Structural transformation of Al-Fe alloys analysed by neutron diffraction and Mössbauer spectroscopy

The mechanical treatment of Al-25 at.% Fe and Al-34 at.% Fe mixtures of pure elemental powders in a high-energy mixer-mill induces solid solution formation and substantial solid state reactivity. Neutron diffraction data were collected over a wide Q-range. A quantitative phase evaluation was carried out by the Rietveld method which enabled us to overcome problems of peak overlapping between the two constituent phases. Changes of lattice parameters, as well as lattice strain and average crystallite size were also determined versus the processing time. The local magnetic environment around the iron atoms was simultaneously monitored by means of the Mossbauer spectroscopy. The formation of an extended bcc solid solution for prolonged treatment times is confirmed even for the present Al-rich compositions. Neutron diffraction experiments after annealing of the Al-25 at.% Fe mechanically alloyed end products revealed a polymorphous phase transformation after 85 minutes at 583 K, accounting for the metastable character of these materials. The transformation product, Al5Fe2, is orthorhombic and stoichiometrically near to the nominal composition. The short interatomic Al-Al distances, resulting from the analysis of the unit cell properties, suggests that oxygen impurities may be present in the system and may play a role for the observed reactivity. In the case of Al-34 at.% Fe composition the products are the Al5Fe2 and FeAl intermetallic phases.