稀有金属(英文版) 2015,34(02),89-94

Desorption–recombination behavior of as-disproportionated NdFeCoB compacts by reactive deformation

School of Materials Science and Engineering, Sichuan University

摘 要：

The disproportionated phases of Nd Hx, Fe2B,and a-Fe from Nd2Fe14B were applied to prepare Nd Fe B magnets by two different routes. The results show that the route of annealing in horizontal vacuum sintering furnace cannot reach the purpose of complete recombination after the hot pressing and hot deformation process due to the lack of dehydrogenation channel. The route of applying low pressure of 4–25 MPa on the as-disproportionated green compact during the desorption recombination process in situ hot deformation in a spark plasma sintering(SPS)system can obtain completely recombined Nd Fe B magnet with good anisotropy and magnetic properties. The maximum magnetic properties,(BH)max= 201 kJ m-3,Br= 1.142 T and Hcj= 469 k A m-1, are obtained after being treated for 15 min at 750 °C under low pressure.

收稿日期：29 May 2014

基金：financially supported by the National Natural Science Foundation of China (No. 51171122);

Desorption–recombination behavior of as-disproportionated NdFeCoB compacts by reactive deformation

Yun-Ping Yu Ying Liu Jun Li Qing Zheng Ren-Quan Wang

School of Materials Science and Engineering, Sichuan University

Abstract：

The disproportionated phases of Nd Hx, Fe2 B,and a-Fe from Nd2Fe14B were applied to prepare Nd Fe B magnets by two different routes. The results show that the route of annealing in horizontal vacuum sintering furnace cannot reach the purpose of complete recombination after the hot pressing and hot deformation process due to the lack of dehydrogenation channel. The route of applying low pressure of 4–25 MPa on the as-disproportionated green compact during the desorption recombination process in situ hot deformation in a spark plasma sintering(SPS)system can obtain completely recombined Nd Fe B magnet with good anisotropy and magnetic properties. The maximum magnetic properties,(BH)max= 201 kJ m-3,Br= 1.142 T and Hcj= 469 k A m-1, are obtained after being treated for 15 min at 750 °C under low pressure.

Keyword：

NdFeB magnets; Hydrogenation– disproportionation–desorption–recombination; Hot deformation; Anisotropy;

Author： Ying Liu e-mail: liuying5536@163.com;

Received： 29 May 2014

1 Introduction

The Nd Fe B-based materials are one of the focuses of permanent magnet research due to their high performance and wide applications, such as generators, motors, and computer devices [1, 2]. The market and consumption for Nd Fe B magnets are rapidly growing, especially when their properties and cost-effectiveness are drastically improved. Several processing routes for the production of Nd Fe B magnets have been developed since their elaboration in 1984 [3–5]. Except usual powder sintering [6, 7], there are two more effective methods to prepare Nd Fe B magnets. One is hydrogenation–disproportionation–desorption–recombination (HDDR) process,and the other is hot pressing and hot deformation process. The HDDR process was developed by Nakayama et al. [8] in 1989as a method for producing high coercivity Nd Fe B powders. It is also a very effective mean of producing the fine grain structure [9]. The HDDR process involves two important chemical reactions. In the hydrogenation–disproportionation(HD) process [10], large-sized Nd Fe B powders are hydrogenated followed by the disproportionation reaction:

In the desorption–recombination (DR) stage, hydrogen is evacuated, and then the recombination reaction occurs to form an ultrafine polycrystalline microstructure:

The average grain size of the Nd₂Fe₁₄B phase in the final product can decrease to 300 nm [11], which is very close to the single-domain grain size of this hard magnetic phase.

Meanwhile, the hot pressing and hot deformation process [12–15] has been used for producing full dense anisotropic magnets in commercial quantity for many years. The original powder used in this process can be quenched powder [16, 17], HDDR powder [18, 19], or mechanically alloyed powder [20]. The process is pided into two steps: the first is to produce full dense isotropic magnet by hot pressing; and the second is to obtain anisotropic magnet by hot deformation.

Many researchers studied the two preparation methods isolatedly to improve the magnetic properties of anisotropic magnets. But there are few studies on the combination of the HD process and hot deformation process. In the present study, the HD process and the hot deformation process were completely combined together. The as-disproportionated compacts were completely recombined during the hot deformation process in the spark plasma sintering (SPS)system, and the anisotropic magnets were obtained.

