Reaction synthesis of FeAl alloys

Reaction synthesis of several FeAl alloys has been observed in an evacuated quartz tube under a vacuum of 10^-4 Pa. The compositions studied include 25, 30, 35.8, 45, and 54 at.% Al. Direct observation of the progression of the reaction of compacts formed from 45 μm Fe and 10 μm Al powders revealed that an unusual "two-stage" rection exists under these conditions. The occurrence of this two-stage reaction is sensitive to the composition, density, and volume of the powder compact. The first stage, which lasts several seconds, starts at around 650 °C. After it passes through the powder compact, the temperature continues to rise, and the second reaction begins at about 900 °C. The second reaction reaches temperatures between 1250 and 1350 °C. Electron microprobe analysis of samples quenched during the first reaction stage revealed the presence of large amounts of FeAl₃ together with unalloyed Fe and small amounts of an aluminum-rich phase (∼ 95 at.% Al). Samples allowed to react through the second stage contained primarily FeAl and Fe₃Al, with very small amounts of FeAl₂. The occurrence of the "two-stage" reaction is explained in terms of thermodynamics and heat transfer. © 1995.