REACTION SYNTHESIS OF FE-AL ALLOYS

Reaction synthesis of several Fe-Al 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 mu m Fe and 10 mu m 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 degrees C. After it passes through the powder compact, the temperature continues to rise, and the second reaction begins at about 900 degrees C. The second reaction reaches temperatures between 1250 and 1350 degrees 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 (similar to 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.