Hot deformed anisotropic nanocrystalline NdFeB based magnets prepared from spark plasma sintered melt spun powders

Materials Science and Engineering B 178 (2013) 990–997

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Hot deformed anisotropic nanocrystalline NdFeB based magnets prepared from spark plasma sintered melt spun powders Y.H. Hou a,b , Y.L. Huang a,b , Z.W. Liu b,∗ , D.C. Zeng b , S.C. Ma a , Z.C. Zhong a a b

School of Materials Science and Engineering, Nanchang Hangkong University, Nanchang 330063, PR China School of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, PR China

a r t i c l e

i n f o

Article history: Received 27 December 2012 Received in revised form 16 May 2013 Accepted 10 June 2013 Available online 22 June 2013 Keywords: NdFeB magnet Hot deformation Microstructure evolution Coercivity mechanism Non-uniform plastic deformation

a b s t r a c t Anisotropic magnets were prepared by spark plasma sintering (SPS) followed by hot deformation (HD) using melt-spun powders as the starting material. Good magnetic properties with the remanence Jr > 1.32 T and maximum of energy product (BH)max > 303 kJ/m3 have been obtained. The microstructure evolution during HD and its influence on the magnetic properties were investigated. The fine grain zone and coarse grain zone formed in the SPS showed different deformation behaviors. The microstructure also had an important effect on the temperature coefficients of coercivity. A strong domain-wall pinning model was valid to interpret the coercivity mechanism of the HDed magnets. The increase of stray field and weakening of domain-wall pinning effects were the main reasons of the decrease of the coercivity with increasing the compression ratio. The influences of non-uniform plastic deformation on the microstructure and magnetic properties were investigated. The polarization characteristics of HDed magnets were demonstrated. It was found out that the HDed magnets had better corrosion resistance than the counterpart sintered magnet. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved.

1. Introduction Anisotropic NdFeB magnets are generally prepared by two different processes. One is pressing in a magnetic field followed by sintering [1]; the other process is to induce a c-axis crystallographic alignment by plastic deformation, such as hot deformation (HD) like extrusion, compression or rolling [2]. Hot deformed (HDed) NdFeB magnets have attracted much attention not only because of their good magnetic properties but also due to their exceptional corrosion resistance, thermal stability, and fracture toughness [3–6]. Up to now, the largest reported value of (BH)max for hot deformed magnets is 433 kJ/m3 , which was achieved in a Nd13.5 (Fe, Co)80 Ga0.5 B6 alloy [7]. The deformation mechanism, which is not capable of classic plastic flow through dislocation mechanism, is believed to be a combination of stress-assisted grain growth via mass transport and grain boundary sliding [8,9]. Experimental studies suggest that magnetic hardening would arise from the domain-wall pinning at grain boundaries [10,11]. The domain walls run continuously from grains to grains, and are pinned where they encounter the intergranular Nd-rich phase [10]. Spark plasma sintering (SPS) has been developed rapidly in recent years [12]. It has been found out that SPS can effectively

∗ Corresponding author. Tel.: +86 20 22236906 E-mail address: [email protected] (Z.W. Liu).

inhabit the growth of grains by applying high heating rate, short holding time, and low sintering temperature. It, therefore, has been frequently employed to prepare nanocrystalline materials [13], composites [14], and functionally graded materials [15] and so on. In this work, SPS was employed as the pre-process for obtaining nanocrystalline bulk NdFeB precursors from melt-spun powders. To achieve enhanced magnetic properties, temperature stability, and corrosion resistance in HDed magnets, the hot deformation processing has to be optimized for improving c-axis crystallographic alignment of the Nd2 Fe14 B grains. In the present work, investigated in details were the influences of hot deformation processing on the microstructure, magnetic properties, temperature stability, mechanical properties, and corrosion resistances. The influences of non-uniform deformation on the microstructure and magnetic properties were analyzed. The coercivity mechanism of hot deformed magnets was also discussed.

