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Effects of hot pressing temperature on the alignment and phase composition of hot-deformed nanocrystalline Nd-Fe-B magnets
Journal of Magnetism and Magnetic Materials 488 (2019) 165353
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Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Research articles
Effects of hot pressing temperature on the alignment and phase composition of hot-deformed nanocrystalline Nd-Fe-B magnets ⁎
T
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Zheng Jing, Zhaohui Guo , Younian He, Mengyu Li, Meiling Zhang, Minggang Zhu, Wei Li Division of Functional Materials, Central Iron & Steel Research Institute, Beijing 100081, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Hot pressing temperature Hot deformation Nd-Fe-B Alignment Phase composition
The magnetic properties, alignment and phase composition of nanocrystalline Nd-Fe-B magnets hot deformed (HD) at 830 °C employing different hot-pressed precursors fabricated at various hot pressing (HP) temperatures were investigated. Both the remanence and coercivity simultaneously increased first and then decreased with the HP temperature from 450 °C to 650 °C. The optimum magnetic properties of HD magnets were obtained at HP temperature of 550 °C with remanence and coercivity of 1.40 T and 924 kA/m, respectively. The usage of the HP temperatures lower than 500 °C was resulted in high porosity and low crystallization degree for hot pressed magnets, causing aggregation of the RE-rich phase and small amount of residual amorphous after hot deformation. This procedure increased the volume fraction of nonmagnetic phase and reduced magnetic properties. Furthermore, the platelet-shaped grains were separated by thick triple junctions, which deteriorated the alignment of grains. When HP temperature was 550 °C, the hot-deformed magnet gained the optimum alignment, and the stacked platelet-shaped grains were separated by smooth thin grain boundary. However, at HP temperature of 650 °C equiaxed coarse grains were dominant instead of platelet-shaped grains in the hot-deformed magnet. The high HP temperature led to apparent grain growth, and then the nucleated grains further grew and were hard to be deformed, developing a poor alignment. A schematic model of the microstructure evolution for hot-deformed magnet is proposed.
1. Introduction Since hot-deformed method was reported by Lee first [1], this process has been an important technique for preparing anisotropic NdFe-B magnets with high remanence comparable to that of commercial sintered magnets [2–4]. Over the past three decades, the hot deformation is still the only practical technique of inducing texture in nanocrystalline hard magnetic materials. The distinct advantages of hot deformation process are near net-shape and time-saving comparing with classical sintering process [5–7]. Usually, the raw material for hot deformation is melt-spun ribbons composed of amorphous and nanocrystalline. The isotropic randomly oriented nanocrystals in melt-spun powder are subjected to form laterally grown grains during hot deformation. The c-axes of these grains are parallel to the pressing direction [8,9]. The alignment and microstructure of hot-deformed magnets are determined not only by the raw melt-spun ribbons, but also by the process routes. Because nanocrystalline is very sensitive to process parameters such as temperatures for hot pressing and hot deforming. Hence the deformation behavior of nanocrystalline is hard to be controlled.
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The Nd-Fe-B magnets are mainly composed of Nd2Fe14B phase and a small amount of RE-rich phase. Although the amount of the RE-rich phase is small, the function of which is very important and indispensable especially for hot-deformed magnets [10,11]. The liquid Ndrich phase provides a good channel for elements diffusion and acts as a lubricant allowing the grains to slide over each other to form c-axis textured anisotropic magnets [12–14]. Moreover, the Nd-rich phase contributes to facilitating the grain boundary sliding and the grain migration. Thereby misoriented grains rotate toward the preferential press direction, and alignment of Nd2Fe14B grains is further enhanced [13,15,16]. The distribution of the Nd-rich phase shows a great difference in both wheel side and free side of melt-spun ribbons [14]. This inhomogeneity of melt-spun ribbons would affect the deformation process of Nd2Fe14B grains and brought negative effects on the final properties of hot-deformed magnets. Crushing the melt-spun ribbons into powders is effective to limit the influence to the lower extent, however, it cannot be avoided completely. Therefore, it is quite necessary to optimize the technical process to obtain good alignment of Nd2Fe14B grains and microstructure. The magnetic properties and microstructure of hot-deformed magnets usually seem to be dependent on
Corresponding authors. E-mail addresses: [email protected] (Z. Guo), [email protected] (W. Li).
https://doi.org/10.1016/j.jmmm.2019.165353 Received 24 December 2018; Received in revised form 16 May 2019; Accepted 22 May 2019 Available online 23 May 2019 0304-8853/ © 2019 Elsevier B.V. All rights reserved.
Journal of Magnetism and Magnetic Materials 488 (2019) 165353
Z. Jing, et al.
Fig. 1. Magnetic properties of hot-deformed magnets prepared from different hot-pressed precursors; (a) initial magnetization curves and hysteresis loops, (b) remanence and coercivity as a function of hot pressing temperature.
equipped with a 9 T vibrating sample magnetometer (VSM) without demagnetization correction. The alignment and phase composition of the magnets were characterized by x-ray diffraction (XRD) using Cu Kα radiation with a scanning speed of 4°/min. The cylindrical sample dimensions for XRD detection was 6 mm in diameter and 3 mm in height. Oxygen contents of these hot-deformed magnets were determined using a IRO-Ⅱ nitrogen/oxygen analyzer produced by the NCS Testing Technology Co., Ltd. Backscattered electron (BSE) scanning electron microscopy (SEM) observations were made on the bulk samples using a FEI Quanta 650F. Transmission electron microscopy (TEM) was performed using a JEOL JEM-2100.
