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Improved corrosion resistance of low rare-earth Nd–Fe–B sintered magnets by Nd6Co13Cu grain boundary restructuring
Journal of Magnetism and Magnetic Materials 379 (2015) 186–191
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Improved corrosion resistance of low rare-earth Nd–Fe–B sintered magnets by Nd6Co13Cu grain boundary restructuring Pei Zhang, Tianyu Ma n, Liping Liang, Xiaolian Liu, Xuejiao Wang, Jiaying Jin, Yujing Zhang, Mi Yan n Department of Materials Science and Engineering, State Key Laboratory of Silicon Materials, Key Laboratory of Novel Materials for Information Technology of Zhejiang Province, Zhejiang University, Hangzhou 310027, China
art ic l e i nf o
a b s t r a c t
Article history: Received 21 July 2014 Received in revised form 10 November 2014 Accepted 16 December 2014 Available online 17 December 2014
Aiming to improve the corrosion resistance of Nd–Fe–B sintered magnets, Nd6Co13Cu powders are introduced into a near-stoichiometric Nd2Fe14B magnet by grain boundary restructuring. The restructured magnet containing only 28.46 wt% (Pr, Nd) shows much better corrosion resistance against hot/humid, acidic and saline environments than the un-restructured magnet with equivalent rare earth content (REC). It results mainly from the formation of higher electrode potential Co-rich phase and Cu-rich interface layers in the intergranular regions. Besides, magnetic properties of the restructured magnet are well retained with coercivity Hcj above 12.5 kOe, remanence Br above 14.0 kGs and maximum magnetic energy product (BH)max above 48.5 MGOe respectively. It can be ascribed to the formation of more continuous grain boundary phase that isolates well the neighboring Nd2Fe14B grains. & 2014 Published by Elsevier B.V.
Keywords: Permanent magnet Microstructure Magnetic property Corrosion
1. Introduction Since discovered in 1984, Nd–Fe–B sintered magnets have drawn extensive research interests due to their outstanding magnetic performance [1–3]. However, they exhibit high susceptibility to corrosion attack, limiting their applications in corrosive environments. The poor corrosion resistance is originated from the multiple-phase morphology, where the standard electrode potential of Nd-rich intergranular phase is much lower than the Nd2Fe14B matrix, being the main driving force for electrochemical corrosion [4–8]. Therefore, many efforts have been taken to improve the corrosion resistance of sintered Nd–Fe–B magnets. Normally, reducing the volume fraction of Nd–rich phase by decreasing the rare earth content (REC) is effective to improve the corrosion resistance [9–12]. Unfortunately, the near-stoichiometric magnets cannot reach full densification because of the insufficient liquid-phase during sintering, thus leading to poor magnetic performance. Alternatively, minimizing the difference of electrode potential between the matrix phase and Nd-rich intergranular phase by either intergranular addition or grain boundary restructuring is proved to be more feasible for improving corrosion resistance without declining magnetic properties [5,13–17]. Through introducing extra elements, they facilitate the formation n
Corresponding authors. Fax: þ 86 571 87952366. E-mail addresses: [email protected] (T. Ma), [email protected] (M. Yan).
http://dx.doi.org/10.1016/j.jmmm.2014.12.044 0304-8853/& 2014 Published by Elsevier B.V.
of more stable intergranular phase with higher electrode potentials. Generally, intergranular adding rare-earth-free additives, such as Cu, Ta, MgO, SiO2 etc., always requires a high REC above 30 wt% in the starting magnet [13–16], providing sufficient Ndrich phase to ensure magnet densification. Differently, utilizing the recently developed grain boundary restructuring approach, adding high potential Nd6Fe13Cu into a slightly off-stoichiometric Nd2Fe14B-type (2:14:1 phase) alloy can improve corrosion resistance in low-rare-earth Nd–Fe–B magnets [17]. This approach is capable of improving wettability with the 2:14:1 phase grains, changing morphologies and chemical composition of the intergranular regions, and enhancing the Nd-rich phase stability, resulting in better corrosion resistance and reduction in REC simultaneously. In our previous work [17], the restructured magnets by introducing 3–12 wt% Nd6Fe13Cu into (Pr, Nd)28.30FebalB1.03 (wt%), still contain a relatively high volume fraction of rare-earthrich (RE-rich) intergranular phase. Therefore, here we investigated the slight introducing effects on the corrosion resistance and magnetic performance by a similar alloy Nd6Co13Cu (1–3 wt%) to further reduce the volume fraction of RE-rich phase as well as the REC. Besides, since the standard electrode potential of Co ( 0.277 V) is higher than both Fe (0.440 V) and Nd ( 2.431 V) [5], further decrease in the electrode potential difference is also expected. More comprehensive corrosion resistance against hot/humid, acidic and saline environments is achieved by employing only 2 wt% Nd6Co13Cu, where the rare-earth consumption is 28.46 wt%. Well retained
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magnetic properties with Br ¼14.03 kGs, Hcj ¼ 12.62 kOe, and (BH)max ¼48.55 MGOe are obtained simultaneously.
