New aspects of Nd–Fe–B-based hydrogenation-disproportionation-desorption-recombination powders and anisotropic bonded magnets made from them: Microstructure and magnetic properties (invited)

Abstract

Recent progress in the understanding of the texture formation mechanism in the hydrogenation-disproportionation-desorption-recombination (HDDR) process, improvements of stability with regard to thermal and structural losses, and the state-of-the-art performance of both compression and injection molded magnets made from the newly developed anisotropic HDDR powders are reported. Transmission electron microscopy observations of disproportionated Nd–Fe–Co–Ga–Zr–B alloys have revealed the existence of finely dispersed crystallites of Nd2(Fe,Co,Ga)14B which have a common crystallographic orientation. It is proposed that, upon removal of hydrogen, the hydrogen-disproportionated structure recombines from these crystallites to form textured submicron crystallites of the 2:14:1 phase. Using highly anisotropic HDDR powders, energy products (BH)max exceeding 170 kJ/m3 (21 MGOe) have been obtained on compression-molded resin-bonded magnets and 130 kJ/m3 (16 MGOe) on injection-molded ones. High coercivity HDDR powders with an intrinsic coercivity (HcJ) exceeding 1.27 MA/m (16 kOe) have also been obtained by replacing part of Nd with Dy, which enabled improvement of thermal stability. It is shown that the degradation of magnetic performance of resin-bonded HDDR magnets is prevented by eliminating pore formation during the molding process.