Ordering of Nanocrystalline FeAl produced by cluster condensation

The ordering kinetics of nanocrystalline Fe₅₀Al₅₀ alloys the specimens were prepared by sputtering, crystallite condensation in an Ar atmosphere, and subsequent compaction (2–4 GPa) at room temperature. The lattice structure is B2 as in coarse grained FeAl but with a significantly lower degree (≤ 0.5) of order. The crystallite size is about 20 nm and strains exceed 1.5 % in the {110} direction and are higher than in nanocrystalline Fe. The ordering process takes place mainly in the temperature range from 400 to 500 K, which is measured by the x-ray intensities of the superlattice reflections. This may indicate a higher vacancy diffusivity in the disordered than in the ordered state. The crystallite site remains smaller than 50 nm even after annealing at 0.8 T_m (melting temperature T_m = 1488K) as evidenced by transmission electron microscopy. Furthermore the strains decrease during annealing.