2. Experiments Starting magnet with near-stoichiometric composition of (Pr, Nd)28.00FebalB1.03 (wt%) was prepared by vacuum induction melting of Fe (purity 99.95%), Pr–Nd alloy containing 20 at% Pr and Fe– B alloy containing 20 at% B under the protection of high-purity argon atmosphere. The ingots were subsequently subjected to strip casting with a copper wheel. The flakes were pulverized into 4 μm powders by hydrogen decrepitated (HD) and jet-milling processes. The Nd6Co13Cu alloy was prepared by arc melting for 5 times to ensure homogenization in a vacuum furnace. The alloy ingots were crushed and ball-milled into fine powders with an average particle size of 1.7 μm. The two kinds of powders were then mixed under the protection of nitrogen. Afterward, the mixed powders were aligned in a magnetic field of 1.8 T under 6 MPa, followed by isostatic compressing under 200 MPa. The green compacts were sintered for 4 h at 1075 °C, followed by a two-step annealing treatment, which was performed for 2 h at 890 °C and for 3.5 h at 500 °C, respectively. For comparison, a magnet (Pr, Nd)28.46FebalB1.03 with equivalent REC (thereafter, defined as the un-restructured magnet) to the 2 wt% Nd6Co13Cu added magnet was prepared by the same liquid-phase sintering processing. Magnet density was determined according to the Archimedes principal. Thermal analysis of the Nd6Co13Cu alloy and the restructured magnets was conducted by the differential scanning calorimetry (DSC) upon heating at 10 K/min. The chemical compositions of final magnets were determined by the inductively coupled plasma (ICP) analysis. Accelerated corrosion test was performed by placing cubic samples (10 10 10 mm3) in 120 °C, 2 bar and 100% relative humid atmosphere. Polarization curves were measured with a CHI604B electrochemistry analyzer to characterize the electrochemical stability. All experiments were polished with 1000# sandpapers and performed in a standard three electrode cell consisting of Nd–Fe–B working electrode (10 10 mm2), saturated calomel reference electrode and Pt counter electrode. Each measurement was conducted at 25 70.1 °C in a 3.5% NaCl aqueous solution. Sulfuric acid corrosion test was performed to characterize the chemical stability of intergranular phase. The samples (10 10 10 mm3) were grinded by 400# sandpaper and immersed in 0.1 M H2SO4 aqueous solution for 1800 s. The amount of released hydrogen was measured by a universal gas flowmeter (ADM 2000) under one bar pressure. Magnetic properties were measured using an AMT-4 magnetic measurement device. Microstructures were observed under a field-emission scanning electron microscopy (FE-SEM SIRION-100) equipped with an energy dispersive X-ray spectroscopy (EDS).
3. Results and discussion Fig. 1 reveals the different corrosion properties of the starting magnet, restructured magnets and un-restructured magnet in hot/ humid atmosphere, acidic and saline environments. Due to the low electrode potential and active chemical property, the intergranular Nd-rich phase can be preferentially corroded in corrosive environments, leading to the degradation of the matrix grains finally. Consequently, the magnets exhibit mass loss in hot/humid atmosphere and release hydrogen in acidic solutions [14]. In saline solution, the intergranular Nd-rich phase acts as the anode and Nd2Fe14B phase acts as the cathode. From the polarization curves in Fig. 1c, it can be seen that the initial anode side of the polarization curves is contributed by active dissolution reactions and
Fig. 1. Corrosion behaviors of the starting magnet, un-restructured magnet, and restructured magnets in different corrosive environments. Mass loss in 120 °C, 2 bar and 100% relative humid atmosphere (a), volume of released hydrogen in 0.1 M H2SO4 aqueous solution for 1800 s under one bar pressure (b), and potentiokinetic polarization curves in 3.5% NaCl aqueous solutions (c).