2. Experimental Commercial melt-spun ribbons with nominal composition of Nd13.5 Co6.7 Ga0.5 Fe73.5 B5.6 were consolidated to dense isotropic magnets by SPS (the facility of SPS-825 (Sojitz Machinery Co.)) using the optimized processing parameters obtained previously (TSPS = 700 ◦ C, tSPS = 5 min and PSPS = 50 MPa) [16]. The SPSed magnets were subjected to hot deformation using a vacuum hot deformation facility (HP-12 × 12 × 12, Centorr Vaccum Industries,

0921-5107/$ – see front matter. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mseb.2013.06.009

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3. Results and discussion 3.1. Structure and phase constitution

Fig. 1. XRD patterns for the SPSed and HDed magnets at various deformation compression ratios and temperatures: (a) SPSed magnet; (b) 750 ◦ C – 51%; (c) 750 ◦ C – 68%; (d) 800 ◦ C – 73%; (e) 800 ◦ C – 80%.

USA) at 750 ◦ C and 800 ◦ C with true strain rate of 0.001 S−1 . The temperature dependence of Hcj was obtained from hysteresis loops run at approximately 50 K intervals between 100 and 550 K. The density of the magnets was measured based on the Archimedes principle. X-ray diffraction patterns were obtained with X-ray diffractometer (XRD, Philip X-pert) using Cu-K␣ radiation. To characterize the microstructure, the SPSed and HDed magnets were carefully broken off manually from an indentation and the fracture surfaces were examined by SEM (Nano430, FEI Co.). Magnetic properties were characterized by physical properties measurement system (PPMS-9 Quantum Design, USA) equipped with a 9 T vibrating sample magnetometer (VSM). The microhardness of the magnets was measured by a microhardness tester at a load of 200 g with 10 s dwell time. At least six measurements were taken on each sample to obtain the average values and standard deviation. The anti-corrosion properties of the magnets in electrolytes were investigated by using conventional electrochemical methods. Anodic potentiodynamic polarization experiments were carried out in 3 wt.% NaCl electrolytes with a CHI604C potentiostat at 25 ◦ C. The working electrode with an operating surface area of S = 0.25 cm2 perpendicular to the pressing direction was mechanically grounded with SiC-paper (grade 3201000) and polished with diamond paste (0.25 ␮m). A standard three electrode method was applied to perform the test, which include a NdFeB working electrode, a saturated calomel reference electrode, and a platinum counter electrode. The open-circuit potential was measured with the scanning rate of 2 mV/s to obtain the anode polarization curves.

Fig. 1 shows the XRD patterns of SPSed and HDed magnets obtained from the surface perpendicular to the pressing direction at the various compression ratios and temperatures. For the SPSed isotropic magnet, all peaks are attributed to the tetragonal hard magnetic Nd2 (FeCo)14 B phase, as shown in Fig. 1(a). For the HDed magnets, the (0 0 L) peaks and peaks with direction close to (0 0 L), such as (0 0 4), (0 0 6), (0 0 8), and (1 0 5) peaks, have enhanced intensity, indicating a slight c-axis crystallographic alignment of the magnets, though the alignment is not so strong as the conventional anisotropic sintered NdFeB magnets [17]. With increasing the compression ratio from 51% to 80%, the relative intensities of the (0 0 4), (0 0 6), (0 0 8) and (1 0 5) diffraction peaks become stronger gradually, as shown in Fig. 1(b)–(e), which indicates the increase of c-axis crystallographic alignment. For quantitative analysis, a quantitative evaluation method was employed to determine the alignment degree of NdFeB magnets [18]. Obviously, the alignment degree is enhanced from 74.1% to 88.9% with the compression ratio increasing from 51% to 80%, as listed in Table 1. The value of alignment degree for SPSed magnets is only 56.7% due to the random distribution of the c-axis. The degree of texture [(Jr − Jr⊥ )/Jr ] can be used to characterize the mean alignment of c-axis texture of the Nd2 Fe14 B grains along the pressing axis. Here Jr and Jr⊥ are the values of the magnetic remanences measured parallel and perpendicular to the pressing direction respectively. The degree of texture increases with increasing compression ratio as listed in Table 1. The variation is in consistence with the XRD results. The two distinct zones with different grain sizes were noticed in SPSed NdFeB magnets [16]. The coarse grain zones correspond to the particle boundary area of ribbons and the fine grain zones correspond to the interior of the particles. The formation mechanism of the two-zone structure was discussed in our previous work [16]. The microstructure of the coarse grain zones and fine grain zones were composed of the equiaxed grains for SPSed isotropic magnets. For HDed anisotropic magnets, the two-zone microstructure was still maintained as shown in Fig. 2(a). It is observed that the microstructure of the fine grain zones consists of small plateletshaped Nd2 Fe14 B grains due to the deformation of the grains, as shown in Fig. 2(a) and (b). Within each grain the crystallographic c-axis, the magnetic easy axis, is perpendicular to the platelet. In the coarse grain zones, however, the microstructure of equiaxed grains was maintained, as shown in Fig. 2(a), indicating that large grains are hard to be deformed. This is because the fine grains need a shorter diffusion distance for preferential growth of Nd2 Fe14 B grain and a high driving force exists for the growth of small grains