Table 1 Remanence of hot-pressed precursors (Br-HP), hot-deformed magnets (Br-HD), ratio of Br-HD to Br-HD (Br-HD/Br-HP) and density of hot-pressed precursors (ρHP). HP Temperature (℃)
450
500
550
600
650
Br-HP (T) Br-HD (T) Br-HD/Br-HP (%) ρHP (g/cm3)
0.81 1.29 159.2% 7.05
0.82 1.35 164.6% 7.26
0.84 1.40 166.7% 7.46
0.85 1.39 163.5% 7.51
0.83 0.92 110.8% 7.59
hot pressing temperature, hot deformation temperature and hot deformation rate. The purpose of hot pressing process is not only to densify, but also to crystallize the original amorphous in melt-spun ribbons. Meanwhile, aggregation of the RE-rich phase should be avoided. Obviously, if the hot pressing temperature significantly exceeds the melting point of the RE-rich phase, and then the melted RErich phase would be squeezed into the gaps between the ribbons more easily under high pressure. This affects the final properties of hot-deformed magnets [14,17]. Many efforts have been made to study the effects of hot deformation rate and temperature on hot-deformed magnets [18–20], whereas the study about the effect of the hot pressing temperature on the hot-deformed magnets is still insufficient. Therefore, it is necessary to explore the hereditary effects from hot pressing to hot deformation. This will contribute to an understanding of the overall function of the RE-rich phase in densification during hot pressing and in assisting the deformation of the main phase grains. In this paper, the effects of hot pressing temperature on hot-deformed bulk nanocrystalline Nd-Fe-B magnets have been discussed by analyzing the magnetic properties, phase composition, and microstructure of hot-deformed magnets.
3. Results and discussion The magnetic properties of hot-deformed magnets prepared from different hot-pressed precursors are shown in Fig. 1. Both the remanence and coercivity increased first, and then decreased with the hot pressing temperature from 450 °C to 650 °C [Fig. 1(b)]. When the hot pressing temperature was below 600 °C, all the samples have good squareness with single phase characteristics, indicating exchange coupled neighboring magnetic grains. However, an obvious loss of squareness can be found in the HP650 + HD sample [Fig. 1(a)]. The enhanced squareness is possibly due to the improved uniformity of the microstructure for the magnets prepared at hot pressing temperature below 600 °C, which has also been demonstrated by the microstructure analysis, as discussed later. When the hot pressing temperature was 650 °C, the magnetic properties of HP650 + HD sample deteriorated drastically, especially for remanence. This means that the degree of alignment of Nd2Fe14B grains is decreased. The remanence of hot-deformed magnets (Br-HD), hot pressed precursors (Br-HP), the ratio of the two remanences (Br-HD/Br-HP) and the density of these hot-pressed precursors (ρHP) are summarized in Table 1. The HP550 + HD sample shows the highest value of Br-HD/Br-HP, which indicates that the HP temperature of 550 °C is suitable for achieving the higher c-axis texture along the deformation direction. It can be seen that the densification was enhanced monotonously by increasing the hot pressing temperature, and the density of HP650 magnet nearly reached the theoretical value. The phase compositions of all the hot-deformed magnets were confirmed by XRD patterns reflected from the surface perpendicular to the pressing direction. Fig. 2(a) shows the XRD patterns of all the hotdeformed magnets. It is found that all samples contain Nd2Fe14B main phase. The peak intensity ratio of (0 0 6) to (1 0 5), R(0 0 6)/(1 0 5), is usually used to characterize the texture degree due to Nd2Fe14B grains deformation, where all the stacked platelet-shaped grains rotated their c-axes parallel to deforming direction. The values of R(0 0 6)/(1 0 5) were
2. Experimental procedure The raw melt-spun ribbons with a nominal composition of (Nd0.8Pr0.2)29FebalCo4Ga0.42B0.92 (wt.%) were pulverized into powder with a diameter of 200–450 μm. The powders were pressed in vacuum at different temperatures (450 °C, 500 °C, 550 °C, 600 °C and 650 °C) under 500 MPa for 70 s to obtain nanocrystalline isotropic bulk magnets of ∼13 mm in diameter and ∼25 mm in height, and subsequently all the precursors were subjected to hot deformation at the elevated temperature of 830 °C until the height reduction reaching ∼72%. The hot deformation speed was 10 mm/min. For convenience of description, the hot-deformed magnets prepared from different hot-pressed precursors were denoted as HP450 + HD, HP500 + HD, HP550 + HD, HP600 + HD and HP650 + HD. Magnetic properties of the magnets were measured with a physical property measurement system (PPMS) 2
Journal of Magnetism and Magnetic Materials 488 (2019) 165353
Z. Jing, et al.
Fig. 2. (a) XRD patterns of hot-deformed samples; (b) local 2θ profile of XRD patterns in the 2θ range of 26-37° [red rectangle region in Fig. 2(a)]; (c) half Gaussian fitted curves of orientation deviation of the hot-deformed samples.
Fig. 3. BSE SEM images of hot-pressed precursors prepared at different temperatures. The pressure direction (PD) is in plane, (a) HP450, (b) HP550 and (c) HP650.
could be observed in the HP450 + HD, HP500 + HD, HP650 + HD. The detected peak intensity of the RE-rich phase was stronger in HP450 + HD than that of the other two samples. Furthermore, a slight RE2O3 (bcc) and metallic Nd phase (dhcp) were detected in the samples
1.57, 1.78, 2.32, 2.29 and 0.92 for HP450 + HD, HP500 + HD, HP550 + HD, HP600 + HD, and HP650 + HD, respectively. The results are consistent with the magnetic properties. The results of the local 2θ from 26° to 37°are shown in Fig. 2(b). A little RE-rich phase 3
Journal of Magnetism and Magnetic Materials 488 (2019) 165353
Z. Jing, et al.
Fig. 4. BSE SEM images of HP450 + HD, HP550 + HD and HP650 + HD magnets, c-axis is in plane; (a), (c) and (e) are low magnification images of HP450 + HD, HP550 + HD and HP650 + HD, respectively; (b), (d) and (f) are high magnification images of the regions marked by dotted rectangle in (a), (c) and (e), respectively.
