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In order to find an effective means of production of the interstitial Sm2Fe17Nx‐type nitride, an alloy modification by the addition of small amount of alloying element (4 at.% Nb) and a hydrogenation treatment prior to the nitriding process have been employed. The effect of previous hydrogenation on the nitride formation of the Sm2Fe17(4 at. % Nb) alloy has been investigated systematically by means of thermopiezic analysis, thermomagnetic analysis, and vibrating sample magnetometer. It has been found that the previous hydrogen treatment facilitates significantly the formation of the nitride, and this may be due to the clean surface and the finer size of the particles caused by the hydrogen decrepitation. It has also been found that the combination of the alloy modification with the addition of Nb and the previous hydrogen treatment can be utilized effectively for the production of a Sm2Fe17Nx‐type nitride.

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.

Hydrogen decrepitation is a very effective method of pulverizing Nd‐Fe‐B‐type alloys for sintered magnets. Jet‐mill response of hydrided materials is much better than that of unhydrided materials, i.e., shorter milling time is required and finer particle size is obtained for hydrided materials. The sinterability of hydrided powder is also improved from that of unhydrided powder. The minimum sintering temperature for obtaining full density (≳98% of theoretical density) magnets made from hydrided powders ranges between 900 and 950 °C, while that for magnets made from unhydrided powders ranges between 1050 and 1100 °C. Magnets made from hydrided powder sintered at low temperatures usually exhibit high coercivity comparable to or greater than that of the magnets made from unhydrided powder sintered at high temperatures followed by subsequent heat treatment. The enhancement of sinterability and the increase in coercivity of Nd‐Fe‐B magnets are discussed.

The high (BH)max potential of Nd‐Fe‐B materials make them excellent candidates for bonded magnets. We have previously shown that anisotropic bonded magnets can be made from hot deformed rapidly solidified materials. The purpose of this paper is to show how hot deformed materials can be pulverized into desired particle size ranges by using specific hydrogen decrepitation (HD) cycles. Specific temperatures for hydriding and desorbing cycles to achieve desired particle size ranges and magnetic values are given. The data shows that there are optimum times and temperatures to achieve the desired magnetic properties. TEM and SEM photomicrographs of mechanically and HD pulverized materials are shown. TEM studies show that stacking faults generated during HD are relieved after desorption. With proper control of HD and desorption, bonded magnets have been produced having (BH)max values as high as 18.9 MGOe.

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.

The magnetic properties of Nd17Fe76.5B5Cu1.5‐type sintered magnets, produced using a combination of hydrogen decrepitation and milling, have been investigated. An isothermal anneal or a step anneal, both performed after homogenization, resulted in a marked improvement of magnetic properties of the sintered magnets. Thermomagnetic analysis (TMA) of the sintered magnets showed that, in addition to a matrix phase with a Tc = 314 ± 5 °C, there was another ferromagnetic phase with a Tc = 54 ± 5 °C. The microstructural investigations showed a considerable amount of free α‐iron in the cast material, whereas no free iron was observed in the microstructure of the sintered magnets. However in very isolated areas within the sintered magnets, a Nd‐Fe phase with Fe/Nd∼8 was identified (probably Nd2Fe17‐type phase). The observation of this phase was consistent with results of TMA and the possible role of this phase in coercivity behavior is discussed.

High‐energy product anisotropic Nd‐Fe‐B permanent magnets can be prepared by consolidation and hot‐deforming melt‐spun material. One feature of this type of magnet is that it can be converted into stable powders for subsequent processing into anisotropic bonded magnets. The stability of this powder is attributed to the much smaller crystalline size of the rapidly solidified materials compared to that of sintered magnets. In this work, the properties of anisotropic bonded magnets made from powders produced by hydrogen decrepitation and by a mechanical crushing process are given. Energy products of 15–18 MGOe have been achieved by compression molding techniques. Long‐term thermal stability at 100 and 125 °C of bonded magnets using various binders are given. Transmission electron microscopy studies of the anisotropic powder indicate the microstructure to be similar to the hot‐deformed precursor material.

Large particle size Sm2Fe17Nx with a low oxygen content has been produced by treating Sm2Fe17 with hydrogen before nitrogenation. Large Sm2Fe17Nx particles of more than 2 mm include nearly 3.0‐wt. % nitrogen and a magnetization level of 140 emu/g. The hydrogen decrepitation behavior was not observed for this alloy and the size of samples were not changed after hydrogen treatment and nitrogenation. It is thought that hydrogen absorption and desorption increase the gas‐solid reaction area significantly. The oxygen content of these particles is less than 200 ppm because the fine pulverization step was skipped. The relation between the nitrogen content and magnetization of Sm2Fe17Nx will be presented.

This paper reports on the characterization of giant magnetostrictive alloys, of the general formula Tb0.27Dy0.73Fe2−x (0

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...

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.

Abnormal grain growth was found in sintered Nd‐Fe‐B magnets which are made from powder milled after hydrogen decrepitation. The abnormal grain size in the magnets sintered at 1000 °C for 1.5 h was as large as 400 μm. The formation of abnormally large grains reduced the magnetic properties. As sintering temperature increased, the number of large grains increased and coercivity reduced. However, abnormal grain growth was rarely found when the magnets were fabricated without hydrogen decrepitation. A grain growth mechanism is discussed in relation to the effect of hydrogenation and sintering process.

The hydrogen absorption/desorption behavior of giant magnetostrictive alloys represented by the general formula Tb0.27Dy0.73Fe2−X (0

The effects of hydrogen contents after the process of hydrogen decrepitation on the magnetic properties and corrosion resistance of sintered Nd–Fe–B permanent magnets have been studied. The intrinsic coercivity of the magnets increases and the remanence decreases with increasing hydrogen content. High residue hydrogen content in the starting ingot leads to the reaction of hydrogenation, disproportionation, desorption and recombination in the sinter process resulting in magnets with low remanence. The results of highly accelerated temperature/humidity stress test indicate that excessive hydrogen contents are harmful to the corrosion resistance of the magnets. The influences of residue hydrogen contents on the corrosion resistance of magnets dominate only when the residue hydrogen content exceeds 1300 parts per million.

We examine the effects of surface tension and surface modulus on diffusion-induced stresses within spherical nanoparticles. We show that both the magnitude and distribution of stresses can be significantly affected by surface mechanics if the particle diameter is in the nanometer range. In particular, a tensile state of stress may be significantly reduced in magnitude or even be reverted to a state of compressive stress with decreasing particle radius. This reduction in tensile stress may be responsible for the observed resilience to fracture and decrepitation of nanoparticles used in various industrial applications.