Fig. 2. Microstructure of hot deformed magnets with the compression ratio of 73%: (a) overview; (b) fine grain zone.

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Table 1 Magnetic properties and degree of texture of SPSed and HDed magnets parallel and perpendicular to the pressing direction. Process of sample

◦

700 C – SPS 750 ◦ C – 51% 750 ◦ C – 68% 800 ◦ C – 73% 800 ◦ C – 80%

Jr (T)

(BH)max (kJ/m3 )

Hcj (kA/m)

Jr

Jr⊥

0.82 1.19 1.29 1.32 1.34

0.81 0.39 0.30 0.24 0.23

Hcj

Hcj⊥

1516 1239 995 847 731

1517 1095 773 638 585



c-Axis alignment

(Jr − Jr⊥ )/Jr

56.7% 74.1% 80.1% 82.2% 88.9%

0.012 0.670 0.768 0.820 0.823

⊥

(BH)max

(BH)max

116 259 293 303 285

113 25 14 9 8

Fig. 3. Microstructure of HDed magnets parallel to the pressing direction at various compression ratios and temperatures: (a), (b) 750 ◦ C – 51%; (c), (d) 750 ◦ C – 68%; (e), (f) 800 ◦ C – 73%; (g), (h) 800 ◦ C – 80%, the insets are the microstructure perpendicular to the pressing direction respectively.

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Fig. 4. The width of fine grain zone and coarse grain zone for HDed magnets under various compression ratios and temperatures.

compared to the large grains [19]. Therefore, the results imply that the microstructure with large grain size is not beneficial in obtaining anisotropic magnets by plastic deformation. Our latest results indicate that the platelet-shaped grain with c-axis texture can be obtained by increasing deformation time for the coarse grain zone. 3.2. The effect of hot deformation process on the microstructure Fig. 3 shows the microstructure of the HD magnets obtained at various deformation conditions. The coarse grain zones in Fig. 3(a), (c), (e), and (g) correspond to the particle boundary area of ribbons and the platelet-shaped grain zone correspond to the interior of the particles. With the compression ratio from 51% to 80%, the average width of the coarse grain area increases from ∼3.9 ␮m to ∼5.4 ␮m, and the average width of the fine grain area decreases from ∼11.2 ␮m to ∼4.1 ␮m. For a quantitative analysis, the widths of the coarse grain zone and the fine grain zone for all samples were determined. Fig. 4 shows the dependences of the average width of the fine grain zone and coarse grain zone on the compression ratios and temperatures. It is clear that the width of the fine grain zone decreases with increasing the compression ratio and temperature. Under the same strain rate of 0.001 S−1 , the deformation time is 710 s, 1140 s, 1310 s, and 1610 s for the HDed magnets with the compression ratio of 51%, 68%, 73%, and 80%, respectively. The increase of deformation time can cause more mass transport along the direction perpendicular to the pressing in the fine grain zone. The increasing deformation temperature, which can improve the plastic deformability of the fine grain zone, leads to more mass transport occurred in the fine grain zone. Therefore, the preferential growth of Nd2 Fe14 B grains under stress along perpendicular to the pressing direction contributes to the decrease of the fine grain area width. However, no preferential growth of Nd2 Fe14 B grains is observed in the coarse grain zone and the width of the coarse grain zone is enhanced by increasing deformation time and temperature. The mean dimensions of the grains perpendicular and parallel to the pressing direction, defined as w and h (as inset in Fig. 5), in the fine grain zone for HD magnets prepared at various compression ratios are plotted in Fig. 5. Both w and h increase with enhancing the compression ratio. The increase of grain dimension perpendicular to the pressing direction, i.e. the value of w, was attributed to the increases of compression ratio and time. The large compression ratio can enhance grain rotation and deformation. The increase of deformation time leads to more mass transport caused by diffusion [8,9]. Both of them can increase the preferential growth of Nd2 Fe14 B grains along perpendicular to the pressing direction. This is consistent with the XRD results. This characteristic microstructure has an important impact on the magnetic properties.