pressed precursors prepared at different temperatures, where the pressure direction (PD) is in plane. At HP temperature of 450 °C the gaps between ribbons, marked by the yellow arrows, were very apparent [Fig. 3(a)], resulting in a high porosity. As HP temperature further increased to 550 °C it could be seen that a small amount of the RE-rich phase was squeezed into part of the gaps [Fig. 3(b)], and the interval of the gaps were narrowed being coupled with reduced pores. However, when the HP temperature came up to 650 °C, all the ribbons were enveloped by the RE-rich phase [Fig. 3(c)]. The high HP temperature accelerates the aggregation of the RE-rich phase at gaps between ribbons. Usually, the periodic RE-rich layers were only found in hot-deformed magnets [11,14]. Our previous study [17] has demonstrated that the presence of the periodic RE-rich layers was just affected by temperature rather than process steps. The RE-rich phase usually is believed to facilitate grain growth by supplying diffusion path of elements and to contribute to grain deformation by providing good wettability. Apparently, the microstructure shows great difference after hot pressing, which would affect the final alignment of grains as a consequence. Fig. 4(a), (c) and (e) show the low magnification BSE SEM images of HP450 + HD, HP550 + HD and HP650 + HD magnets, respectively, where c-axis is in plane. The periodic RE-rich layers is perpendicular to the press direction can be observed clearly in the HP450 + HD and HP550 + HD magnets, whereas large area of aggregated RE-rich phase occurred in the HP650 + HD magnet which distribution seems to be random and irregular. This means the RE-rich phase originally located in the interior of melt-spun ribbons was also squeezed out. By comparing Fig. 4(a) with (c), it is found that severe aggregation of the RE-
besides HP550 + HD. The observed RE-rich phase diffraction peak should be the result of the uneven aggregation of RE-rich phase. The low HP temperatures (< 550 °C) led to high porosity for hot-pressed precursors. Subsequently, at the beginning stage of hot deformation the liquid RE-rich phase would be squeezed into the pores for the magnets prepared at low hot pressing temperature, which caused the aggregation of the RE-rich phase. However, when the HP temperature was high (650 °C), the RE-rich phase has already been squeezed into the pores and some of which was squeezed out to the surface of magnet even during hot pressing process. Therefore, the peak intensity of the RE-rich phase for HP650 + HD was weaker than those of hot-deformed magnets prepared at relative low HP temperatures. The degree of c-axis alignment of Nd2Fe14B grains was characterized by fitting the standard deviation of a half Gaussian distribution for the relative intensity versus the angle between the normal of (hkl) and the c-axis of Nd-Fe-B magnets [Fig. 2(c)] [11,21]. According to this method, the orientation deviation, magnet powder σ, can be obtained by calculating the ratio of Ihkl to Ihkl , which can be expressed as the following formula [21]: magnet Ihkl powder Ihkl
= A × exp(−
φ2 ) 2σ 2
(1)
where σ is the orientation deviation of Nd2Fe14B grains, φ is the angle between the normal of the (hkl) plane and the c-axis of Nd-Fe-B magnet, magnet I is the intensity of diffraction peaks of the samples (Ihkl ) and that of powder the ideally isotropic powder (Ihkl ). Small value of σ indicates the high degree of alignment, and the value of σ is consistent with the result of R(0 0 6)/(1 0 5). Fig. 3 shows the backscattered electron (BSE) SEM images of hot4
Journal of Magnetism and Magnetic Materials 488 (2019) 165353
Z. Jing, et al.
Fig. 5. (a) and (b): Bright-field TEM image of the HP450 + HD and HP550 + HD, (c): HR TEM image of a grain boundary marked in the dotted rectangle in (a), (c) and (d): HR TEM images and FFT patterns of the selected area marked with A and B in (c). Table 2 The Oxygen content of hot-deformed magnets prepared from different hot-pressed precursors. Sample
HP450 + HD
HP500 + HD
HP550 + HD
HP600 + HD
HP650 + HD
Oxygen content (ppm)
773
647
564
558
562
HP650 + HD magnet. The equiaxed coarse grains are much like that of sintered Nd-Fe-B magnets [22,23]. High HP temperature (650 °C) led to an apparent grain growth after hot pressing process, and consequently the nucleated grains further grew and were hard to be deformed. Furthermore, the equiaxed coarse grains could not create the fine plateletshaped grains, developing a poor microstructure. The poor microstructure deteriorates the squareness of HP650 + HD magnet [Fig. 1(a)]. The SEM results are corresponding pretty much to the changes in alignment and magnetic properties. Fig. 5(a) and (b) are the typical bright-field TEM images of HP450 + HD and HP550 + HD magnets, respectively, showing platelet-shaped grains and grain boundary phase. It is found that large area of triple junctions and thick grain boundary phase existed in the HP450 + HD magnet [Fig. 5(a)]. However, the grain boundary was
rich phase presented at the gaps between ribbons in the HP450 + HD magnet. This difference originates from the high porosity of the HP450 precursor [Fig. 3(a)]. At the beginning stage of the hot deformation the external pressure is relatively low and the grain size is fine. This condition gives more space for the RE-rich phase to flow to the pores. Therefore, the pores and gaps of ribbons were first filled by the RE-rich phase. The high magnification BSE SEM images of the regions marked by dotted rectangle in Fig. 4(a), (c) and (e) are shown in Fig. 4(b), (d) and (f), respectively. Clearly, the HP450 + HD and HP550 + HD magnets present a microstructure of stacked platelet-shaped grains. However, the microstructure of HP650 + HD magnet show a great difference compared with that of the other two magnets. The equiaxed coarse grains were dominant instead of platelet-shaped grains and fine platelet-shaped grains were also observed between coarse grains in the 5
Journal of Magnetism and Magnetic Materials 488 (2019) 165353
Z. Jing, et al.
Fig. 6. Schematic image of microstructure with grains and grain boundary of hot-deformed Nd-Fe-B magnets prepared from different hot-pressed precursors. (a) HP450 + HD, (b) HP550 + HD and (c) HP650 + HD.
