Cerium and lanthanum substitution in Nd2Fe14B-based hard magnetic alloys for balanced utilization of rare-earth resources

Abstract

The Nd2Fe14B-based permanent magnets are fabricated by rapid solidification, powder metallurgy and hydrogen treatment methods, production relaying heavily on strategic rare-earth raw materials. Considerably cheaper due to lower demand, Ce and La drew attention as substitutes for resource-critical Nd in Nd2Fe14B-based alloys to obtain cost-efficient magnets of intermediate performance, between hard ferrites and Nd2Fe14B. The isomorphous Ce2Fe14B and La2Fe14B tetragonal compounds have inferior intrinsic magnetic properties therefore substitution of Ce or La for Nd in the Nd2Fe14B compound lowers the maximum achievable, theoretical performances. Furthermore, in practical alloys, depending on concentration, solubility, solidification rate and processing regimes, replacing Nd with Ce or La induces changes in the Nd2Fe14B phase (Φ-phase) crystal lattice and in the alloy phase composition and microstructure, affecting the intrinsic (composition-dependent) and extrinsic (microstructure-dependent) magnetic properties. The partitioning of the substitution element between the Φ-phase and the intergranular phase(s) impacts the saturation magnetization and the Curie temperature of the Φ-phase. Through the variation of the Nd2Fe14B cell constants the interatomic distances change and impact on the magnetic Fe-Fe ion exchange interaction with effects on the Curie temperature. Segregations like CeFe2 Laves-type phase and primary α-Fe occurring in the alloys reduce the Φ-phase relative fraction which decreases the remanent magnetization. Microstructure alterations in grain structure like excessive grain growth and in the distribution of the intergranular material like a discontinuous intergranular phase negatively impact the coercivity. In this thesis, rapid solidification by melt-spinning and strip-casting was chosen for alloy synthesis, followed by melt-spun powder hot-working (hot-pressing followed by hot-deformation) and hydrogen treatment of strip-cast alloys (decrepitation and HDDR (hydrogenation disproportionation desorption recombination)). High solidification rates restrict phase segregations and produce fine grained microstructures (nanosized in melt-spun ribbons and down to a few microns in strip-cast flakes). Hot-working is performed for densification and crystallographic c-axis texture development through grain deformation to enhance the remanence. At this stage, the segregation of the substitution element in the Nd-rich intergranular eutectic phase is shown to change its melting behavior thus influencing the melt-spun alloy's deformability. The HDDR treatment was employed for grain refinement and texture inducement to produce anisotropic powders. The phase structure and microstructure evolutions through processing stages, from as-cast to hydrogen decrepitated, disproportionated and recombined states are comprehensively analyzed in relation to the Ce concentration in the strip-cast alloys, with a focus on the grain boundary processes. The transformation of the CeFe2 intergranular segregations to amorphous CeFe2Hx upon hydrogen absorption, decomposition into CeHx and α-Fe upon heating and redistribution among the hard matrix phase play a supportive role in coercivity development through HDDR treatment.