Synthesis of iron aluminides from elemental powders: Reaction mechanisms and densification behavior
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INTRODUCTION
IRON aluminides based on Fe3A1 and FeA1 are candidates for a variety of structural applications. [~] The combination of low density, excellent oxidation and sulfidation resistance, and lack of strategic alloying elements makes these alloys particularly attractive. A variety of fabrication methods have been employed in the study of intermetallic compounds; powder metallurgy processing is becoming increasingly important for obtaining desirable microstructures, improved properties, and near net shape manufacturing capabilities. [2j Most powder processing routes for intermetallics utilize rapidly solidified prealloyed powders or ribbons as starting materials, and consolidation is carried out by hot isostatic pressing or hot extrusion. Although successful, these methods involve many processing steps and considerable expense. High costs may be justified for certain applications by improvements in performance; however, many potential uses for these materials will be realized only if lower cost processing methods emerge. An alternative powder processing method applicable to intermetallic comPOunds has received recent attention. This approach, known as reaction sintering, combustion synthesis, or self-propagating high-temperature synthesis, utilizes an exothermic reaction between powder constituents to synthesize compounds, t3,4,51Process advantages include the use of inexpensive and easily compacted elemental powders, low processing temperatures, short B.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. Manuscript submitted March 16, 1990. METALLURGICAL TRANSACTIONS A
processing times, and considerable flexibility in terms of compositional and microstructural control. Depending upon thermodynamic properties and phase diagram features, a variety of reaction products are possible, ranging from highly porous to fully densified cast materials. Recent studies have demonstrated the success of this approach for fabricating nickel aluminides, t6'7's] Near full-density Ni3A1 alloys were achieved by pressureless reaction sintering of elemental powder mixtures. It was shown that sintering was controlled by the transient liquid phase that formed during rapid exothermic heating. Elemental iron-aluminum mixtures represent a particular challenge for powder processing because extensive compact swelling has been observed, tg'~~ Figure 1 shows the iron-aluminum phase diagram, m] Swelling is predicted based upon phase diagram features; t4,~2,x3]notably, there is a large solubility for aluminum in iron, low reverse solubility, and a large melting point difference, suggesting imbalanced diffusion rates. Systems that exhibit a large driving force for compound formation are particularly susceptible to the formation of porosity during alloying, tt4'~SI The amount of swelling observed in such systems depends upon a number of processing variables including composition, particle sizes, heating rate, green densi
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