Microstructures of Rapidly Solidified Fe-Al-Zr Alloys
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Kear,
B.C.
Giessen,
and
M. Cohen,
259
editors
MICROSTRUCTURES OF RAPIDLY SOLIDIFIED Fe-Al-Zr ALLOYS
A. N. PATEL, L. F. VASSAMILLET, AND A. H. CLAUER Battelle Columbus Laboratories, 505 King Avenue, Columbus,
Ohio, USA
ABSTRACT Rapidly quenched short fibers of Fe-8Al and Fe8AI-XZr (X = 0.1, 0.2, 0.4, and 1) alloys were produced by the Pendant Drop Melt Extraction Process, (PDME) to explore the feasibility of producing intermediate temperature, oxidation resistant iron base alloys. The fibers had a diameter of 20 to 50 Um and ranged in length from 2-5 mm. The cooling rate was insufficient to retain zirconium in solution in the Zr-containing alloys, resulting in precipitates of the type Fex(Zr,Al) having differing compositions at the grain boundaries and within the matrix. No precipitates were observed in the FeAl alloys. The precipitates were uniform in distribution and size (nominally 0.1 to 0.3 pm diameter). Grain boundary precipitation and precipitate size both decreased with decreasing Zr contents.
INTRODUCTION Oxidation-resistant, high-temperature alloys typically contain large amounts of nickel, chromium, and in many cases, cobalt. Supplies of these elements are subject to both increasing cost and uncertainty. Since many high technology applications rely on these alloys, there is a need to develop less expensive, iron-rich alloys containing less of these elements as replacement alloys. Aluminum, chromium, and/or silicon have been alloyed with the iron to improve the oxidation resistance but the strength of these alloys is poor at elevated temperatures. Precipitation hardening approaches usually suffer from the solutioning or coarsening of the strengthening phases at high temperatures. The use of inert particles addresses these problems but this approach is often limited by inadequate control over the distribution of the dispersoid particles when using conventional powder metallurgy processing techniques. One approach for improving the high temperature strength of iron-base alloys may be to use zirconium nitride as a strengthening phase. Zirconium nitride is a stable precipitate in iron-base alloys when exposed to high temperature, stressed conditions. If a stable, finely dispersed phase such as zirconium nitride can be obtained, the precipitation hardening alloy could have advantages over a similar alloy that has been dispersion hardened. However, zirconium has very low solid solubility in iron, and when present in conventionally cast and processed alloys, it precipitates as coarse, blocky Fe 2 Zr or ZrN. For effective strengthening, the precipitate must form initially as a very finely dispersed phase and then possess sufficient stability to keep it that way. By producing the alloy through rapid quenching from the melt, the extremely short solidification times could suppress the blocky Fe 2 Zr phase and either precipitate Fe 2Zr in a finely dispersed form or retain the Zr in solution. In either case it was considered likely that subsequent nitriding or heat treatment could result in the desired micro
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