Near equiatomic FeCo alloys: constitution, mechanical and magnetic properties

Alloys based on the near-equiatomic FeCo offer exceptional magnetic properties. The equiatomic alloy was ‘invented’ in 1929, it offers a saturation hardly lower than that of the maximum obtained for Fe-0.35Co, with higher permeability and a lower coercivity than the latter. However, this alloy remained without industrial application, mainly because of its extreme brittleness. Only with the addition of a third element did it become possible to impart sufficient ductility for cold-rolling, and develop applications as laminated products. The addition of 2 wt% vanadium (1932) led to the ubiquitous FeCo-2V, or Permendur. It not only imparts, given appropriate heat-treatments, sufficient ductility, but also increases significantly the resistivity of the alloy while having little impact on the saturation. From a scientific point of view, the FeCo alloys, with their B2 structure below 730 ◦C, fall in the interesting category of ordered compounds. The ordering reaction has significant influence on the mechanical and magnetic properties and has therefore prompted a number of investigations. Not surprisingly, the vast majority of the work published to date concerns the FeCo-2V or its variants, rather than the binary alloy or other ternary systems. Recently though, alternative compositions, or improvement on the basic FeCo-2V have been put forward. This review attempts to summarise the current knowledge about the constitution, mechanical and magnetic properties of these alloys, focussing on the general properties of bulk FeCo and FeCo-X alloys (developped for applications such as rotor or stator laminations in motors). Recent developement of nanocomposite and