anisotropic Nd-Fe-B magnets was observed after grain boundary diffusion [28]. Akiya et al. [28] reported the decreased remanence and decrease of grain alignment of Nd-Fe-B magnets attributed to the diffusion of Nd-Cu liquid phase. This is similar to the result of HP450 + HD magnet to some extent. The possible reasons for aggregation of the RErich phase should be attributed to the high porosity at the first stage of hot deformation. By increasing HP temperature properly to 550 ℃, the final HP550 + HD magnet obtains a good alignment, nearly all the platelet-shaped grains were separated by smooth grain boundary coupled with few triple junctions [Figs. 4(d), 5(b) and 6(b)]. Therefore, the HP550 + HD magnet shows the best magnetic properties (Fig. 1) and alignment [Figs. 2 and 6 (d)]. However, the morphology of HP650 + HD magnet [Figs. 4(f) and 6(c)] is totally different from that of the magnets prepared at low HP temperatures. The grains are substantially equiaxed instead of platelet-shaped grains. These equiaxed grains lead to poor magnetic properties. Therefore, appropriate HP temperature plays a very important role in optimizing phase compositions and in homogenizing the distribution of the RE-rich phase. Thereby lays a foundation of obtaining high performance hot-deformed magnets.
relatively thin and smooth for the HP550 + HD magnet [Fig. 5(b)]. Fig. 5(c) shows the low magnification high resolution TEM image of a typical triple grain boundary marked by the dotted rectangle in Fig. 5(a), which clearly shows the intergranular phase with a mean thickness of about 20–30 nm. The phase compositions and crystal structure of the grain boundary marked by yellow dotted rectangle A and B in Fig. 5(c) were further identified by performing local Fast Fourier Transformation (FFT). The results are shown in Fig. 5(d) and (e). Area-B shows the structure of bcc-Nd2O3 with a = 1.105 nm [Fig. 5(e)]. Area-A could be divided into two regions, whose boundary was marked by a dotted line [Fig. 5(d)]. Region 1 also shows the structure of bcc-Nd2O3, whereas region 2 corresponds to amorphous phase. The phase composition of thin triple junctions for HP550 + HD magnet, which is not shown here, is nearly similar to that for HP450 + HD magnet. Increasing HP temperature may promote elimination of the oxygen absorption in the melt-spun ribbons. Therefore, the RE-rich phase may not be oxidized. Thereby the RE-rich phase could maintain a good fluidity, facilitating the formation of thin and continuous grain boundary rather than aggregated triple junctions. The oxygen contents of all the hot-deformed magnets were analyzed by a nitrogen/oxygen analyzer. The results are summarized in Table 2. It is found that the HP450 + HD magnet possessed the highest oxygen content. Moreover, the oxygen content decreased with the HP temperature from 450 °C to 550 °C, and after the HP temperature exceeding 550 °C the oxygen content was almost the same. The oxygen content of hot-deformed magnets is lower than that of sintered Nd-Fe-B magnets (> 1400 ppm) due to the process without milling to fine powders about 3–5 μm [24]. Our previous [17] work showed that the amount of residual amorphous decreased with increasing the HP temperature, however it could not be eliminated by hot pressing even though HP temperature was as high as 650 °C. Therefore, some of the amorphous phase in the grain boundary might result from the original melt-spun ribbons. Fig. 6 shows the schematic image of microstructure with grains and grain boundary of hot-deformed Nd-Fe-B magnets prepared from different hot-pressed precursors. By analyzing the relationship between magnetic properties and microstructure, it is found that the HP550 + HD magnet shows the highest performance among all the magnets by creating an optimum microstructure. Obviously, in order to obtain good alignment of grains it is necessary first to impede the aggregation of the RE-rich phase. The RE-rich phase on the c-planes of the grains increases misalignment of the grains by the excess grain boundary diffusion [11]. Especially many triple junctions with large area impede the c-axis alignment of grains [Figs. 4(a) and 6(a)]. The reduced remanence and deteriorated alignment were also found in hotdeformed magnets treated by grain boundary diffusion process of RE-Cu eutectic alloy, such as Nd-Cu, Pr-Cu [25,26], and Nd-Dy-Cu [27]. Furthermore, obvious expansion in the c-axis direction of hot-deformed
4. Conclusions The effects of hot pressing (HP) temperature on the alignment and phase composition of hot-deformed (HD) magnets have been investigated. By changing the HP temperature from 450 °C to 650 °C, the HD magnet prepared from precursor fabricated at 500 °C showed the highest remanence of ∼1.40 T. The alignment of HD magnets improved first and then deteriorated with increasing the HP temperature. The lower HP temperature (below 500 °C) was resulted in the higher porosity of hot-pressed precursors. Therefore during hot deformation the pores were first filled by the liquid RE-rich phase. This caused the aggregation of the RE-rich phase, and consequently the amount of the liquid RE-rich phase used for facilitating grains deformation was reduced. The reduction of the RE-rich phase led to the deterioration of alignment. Furthermore, low HP temperature also led to the increased volume fraction of non-magnetic phase, such as amorphous and Ndoxide, which decreased the remanence. However, at high HP temperature of 650 °C, the grains exceed critical size for performing hot deformation. These grains further grew to equiaxed coarse grains during hot deformation, developing a poor alignment. Clearly, appropriate HP temperature plays a very important role in optimizing phase composition and in homogenizing distribution of the RE-rich phase. Thereby lays a foundation of obtaining high performance hot-deformed magnets.