993

Fig. 5. The mean dimension of grain in the fine grain zone perpendicular and parallel to the pressing axis for HDed magnets at various compression ratios and temperatures.

3.3. The effect of hot deformation process on the magnetic properties Fig. 6 shows the magnetic hysteresis loops of the SPSed and HDed magnets parallel and perpendicular to the pressing direction at various deformation conditions. As listed in Table 1, the degree of texture [(Jr − Jr⊥ )/Jr ] for the SPSed magnet is only 0.012, which indicates it is isotropic. For the magnetic properties of the HDed magnets parallel to the pressing direction, the remanence (Jr ) increases from 1.19 T to 1.34 T with the compression ratio increasing from 51% to 80%, implying the increase of the c-axis orientation degree, which also agrees with the XRD results. The coercivity (Hcj ), however, decreases from 1239 kA/m to 731 kA/m. The different dependences of remanence and coercivity on the compression ratio lead to the maximizing energy product ((BH)max ) at the compression ratio of 73%. The optimal combination of the magnetic properties are Jr = 1.32 T, Hcj = 847 kA/m, and (BH)max = 303 kJ/m3 . With the formation and enhancement of c-axis texture along the pressing direction as the compression ratio increases, the remanence perpendicular to the pressing direction decreases from 0.39 T to 0.23 T. The magnetic properties are directly related to the microstructure, so the dimension w of the platelet-shaped grain is related to the remanence and coercivity. Table 1 shows the larger the values of w, the higher the remanences, which can be attributed to the improved anisotropy. Since the increase of grain dimension w of the platelet-shaped grain enhances the irregular degree of grain, which possibly increases the stray field acting on the grains. Therefore, the increase of stray field is one possible reason for the reduction of the coercivity. 3.4. The effect of hot deformation process on the temperature stability The elevated temperature magnetic properties are critical for NdFeB magnets to be used above the room temperature. The magnetic properties at 293 K and 393 K for the SPSed and HDed magnets at various deformation conditions are shown in Table 2. The remanence and coercivity decrease with increasing temperature due to the temperature dependence of magnetization and anisotropic field. The temperature stability of permanent magnets in the temperature range To − T can be expressed by the temperature coefficient of remanence (˛) and temperature coefficient of coercivity (ˇ), where ˛ = ((Jr (T) − Jr (To ))/Jr (To )(T − To )) × 100% and ˇ = ((j Hc (T ) − j Hc (To ))/j Hc (To )(T − To )) × 100% [20]. The calculated temperature coefficient in the temperature of 293–393 K for the SPSed and HDed magnets are listed in Table 2. The temperature coefficient of coercivity decreases from −0.584% K−1 to −0.703% K−1 ,

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Fig. 6. Magnetic hysteresis curves of the SPSed and HDed magnets parallel to the pressing direction (a) and perpendicular to the pressing direction (b).

Table 2 Magnetic properties at 293 and 393 K, temperature coefficient of remanence (˛), and temperature coefficient of coercivity (ˇ) for SPSed and HD magnets. Process of sample

700 ◦ C – SPS 750 ◦ C – 51% 750 ◦ C – 68% 800 ◦ C – 73% 800 ◦ C – 80%

Jr (T)

(BH)max (kJ/m3 )

Hcj (kA/m)