2 Experimental

Commercial melt-spun ribbons with nominal composition of Nd_13.5Fe₇₃Co_7.5B₆were used as starting material in the experiment. Firstly, the matrix Nd₂Fe₁₄B phase disproportionated into Nd H_x, Fe₂B, and a-Fe during heat treatment in the hydrogen atmosphere with pressure of0.15 MPa at 700 °C for 30 min. Then two different routes were used to produce anisotropic Nd₂Fe₁₄B magnets.Route I: the as-disproportionated powders were first hotpressed at 650 °C and hot-deformed at 780 °C in SPS system, and then the desorption–recombination (DR) process might be proceeded by annealing at 700–850 °C in high vacuum (1 9 10^-3Pa) in horizontal vacuum sintering furnace. Route II: the as-disproportionated powders were firstly compacted into a green cylindrical compact (10 mm in diameter and 12 mm in height), and then it was hotdeformed in the SPS system under low pressure of4–25 MPa in vacuum while the DR process was proceeding at the same time. The phases of magnets were examined by X-ray diffraction (XRD) with Cu Ka radiation. The morphologies were observed by field emission scanning electron microscope (FESEM). Magnetic properties were measured by vibrating sample magnetometer (VSM),and the accurate phase compositions were measured by room temperature transmission Mo¨ ssbauer spectroscopy(MS).

3 Results and discussion

The XRD patterns of raw material and the as-disproportionated powders are shown in Fig. 1. Figure 1 presents that all peaks of the raw material are attributed to the tetragonal hard magnetic Nd₂Fe₁₄B phase, and the matrix phase Nd₂Fe₁₄B totally turns into disproportionated phases composed of Nd H_x, Fe₂B, and α-Fe after the HD reaction.

3.1 Route I

Figure 2 shows the XRD patterns of magnets after the hot pressing and hot deformation process, where the diffractive surface is parallel to the direction of deformation. It can be seen that there are few changes in the phases. The XRD patterns in Fig. 3 show the phases of powders obtained from the compacts treated for 60 min at various temperatures in high vacuum. It shows that only a few disproportionated phases recombine to Nd₂Fe₁₄B phase at 700 °C and the amount of Nd₂Fe₁₄B phase increases a little with the increase of temperature. This indicates that the deformed magnets could not be fully recombined even when the annealing temperature is remarkably elevated.Figure 4 shows the XRD patterns of magnetic powders obtained from the compacts after annealing in high vacuum at 800 °C for different time. It can be seen that the peaks’intensity of the Nd₂Fe₁₄B phase becomes more remarkable with the increase of time, but the as-disproportionated phases are still the main phases. This demonstrates that the deformed magnets cannot be fully recombined to Nd₂Fe₁₄B phase by means of extending the heat-treatment time. As it is shown in Fig. 5 that the grains are fused together after annealing, and as shown in Fig. 5a–d, the higher temperature will result in stronger fusion. Thus,there are few channels for hydrogen to escape out of the magnet. Consequently, it would prevent the recombination reaction process greatly. Figure 5e–g shows that annealing for longer time would lead to a stronger fusion among the grains, resulting in less dehydrogenation channels, so that the recombination reactions cannot be processed completely. As the as-disproportionated phases transform only a little into Nd₂Fe₁₄B by Route I, the magnetic properties of the magnets are extremely low, as shown in Table 1.

Fig. 1 XRD patterns of raw material and as-disproportionated powder

Fig. 2 XRD patterns of compacts after hot pressing and hot deformation process

Fig. 3 XRD patterns of magnetic powders crushed from compacts after annealing at various temperatures

Fig. 4 XRD patterns of magnetic powders crushed from compacts after annealing at 800 °C for different time

In Route I, the deformed magnets cannot recombine completely and the phases of magnets are still basically the as-disproportionated phases through extending the time or elevating the temperature of heat treatment. It is because of a strong grain fusion during the hot pressing and hot deformation process which results in disconnected hollow.

Fig.5 FESEM images of fracture surfaces of magnets perpendicular to orientation of deformation:annealing for 60 min at a 700°C,b 750°C,c 800°C,and d 850°C;annealing at 800°C for e 30 min,f 60 min,and g 120 min

As a result, the hydrogen produced in DR process is difficult to escape out from magnet, and the HD reaction may occur again when the hydrogen concentration reaches a certain value in the disconnected hollow. Therefore, in order to overcome the defect in Route I, another research route was carried out.

