Grain growth of nanocrystalline cryomilled Fe-Al powders
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I.
INTRODUCTION
CONSIDERABLE recent evidence has indicated that fully dense nanocrystalline alloys may provide mechanical and electrical properties superior to those of their coarse-grained counterparts.[1,2,3] These findings have fueled an ongoing search for a practical method of producing monolithic nanocrystalline material on a kilogram scale. Although significant progress has been achieved in the synthesis of nanocrystalline powder precursors, extensive grain growth is typically found to occur during conventional elevated temperature consolidation. In response, recent studies have investigated mechanisms through which the thermal stability of nanocrystalline microstructures might be enhanced. For example, stabilization against grain growth by solute segregation in a supersaturated solution has been considered theoretically[4] and has been experimentally observed in ball-milled Pd(Zr)[5] and Fe(N).[6] Also, the formation of an ordered phase has been reported to be effective in suppressing thermally activated grain growth.[7,8] Finally, the pinning of nanocrystalline grain boundaries by a dispersion of second-phase particles has been reported.[9,10] Since an underlying requirement for effective grain boundary pinning is a particle size that is significantly smaller than the average grain size,[11] a suitably fine dispersion is perhaps best formed by the promotion of in situ reaction within the nanocrystalline matrix. For example, Luton et al. demonstrated that a dispersion of nanometer-scale aluminum oxy-nitride particles could be formed in Al powders during ball milling in liquid N (i.e., cryomilling).[9] The presence of this stable, dispersed second phase was found to effectively stabilize the Al grains against thermally activated growth.[9] Perez et al. recently demonstrated that cryogenic mechanical alloying may be successfully applied to the synthesis of nanocrystalline Fe-10 wt pct Al powders.[12] Using R.J. PEREZ, formerly Postdoctoral Student, Department of Chemical Engineering and Materials Science, University of California, is Technical Staff Member, Material & Processes Engineering Space System Division, Boeing, Downey, CA. H.G. JIANG, formerly Postdoctoral Student, Department of Chemical Engineering and Materials Science, University of California, is Research Associate, Los Alamos National Laboratory, Los Alamos, NM. E.J. LAVERNIA, Professor and Chair, is with the Department of Chemical Engineering and Materials Science, University of California, Irvine, Irvine, CA 92697-2575. C.P. DOGAN, Researcher, is with the United States Bureau of Mines, Albany, OR 97321-2198. Manuscript submitted March 31, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
transmission electron microscopy (TEM), it was observed that the rate of grain growth during annealing was slowed significantly relative to that of nanocrystalline Fe prepared in the same manner. Evidence was presented that indicated the presence of finely dispersed oxide and nitride phases. The present study expands upon these findings by investigating the g
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