Microstructure and properties of an oxide dispersion-strengthened iron aluminide

Oxide dispersion-strengthened (ODS) iron aluminide alloys based on Fe,AI ha ve been formed by reaction synthesis from elemental powders followed by hot extrusion. The resulting alloys have approximately 2.5% by volume Al2O3 pa rticles dispersed throughout the material. A proper combination of extrusio n temperature, extrusion ratio, and post-consolidation heat treatment resul ts in a secondary recrystallized microstructure with grain sizes greater th an 25 mm. ODS material with 5%, Cr addition exhibits approximately one orde r of magnitude increase in time to failure at 650 degrees C compared to a s imilar 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 c reep exponent is approximately 9.8. Transmission electron microscopy examin ation of the substructure of deformed samples indicates some formation of l ow angle dislocation boundaries, however, most of the dislocations are pinn ed at particles. The TEM observations and the value of the creep exponent a re indicative of dislocation breakaway from particles as the rate controlli ng deformation mechanism. The TEM results indicate that particles smaller t han approximately 50 nm and larger than approximately 500 nm do not contrib ute significantly to dislocation pinning. Addition of Nb and Mo along with Cr results in decreased minimum creep rates, however, the time to rupture i s greatly reduced due to fracture at low strains initiated at large Nb part icles that were not put into solution. (C) 1998 Elsevier Science S.A. All r ights reserved.