X-ray powder diffraction and Mossbauer study of nanocrystalline Fe-Al prepared by mechanical alloying

Iron-aluminium alloys of composition Fe50Al50 and Fe75Al25 were produced by mechanical alloying (MA) of the pure elemental powders. A structural refinement of X-ray powder data on the mechanically alloyed products according to the Rietveld method has detailed the progressive dissolution of aluminium into the lattice of alpha-iron as a function of MA time. With respect to pure iron, a volume expansion of approximate to 3% is measured in both compositions mechanically alloyed for 32 k In the iron and aluminium phases, the Debye-Waller static disorder increases as a function of MA time and the intrinsic shape of the peak profiles becomes predominantly Cauchy. These changes are accompanied by an increase in the average microstrain and by a reduction in the average crystallite size (which includes also the effect of dislocations). The Mossbauer spectra show that in the equiatomic case the initial sharp magnetic sextet of alpha-iron is progressively reduced and is replaced by a doublet, while for the Fe75Al25 composition a broad magnetic sextet is eventually obtained. Thermal scans at 600 degrees C of the specimens mechanically alloyed for 2, 4 and 8 h precipitate essentially the Al5Fe2 phase. In the case of the Fe50Al50 specimens, annealing of the powders mechanically alloyed for 16 and 32 h precipitates mainly the partially ordered FeAl intermetallic compound, whilst no ordering is obtained in the Fe75Al25 case. Copyright (C) 1996 Acta Metallurgica Inc.