A high temperature X-ray study of ordering in iron-aluminium alloys

The variation with temperature of long-range order in iron-aluminium solid solutions was studied using a high temperature diffractometer, and short-range order was studied at room temperature by measuring diffusely scattered X-rays. Compositions in the range 19–50 at. per cent^? aluminium were investigated. The atomic configurations discovered by Bradley and Jay were largely confirmed, including the structural discontinuity at the stoichiometric Fe₃Al composition. The Fe₃Al-type superlattice exists over the range 23–35 per cent Al approximately, and the order disappears gradually during heating, with no sudden drop at the critical temperature T_c. The degree of order at a given temperature below T_c depends on the thermal history by which that temperature was approached. This anomaly is explained in terms of a competition between directional order, of ferromagnetic origin, and long-range order; this model is also used to interpret certain anomalous magnetic properties at high temperature.

Alloys with less than 23 per cent aluminium possess various kinds of short-range order. An alloy containing 20.5 per cent aluminium has a structure which is unaffected by heat treatment, but all the other alloys investigated are sensitive to heat treatment. A tentative equilibrium diagram is proposed.