293 K

393 K

293 K

393 K

293 K

393 K

0.83 1.20 1.30 1.33 1.36

0.74 1.07 1.16 1.19 1.22

1573 1308 1054 907 788

813 544 378 290 234

121 263 298 309 301

91 187 192 163 146

but the temperature coefficient of remanence has no significant change with increasing compression ratio. The optimal combination of magnetic properties are Jr = 1.16 T, Hcj = 378 kA/m, and (BH)max = 197 kJ/m3 at the temperature of 393 K for the HDed magnet with the compression ratio of 68%, which is very useful for high temperature applications. For the HDed magnet with the compression ratio of 73%, although the optimal magnetic properties with Jr = 1.33 T, Hcj = 907 kA/m, and (BH)max = 309 kJ/m3 were obtained at 293 K, its properties at 393 K are not the best due to relatively low temperature coefficient of coercivity. With no addition of heavy rare earth elements and low Nd content, the obtained optimal magnetic properties of Hcj = 847 kA/m, Jr = 1.32 T, and (BH)max = 303 kJ/m3 are indeed very promising, comparing to those of the sintered NdFeB magnets with higher Nd contents [21,22]. The results also indicate that the microstructure plays an important role in the temperature stability, especially for the temperature coefficient of coercivity. The SPSed magnet has the microstructure of fine equiaxed grains, while the HDed magnets are composed of irregular and coarse platelet-shaped grains, as shown in Figs. 2 and 3. Table 2 indicates a low temperature coefficient ˇ in the SPSed magnets and the increase of the absolute value of ˇ for the HDed magnets with the increase of the compression ratio, which indicates that fine and regular grain microstructure is beneficial in improving the temperature coefficient of coercivity.

3.5. Coercivity mechanism For the coercivity mechanism of HDed magnets, a variety of experimental studies suggest that magnetic hardening would arise from domain-wall pinning at grain boundaries [10,11]. The domain walls run continuously from grains to grains, and are pinned where they encounter the intergranular Nd-rich phase. Graunt proposed a strong domain-wall pinning model by a random array of pinning sites considering the effect of thermal activation of domain walls over pinning barriers [23]. The model was

˛ (% K−1 )

ˇ (% K−1 )

−0.108 −0.108 −0.107 −0.109 −0.103

−0.483 −0.584 −0.642 −0.681 −0.703

Fig. 7. Hcj as a function of temperature for HDed magnet with compression ratio of 68%.

developed by Pinkerton [24], who derived the model prediction that (Hcj /HA )1/2 varies linearly with (T/)2/3 , i.e.



Hcj HA

1/2 = Co − C1

 T 2/3 

(1)

where HA and  are the magnetocrystalline anisotropic field and the domain wall energy per unit area of the Nd2 Fe14 B phase, respectively. The values of Hcj were obtained from hysteresis loops run at approximately 50 K intervals between 100 and 550 K. Ms for each temperature were obtained by extrapolating the J ∝ 1/H2 plot to H = ∞. The value of  are calculated from A (exchange stiffness) [25], HA [26], and Ms . Fig. 7 shows the temperature dependence of coercivity for the HDed magnet with 68% compression ratio, whose magnetic properties are listed in Table 1. The plot of (Hcj /HA )1/2 vs. (T/)2/3 for the HDed magnet are shown in Fig. 8. The data are successfully described by a straight line over the temperature range from 100 to 500 K, and the constants Co = 0.106 (cm2 erg−1 )1/2 ,

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Table 3 Magnetic properties of different deformation zone for HDed magnet with 73% compression ratio.

Fig. 8. (Hcj /HA )1/2 versus (T/)2/3 for the HDed magnet with the compression ratio of 68%. The solid circles indicate experimental data points at approximately 50 K intervals between 100 and 550 K. The line is the linear fit to the experimental data points between 100 and 500 K.

