Hydrogen Decrepitation Process Research Articles

Study of desorbed hydrogen-decrepitated anisotropic Nd-Fe-B powder using x-ray diffraction

The intrinsic magnetic coercivity (Hci) of Nd-Fe-B-based permanent magnet material is profoundly affected by hydrogen absorbed during the hydrogen decrepitation (HD) process for producing anisotropic powders from bulk anisotropic hot-deformed MAGNEQUENCH (MQ) magnets. Hydrogen (H) content and x-ray diffraction measurements clarify the effects of H and desorption temperature (Td) on the intrinsic magnetic anisotropy (IMA) of the Nd2Fe14B-type phase and the nature of the intergranular phases, both of which are crucial for high Hci. The Nd-rich intergranular phase disproportionates during HD, initially forming a microcrystalline Nd-hydride phase, possibly Nd2H5. For Td≤220 °C, H remains in the Nd2Fe14B-type phase, severely degrading the IMA, which causes a low Hci. For 220 °C≤Td≤250 °C, enough H desorbs from the Nd2Fe14B-type phase and the IMA recovers its large prehydrided value, and the microstructure supports a high Hci≤10 kOe in spite of the H disproportionated intergranular phase. Only for Td≳250 °C is Hci degraded by the microstructure, corresponding to further H desorption and the microcrystalline Nd-hydride phase becoming well-crystallized NdH2. The NdH2 phase decomposes with continued H desorption and at Td≳580 °C recombines to re-form the Nd-rich intergranular phase of prehydrided MQ material. H is completely desorbed above 580 °C and Hci≳11 kOe, nearly that of the prehydrided MQ magnets.

Microstructural and magnetic studies of Pr-Fe-B-Cu HD sintered magnets

Recent work has shown that the coercivity of Pr 20.5Fe 73.8B 3.7Cu 2-type sintered magnets produced by the hydrogen decrepitation (HD) process can be enhanced substantially by a high-temperature post-sintering heat treatment. In the present work, the microstructures of both as-sintered and annealed magnets have been investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) in an attempt to reveal the reason for this increase. Backscattered electron image on SEM and energy dispersive X-ray analysis (EDX) indicated the presence of Pr 2Fe 17 (Fe:Pr (at%) ≈ 8.2) in both magnets. The presence of this phase has been confirmed by thermomagnetic analysis and TEM investigations. The amount of this phase diminished after heat treatment. A Pr 34Fe 62Cu 4 phase has also been observed by TEM. The increase in the coercivity on annealing has been attributed to the improved magnetic isolation of the Pr 2Fe 14B grains, to the diminution in the amount of Pr 2Fe 17 phase, and to the formation of individual and isolated grains of this phase.

Hydrogen decrepitated anisotropic Nd-Fe-B powder: Hydrogen desorption and magnetic properties

The hydrogen decrepitation (HD) process easily pulverizes rapidly solidified, hot-deformed Nd-Fe-B (MQ3) magnets but degrades the magnetic properties. Recovery of high-performance magnetism in HD MQ3 powder has always required hydrogen desorption at temperatures above 600 °C. It is reported that desorption at the relatively low temperature of 240 °C restores magnetic coercivity Hci to above 10 kOe despite residual hydrogen in the material. Desorption between 240 and 580 °C significantly degrades Hci but not the intrinsic magnetic anisotropy. Hydrogen is reabsorbed when the desorbed powder is cooled from above 600 °C to room temperature in the presence of small amounts of residual hydrogen gas. This hydrogen reabsorption strongly degrades the coercivity but does not affect the intrinsic magnetic anisotropy.