the anodic current density increases quickly with the enhanced anode potential. Both the corrosion potential Ecorr and current density icorr can be calculated by Tafel slope extrapolation method [18]. More positive Ecorr and lower icorr indicate much better electrochemical stability of the magnets [14]. The REC of our starting magnet is 28.00 wt%, just 1.33 wt% higher than 26.67 wt% for the stoichiometric Nd2Fe14B. Although it exhibits good corrosion resistance due to the low REC, the
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Fig. 2. Demagnetization curves of the starting magnet, un-restructured magnet and restructured magnets with 1 wt%, 2 wt% and 3 wt% Nd6Co13Cu additions.
magnetic properties are quite poor with Br ¼13.97 kGs, Hcj ¼10.62 kOe and (BH)max ¼43.12 MGOe, respectively (Fig. 2). It is because such low rare earth content cannot supply sufficient liquid phase during sintering, leading to low magnet density (7.42 g/cm3) and discontinuous Nd-rich intergranular phase (Fig. 3a). Directly increasing REC to 28.46 wt% (the un-restructured magnet), promoted density of 7.54 g/cm3 and improved magnetic properties with Br ¼13.85 kGs, Hcj ¼ 11.24 kOe and (BH)max ¼ 46.38 MGOe can be achieved accordingly. However, corrosion resistance deteriorates severely due to the enlarged volume fraction of low electrode potential RE-rich phase (Fig. 3b). The mass loss after exposing for 96 h in hot/humid atmosphere increases drastically from 0.23 to 1.06 mg/cm2, volume of released hydrogen in 0.1 M H2SO4 solution for 1800 s rises from 12.14 to 29.42 ml, and Ecorr in 3.5% NaCl solution decreases from 0.691 to 0.711 V, respectively. Meanwhile, for the 2 wt% Nd6Co13Cu restructured magnet with equivalent REC, much better corrosion resistance in the hot/humid atmosphere, acidic and saline environments is obtained, where the corresponding mass loss, volume of released hydrogen and Ecorr are 0.28 mg/cm2, 18.52 ml and 0.705 V, respectively. Supposing that they contain equivalent RE-rich phase volume fraction, it is inferred that the remarkable improvement of corrosion resistance could be ascribed to the electrode potential change of the RE-rich phase. Besides, magnetic properties of the restructured magnet are also superior to the un-restructured one with Br ¼14.03 kGs, Hcj ¼12.62 kOe, and (BH)max ¼ 48.55 MGOe (Fig. 2). Among the restructured magnets, although there is a slight deterioration in corrosion resistance when 3 wt% Nd6Co13Cu is added, it still shows much better corrosion resistance in different environments and superior magnetic properties than the un-restructured one. In comparison with our previously restructured magnets with 3 wt% Nd6Fe13Cu addition, where the mass loss is 0.60 mg/cm2 [17], the Nd6Co13Cu restructured magnet possesses much lower mass loss of only 0.30 mg/cm2, further reducing by 50%. In addition, the corresponding magnetic properties are also superior, especially the coercivity reaches 11.35 kOe, 25.6% higher than 9.04 kOe for the Nd6Fe13Cu case. As proved by many previous literatures [4–8], the large potential difference between the Nd-rich phase and the matrix leads to the preferentially dissolution of Nd-rich phase in corrosive environments. The reduced REC in 1 wt% Nd6Co13Cu restructured magnet does bring continuously improvement in corrosion resistance, where the mass loss and volume of released hydrogen further reduces to 0.25 mg/cm2, and 16.52 ml, respectively. To understand the superior corrosion resistance of the restructured
Fig. 3. SEM-back scattered images of the starting magnet (a), un-restructured magnet (b), and magnet with 2 wt% Nd6Co13Cu addition (c). The dark contrast refers to the matrix 2:14:1 phase, the gray corresponds to the intergranular RE-rich phase.