C1 = 4.20 × 10−3 (erg K−4 cm−2 )1/6 . Large deviations from linearity are observed at the temperature of 550 K, which is because the strong pinning model is no longer valid once the temperature is close to the Curie temperature. This result confirms that the magnetic hardening does arise from a domain wall pinning, being consistent with the experimental evidences [10,11]. Similar results can be obtained for the HDed magnets with the other compression ratios. Presented in Fig. 9(a) are the initial magnetization curves of the demagnetized samples at various deformation conditions. For the HDed magnet with the 51% compression ratio, the initial curve shows an obvious stepwise shape. With increasing compression ratio, the characteristics of stepwise shape degenerate gradually. Eventually, there was no stepwise characteristic for the HDed magnet with the 80% compression ratio. It is believed that the magnetic domain wall pinning mechanism should be responsible for such behavior of the initial magnetization curves for the HDed magnets. Fig. 9(b) shows the differential susceptibility curves of the HDed magnets. One or two peaks can be observed on each curve, corresponding to the stepwise characteristics. The external field at the peak position is close to the respective value of coercivity. With increasing compression ratio, the peak gets weaken. For the magnet with 80% compression ratio, the peak was not observed, corresponding to the disappearance of stepwise shape in the initial magnetization curve as shown in Fig. 9(a). The variation of domain wall pinning effects induced by microstructure evolution should be responsible for the results. The domain walls are pinned where they encounter the intergranular Nd-rich phase. With the increase of compression ratio, the dimensions of platelet-shaped grain increase, and, hence, the volume fraction of grain boundary decreases. This induced the weakening of domain wall pinning effects in the intergranular Nd-rich phase. Therefore, the weakening of domain wall pinning effects should be responsible to the evolution of initial magnetization and differential susceptibility curves. The decrease of the coercivity with an increase of the compression ratio should be related to the weakening of domain wall pinning effects. Meanwhile, the stray field is possibly also contribution to the reduction of the coercivity, as discussed earlier. 3.6. Non-uniform plastic deformation of NdFeB magnet According to the plastic deformation theory, the inhomogeneous chemical composition, microstructure, inhomogeneous temperature distribution, and friction between punch and sample can induce the non-uniform plastic deformation during HD. The friction between punch and sample is possibly the main reason

Deformation zone

Jr (T)

Hcj (kA/m)

(BH)max (kJ/m3 )

I II III

1.21 1.29 1.26

978 995 1048

251 293 274

of the non-uniform plastic deformation. According to the difference of stress distribution, the sample under HD can be divided into three zones, the conical contact zone, the central zone, and the vertical edge zone, respectively, indicating by I, II and III, as shown in Fig. 10(a). The conical contact zone is the most difficult to be deformed under the intense three-directional compressive stress. The central zone has the most serious deformation since it is far away from the punch in spite of the three-directional compressive stress. The plastic deformability of the vertical edge zone lies between the conical contact zone and the central zone due to under two-directional tensile stress and one-directional compressive stress in the process of the deformation. Fig. 10(b)–(d) shows the microstructures of the conical contact zone, the central zone, and the vertical edge zone after deformation. For the conical contact zone, some un-deformed grains are observed, as shown in the inset of Fig. 10(b), and the direction of oriented growth of Nd2 Fe14 B grains is not consistent with each other indicated by the arrow due to the difference of stress distribution. In Fig. 10(c), the central zone consists of platelet-shaped grains, and the Nd2 Fe14 B grains are well oriented with c-axis along the pressing direction due to the homogeneous distribution of stress. For the vertical edge zones, although the microstructure also consists of platelet-shaped grains, the direction of oriented growth of Nd2 Fe14 B grains is not perpendicular to the pressing direction, as shown in Fig. 10(d). Different tilting degrees of oriented growth direction away from that perpendicular to the pressing direction are observed. This is not beneficial in improving magnetic properties. The magnetic properties for three zones with different microstructures are listed in Table 3. The optimal magnetic properties with Jr = 1.29 T, Hcj = 995 kA/m, and (BH)max = 293 kJ/m3 were obtained in the central zone due to the optimal c-axis crystallographic alignment. For the conical contact zone, the relatively low magnetic properties with Jr = 1.21 T, Hcj = 978 kA/m, and (BH)max = 251 kJ/m3 were obtained because of the inconsistent directions of the oriented growth and the existence of un-deformed grains.

