Pavel A. Prokofev

Electron microscopy, X-ray fluorescence, gas, and X-ray diffraction analyses, and magnetometry are used to study the chemical and phase compositions and magnetic properties of a powder prepared by crushing and subsequent milling of end-of-life Nd–Fe–B sintered magnets, which is performed in an isopropyl alcohol medium. The milling processes used for the preparation of powder are among those comprising the manufacturing procedure of sintered NdFeB permanent magnets by powder metallurgy and should not change the phase composition of the alloy. The maximum powder particle size obtained after milling of the waste sintered permanent magnets is 50 μm. The prepared powder is found to be characterized by an increased oxygen content. The phase composition (wt %) comprises 82.0 % Nd₂Fe₁₄B, 9.7% Nd(OH)₃, 8.0% α-Fe, and 0.3% Nd₂O₃. The oxide phases are present at the powder particle surface. The powder material is shown to have the sufficiently high level of magnetic properties. The possibility of application of the powder in manufacturing sintered permanent magnets, in particular, by 3D printing technology is discussed.

Hard magnetic (Sm,Zr)(Co,Fe,Cu)_z alloys intended for application as powder fillers in bonded magnets are considered. The microstructure and intrinsic coercive force of Sm_0.87Zr_0.13(Co_0.702Cu_0.088Fe_0.210)_6.7, Sm_0.81Zr_0.19(Co_0.702Cu_0.088Fe_0.210)_6.3, and Sm_0.85Zr_0.15(Co_0.690Cu_0.070Fe_0.240)_6.8 alloys, the structural and magnetic states of which are formed at different aging temperatures and cooling rates, are studied. A high coercive force and adequate squareness of the magnetic hysteresis loop of the alloy powders under study are observed at grain and subgrain sizes of the 2 : 17 phase of 10–20 μm. It is shown that the anisotropic powders of the Sm_0.87Zr_0.13(Co_0.702Cu_0.088Fe_0.210)_6.7 alloy can be prepared for manufacturing bonded magnets with a coercive force of 14–20 kOe or higher, which are intended for application in electric machines.

The finite element micromagnetic simulation is used to study the role of complex composition of 2:17R-cell boundaries in the realization of magnetization reversal processes of (Sm, Zr)(Co, Cu, Fe)_z alloys intended for high-energy permanent magnets. A modified sandwich model is considered for the combinations of 2:7R/1:5H phase and 5:19R/1:5H phase layers as the 2:17R-cell boundaries in the alloy structure. The results of the simulation represented in the form of coercive force vs. total width of cell boundary showed the possibility of reaching the increased coercivity at the expense of 180°-domain wall pinning at the additional barriers within cell boundaries. The phase and structural states of the as-cast Sm_1-xZr_x(Co_0.702Cu_0.088Fe_0.210)_z alloy sample with x = 0.13 and z = 6.4 are studied, and the presence of the above phases in the vicinity of the 1:5H phase was demonstrated.

Observations of the surface domain structure (Kerr-effect), optical metallography, scanning electron microscopy (SEM-SE), and electron microprobe analysis (EPMA-SEM), measurements of major and minor magnetic hysteretic loops were used to study pseudo-single-crystal samples of (Sm,Zr)(Co,Cu,Fe)_z alloys subjected to heat treatments to the high-coercivity state, which are used in fabricating sintered permanent magnets. Correlations between the chemical composition, hysteretic properties, structural components, domain structure, and phase state were determined for the concentration ranges that ensure wide variations of 4f-/4d-/3d-element ratio in the studied samples. The phase state formed by collinear and coherent phase components determines the high coercive force and ultimate magnetic hysteresis loops of the pseudo-single crystals. It was found that the 1:5 phase with the hexagonal structure (P6/mmm) is the matrix of the alloys for (Sm,Zr)(Co,Cu,Fe)_z permanent magnets; the matrix undergoes phase transformations in the course of all heat treatments for the high-coercivity state. The heterogeneity observed with optical magnifications, namely, the observation of main structural components A and B, is due to the alternation, within the common matrix, of regions with modulated quasi-spherical precipitates and regions with hexagonal bipyramids (cellular phase) although, traditionally, many investigators consider the cellular phase as the matrix. It is shown that the relationship of volume fractions of structural components A and B that account for more than 0.9 volume fraction of the total, which is due to the integral chemical composition of the alloys, determines the main hysteretic performances of the samples. The Zr-rich phases, such as 5:19, 2:7, and 6:23, and a structural component with the variable stoichiometry (Sm(Co,Cu,Fe)_3.5–5) that is almost free of Zr and contains up to 33 at% Cu, were found only within structural component A in quantities sufficient for EPMA analysis.

