Hydrogenation Disproportionation Desorption Recombination Treatment Research Articles

Anisotropy optimization of HDDR magnetic powders by precursor alloy annealing

The anisotropy of hydrogenation–disproportionation–desorption–recombination (HDDR) method to form Nd2Fe14B powders was effectively improved by annealing the strip casting (SC) alloy precursors at 1080 °C. According to the analysis of the evolution of microstructure, the excellent anisotropy was attributed to the nearly consistent orientation of the nanoscale Nd2Fe14B grains in the HDDR magnetic powders, which perfectly inherited that of the coarse grains in the treated SC alloy flakes. In contrast, for the untreated specimen, more nanoscale clusters of different orientations were generated after smashing and HDDR treatment of fine columnar Nd2Fe14B grains with different orientations in the original SC alloy, severely affecting the anisotropy of the magnetic powders. This study provides new insights into the development of high-anisotropy and high-performance Nd-Fe-B magnets.

High Coercive RE-Fe-B Powders Recycled via HD and HDDR Process from Waste Magnets

In order to fabricate high-coercivity hard magnetic RE-Fe-B powders from the waste bulk magnets, the hydrogen decrepitation (HD) and hydrogenation-disproportionation-desorption-recombination (HDDR) processes were employed to the waste RE-Fe-B magnets. The optimum HD and HDDR conditions for the waste RE-Fe-B magnets were established through the systematic investigation on the effect of HD and HDDR treatment temperature on the magnetic and microstructural properties of final recycled powders. We found that the waste magnets were crushed into the flakes with a uniform size distribution when the HD treatment was carried at 200 ℃. By applying the HDDR process to the HD treated flakes at 920 ℃, high coercivity of 11.2 kOe could be achieved in the recycled RE-Fe-B HDDR powders. Based on the results from the microstructural characterization, the underlying reasons for the magnetic and microstructural changes of the recycled powders as a function of the HD and HDDR treatment temperature is discussed in detail.

Microstructure evolution and coercivity mechanism of hydrogenation-disproportionation-desorption-recombination (HDDR) treated Nd-Fe-B strip cast alloys

In this paper, we systematically investigated the microstructure evolution and coercivity mechanism of hydrogenation-disproportionation-desorption-recombination (HDDR) treated Nd-Fe-B strip cast alloys by transmission electron microscopy (TEM) and three-dimensional atom probe (3DAP) analyses. The rod-like NdH2+x phases with diameters of 10–20 nm are embedded into α-Fe matrix, which hereditarily leads to textured grains in HDDR alloy. The migration of NdH2+x from Nd-rich region to α-Fe matrix during hydrogen absorption process contributes to the uniform redistribution of Nd-rich phases after HDDR treatment. The HDDR alloy with single domain grain sizes of 200–300 nm exhibits relatively low coercivity of 1.01 T that arises from pinning magnetic domain motion. The weak c-axis orientation of HDDR alloy results in a lower reverse magnetic field (coercivity) to reduce remanence to 0. Moreover, the direct contact of Nd2Fe14B grains and the high concentration of ferromagnetic elements (Fe content ≈ 66.06 at%, Co content ≈ 0.91 at%) in Nd-rich grain boundary layer lead to strong magnetostatic coupling effect among Nd2Fe14B grains. The nano-sized α-Fe inside Nd2Fe14B matrix makes the magnetization reversal easily and decreases the coercivity of HDDR alloy.

Stoichiometric study of metastable TbCu7 type Sm-Fe phase synthesized by HDDR treatment

• Compositional variety of HDDR-synthesized TbCu 7 type Sm-Fe alloys is examined. • HDDR treatment is carried out for raw Sm-Fe alloys with various Fe-rich compositions. • TbCu 7 type Sm-Fe alloys synthesized by HDDR behaves like stoichiometric SmFe 8.5 alloy. • Transition from Fe-rich compositions to SmFe 8.5 occurs in a stage of HD reaction. Hydrogenation-disproportionation-desorption-recombination (HDDR) treatment of Fe-rich SmFe x alloys with x > 8.5 was conducted in order to investigate the possibility that the HDDR treatment can synthesize metastable TbCu 7 Sm-Fe phases with a Fe-rich composition, which are expected to provide large saturation magnetization . Fe-rich alloys prepared by melt spinning were HDDR-treated at 600 °C. As a result, the constituent phases of all the alloys transformed into non-Fe-rich TbCu 7 type SmFe 8.5 and iron phases in spite of the Fe-rich compositions of the original Sm-Fe phases. Thus, the TbCu 7 type Sm-Fe phase synthesized by HDDR treatment behaved as the stoichiometric compound of SmFe 8.5 . Electron microscope analysis revealed that the exclusion of excess iron from the original Sm-Fe phase occurred in the stage of disproportionation. In addition, the Sm-Fe phase prepared via a combination process of melt spinning and HDDR treatment had a very refined microstructure with grain sizes smaller than 100 nm, resulting in a large coercivity of 14.1 kOe after nitrogenization.

