Coherent cluster atomic ordering in the Fe-27Al intermetallic compound

Abstract

Neutron diffraction studies of the Fe_0.735Al_0.265 compound are performed in a wide temperature range (20–900°C) in order to determine its structural states and the mechanism of ordering of atoms. The combination of high-resolution diffraction and the real-time detection of diffraction spectra makes it possible to establish that, in contrast to traditional notions, the structure of this compound at room temperature is a phase with only a partially ordered arrangement of Fe and Al in a unit cell. A completely ordered phase (such as Fe₃Al) is present in the form of mesoscopic (~200Å) clusters coherently incorporated into the disordered matrix of the main phase. After the transition of the sample to a disordered state (T> 740°С) and slow cooling to room temperature, the size of structurally ordered clusters increases to ~900 Å. A high contrast in the coherent neutron scattering lengths of iron and gallium nuclei allows the accurate determination of the temperature dependence of the occupancy factors of sites by Fe and Al atoms up to a phase transition to the disordered state.