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Journal of Magnetism and Magnetic Materials 488 (2019) 165353
Z. Jing, et al.
Acknowledgments
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Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Research articles
Effects of hot pressing temperature on the alignment and phase composition of hot-deformed nanocrystalline Nd-Fe-B magnets ⁎
T
⁎
Zheng Jing, Zhaohui Guo , Younian He, Mengyu Li, Meiling Zhang, Minggang Zhu, Wei Li Division of Functional Materials, Central Iron & Steel Research Institute, Beijing 100081, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Hot pressing temperature Hot deformation Nd-Fe-B Alignment Phase composition
The magnetic properties, alignment and phase composition of nanocrystalline Nd-Fe-B magnets hot deformed (HD) at 830 °C employing different hot-pressed precursors fabricated at various hot pressing (HP) temperatures were investigated. Both the remanence and coercivity simultaneously increased first and then decreased with the HP temperature from 450 °C to 650 °C. The optimum magnetic properties of HD magnets were obtained at HP temperature of 550 °C with remanence and coercivity of 1.40 T and 924 kA/m, respectively. The usage of the HP temperatures lower than 500 °C was resulted in high porosity and low crystallization degree for hot pressed magnets, causing aggregation of the RE-rich phase and small amount of residual amorphous after hot deformation. This procedure increased the volume fraction of nonmagnetic phase and reduced magnetic properties. Furthermore, the platelet-shaped grains were separated by thick triple junctions, which deteriorated the alignment of grains. When HP temperature was 550 °C, the hot-deformed magnet gained the optimum alignment, and the stacked platelet-shaped grains were separated by smooth thin grain boundary. However, at HP temperature of 650 °C equiaxed coarse grains were dominant instead of platelet-shaped grains in the hot-deformed magnet. The high HP temperature led to apparent grain growth, and then the nucleated grains further grew and were hard to be deformed, developing a poor alignment. A schematic model of the microstructure evolution for hot-deformed magnet is proposed.
1. Introduction Since hot-deformed method was reported by Lee first [1], this process has been an important technique for preparing anisotropic NdFe-B magnets with high remanence comparable to that of commercial sintered magnets [2–4]. Over the past three decades, the hot deformation is still the only practical technique of inducing texture in nanocrystalline hard magnetic materials. The distinct advantages of hot deformation process are near net-shape and time-saving comparing with classical sintering process [5–7]. Usually, the raw material for hot deformation is melt-spun ribbons composed of amorphous and nanocrystalline. The isotropic randomly oriented nanocrystals in melt-spun powder are subjected to form laterally grown grains during hot deformation. The c-axes of these grains are parallel to the pressing direction [8,9]. The alignment and microstructure of hot-deformed magnets are determined not only by the raw melt-spun ribbons, but also by the process routes. Because nanocrystalline is very sensitive to process parameters such as temperatures for hot pressing and hot deforming. Hence the deformation behavior of nanocrystalline is hard to be controlled.
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The Nd-Fe-B magnets are mainly composed of Nd2Fe14B phase and a small amount of RE-rich phase. Although the amount of the RE-rich phase is small, the function of which is very important and indispensable especially for hot-deformed magnets [10,11]. The liquid Ndrich phase provides a good channel for elements diffusion and acts as a lubricant allowing the grains to slide over each other to form c-axis textured anisotropic magnets [12–14]. Moreover, the Nd-rich phase contributes to facilitating the grain boundary sliding and the grain migration. Thereby misoriented grains rotate toward the preferential press direction, and alignment of Nd2Fe14B grains is further enhanced [13,15,16]. The distribution of the Nd-rich phase shows a great difference in both wheel side and free side of melt-spun ribbons [14]. This inhomogeneity of melt-spun ribbons would affect the deformation process of Nd2Fe14B grains and brought negative effects on the final properties of hot-deformed magnets. Crushing the melt-spun ribbons into powders is effective to limit the influence to the lower extent, however, it cannot be avoided completely. Therefore, it is quite necessary to optimize the technical process to obtain good alignment of Nd2Fe14B grains and microstructure. The magnetic properties and microstructure of hot-deformed magnets usually seem to be dependent on
Corresponding authors. E-mail addresses: [email protected] (Z. Guo), [email protected] (W. Li).
https://doi.org/10.1016/j.jmmm.2019.165353 Received 24 December 2018; Received in revised form 16 May 2019; Accepted 22 May 2019 Available online 23 May 2019 0304-8853/ © 2019 Elsevier B.V. All rights reserved.
Journal of Magnetism and Magnetic Materials 488 (2019) 165353
Z. Jing, et al.
Fig. 1. Magnetic properties of hot-deformed magnets prepared from different hot-pressed precursors; (a) initial magnetization curves and hysteresis loops, (b) remanence and coercivity as a function of hot pressing temperature.
equipped with a 9 T vibrating sample magnetometer (VSM) without demagnetization correction. The alignment and phase composition of the magnets were characterized by x-ray diffraction (XRD) using Cu Kα radiation with a scanning speed of 4°/min. The cylindrical sample dimensions for XRD detection was 6 mm in diameter and 3 mm in height. Oxygen contents of these hot-deformed magnets were determined using a IRO-Ⅱ nitrogen/oxygen analyzer produced by the NCS Testing Technology Co., Ltd. Backscattered electron (BSE) scanning electron microscopy (SEM) observations were made on the bulk samples using a FEI Quanta 650F. Transmission electron microscopy (TEM) was performed using a JEOL JEM-2100.
