X-RAY-POWDER DIFFRACTION AND MOSSBAUER STUDY OF NANOCRYSTALLINE FE-ALPREPARED BY MECHANICAL ALLOYING

Iron-aluminium alloys of composition Fe50Al50 and Fe75Al25 were produc ed by mechanical alloying (MA) of the pure elemental powders. A struct ural refinement of X-ray powder data on the mechanically alloyed produ cts according to the Rietveld method has detailed the progressive diss olution of aluminium into the lattice of alpha-iron as a function of M A time. With respect to pure iron, a volume expansion of approximate t o 3% is measured in both compositions mechanically alloyed for 32 k In the iron and aluminium phases, the Debye-Waller static disorder incre ases as a function of MA time and the intrinsic shape of the peak prof iles 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). Th e Mossbauer spectra show that in the equiatomic case the initial sharp magnetic sextet of alpha-iron is progressively reduced and is replace d by a doublet, while for the Fe75Al25 composition a broad magnetic se xtet is eventually obtained. Thermal scans at 600 degrees C of the spe cimens mechanically alloyed for 2, 4 and 8 h precipitate essentially t he Al5Fe2 phase. In the case of the Fe50Al50 specimens, annealing of t he powders mechanically alloyed for 16 and 32 h precipitates mainly th e partially ordered FeAl intermetallic compound, whilst no ordering is obtained in the Fe75Al25 case. Copyright (C) 1996 Acta Metallurgica I nc.