P.J. McGuiness

Hydrogenation, disproportionation and desorption (HDD): An effective processing route for NdFeB-type magnets

Abstract Polymer-bonded magnets have been made from powdered material of the alloys Nd16Fe76B8 and Nd12.3Fe81.9B5.8 (Nd2.1Fe14B) by a hydrogenation-disproportionation-desorption (HDD) process. The HDD process resulted in the production of fine-grained homogeneous materials (from the coarse-grained, non-homogeneous cast alloy) which exhibited appreciable coercivities and remanences characteristic of isotropic materials. Optimum annealing conditions were established for both alloys and superior coercivities were observed for the alloy richer in neodymium.

Disproportionation in Nd16Fe76B8-type hydrides

Abstract This work has shown that heating Nd 16 Fe 76 B 8 -type materials in a hydrogen atmosphere, with subsequent vacuum annealing at greater than 750°C produces coercive powders which are suitable for the production of polymer-bonded magnets. A study of the material at different stages of the process has confirmed that the Nd-Fe-B alloy disproportionates into iron, Fe 2 B and neodymium hydride before recombining during the vacuum annealing stage to produce the fine-grained microstructure necessary for highly coercive material. The grain size of the final microstructure has been found to be very dependent on the temperature at which the hydrogenation-disproportionation-desorption cycle is carried out with optimum properties for the Nd 16 Fe 76 B 8 alloy being obtained for a desorption temperature of 785°C for 1 h.

Hydrogen: its use in the processing of NdFeB-type magnets

Abstract In this paper we describe and discuss the use of hydrogen in the processing ofNdFeB-type magnets. The roles of hydrogen in the hydrogen decrepitation (HD) and hydrogenation, disproportionation, desorption and recombination (HDDR) processes are considered together with the characteristics of the magnets produced by these routes.

The production of a Nd-Fe-B permanent magnet by a hydrogen decrepitation/attritor milling route

A bulk ingot of a Nd-Fe-B alloy has been powdered by a combination of hydrogen decrepitation and attritor milling. The powder was aligned and pressed in the hydrided condition and the green compact sintered at 1080‡ C for 1 h after an appropriate heating rate. Excellent densities were achieved after this procedure and the magnets produced by this method exhibited energy products in the region of 250 kJm−3 (32 M GOe).

A study of Nd-Fe-B magnets produced using a combination of hydrogen decrepitation and jet milling

A combination of hydrogen decrepitation (HD) and jet milling (JM) has been used to produce powder for the processing of permanent magnets. The procedure has proved to be very successful for both NdFeB (“Neomax”) alloys and the NdDyFeNbB high coercivity alloys. The magnets produced by the HD/JM process showed excellent coercivities when sintered between 980°C and 1040°C, at higher temperatures, excessive grain growth reduced the coercivity values significantly.

The production and characterization of bonded, hot-pressed and die-upset HDDR magnets

The recently developed hydrogenation, disproportionation, desorption and recombination (HDDR) process has now been extended to the production of hot-pressed isotropic magnets. These magnets have been produced from cast alloy with a composition Nd16Fe76B8 and exhibit intrinsic coercivities of about 1200 kA m−1, remanences of around 690 mT and BH (max) values of approximately 90 kJ m−3. Subsequent die upsetting indicates that increased remanences can be obtained by using a suitable height reduction ratio. Careful studies of the microstructures of the isotropic hot-pressed magnets reveal that they are similar to the HDDR material in its powder form, i.e. they consist of Nd2Fe14B grains predominantly in the range 0.1⩽ × ⩽1.0 μm with a very small number of grains in the range 10 ⩽ × ⩽ 35 μm. These larger grains exhibit a very regular morphology and appear to be the result of a very rapid growth of some of the smaller submicron grains. The starting composition of the cast material ensures that the magnets have about 12% neodymium rich intergranular material and this low melting point constituent has been found to be of great assistance in the densification process. However, scanning and transmission electron microscopy studies show that the neodymium rich material is very coarsely distributed throughout the hot-pressed magnet and little evidence can be found for its presence between the grains of Nd2Fe14B. The addition of small amounts of zirconium to the basic Nd16Fe76B8 alloy was investigated to determine its effect on coercivity and grain orientation. Highly oriented powder with good coercivity has been produced from material containing 0.1 at.% Zr (Nd16Fe76.9B8Zr0.1). Larger zirconium additions resulted in significant losses in coercivity. Bonded magnets with a remanence of about 150 mT, an intrinsic coercivity of 740 kA m−1 and an energy product of about 150 kJ m−3 have been produced from the anisotropic HDDR powder.