Table 1 Remanence of hot-pressed precursors (Br-HP), hot-deformed magnets (Br-HD), ratio of Br-HD to Br-HD (Br-HD/Br-HP) and density of hot-pressed precursors (ρHP). HP Temperature (℃)
450
500
550
600
650
Br-HP (T) Br-HD (T) Br-HD/Br-HP (%) ρHP (g/cm3)
0.81 1.29 159.2% 7.05
0.82 1.35 164.6% 7.26
0.84 1.40 166.7% 7.46
0.85 1.39 163.5% 7.51
0.83 0.92 110.8% 7.59
hot pressing temperature, hot deformation temperature and hot deformation rate. The purpose of hot pressing process is not only to densify, but also to crystallize the original amorphous in melt-spun ribbons. Meanwhile, aggregation of the RE-rich phase should be avoided. Obviously, if the hot pressing temperature significantly exceeds the melting point of the RE-rich phase, and then the melted RErich phase would be squeezed into the gaps between the ribbons more easily under high pressure. This affects the final properties of hot-deformed magnets [14,17]. Many efforts have been made to study the effects of hot deformation rate and temperature on hot-deformed magnets [18–20], whereas the study about the effect of the hot pressing temperature on the hot-deformed magnets is still insufficient. Therefore, it is necessary to explore the hereditary effects from hot pressing to hot deformation. This will contribute to an understanding of the overall function of the RE-rich phase in densification during hot pressing and in assisting the deformation of the main phase grains. In this paper, the effects of hot pressing temperature on hot-deformed bulk nanocrystalline Nd-Fe-B magnets have been discussed by analyzing the magnetic properties, phase composition, and microstructure of hot-deformed magnets.
3. Results and discussion The magnetic properties of hot-deformed magnets prepared from different hot-pressed precursors are shown in Fig. 1. Both the remanence and coercivity increased first, and then decreased with the hot pressing temperature from 450 °C to 650 °C [Fig. 1(b)]. When the hot pressing temperature was below 600 °C, all the samples have good squareness with single phase characteristics, indicating exchange coupled neighboring magnetic grains. However, an obvious loss of squareness can be found in the HP650 + HD sample [Fig. 1(a)]. The enhanced squareness is possibly due to the improved uniformity of the microstructure for the magnets prepared at hot pressing temperature below 600 °C, which has also been demonstrated by the microstructure analysis, as discussed later. When the hot pressing temperature was 650 °C, the magnetic properties of HP650 + HD sample deteriorated drastically, especially for remanence. This means that the degree of alignment of Nd2Fe14B grains is decreased. The remanence of hot-deformed magnets (Br-HD), hot pressed precursors (Br-HP), the ratio of the two remanences (Br-HD/Br-HP) and the density of these hot-pressed precursors (ρHP) are summarized in Table 1. The HP550 + HD sample shows the highest value of Br-HD/Br-HP, which indicates that the HP temperature of 550 °C is suitable for achieving the higher c-axis texture along the deformation direction. It can be seen that the densification was enhanced monotonously by increasing the hot pressing temperature, and the density of HP650 magnet nearly reached the theoretical value. The phase compositions of all the hot-deformed magnets were confirmed by XRD patterns reflected from the surface perpendicular to the pressing direction. Fig. 2(a) shows the XRD patterns of all the hotdeformed magnets. It is found that all samples contain Nd2Fe14B main phase. The peak intensity ratio of (0 0 6) to (1 0 5), R(0 0 6)/(1 0 5), is usually used to characterize the texture degree due to Nd2Fe14B grains deformation, where all the stacked platelet-shaped grains rotated their c-axes parallel to deforming direction. The values of R(0 0 6)/(1 0 5) were
2. Experimental procedure The raw melt-spun ribbons with a nominal composition of (Nd0.8Pr0.2)29FebalCo4Ga0.42B0.92 (wt.%) were pulverized into powder with a diameter of 200–450 μm. The powders were pressed in vacuum at different temperatures (450 °C, 500 °C, 550 °C, 600 °C and 650 °C) under 500 MPa for 70 s to obtain nanocrystalline isotropic bulk magnets of ∼13 mm in diameter and ∼25 mm in height, and subsequently all the precursors were subjected to hot deformation at the elevated temperature of 830 °C until the height reduction reaching ∼72%. The hot deformation speed was 10 mm/min. For convenience of description, the hot-deformed magnets prepared from different hot-pressed precursors were denoted as HP450 + HD, HP500 + HD, HP550 + HD, HP600 + HD and HP650 + HD. Magnetic properties of the magnets were measured with a physical property measurement system (PPMS) 2
Journal of Magnetism and Magnetic Materials 488 (2019) 165353
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Fig. 2. (a) XRD patterns of hot-deformed samples; (b) local 2θ profile of XRD patterns in the 2θ range of 26-37° [red rectangle region in Fig. 2(a)]; (c) half Gaussian fitted curves of orientation deviation of the hot-deformed samples.
Fig. 3. BSE SEM images of hot-pressed precursors prepared at different temperatures. The pressure direction (PD) is in plane, (a) HP450, (b) HP550 and (c) HP650.
could be observed in the HP450 + HD, HP500 + HD, HP650 + HD. The detected peak intensity of the RE-rich phase was stronger in HP450 + HD than that of the other two samples. Furthermore, a slight RE2O3 (bcc) and metallic Nd phase (dhcp) were detected in the samples
1.57, 1.78, 2.32, 2.29 and 0.92 for HP450 + HD, HP500 + HD, HP550 + HD, HP600 + HD, and HP650 + HD, respectively. The results are consistent with the magnetic properties. The results of the local 2θ from 26° to 37°are shown in Fig. 2(b). A little RE-rich phase 3
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Fig. 4. BSE SEM images of HP450 + HD, HP550 + HD and HP650 + HD magnets, c-axis is in plane; (a), (c) and (e) are low magnification images of HP450 + HD, HP550 + HD and HP650 + HD, respectively; (b), (d) and (f) are high magnification images of the regions marked by dotted rectangle in (a), (c) and (e), respectively.