Table 1 Magnetic properties of magnets obtained by Route I  下载原图

Table 1 Magnetic properties of magnets obtained by Route I

3.2 Route II

Figure 6 shows the XRD patterns of magnetic powders obtained from the compacts after annealing at 750°C for different time in SPS system with hot deformation pressure of 4–25 MPa.It can be seen that the as-disproportionated phases almost recombine to Nd₂Fe₁₄B magnetic phase when annealing for 15 min.And after 30 min,the as-disproportionated phases recombine totally.Figure 7 shows the XRD patterns of magnets surface perpendicular to the pressure direction.It is observed that peaks such as(004),(006),(105),and(008)become dominant for all deformed magnets,indicating c-axis crystallographic alignment in the magnets.As the time increases,the c-axis crystallographic texture in the magnets enhances gradually,indicating that the texture becomes stronger.However,longer annealing time would result in the deterioration of texture.

The Mo¨ ssbauer spectra in Fig. 8 are obtained from the powders by crushing the magnets to understand the effect of time on the recombination evolution of phase compositions in the magnets. The relative intensity of Nd₂Fe₁₄B contributions increases, while those of Fe₂B and a-Fe decreases remarkably after reactive deformation treatment from 1 to15 min. Additionally, calculated by the spectra, the area ratios corresponding to Nd₂Fe₁₄B phase for reactive deformation treated samples are approximately 78.7 % (1 min),82.3 % (5 min), 85.9 % (15 min), 100.0 % (30 min), and100.0 % (45 min), respectively. The results suggest that the fraction of Nd₂Fe₁₄B phase increases steadily with time increasing under pressure, confirming that a full recombination is achieved by treatment for 30 min under pressure.

Figure 9 shows the FESEM images of fracture surface of the magnets perpendicular to the orientation of deformation. When annealing for 1 min, the grain is basically in a equiaxed arrangement. And for 5 min, the grains begin to show c-axis crystallographic alignment. The grains present an obvious orientation arrangement after 15 min. When annealing for 30 min, the grains still present an obvious orientation arrangement while abnormal grown grains appear. After annealing for 45 min, the alignment decreases and abnormal grown grains become more obvious. The alignment results are well matched with the XRD patterns in Fig. 7.

Fig. 6 XRD patterns of magnetic powders crushed from compacts after annealing at 750 °C for different time in SPS system

Fig. 7 XRD patterns of magnets surface after annealing at 750 °C for different time in SPS system

Figure 10 shows the magnetic properties of the magnets measured by VSM. It is shown that the properties are extremely higher compared with those in Route I, and the magnetic properties reach the maximum when annealing for 15 min: (BH)_max= 201 k J m^-3, B_r= 1.142 T, and H_cj= 469 k A m^-1. This is because that at the first minute,the recombination reaction is not complete and the asdisproportionated phases damage the properties. And with the annealing time increasing, recombination reaction tends to be complete and the grain orientation becomes better, which are both beneficial to the properties. Then after 15 min, with the time increasing, the magnetic properties decrease to some extent, which is the result of the appearance and the increase of abnormal grown grains.

In Route II, the as-disproportionated green compact can completely recombine to form the Nd₂Fe₁₄B anisotropic magnet during the hot deformation process in vacuum. The desorption reaction occurs in the as-disproportionated green compact, and hydrogen escapes out rapidly because the magnet is not dense enough at the beginning of the hot deformation. Then the recombination reaction takes place in the hot deformation process. Simultaneously, the newly generated Nd₂Fe₁₄B grains preferentially grow under the hot deformation pressure, which can result in good magnetic properties. The magnetic properties decrease as the Nd₂Fe₁₄B grains grow too much with time increasing.

Fig.8 MS results of magnets after annealing at 750°C for various time:a 1 min,b 5 min,c 15 min,d 30 min,and e 45 min.4c,4e,8j₁,8j₂,16k₁,and 16k₂corresponding to 6 positions occupied by 56 Fe atoms of Nd₂Fe₁₄B crystal

Fig.9 FESEM images of fracture surface of magnets after annealing under low pressure in SPS system for different time:a 1 min,b 5 min,c 15 min,d 30 min,and e 45 min

Fig. 10 Magnetic properties of magnets measured by VSM: a (BH)max, b Br, and c Hcj

4 Conclusion

Two different methods were applied to prepare Nd Fe B magnets from the as-disproportionated powders. Owing to the lack of dehydrogenation channel, the route of annealing in vacuum after the hot pressing and hot deformation process(Route I) cannot reach the purpose of complete recombination even in high temperature or for long time. The route of applying hot deformation pressure on the as-disproportionated green compact during the desorption recombination process in SPS system in vacuum (Route II) can obtain completely recombined anisotropic Nd Fe B magnet with good magnetic properties of(BH)_max=201 k J m^-3,B_r=1.142 T,and H_cj=469 k A m^-1.