Crystallographic alignment of Fe2B and Nd2Fe14B for texture memory in hydrogenation–disproportionation–desorption–recombination-processed Nd–Fe–B powders

Texture development in hydrogenation–disproportionation–desorption–recombination (HDDR)-processed NdFeB magnets is discussed with a focus on the crystallographic alignment of the Nd2Fe14B and Fe2B phases. By careful control of the dynamic HDDR-processing parameters, shape changes in the HDDR-treated samples were minimized by suitable modifications to the sample dimensions to trace the orientation relation between the c-axes of the Nd2Fe14B and Fe2B phases. The magnetic anisotropy was significantly dependent on the hydrogen pressure during the HD process and highly aligned Fe2B phases, with the same c-axis orientation as the initial Nd2Fe14B phase, were formed under the dynamic HDDR treatment at a hydrogen pressure of 30 kPa under controlled HD conditions. During the DR reaction following the HD process, the orientation of the initial Nd2Fe14B was recovered by a highly aligned Fe2B phase. It was demonstrated that the Fe2B phase could mediate texture during the HDDR processing owing to the atomic configuration and crystallographic relation between the Nd2Fe14B and Fe2B phases.

Direct observation of texture memory in hydrogenation–disproportionation–desorption–recombination processed Nd-Fe-B magnets using electron backscatter diffraction

Memory of texture in hydrogenation–disproportionation–desorption–recombination (HDDR) processed NdFeB magnets was directly observed using electron backscatter diffraction (EBSD). By precise control of dynamic HDDR processing conditions, shape changes of samples were minimized by proper modifications of dimensions of samples subjected to HDDR treatment. EBSD analysis shows the preservation of texture before and after the HDDR process and biaxial texture in initial Nd2Fe14B grains after the dynamic HDDR treatment at a hydrogen pressure of 30kPa. It is clearly demonstrated that the c-axis of initial Nd2Fe14B matches the c-axis of recombined Nd2Fe14B and it recovers after an anisotropic HDDR process.

Effect Of The Desorption-Recombination Temperature On The Microstructure And Magnetic Properties Of HDDR Processed Nd-Fe-B Powders

Abstract The effect of the desorption-recombination temperature on the microstructure and magnetic properties of hydrogenation-disproportionation-desorption-recombination (HDDR) processed Nd-Fe-B powders was studied. The NdxB6.4Ga0.3Nb0.2Febal (x=12.5-13.5, at.%) casting alloys were pulverized after homogenizing annealing, and then subjected to HDDR treatment. During the HDDR process, desorption-recombination (DR) reaction was induced at two different temperature, 810°C and 820°C. The higher Nd content resulted in enhanced coercivity of the HDDR powder, and which was attributed to the thicker and more uniform Nd-rich phase along grain boundaries. But this uniform Nd-rich phase induced faster grain growth. The remanence of the powder DR-treated at 820°C is higher than that DR-treated at 810°C. In addition, it was also confirmed that higher DR temperature is much more effective to improve squareness.

Microstructural evolutions in NdFe10.5Mo1.5NX and Pr13Fe80B7 compounds during hydrogenation-disproportionation desorption recombination process

Microstructural changes of NdFe10.5Mo1.5 and Pr13Fe80B7 compounds during the solid-hydrogenation disproportionation desorption recombination process are investigated in this paper. It is found that the early disproportionated microstructure of NdFe10.5Mo1.5 and Pr13Fe80B7 alloys shows similar rodlike morphology. All the NdH2+X and PrH2+X rods have the same orientation and grow with a definite orientation related to the α-Fe matrix. However, it is different that Mo atoms are found to dissolve in the α-Fe matrix of the NdFe10.5Mo1.5 disproportionated products while the PrH2+X rods are enriched with B in the case of Pr13Fe80B7 alloys. No special crystallographic relationship between the recombined NdFe10.5Mo1.5 or Pr13Fe80B7 and its disproportionated products is observed. A preferred nucleation and growth of recombined Pr2Fe14B grains is found while that of recombined NdFe10.5Mo1.5 grains is not. Thus, the final hydrogenation-disproportionation desorption recombination (HDDR) NdFe10.5Mo1.5NX powder is isotropic while the HDDR Pr13Fe80B7 powder has a strong anisotropy under the same HDDR treatment conditions. In the end, the formation of anisotropy in the HDDR Pr13Fe80B7 and NdFe10.5Mo1.5NX powders is discussed.

