Hydrogen as a working atmosphere for manufacturing permanent magnets based on rare-earth metals

Abstract

The phase compositions and magnetic properties of permanents magnets of the systems Sm – Co and Nd – Fe – B are analyzed. Features of the hydrogenation–disproportionation–desorption– recombination (HDDR) process in the Nd 2 Fe 14 B intermetallic are considered. Using the Dd – Fe – B system as an example, we assess stages of manufacture of commercial permanent magnets and show the prospect of using hydrogen as a working atmosphere for the manufacture of magnetic powder. It is established that the HDDR of Dd – Fe – B alloys leads to their homogenization, grain refinement to a grain size of 0.2 to 0.5 μm, and an increase in the volume content of the main ferromagnetic phase Dd 2 Fe 14 B. By optimizing such a treatment, we managed to increase the magnetic energy (by 10%) and the lift force (by 25 – 27%) of Dd – Fe – B commercial permanent magnets. In modern materials science, a new trend in chemicothermal treatment of metals that consists of using hydrogen as a working atmosphere is being intensively developed. Developed hydrogen technologies are based on the regularities of its effect on the phase transformations, specifically, atomic ordering and hydride formation, in metals [1 – 3]. Earlier, magnetic materials were treated in a hydrogen atmosphere to clear them from impurities by annealing at high temperatures (1100 – 1300°C) [4]. However, in the manufacture of permanent magnets based on rare-earth metals (REMs), the possibility of using hydrogen both during their milling (hydride embrittlement) and for the change of the phase–structural state arose. Optimizing conditions for such a treatment, one can increase the operating characteristics of magnets (the coercive force Hc and residual induction Br ) and decrease the consumption of energy in their manufacture. Rare-earth magnets based on samarium were first obtained in the early 1970s. Alloys of the samarium–cobalt (Sm – Co) system have high saturation magnetization, coercive force, heat resistance, and corrosion resistance. However, due to the high cost, their use in industry is limited to a range of elevated temperatures (150 – 500°C). At present, magnets based on the Nd – Fe – B system are most extensively used. Of all known ferromagnets they exhibit the highest value of the energy product ( BH ) max (to 50 MG ⋅ Oe), which is five times higher than those of the best Alnico type magnets (Al – Ni – Co) [4]. Moreover, the determining advantage of these magnets over other materials is the relatively low cost calculated per unit magnetic energy (Table 1).