The effect of ingot heat treatment on the magnetic properties of PrFeBCu hydrogen decrepitation sintered magnets

The as-cast ingot microstructure of the alloy Pr 20.5Fe 73.8B 3.7Cu 2 has been modified by a homogenization heat treatment of 1000°C for 24 h. For particular processing conditions, sintered magnets prepared from the homogenized alloy using the hydrogen decrepitation (HD) process exhibit superior remanence and energy product to those prepared from the as-cast ingot. The squareness factor has also been improved considerably in the magnets prepared from the homogenized alloys. This work also showed that the influence of the initial microstructure on the intrinsic coercivity of PrFeBCu HD sintered permanent magnets diminishes rapidly with increasing milling time and is a minimum when the coercivity reaches the maximum value of about 20 kOe.

Magnetic properties of giant magnetostrictive Terfenol-D processed by hydrogen decrepitation

The magnetic properties of a magnetostrictive Terfenol-D rod produced by the hydrogen Decrepitation (HD) process from cast material of composition RFe 1.5 (R=Tb 0.27Dy 0.73) are compared with those of cast and zoned rods of composition RFe 1.9. The cast RFe 1.5 alloy disproportionates into α-iron and rare earth hydrides on exposure to hydrogen, the material re-combining on desorption of hydrogen and subsequent sintering. Magnetisation measurements show that the HD processed rod was more difficult to saturate than the cast and zoned rods. Measurements of the magneto-mechanical coupling factor k 33 also show that the HD rod requires a higher optimum dc bias field than the cast or zoned rods, giving a k 33 value of 0.28 at the highest bias field of 43 kA/m, having not yet reached its maximum value.

High coercivity sintered PrFeBCu magnets using the hydrogen decrepitation process

Sintered permanent magnets based on the compositions Pr 20.5Fe 73.8Cu 2 and Pr 16.9Fe 79.3B 4 have been prepared using the hydrogen decrepitation process. For particular processing conditions, annealing the magnets at 1000°C resulted in an increase in i H c from 11 to around 20 kOe for both alloys. Sintered and furnace-cooled magnets exhibited higher intrinsic coercivities than the quenched magnets. The possibility of changing the easy direction of magnetization from the tetragonal c-axis to easy basal plane during hydrogen absorption has also been investigated as a means of producing radially anisotropic permanent magnets using the isostatic pressing method.

Controlled solidification and magnetic properties of Pr-Fe-B-Cu and Nd-Fe-B alloys

The as-cast microstructure of the alloys Pr20.5Fe73.8B3.7Cu2 and Nd16Fe76B8 have been modified using a vertical floating-zone (VFZ) technique. By annealing the as-cast alloys and VFZ material at 1000 °C, the magnetic properties increased substantially for Pr-Fe-B-Cu alloy. VFZ growth rates up to 38 cm/h have not been sufficient to reproduce the as-cast ingot microstructure of the Pr-Fe-B-Cu alloy. Cast magnets prepared in a very small chill mold showed the best magnetic properties, with a (BH)max=98 kJ/m3. Sintered magnets prepared using the hydrogen decrepitation process from the large grained Nd-Fe-B zoned alloys exhibit inferior squareness factors to those prepared from the as-cast ingot.

Hydrogen absorption/desorption studies on some giant magnetostrictive REFe 2 alloys and their powder metallurgical processing by hydrogen decrepitation

The hydrogen decrepitation (HD) process has been shown to offer two possible routes for the powder metallurgical production of REFe 2 (RE≡rare earth) type giant magnetostrictive alloys, one from crystalline hydride poweers and one from disproportionated material consisting of rare earth di- and tri-hydrides and α-iron. Differential thermal analysis (DTA) studies have shown the large exothermicity of the reaction of hydrogen with these materials, 50 mg of cast Terfenol-D (Tb 0.27Dy 0.73Fe 1.9) producing a 50°C temperature rise on hydrogeneration. Mass spectrometer studies have indicated the temperatures at which the hydrogenated alloys have completely desorbed hydrogen, and give an indication of the amount of oxidation of the rare-earth-rich phase. Complex microstructures on hydrogenation in these alloys are revealed by metallographic study.

