Near equiatomic FeCo alloys: Constitution, mechanical and magnetic properties

Abstract 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 (developed for applications such as rotor or stator laminations in motors). Recent development of nanocomposite and nanocrystalline materials such as HITPERM are not considered. A review of this developments is available in [McHenry ME, Willard MA, Laughlin DE. Amorphous and nanocrystalline materials for applications as soft magnets. Prog Mater Sci 1999;44:291–433]. An overview is given of work undertaken to date on various FeCo–X ternary system, with emphasis on the influence of these ternary additions on microstructure and characteristics of the phase diagram. The problem of the kinetics of ordering is given particular attention. Magnetic and mechanical properties are then discussed with emphasis on the relationship between microstructure and properties, and the main quantitative theories put forward are assessed again data gathered from the literature. It is shown that, while some points are clearly understood, a number of question remains in different areas which are outlined.