Constitution and magnetic properties of iron-rich iron-aluminum alloys

Abstract

The constitution of high-purity iron-aluminum alloys over the composition range 0–62 atomic per cent aluminum has been investigated, primarily by X-ray diffraction methods. New lattice-parameter data were obtained which differ significantly from the classical results obtained by Bradley and Jay nearly 25 years ago. Diffraction patterns taken at elevated temperatures have shown that the phase diagram needs correction in the region of Fe 3Al, along with the further addition of a sloping transformation boundary beyond 18 per cent aluminum. According to the results of this investigation, alloys in the composition range 18–33 per cent aluminum undergo a classical phase change and, on heating, pass successively from the Fe 3Al type of atomic ordering to the FeAl type. As the temperature is still further raised, anti-phase nuclei form, and finally the atomic arrangement becomes either a random one or one possessing short-range order. Alloys containing more than 37.5 atomic per cent aluminum retain the FeAl type of order all the way up to the melting point. Anomalies in the lattice parameters, magnetic behavior, and thermal expansion of alloys in the region of Fe 3Al can be interpreted in the light of these structural changes. It has been found that cold working alloys containing aluminum in excess of 20 atomic per cent increases the magnetic saturation moment and also the lattice parameter. In the region of Fe 2Al, the resulting increase in volume on cold working approaches 3 per cent, and FeAl, which is virtually non-magnetic at absolute zero, becomes appreciably ferromagnetic after severe grinding. A ferromagnetic face-centered cubic phase structure, probably based on Fe 3AlC, is found associated with alloys lying in the approximate composition range 10–33 atomic per cent aluminum. Lattice-parameter measurements indicate the possibility of an epitaxial relationship between the body-centered cubic matrix and the face-centered structure. There are indications that this “extra” phase structure undergoes an order-disorder transformation. The high-temperature Fe 3Al 5 ϵ-phase which exists above 1103°C was found to be orthorhombic.