Effect of crystal orientation relationship on inducement of anisotropy in HDDR-treated Nd-Fe-B alloys

Using anisotropic sintered bodies, the crystal orientation relationship between the parent Nd/sub 2/Fe/sub 14/B phase and the disproportionated mixture during the hydrogenation disproportionation desorption recombination (HDDR) treatment was investigated by using X-ray diffraction and transmission electron microscopy. It can be concluded that retaining the crystallographic orientation relationship such as /sub /spl alpha/-Fe/ and (NdH/sub 2/)// (Nd/sub 2/Fe/sub 14/B), [110]/sub /spl alpha/-Fe/ and [110](NdH/sub 2/)//[100](Nd/sub 2/Fe/sub 14/B) during HDDR treatment is important for the inducement of anisotropy.

An improved HDDR treatment for the production of anisotropic Nd–Fe–B ternary powders

The magnetic properties of Nd 12.2Fe 81.8B 6.0 alloys processed using a new HDDR (hydrogenation disproportionation desorption recombination) treatment were investigated. This newly proposed HDDR treatment is a combination of heat treatments at hydrogen pressures close to the recombination pressure of the Nd 2Fe 14B compound, in both the disproportionation and recombination stages. In other words, this new treatment is a combination of the l-HD (heating in a low H 2 pressure during the hydrogenation disproportionation stage), and s-DR (heating in Ar or in a relatively high pressure of hydrogen, at the start of recombination) treatments. In this investigation, the influence of the s-DR conditions on the magnetic properties of anisotropic Nd–Fe–B HDDR-treated powders, were investigated. It was found that an s-DR treatment in which the hydrogen pressure was decreased in steps from 0.1 MPa to 0.5 kPa, enhanced the remanence (1.45 T) and anisotropy (Br/Js=0.94). The rate at which the hydrogen pressure decreases to 6 kPa during s-DR is also considered to be an important factor in obtaining both a high remanence and a high coercivity. In addition, the coercivity was found to increase after an s-DR treatment in which the temperature was decreased in a step by step manner.

Phase transformation and magnetic properties of Nd8Fe84Ti8 alloy during HDDR process

The effects of the hydrogenation-disproportionation-desorption-recombination (HDDR) process on the structure and the magnetic properties of the Nd 8 Fe 84 Ti 8 alloy prepared by mechanical alloying (MA) and its nitride counterpart have been studied in detail. It has been found that Nd(Fe,Ti) 12 H x is formed from 300°C to 550°C. The disproportionation starts at 550°C and is completed at 960°C. The desorption and the recombination are almost synchronized. Nd(Fe,Ti) 7 is formed at 750°C during the HDDR treatment. With increasing temperature of HDDR process, the metastable structure of TbCu 7 type is gradually transformed into the structure of ThMn 12 type. The intrinsic coercivity increases with increasing the temperature of HDDR process.

Influence of M=Al, Ga and Si on microstructure and HDDR-processing of Sm2(Fe,M)17 and magnetic properties of their nitrides and carbides

Microstructural investigations by means of scanning electron microscopy and X-ray diffraction showed that the formation of high amounts of α-Fe in as-cast Sm2(Fe,M)17 alloys can be suppressed by using the substitutions M=Al or Si together with excess Sm. Ga, on the other hand, does not induce this beneficial effect. The hydrogen absorption and desorption behaviour was investigated by temperature–pressure–analysis and hydrogen differential thermal analysis. Decreasing amounts of hydrogen were absorbed in Sm2(Fe,M)17 for increasing contents of all substitutions M=Ga, Al and Si due to their stabilising effect on the 2:17 phase with regard to its disproportionation. Compared with the homogenised alloys, the as-cast materials show a weaker interstitial hydrogen absorption and a stronger disproportionation reaction at lower temperatures due to the higher content of Sm-rich phases in the as-cast alloys. A second cycle of the hydrogenation–disproportionation–desorption–recombination (HDDR) process leads to a faster disproportionation reaction at lower temperatures due to the grain refinement during the first cycle. Magnetic properties of as-cast and homogenised materials were investigated after HDDR treatment and nitrogenation or carburisation of pre-milled powders. Nitrogenated and carburised samples showed coercivities up to 3.0 T and 2.3 T, respectively. There were only slight differences of the magnetic properties of materials prepared from homogenised and from as-cast samples.

Hydrogenation disproportionation desorption recombination in Sm–Co alloys by means of reactive milling

Sm–Co-type alloys were disproportionated by milling in hydrogen at enhanced temperatures. X-ray diffraction confirmed the disproportionation of the SmCo5 and Sm2Co17 phases into Sm hydride and α-Co. This “reactive milling” procedure facilitates the disproportionation of these alloys which are characterized by a very high thermodynamic stability, and therefore are not available for a standard hydrogenation disproportionation desorption recombination treatment. Recombination of the reactively milled powders leads to the formation of the original phases, now with dramatically refined grain sizes of around 25 nm and significant coercivities such as μ0JHC=3.7 T in the case of the SmCo5 alloy. Exchange coupling between the nanoscaled grains resulted in magnetically single phase behavior despite a multiphase microstructure. In particular, for the Sm2Co17 alloy, a remanence enhancement was observed for recombination temperatures ⩽700 °C.