Microstructure and properties of an oxide dispersion-strengthened iron aluminide

Oxide dispersion-strengthened (ODS) iron aluminide alloys based on Fe₃Al have been formed by reaction synthesis from elemental powders followed by hot extrusion. The resulting alloys have approximately 2.5% by volume Al₂O₃ particles dispersed throughout the material. A proper combination of extrusion temperature, extrusion ratio, and post-consolidation heat treatment results in a secondary recrystallized microstructure with grain sizes greater than 25 mm. ODS material with 5% Cr addition exhibits approximately one order of magnitude increase in time to failure at 650°C compared to a similar alloy without the oxide dispersion. The activation energy for creep in the 5% Cr ODS material is of the order of 220 kJ/mol and the power law creep exponent is approximately 9.8. Transmission electron microscopy examination of the substructure of deformed samples indicates some formation of low angle dislocation boundaries, however, most of the dislocations are pinned at particles. The TEM observations and the value of the creep exponent are indicative of dislocation breakaway from particles as the rate controlling deformation mechanism. The TEM results indicate that particles smaller than approximately 50 nm and larger than approximately 500 nm do not contribute significantly to dislocation pinning. Addition of Nb and Mo along with Cr results in decreased minimum creep rates, however, the time to rupture is greatly reduced due to fracture at low strains initiated at large Nb particles that were not put into solution.