The wide application of Nd-Fe-B permanent magnets, in addition to rare-earth metal resource constraints, creates the necessity of the development of efficient technologies for recycling sintered Nd-Fe-B permanent magnets. In the present study, a magnet-to-magnet recycling process is considered. As starting materials, magnets of different grades were used, which were processed by hydrogen decrepitation and blending the powder with NdH_x. Composition inhomogeneity in the Nd₂Fe₁₄B-based magnetic phase grains in the recycled magnets and the existence of a core-shell structure consisting of a Nd-rich (Dy-depleted) core and Nd-depleted (Dy-enriched) shell are demonstrated. The formation of this structure results from the grain boundary diffusion process of Dy that occurs during the sintering of magnets prepared from a mixture of Dy-free (N42) and Dy-containing magnets. The increase in the coercive force of the N42 magnet was shown to be 52%. The simultaneous retention of the remanence, and even its increase, were observed and explained by the improved isolation of the main magnetic phase grains as well as their alignment.

The Ce₂Fe₁₄B intermetallic, like Nd₂Fe₁₄B, has the tetragonal Nd₂Fe₁₄B-type structure (space group P4₂/mnm), in which Ce ions have a mixed-valence state characterized by the coexistence of trivalent 4f¹ and tetravalent 4f⁰ electron states. Despite the fact that the saturation magnetization, magnetic anisotropy field, and Curie temperature of the Ce₂Fe₁₄B intermetallic are substantially lower than those of Nd₂Fe₁₄B and Pr₂Fe₁₄B, Ce₂Fe₁₄B retains the capacity of being able to be used in the manufacturing of rare-earth permanent magnets. Moreover, at low temperatures, the anisotropy field of Се₂Fe₁₄B is higher than that of Nd₂Fe₁₄B, and Се₂Fe₁₄B does not undergo the spin-reorientation transition. In this respect, studies of (Nd, Ce)-Fe-B alloys, which are intended for the improvement of the service characteristics-to-cost ratio, are very relevant. A model and algorithm for calculating the hysteresis loops of uniaxial hard magnetic materials with allowance for the K₁ and K₂ (K₂ > 0 and K₁ > 0 and K₁ < 0) magnetic anisotropy constants were developed and allowed us to obtain data on their effect on the parameters of hysteresis loops for a wide temperature range (0–300 K). The simulation and analysis of hysteresis loops of the quasi-ternary intermetallics (Nd_1−хСе_х)₂Fe₁₄B (х = 0–1) was performed. Results of the simulation indicate that the alloying of the Nd₂Fe₁₄B intermetallic with Ce to x = 0.94 (1) does not completely eliminate the negative effect of spin-reorientation phase transition on the residual magnetization of the (Nd_1−хCe_х)₂Fe₁₄B intermetallic and (2) slightly decreases the slope of magnetization reversal curve.

The focus of new technologies on the formation of inhomogeneous distributions of heavy rare-earth metals (REMs) in hard magnetic Nd–Fe–B materials is of scientific importance to increase their functional properties, along with preserving existing sources of heavy REMs. This paper focused on the coercivity enhancement of Nd₂Fe₁₄B-based magnets by optimizing the microstructure, which includes the processes of grain boundary structuring via the application of a Dy₃Co_0.6Cu_0.4H_x alloy added to the initial Nd–Fe–B-based powder mixtures in the course of their mechanical activation. We have studied the role of alloying elements in the formation of phase composition, microstructure, the fine structure of grains, and the hysteretic properties of hard magnetic Nd(R)₂Fe₁₄B-based materials. It was shown that the Dy introduction via the two-component blending process (the hydrogenated Dy₃Co_0.6Cu_0.4 compound is added to a powder mixture) resulted in the formation of the core-shell structure of 2–14–1 phase grains. The efficient improvement of the coercivity of Nd(RE)–Fe–B magnets, with a slight sacrifice of remanence, was demonstrated.