Earl G. Brewer

Grain growth and alignment in hot deformed Nd‐Fe‐B magnets

Formation of grain oriented Nd‐Fe‐B magnets from melt‐spun ribbons by hot deformation has been studied using electron microscopy. It is shown that deformation and alignment of Nd2Fe14B magnets result from a combination of plastic deformation, grain boundary migration, and grain boundary sliding. Due to a limited number of available slip systems in this material, samples with large grains deform less easily. Small grain size materials, as encountered in the melt‐spun ribbons, are well suited for die‐upsetting to produce oriented magnets.

High‐remanence rapidly solidified Nd‐Fe‐B: Die‐upset magnets (invited)

The best starting materials for die‐upset magnets are moderately overquenched melt‐spun ribbons of Nd‐Fe‐B alloys with ∼20 at. % more rare earth than stoichiometric Nd2Fe14B. Remanence increases with increasing die‐upset (DU) level, reaching a maximum of 13.5 kG at 70% DU for ternary Nd‐Fe‐B magnets, beyond which cracking limits performance. Remanence is also limited by nonuniform deformation, particularly for moderately die‐upset magnets. Heat treatments, dysprosium substitutions, or low‐level additives can be used to compensate for the decreases in coercivity which accompany increases in die‐upset level. By die upsetting in stages and removing misaligned surfaces, a large Nd‐Fe‐Co‐B‐Ga magnet (∼30 g) has been produced with both a high remanence (14.2 kG) and a high‐energy product (48.5 MG Oe).

Diffusion-alloyed additives in die-upset Nd-Fe-B magnets

In general, the stoichiometry of melt‐spun ribbons and subsequent hot‐worked Nd‐Fe‐B magnets derives from the composition of the starting ingot. We have shown, however, that it is possible to introduce small amounts of powdered metals into the ribbons after the ingot has been melt spun. Many elements, when mixed with the ribbons as a fine powder and then hot pressed, have been observed to diffuse into nearby ribbons, and in some cases (Cd, Cu, Au, Ir, Mg, Ni, Pd, Pt, Ru, Ag, and Zn) the elements diffused fairly evenly throughout the Nd‐Fe‐B ribbon matrix. These eleven elements significantly enhanced the coercivity of die‐upset magnets, with the highest coercivity observed with the addition of zinc (∼16 kOe).

Die-upset Pr-Fe-B-type magnets from melt-spun ribbons

In order to optimize the magnetic properties of die‐upset Pr‐Fe‐B magnets, we have systematically altered alloy compositions, including partial substitution of cobalt and neodymium, and low‐level additions of gallium. In general, the remanences of Pr‐Fe‐B die‐upset magnets were lower by ∼1 kG, and coercivities were higher by ∼4 kOe, than analogous Nd‐Fe‐B magnets. The microstructure of the Pr‐Fe‐B die‐upset magnets was similar to Nd‐Fe‐B magnets. Both systems are two phase, consisting of oriented flat platelet‐shaped grains of 2‐14‐1 separated by a rare earth‐rich intergranular phase. The intergranular phase is thicker for the Pr‐Fe‐B magnets, resulting in enhanced magnetic domain wall pinning and increased coercivity. Even higher coercivities were obtained with small amounts of cobalt substitution (≤5 at. % of the transition metal) and by very low‐level additions of gallium (≤0.5 at. %). The largest remanence (Br=13.7 kG) for a Pr‐based 75% die‐upset magnet was obtained with the alloy (R13TM81B6)0.9975Ga...

Processing effects on the magnetostrictive and physical properties of SmFe2/metal composites

Hot pressed SmFe2/Fe and SmFe2/Al composites combine the large magnetostriction of SmFe2 with the mechanical robustness of the Fe or Al matrix. Here we report the dependence of the magnetostriction, density, and mechanical hardness on processing parameters, specifically on the SmFe2 volume fill fraction and the consolidation temperature. The magnetostriction increases monotonically with increasing SmFe2 content, but both the density and hardness decrease; low fill fraction composites are within 10% of theoretical density and have hardnesses of 40–85 Rockwell B, whereas hot pressed SmFe2 powder alone has high porosity (exceeding 30%) and is too soft to register on the Rockwell B hardness scale. The best combination of properties lies in the range of 40%–60% SmFe2. Similar competition between magnetostriction and physical properties is observed as a function of the temperature used to consolidate the samples. The magnetostriction falls slowly as the hot press temperature increases, probably as a result of c...

Die-upset Nd2Fe14M magnets (M=B and C)

Melt‐spun Nd‐Fe‐B ribbons are limited to remanences of about 8 kG by the random orientation of the Nd2Fe14B grains. Hot deformation (die upsetting) causes these grains to reorient with the c axis aligned along the press direction, resulting in a 50% increase in the remanence (12 kG). Previously others have produced magnetically hard melt‐spun ribbons containing the isostructural phase Nd2Fe14C which, like the boride ribbons, were isotropic. By adjusting alloy stoichiometry and process variables, it is possible to die upset Nd‐Fe‐C magnets with properties similar to Nd‐Fe‐B magnets. As with the boride magnets, the remanences of the die‐upset Nd‐Fe‐C magnets were enhanced about 45% relative to the isotropic precursor. However, the remanences of die‐upset carbide magnets were limited to about 10 kG due to the lower saturation magnetization of the Nd2Fe14C phase and higher levels of impurity phases.

Crystallographic alignment analysis of Nd2Fe14B materials using standard x‐ray powder diffraction spectra

The crystallographic alignment quality of anisotropic magnetic materials made from die‐upset Nd2Fe14B (MQ‐III) materials was determined using a phenomenological analysis of θ‐2θ x‐ray diffraction spectra. This analysis consisted of fitting a calculated diffraction spectrum for aligned Nd2Fe14B to an observed spectrum collected on a bulk MQ‐III slab oriented with the c‐axis alignment parallel to the direction normal to the surface of the slab. The calculated spectrum was then used as a standard for intensity shifts of diffraction peaks corresponding to crystallographic reorientation from random to c‐axis alignment. Intensity shifts for other samples for Nd2Fe14B materials having various degrees of c‐axis alignment were determined and compared to the standard shifts giving an alignment factor representing the nature and extent of the crystallographic alignment in the sample.

Improved strain-anneal crystal growth technique

A new method of growing iron single crystals* of predetermined orientation is described. *U.S. Patent No. 3694269. This technique differs from the conventional strain-anneal method in that pulse heating is used to suppress random nucleation of undesired grains ahead of the growth interface, rather than a traveling “sharp” temperature gradient which is often difficult to attain. The procedures used to grow large α-iron single crystals with the new technique are fully described. A significant advantage of this method is that large oriented single crystals, 25 cm (10 in.) or greater in length, can be grown from rods or strip in a few hours, with a much higher probability of success than can be obtained with the conventional technique, which typically requires several days to obtain crystals of this size.

Hot-pressed Nd/sub 2/Co/sub 14/B magnets

The authors report consolidation of Nd/sub 2/Co/sub 14/B ribbons by hot pressing into nearly fully dense isotropic magnets. The remanence (B/sub r/=5.2 kG) and the energy product ((BH)/sub max/=5.4 MGOe) of the magnetically isotropic hot-pressed magnets were 2-3% less than values for annealed ribbons of the same composition. Five compositions were studied, and the best results were compared to those for a hot-pressed Nd-Fe-B magnet. A slight difference between the normalized demagnetization curves of Nd-Co-B and Nd-Fe-B hot-pressed magnets indicates a larger reversible demagnetization component in the former. There is no difference in the normalized recoil plots between Co-based and Fe-based alloys. >