3.7. Microhardness of HDed magnets Besides the magnetic properties, mechanical properties are also important for the practical applications of NdFeB magnets. Similar to the structural materials, microstructure has important effects on the mechanical properties of the magnetic materials. Fig. 11 shows the microhardness of the SPSed magnet and HDed magnets perpendicular to the pressing direction, prepared under various compression ratios. The SPSed magnet with the maximum value of microhardness, 740 Hv, is due to the fine grain structure [16]. The microhardness of the HDed magnets decreases from 660 to 599 Hv with increasing the compression ratios from 51% to 80%. The increase of mean grain dimensions for the HDed magnets, i.e. the value of w and h (as shown in Fig. 5), should be responsible to the decrease of microhardness. The results of microhardness evolution for the SPSed and HDed magnets aroused by the microstructure changes are in accordance with the Hall–Petch relationship.

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Fig. 9. The initial magnetization curves (a) and differential susceptibility curves (b) for unmagnetized samples under various deformation conditions.

Fig. 10. The non-uniform deformation map of NdFeB magnet (a) and microstructure of various deformation zones for HDed magnet with the compression ratio of 73%: (b) the conical contact zone, (c) the central zone, (d) the vertical edge zone.

Table 4 Electrostatic potential (Ecorr ) and corrosion current density (Icorr ) of sintered, SPSed, and HDed magnets in 3 wt.% NaCl electrolytes.

Ecorr (V) Icorr (mA/cm2 )

Sintering magnet

SPSed magnet

−0.845 8

−0.799 89

3.8. Polarization characteristics of HDed magnets Potentiodynamic polarization curves for the SPSed magnet and HDed magnets, in 3 wt.% NaCl solution are shown in Fig. 12. For comparison, a polarization curve for a conventional sintered NdFeB magnet, which has a composition of Nd13.5 Fe80.5 B6 and magnetic properties of Jr = 1.17 T, Hcj = 836 kA/m, and (BH)max = 263 kJ/m3 , is also shown in Fig. 12. No passivation occurred in the NaCl solutions. The corrosion potential Ecorr and corrosion current density Icorr for all tested magnets are listed in Table 4.

HD magnets (compression ratios) 51%

68%

73%

80%

−0.778 37

−0.782 27

−0.776 13

−0.782 9

The corrosion potential of the HDed magnets is more positive than that of the SPSed and conventional sintered magnet. The corrosion potential of HDed magnets with various compression ratios are almost the same due to the same composition and similar microstructure. Conventional sintered magnets, being more prone to electrochemical corrosion, have the lowest corrosion potential. The corrosion current is related to the microstructure, chemical composition, and electric properties of the Nd-rich phase. In this work, the corrosion current densities as listed in Table 4 show that the large grain size is beneficial in reducing corrosion current densities.

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positive corrosion potential, nevertheless, the large grain size is beneficial in reducing corrosion current density. Acknowledgements This work was supported by the Natural Science Foundation of China (Grant No. 51174094 and 51261022), the Guangdong Provincial Science and Technology Program (Grant Nos. 2010A090200060, and 2012B091000005), the Program for New Century Excellent Talents in University (Grant No. NCET-11-0156), and the State Key Laboratory for Advanced Metals and Materials (Grant No. 2011-ZD05), the National Science and Technology Support Project (Grant No. 2012BAE02B01), the Jiangxi Provincial Science and Technology Project (Grant No. 2010AZX00200), the Science and Technology Plan Projects of Department of Education of Jiangxi Province (Grant No. KJ201109132281), and the PhD Start-up Foundation of Nanchang Hangkong University (Grant No. EA201101314). Fig. 11. Microhardness of the SPSed and HDed magnets.

References

Fig. 12. Polarization curves of the sintered, the SPSed, and the HDed magnets in 3 wt.% NaCl electrolytes.

4. Conclusions Anisotropic magnets were prepared by SPS followed by hot deformation and good magnetic properties with Jr = 1.32 T, Hcj = 847 kA/m, and (BH)max = 303 kJ/m3 were obtained at 300 K. The microstructure evolution during HD and its influence on the magnetic properties were investigated. It is found out that the relative fine and regular grain microstructure is beneficial in improving the temperature coefficient of coercivity. A domain-wall pinning model is valid to interpret the coercivity mechanism of the HD magnet, and to be responsible for the variation of the initial magnetization curves. The increase of stray field and the weakening of domain-wall pinning effects are the main reasons of the reduction of coercivity with increasing the compression ratio. Three different deformation zones with different microstructure and magnetic properties induced by non-uniform plastic deformation were investigated. The SPSed and HDed magnets show more