Microstructure and magnetic domains in sintered NdFeB magnets made by hydrogen decrepitation and conventional techniques

The microstructures of magnets fabricated by the hydrogen decrepitation process and a conventional route have been compared using transmission electron microscopy. The results reveal that magnets produced by the conventional technique contain particles of neodymium-rich and boron-rich phases in the hard magnetic Nd 2Fe 14B grains, whilst this matrix phase is found to be almost free of neodymium-rich particles in the magnets produced by the hydrogen decrepitation route, but the boron-rich particles still exist within the matrix phase. In addition, in both routes the microstructure showed that the Nd 2Fe 14B grains have a clean and faultless structure. Magnetic domain walls have been observed within the Nd 2Fe 14B phase using Lorentz electron microscopy with no special stage. The result shows that all the domain walls end, and are impeded, at the neodymium-rich regions around the grain boundaries, confirming that H ci in the sintered Nd-Fe-B magnets is nucleation-controlled rather than pinning-controlled inside the matrix phase.

Powder metallurgical processing of Tb0.27Dy0.73Fe2−X (0<X≤0.5) by a hydrogen-decrepitation (HD) route

The hydrogen absorption/desorption behavior of giant magnetostrictive alloys represented by the general formula Tb0.27Dy0.73Fe2−X (0<X≤0.5) has been investigated to provide background information for the production of powder by the hydrogen-decrepitation (HD) route. X-ray-diffraction studies indicate a range of hydriding behaviors depending upon the particular conditions. Thus, cubic, rhombohedral, and amorphous hydrides have been observed and under certain conditions disproportionation occurs with the production of iron and rare-earth hydrides. Metallographic examination of the latter material reveals the phases produced by this disproportionation. Sintered compacts have been made from the disproportionated material by the HD process and compacts with up to 96% of theoretical density have been achieved after only 1-h sinters. The static magnetostrictive properties of these compacts have been investigated, and compare well with values obtained by other workers on samples produced using conventional powder metallurgical methods. The HD process appears to offer significant advantages over more conventional powder metallurgical routes for the production of compacts of these materials.

The hydrogen decrepitation behavior of alloys and magnets based on Nd16Fe76B8

The hydrogen decrepitation (HD) behaviors of various forms of the permanent magnet alloy Nd16Fe76B8 and stoichiometric composition Nd11.8Fe82.3B5.9 have been investigated to provide background information on the production of sintered magnets by the HD process. The influence of the initial microstructures of the alloys on the subsequent HD behavior has been examined. Thus the as-cast ingots, homogenized ingots, and sintered magnets have been exposed to hydrogen at 2 bars of pressure at room temperature and the hydrogen absorption behaviors investigated by differential thermal analysis. The morphologies of the HD powders have been studied by scanning electron microscopy and have been related to the initial microstructures.

The structure and magnetic properties of NdFeB magnets produced by conventional and hydrogen decrepitation routes

Comparison between magnets produced by the hydrogen decrepitation (HD) process and a conventional technique has been made by studying the microstructure and the magnetic properties. The conclusions are (i) magnets fabricated via the HD process are found to be free of neodymium-rich particles within the matrix phase (Nd 2Fe 14B) whilst these particles still exist in the matrix when the magnets are produced by the conventional technique (ii) the coercivity values of the magnets produced by the HD process are higher than those produced via the conventional technique at low sintering temperatures (around 1030 °C).

The potential of hydrogen in permanent magnet production

In this article, the application of the hydrogen decrepitation (HD) process to the production of rare earth containing permanent magnets is discussed. It is shown that the HD process can be applied successfully to the production of SmCo 5 − , Sm 2(Co, Fe, Cu, Zr) 17 − and Nd 2Fe 14B-type magnets. The particular HD conditions for each category of permanent magnet are described. The characteristics of the HD powder of all three alloy types are compared with those of the normal milled material and it is seen that significant advantages accrue from the use of the HD powder.