Effect of Nb and Cu on the crystallization behavior of under-stoichiometric Nd–Fe–B alloys

Abstract

In this work, we present a complete study of the influence of Nb and Cu addition on the crystallization behavior of Nd-lean Nd-Fe-B melt-spun alloys. Alloys with compositions Nd 10−x−y Fe 84 B 6 Nb x Cu y (x = 1, y = 0 and x = 0.5, y = 0.5) were melt-spun at different wheel speeds (15-40 m s −1) to obtain samples in amorphous, highly disordered and nanocrystalline structures. The crystallization process, induced by different heat treatments, was studied by means of differential thermal analysis and x-ray powder thermodiffraction. Magnetic properties of as-made and heat-treated ribbons were measured by magnetometry. The as-made amorphous samples showed a crystallization to the 2:14:1 hard magnetic phase at T 1 ~ 350 °C. Doping with Nb results in an increase of T 1 , and addition of Cu lowers T 1. This behavior is explained in terms of an inhibition of grain growth by Nb and a nucleation enhancement by Cu additions. During the crystallization process, a secondary phase (identified as a bcc-Fe-rich phase) is formed. The amount of such a phase increases with the annealing temperature. Coercivity increases upon annealing reaching maxima at 700-750 °C. This can be explained in terms of competition between the two phases formed: the 2:14:1 hard phase and the soft bcc-Fe-rich phase. The highest coercivity of the Nd-lean samples is observed when the microstructure is appropriate and both phases are exchange-coupled.

Nano-crystallisation and magnetic softening in Fe–B binary alloys induced by ultra-rapid heating

“…Among these additives, the roles played by Cu and Nb were studied extensively; Cu is known to enhance heterogeneous nucleation [12] while Nb is believed to inhibit the crystal growth process [13] during nano-crystallisation in Fe-based alloys. These additives have been known to be effective in refining microstructures even in nanocomposite magnets [14]. Hence, it has been a common perception that a certain amount of nonmagnetic additives in addition to glass-forming elements is essential to realize magnetically soft nanostructures in Fe-based alloy ribbons.…”

Magnetic Properties of Tetragonal SmFe12−xMox Alloys in Bulk and Melt‐Spun Ribbons

“…Similarly, this effect was not evidenced in the as‐spun ribbons because secondary phases were not present at that stage. On the other hand, the low remanence on the as‐spun ribbons with respect to their saturation magnetization is a clear indication of a poor magnetic exchange‐coupling in these permanent magnet systems, [ 20 ] mainly due to the presence of a bcc‐Fe secondary phase.…”

“…However, when this phase increases above a certain value, the spring magnet starts to decouple and a constrain loop appears, as shown in Figure 7, which is detrimental to the coercivity in exchange‐coupled magnets. [ 20 ] Before the implementation of these alloys in permanent magnet technology, a better microstructure must be defined to prevent loss of Sm by evaporation during processing. For example, an alternative to enhance the coercivity in these compounds is to carry out heat treatments on the as‐spun ribbons, producing a high‐pressure Sm atmosphere in order to avoid the evaporation of this element from the hard magnetic alloy; in this way, we could prevent the formation of a secondary bcc‐Fe phase.…”

Evolución y proceso de fabricación de imanes “NEO” aplicados a motores de vehículos eléctricos

“…Al disminuir el tamaño del grano se disminuye exponencialmente la posibilidad de multidominio. (Sugimoto, 2011;Salazar et al, 2016) b) Control del dominio. El otro método se basa en el recubrimiento de la pared del grano con la fase Nd-rica que elimina los posibles defectos que puedan aparecer en la superficie del mismo, evitando así los dominios en retroceso.…”