pressed precursors prepared at different temperatures, where the pressure direction (PD) is in plane. At HP temperature of 450 °C the gaps between ribbons, marked by the yellow arrows, were very apparent [Fig. 3(a)], resulting in a high porosity. As HP temperature further increased to 550 °C it could be seen that a small amount of the RE-rich phase was squeezed into part of the gaps [Fig. 3(b)], and the interval of the gaps were narrowed being coupled with reduced pores. However, when the HP temperature came up to 650 °C, all the ribbons were enveloped by the RE-rich phase [Fig. 3(c)]. The high HP temperature accelerates the aggregation of the RE-rich phase at gaps between ribbons. Usually, the periodic RE-rich layers were only found in hot-deformed magnets [11,14]. Our previous study [17] has demonstrated that the presence of the periodic RE-rich layers was just affected by temperature rather than process steps. The RE-rich phase usually is believed to facilitate grain growth by supplying diffusion path of elements and to contribute to grain deformation by providing good wettability. Apparently, the microstructure shows great difference after hot pressing, which would affect the final alignment of grains as a consequence. Fig. 4(a), (c) and (e) show the low magnification BSE SEM images of HP450 + HD, HP550 + HD and HP650 + HD magnets, respectively, where c-axis is in plane. The periodic RE-rich layers is perpendicular to the press direction can be observed clearly in the HP450 + HD and HP550 + HD magnets, whereas large area of aggregated RE-rich phase occurred in the HP650 + HD magnet which distribution seems to be random and irregular. This means the RE-rich phase originally located in the interior of melt-spun ribbons was also squeezed out. By comparing Fig. 4(a) with (c), it is found that severe aggregation of the RE-
besides HP550 + HD. The observed RE-rich phase diffraction peak should be the result of the uneven aggregation of RE-rich phase. The low HP temperatures (< 550 °C) led to high porosity for hot-pressed precursors. Subsequently, at the beginning stage of hot deformation the liquid RE-rich phase would be squeezed into the pores for the magnets prepared at low hot pressing temperature, which caused the aggregation of the RE-rich phase. However, when the HP temperature was high (650 °C), the RE-rich phase has already been squeezed into the pores and some of which was squeezed out to the surface of magnet even during hot pressing process. Therefore, the peak intensity of the RE-rich phase for HP650 + HD was weaker than those of hot-deformed magnets prepared at relative low HP temperatures. The degree of c-axis alignment of Nd2Fe14B grains was characterized by fitting the standard deviation of a half Gaussian distribution for the relative intensity versus the angle between the normal of (hkl) and the c-axis of Nd-Fe-B magnets [Fig. 2(c)] [11,21]. According to this method, the orientation deviation, magnet powder σ, can be obtained by calculating the ratio of Ihkl to Ihkl , which can be expressed as the following formula [21]: magnet Ihkl powder Ihkl
= A × exp(−
φ2 ) 2σ 2
(1)
where σ is the orientation deviation of Nd2Fe14B grains, φ is the angle between the normal of the (hkl) plane and the c-axis of Nd-Fe-B magnet, magnet I is the intensity of diffraction peaks of the samples (Ihkl ) and that of powder the ideally isotropic powder (Ihkl ). Small value of σ indicates the high degree of alignment, and the value of σ is consistent with the result of R(0 0 6)/(1 0 5). Fig. 3 shows the backscattered electron (BSE) SEM images of hot4
Journal of Magnetism and Magnetic Materials 488 (2019) 165353
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Fig. 5. (a) and (b): Bright-field TEM image of the HP450 + HD and HP550 + HD, (c): HR TEM image of a grain boundary marked in the dotted rectangle in (a), (c) and (d): HR TEM images and FFT patterns of the selected area marked with A and B in (c). Table 2 The Oxygen content of hot-deformed magnets prepared from different hot-pressed precursors. Sample
HP450 + HD
HP500 + HD
HP550 + HD
HP600 + HD
HP650 + HD
Oxygen content (ppm)
773
647
564
558
562
HP650 + HD magnet. The equiaxed coarse grains are much like that of sintered Nd-Fe-B magnets [22,23]. High HP temperature (650 °C) led to an apparent grain growth after hot pressing process, and consequently the nucleated grains further grew and were hard to be deformed. Furthermore, the equiaxed coarse grains could not create the fine plateletshaped grains, developing a poor microstructure. The poor microstructure deteriorates the squareness of HP650 + HD magnet [Fig. 1(a)]. The SEM results are corresponding pretty much to the changes in alignment and magnetic properties. Fig. 5(a) and (b) are the typical bright-field TEM images of HP450 + HD and HP550 + HD magnets, respectively, showing platelet-shaped grains and grain boundary phase. It is found that large area of triple junctions and thick grain boundary phase existed in the HP450 + HD magnet [Fig. 5(a)]. However, the grain boundary was
rich phase presented at the gaps between ribbons in the HP450 + HD magnet. This difference originates from the high porosity of the HP450 precursor [Fig. 3(a)]. At the beginning stage of the hot deformation the external pressure is relatively low and the grain size is fine. This condition gives more space for the RE-rich phase to flow to the pores. Therefore, the pores and gaps of ribbons were first filled by the RE-rich phase. The high magnification BSE SEM images of the regions marked by dotted rectangle in Fig. 4(a), (c) and (e) are shown in Fig. 4(b), (d) and (f), respectively. Clearly, the HP450 + HD and HP550 + HD magnets present a microstructure of stacked platelet-shaped grains. However, the microstructure of HP650 + HD magnet show a great difference compared with that of the other two magnets. The equiaxed coarse grains were dominant instead of platelet-shaped grains and fine platelet-shaped grains were also observed between coarse grains in the 5
Journal of Magnetism and Magnetic Materials 488 (2019) 165353
Z. Jing, et al.
Fig. 6. Schematic image of microstructure with grains and grain boundary of hot-deformed Nd-Fe-B magnets prepared from different hot-pressed precursors. (a) HP450 + HD, (b) HP550 + HD and (c) HP650 + HD.
