions of the reinforcements or interconnecting phases. The feasibility of processing Fe–Al alloy/Al2O3 composites by an in situ displacement reaction between Fe–40at.%Al and the iron oxide Fe2O3 was investigated and the results are presented in this paper. Specimens were prepared by hot pressing of the powder mixtures at 1300°C. The final volume 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 in this study. Compression creep tests performed at 700°C showed that composites with 7 and 20 vol.% Al2O3 had higher compressive strengths than monolithic iron aluminide (Fe–28at.%Al). In particular, the compressive strength of the composite with 20 vol.% Al2O3 was three times that of the monolithic iron aluminide. Further increases in compressive strength, as well as significant improvements in bend and fracture strength, were obtained when the particle size of the starting Fe–40%Al powders was less than 10 μm. Improved fracture toughness with the smaller Fe–40%Al powders could be attributed to the ductile failure 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 aluminide and alumina phases was also investigated. The compressive strength of a composite processed with <10-μm diameter iron aluminide powders and 8 at.% Cr was the highest of all the specimens. However, within the range of compositions studied in this investigation, only slight improvements in the mechanical properties could be obtained with these alloying additions.