[1] M. Sagawa, S. Fujimura, N. Togawa, Y. Matsuura, Journal of Applied Physics 55 (1984) 2083–2087. [2] R.W. Lee, E.G. Brewer, N.A. Schaffel, IEEE Transactions on Magnetics 21 (1985) 1958–1963. [3] R.K. Mishra, V. Panchanathan, J.J. Croat, Journal of Applied Physics 73 (1993) 6470–6472. [4] B.M. Ma, D. Lee, B. Smith, S. Gaiffi, B. Owens, H. Bie, G.W. Warren, IEEE Transactions on Magnetics 37 (2001) 2477–2479. [5] Y. Li, Y.B. Kim, M.J. Kim, D.S. Suhr, T.K. Kim, C.O. Kim, IEEE Transactions on Magnetics 36 (2000) 3309–3311. [6] M. Yue, M. Tian, J.X. Zhang, D.T. Zhang, P.L. Niu, F. Yang, Materials Science and Engineering B 131 (2006) 18–21. [7] T. Saito, M. Fujita, T. Kuji, K. Fukuoka, Y. Syono, Journal of Applied Physics 83 (1998) 6390–6392. [8] R.K. Mishra, T.Y. Chu, L.K. Rabenberg, Journal of Magnetism and Magnetic Materials 84 (1990) 88–94. [9] L. Li, C.D. Graham, Journal of Applied Physics 67 (1990) 4756–4758. [10] R.K. Mishra, R.W. Lee, Applied Physics Letters 48 (1986) 733–735. [11] F.E. Pinkerton, D.J. Van Wingerden, Journal of Applied Physics 60 (1986) 3685–3690. [12] H. Ono, N. Waki, M. Shimada, T. Sugiyama, A. Fujiki, H. Yamamoto, M. Tani, IEEE Transactions on Magnetics 37 (2001) 2552–2554. [13] H.B. Zhang, B.-N. Kim, K. Morita, H. Yoshida, J.-H. Lim, K. Hiraga, Journal of the American Ceramic Society 94 (2011) 2981–2986. [14] J.-M. Lee, S.-B. Kang, T. Sato, H. Tezuka, A. Kamio, Materials Science and Engineering A343 (2003) 199–209. [15] M. Radwan, M. Nygren, K. Flodström, S. Esmaelzadeh, Journal of Materials Science 46 (2011) 5807–5814. [16] Z.W. Liu, H.Y. Huang, X.X. Gao, H.Y. Yu, X.C. Zhong, J. Zhu, D.C. Zeng, Journal of Physics D: Applied Physics 44 (2011) 025003. [17] W.J. Mo, L.T. Zhang, A.D. Shan, L.J. Cao, J.S. Wu, M. Komuro, Intermetallics 15 (2007) 1483–1488. [18] Z.Y. Wang, N.J. Gu, J.F. Liu, Journal of Materials Science and Engineering 21 (2003) 482–484. [19] L. Li, C.D. Graham, IEEE Transactions on Magnetics 28 (1992) 2130–2132. [20] Z.W. Liu, H.A. Davies, Journal of Magnetism and Magnetic Materials 290-291 (2005) 1230–1233. [21] A. Fukuno, K. Hirose, T. Yoneyama, Journal of Applied Physics 67 (1990) 4750–4752. [22] S. Pandian, V. Chandrasekaran, G. Markandeyulu, K.J.L. lyer, K.V.S. Rama Rao, Journal of Applied Physics 92 (2002) 6082–6086. [23] P. Gaunt, Philosophical Magazine Part B 48 (1983) 261–276. [24] F.E. Pinkerton, C.D. Fuerst, Journal of Magnetism and Magnetic Materials 89 (1990) 139–142. [25] H. Kronmüller, R. Fischer, M. Seeger, A. Zern, Journal of Physics D: Applied Physics 29 (1996) 2274–2283. [26] R. Grössinger, X.K. Sun, R. Eibler, K.H.J. Buschow, H.R. Kirchmayr, Journal of Magnetism and Magnetic Materials 58 (1986) 55–60.