Microstructure and tensile properties of Fe 3 Al produced by combustion synthesis/hot isostatic pressing
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INTRODUCTION
IRON aluminides based on Fe3A1 are candidates for a variety of structural applications. These materials exhibit a combination of low density, excellent oxidation and sulfidation resistance, good ductility, and high workhardening rates. Compositions near the 3 : 1 atomic ratio of iron-to-aluminum have three possible crystal structures. Above about 1100 ~ a solid solution based on the body-centered cubic (bcc) lattice is stable. Two ordered phases exist at lower temperatures; the B2 structure is in equilibrium above about 550 ~ while the D03 ordered structure is stable below this critical temperature. The structure and properties of these alloys have been subjects of several recent studies, tl-51 It has been found that the B2 to D03 transformation on cooling below the critical temperature is quite sluggish in alloys of Fe3A1; furthermore, alloys quenched to retain the B2 structure have higher room-temperature ductility than those with the equilibrium structure. I1,5,6] It has also been found that alloys slightly rich in aluminum and containing up to 5 pct Cr have the greatest room-temperature tensile elongation.It.S] Most work on iron aluminides has focused on cast and wrought material or powder metallurgy products obtained by hot extrusion of prealloyed powders. An alternative fabrication method involves synthesizing the compound directly from elemental powders, utilizing the exothermic reaction that occurs between the constituB.H. RABIN, Senior Scientist, and R.N. WRIGHT, Scientific Specialist, are with the Materials Technology Group, Idaho National Engineering Laboratory, Idaho Falls, ID 83415-2218. This paper is based on a presentation made in the symposium "Reaction Synthesis of Materials" presented during the TMS Annual Meeting, New Orleans, LA, February 17-21, 1991, under the auspices of the TMS Powder Metallurgy Committee. METALLURGICAL TRANSACTIONS A
ents. This process, known as combustion synthesis, selfpropagating high-temperature synthesis, or reaction sintering, offers some advantages over conventional processing methods including the use of inexpensive, readily available, and easily compacted elemental powders, lower processing temperatures, shorter processing times, and the potential to produce new and unique microstructures. In a recent study, CTlthe reaction mechanisms and densification behavior of iron-aluminum powder mixtures having the Fe3A1 stoichiometry were examined. It was shown that the exothermic reaction was accompanied by the rapid formation and outward spreading of an aluminum-rich liquid from prior aluminum particle sites. This behavior is often observed in eutectic systems that exhibit poor solubility and diffusivity ratios tsl and typically leads to an increase in porosity (i.e., swelling) in powder compacts. Although such swelling behavior was previously observed in the Fe-A1 system, tg,~~ the recent results [7l demonstrated that some densification was possible durihg pressureless sintering by using fine particle sizes, rapid heating rates, and high green densities.
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