Mass spectrometer hydrogen desorption studies on some hydrided NdFeB-type alloys

Abstract The desorption of hydrogen from the alloys Nd 11.8 Fe 82.4 B 5.8 (formula unit Nd 2 Fe 14 B), Nd 15.5 Fe 77.5 B 7 , Nd 12.5 Dy 3 Fe 74.5 V 3 B 7 and Nd 12.5 Dy 3 Fe 69.5 Co 5 V 3 B 7 (the numbers are atomic percentages of the elements) and from 95% pure neodymium has been investigated by mass spectroscopy. These studies indicated that the desorption of hydrogen from the multiphase alloys is a three-stage process: (1) desorption from the matrix Nd 2 Fe 14 B, (2) partial desorption from the neodymium-rich material and (3) complete desorption from neodymium-rich material. The desorption behaviour of the cobaltcontaining alloy indicated a change in the character of the grain boundary phases in the alloy.

The production of high coercivity cast magnets using the HDD process

Nd‐Fe‐B as‐cast alloys in the form of small ingots 4–5 mm thick and with composition Nd2.1Fe14B were transformed into high coercivity magnets using the hydrogenation disproportionation desorption (HDD) process without decrepitation of the blocks taking place. Magnetic measurements revealed that it was possible to produce blocks with coercivity of ∼780 kA m−1, remanence of ∼680 mT and a BHmax of ∼68 kJ m−3 (8.6 MGOe). The high coercivities in these samples were attributed to the disproportionation and subsequent recrystallization process. SEM metallographs of the samples processed at optimum temperature revealed an absence of the original large (∼200 μm) grains of Nd2Fe14B containing areas of free iron; instead an extremely fine grained, submicron, microstructure of single phase Nd2Fe14B grains was observed. In samples processed at temperatures higher than that found to be the optimum, larger Nd2Fe14B grains with extremely rectangular morphologies could be observed growing from surrounding submicron matrix...

HDDR hot-pressed magnets: magnetic properties and microstructure

Abstract The recently developed HDDR process has now been extended to production of hot-pressed magnets. The samples prepared to date exhibit intrinsic coercivities of ≈1200 kA/m, remanence of around 690 mT and BHmax values of ≈90 kJ/m3. Careful studies of the microstructures reveal that the magnets consist of Nd2Fe14B grains predominantly in the range 0.1 μm⩽x⩽1.0 μm with a very small number of grains in the range 10 μm ⩽x⩽35 μm. These larger grains exhibit a very regular morphology and appear to be the result of a very rapid growth of some of the smaller sub-micron grains. The starting composition of the cast material Nd16Fe76B8 ensures that the magnets have ≈12% Nd-rich intergranular material; this low melting point phase has been found to assist in the densification process. However, SEM studies have shown that the Nd-rich material is very unevenly distributed throughout the hot-pressed magnet and little evidence can be found for its presence between the grains of Nd2Fe14B.

Anisotropic hydrogen decrepitation and corrosion behaviour in NdFeB magnets

Abstract A number of Nd 16 Fe 76 B 8 sintered magnets were produced in isotropic (unaligned) and anisotropic (aligned) forms. The magnets were prepared in the conventional way via isostatic pressing and finished with a centreless grinder, no coatings were applied and the samples were left unmagnetised. These magnets were identical in every way apart from the orientation of the Nd 2 Fe 14 B grains. Each magnet was exposed to hydrogen and the decrepitation behaviour observed. The anisotropic samples were found to decrepitate exclusively from the ends of the rods whereas the isotropic magnets were attacked by the hydrogen at all points on their surface. Bulk corrosion studies in steam gave comparable results, the oriented sample suffered severe corrosion at its poles and the isotropic sample was attacked over the whole surface. X-ray diffraction studies on the corrosion product indicated a greatly expanded lattice with the Nd 2 Fe 14 B structure. Both the hydrogen decrepitation and the corrosion which take place in these materials can be attributed to the formation of hydrides.