Abstract: | 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 Tc. The degree of order at a given temperature below Tc 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. |