anisotropic Nd-Fe-B magnets was observed after grain boundary diffusion [28]. Akiya et al. [28] reported the decreased remanence and decrease of grain alignment of Nd-Fe-B magnets attributed to the diffusion of Nd-Cu liquid phase. This is similar to the result of HP450 + HD magnet to some extent. The possible reasons for aggregation of the RErich phase should be attributed to the high porosity at the first stage of hot deformation. By increasing HP temperature properly to 550 ℃, the final HP550 + HD magnet obtains a good alignment, nearly all the platelet-shaped grains were separated by smooth grain boundary coupled with few triple junctions [Figs. 4(d), 5(b) and 6(b)]. Therefore, the HP550 + HD magnet shows the best magnetic properties (Fig. 1) and alignment [Figs. 2 and 6 (d)]. However, the morphology of HP650 + HD magnet [Figs. 4(f) and 6(c)] is totally different from that of the magnets prepared at low HP temperatures. The grains are substantially equiaxed instead of platelet-shaped grains. These equiaxed grains lead to poor magnetic properties. Therefore, appropriate HP temperature plays a very important role in optimizing phase compositions and in homogenizing the distribution of the RE-rich phase. Thereby lays a foundation of obtaining high performance hot-deformed magnets.
relatively thin and smooth for the HP550 + HD magnet [Fig. 5(b)]. Fig. 5(c) shows the low magnification high resolution TEM image of a typical triple grain boundary marked by the dotted rectangle in Fig. 5(a), which clearly shows the intergranular phase with a mean thickness of about 20–30 nm. The phase compositions and crystal structure of the grain boundary marked by yellow dotted rectangle A and B in Fig. 5(c) were further identified by performing local Fast Fourier Transformation (FFT). The results are shown in Fig. 5(d) and (e). Area-B shows the structure of bcc-Nd2O3 with a = 1.105 nm [Fig. 5(e)]. Area-A could be divided into two regions, whose boundary was marked by a dotted line [Fig. 5(d)]. Region 1 also shows the structure of bcc-Nd2O3, whereas region 2 corresponds to amorphous phase. The phase composition of thin triple junctions for HP550 + HD magnet, which is not shown here, is nearly similar to that for HP450 + HD magnet. Increasing HP temperature may promote elimination of the oxygen absorption in the melt-spun ribbons. Therefore, the RE-rich phase may not be oxidized. Thereby the RE-rich phase could maintain a good fluidity, facilitating the formation of thin and continuous grain boundary rather than aggregated triple junctions. The oxygen contents of all the hot-deformed magnets were analyzed by a nitrogen/oxygen analyzer. The results are summarized in Table 2. It is found that the HP450 + HD magnet possessed the highest oxygen content. Moreover, the oxygen content decreased with the HP temperature from 450 °C to 550 °C, and after the HP temperature exceeding 550 °C the oxygen content was almost the same. The oxygen content of hot-deformed magnets is lower than that of sintered Nd-Fe-B magnets (> 1400 ppm) due to the process without milling to fine powders about 3–5 μm [24]. Our previous [17] work showed that the amount of residual amorphous decreased with increasing the HP temperature, however it could not be eliminated by hot pressing even though HP temperature was as high as 650 °C. Therefore, some of the amorphous phase in the grain boundary might result from the original melt-spun ribbons. Fig. 6 shows the schematic image of microstructure with grains and grain boundary of hot-deformed Nd-Fe-B magnets prepared from different hot-pressed precursors. By analyzing the relationship between magnetic properties and microstructure, it is found that the HP550 + HD magnet shows the highest performance among all the magnets by creating an optimum microstructure. Obviously, in order to obtain good alignment of grains it is necessary first to impede the aggregation of the RE-rich phase. The RE-rich phase on the c-planes of the grains increases misalignment of the grains by the excess grain boundary diffusion [11]. Especially many triple junctions with large area impede the c-axis alignment of grains [Figs. 4(a) and 6(a)]. The reduced remanence and deteriorated alignment were also found in hotdeformed magnets treated by grain boundary diffusion process of RE-Cu eutectic alloy, such as Nd-Cu, Pr-Cu [25,26], and Nd-Dy-Cu [27]. Furthermore, obvious expansion in the c-axis direction of hot-deformed
4. Conclusions The effects of hot pressing (HP) temperature on the alignment and phase composition of hot-deformed (HD) magnets have been investigated. By changing the HP temperature from 450 °C to 650 °C, the HD magnet prepared from precursor fabricated at 500 °C showed the highest remanence of ∼1.40 T. The alignment of HD magnets improved first and then deteriorated with increasing the HP temperature. The lower HP temperature (below 500 °C) was resulted in the higher porosity of hot-pressed precursors. Therefore during hot deformation the pores were first filled by the liquid RE-rich phase. This caused the aggregation of the RE-rich phase, and consequently the amount of the liquid RE-rich phase used for facilitating grains deformation was reduced. The reduction of the RE-rich phase led to the deterioration of alignment. Furthermore, low HP temperature also led to the increased volume fraction of non-magnetic phase, such as amorphous and Ndoxide, which decreased the remanence. However, at high HP temperature of 650 °C, the grains exceed critical size for performing hot deformation. These grains further grew to equiaxed coarse grains during hot deformation, developing a poor alignment. Clearly, appropriate HP temperature plays a very important role in optimizing phase composition and in homogenizing distribution of the RE-rich phase. Thereby lays a foundation of obtaining high performance hot-deformed magnets.
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Journal of Magnetism and Magnetic Materials 488 (2019) 165353
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Acknowledgments
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