magnets, we focus in the following on the electrode potential change of the intergranular phase. The chemical composition analysis by EDS in Fig. 4 show that Co and Cu, both of which have higher electrode potential than Nd and Fe, dissolve into the RErich phase. Co is detected in both the intergranular and the matrix regions. Comparably, Co is richer in the center of the intergranular region than the matrix phase due to the limited sintering and
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Fig. 4. Mapping of Nd, Fe and Co elements in the selected area for the magnet with 3 wt% Nd6Co13Cu (a). (Prþ Nd), Fe, Co, and Cu distributions across the Nd2Fe14B matrix and triple junction phase, as indicated by the red line in a (b). The dash lines in b distinguish the matrix from triple junction phase schematically. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
annealing time for diffusion. This is beneficial to enhance the electrode potential of the intergranular region. Different from Co, Cu is enriched in the interface between the matrix and intergranular phases but excluded in the matrix due to the low solubility of Cu in the 2:14:1 phase, which is in agreement with Sagawa and Hono's work [19,20]. The higher concentration of Cu at the interface is ascribed to the diffusion of Co into the matrix from the interface region. The high electrode potential Cu-rich interface layers can avoid the direct contact between the matrix and intergranular phase, thus decreasing the corrosion driving force and improving the corrosion resistance. The distributions of (Pr, Nd), Fe, Co and Cu across the matrix phase and the triple junction
phase are shown in Fig. 4b. In the center of the triple junction region, the composition is approximately to be 30 at% (Pr, Nd), 65 at% (Fe, Co) and 5 at% Cu, which is consistent with that of Nd6Co13Cu. Therefore, the existence of high electrode potential Nd6Co13Cu in the triple junctions companied with the Cu-rich layers at the interface can effectively decrease the driving force for electrochemical corrosion. The improved magnetic properties for the restructured magnets can be attributed mainly to the change in morphology. As proved, the continuous grain boundary phase isolating the ferromagnetic grains is a prerequisite for high coercivity of magnets due to the decoupling effect and the reduced defects [21,22]. In the
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To further understand the morphology change and Co distribution, the influences of Nd6Co13Cu addition on the sintering process should be stressed. It has been reported that during liquidphase sintering, a transition from partial wetting to complete wetting of solid Nd2Fe14B particles occurs with increasing temperature [27]. The Nd-rich liquid phase starts to wet the Nd2Fe14B phase grains at 690 710 °C, a little higher than the eutectic temperature 655 °C for the three-phases Nd2Fe14B, Nd1.1Fe4B4, and liquid Nd-rich, and reaches a complete wetting at 1150 710 °C [28]. In Fig. 5b, the DSC measurement reveals that Nd6Co13Cu alloy starts to melt at 605 °C, which is lower than the melting temperature of Nd-rich phase. Then, the addition of Nd6Co13Cu reduces the starting wetting temperature when compared to the unstructured magnet. Besides, addition of Nd6Co13Cu also enlarges the volume fraction of liquid phase during sintering, increasing the direct contact area between grain boundary phases and the matrix grains. Therefore, better wettability between the intergranular RErich and the 2:14:1 phases can be achieved, enabling the formation of continuous grain boundary phase and promoted densification. It is also noted that the wider temperature scope from the liquid phase precipitation to the sintering allows the diffusion of partial Co atoms towards the Nd2Fe14B phase. In our previous work [17,29], Nd–Fe–Cu and Dy–Ni, which start to melt at 506 °C and 693 °C respectively, have also been introduced into near-stoichiometric (Pr, Nd)2Fe14B magnets to improve the corrosion resistance and magnetic performance efficiently. This work further demonstrates that introducing RE-contained alloys with melting point below or close to the starting wetting temperature can be a distinct approach to prepare high performance magnets with low rare earth content.
4. Conclusions
Fig. 5. DSC curves for the restructured magnets with different Nd6Co13Cu additions (curves from the bottom to the top refer to the starting magnet, un-restructured magnet, 1 wt%, 2 wt% , and 3 wt% Nd6Co13Cu added magnets respectively) (a) and for the alloy Nd6Co13Cu (b). Each curve is measured upon heating at 10 °C/min. Curie transition temperature TC is indicated by arrows.
Nd6Co13Cu restructured magnets, continuous grain boundary phase are formed between the neighboring 2:14:1 phase grains in Fig. 3c. It is also noted that, in Fig. 4a, Co partially enters the matrix phase, which will change the intrinsic magnetic properties. It is evident that the Curie temperature TC increases gradually from 307.9 to 314.6 °C (Fig. 5a) when the addition amount increases from 0 to 3 wt%. This may be due to the strengthened interaction of the 3d sub-lattice by Co substituting for Fe [23–25]. It is also reported that moderate replacement of Co for Fe (below 10 at%) in the 2:14:1 phase can slightly increase its saturation magnetization Ms because of the electron transfer from Co to Fe atoms, filling primarily the 3d up-band and raising the total magnetic moment accordingly [26]. For the magnet with 3 wt% Nd6Co13Cu addition, the EDS results (Fig. 4) show that the content of Co at the interface between the matrix phase and grain boundary phase is about 7 at%, which is less than 10 at%, indicating that saturation magnetization of the 2:14:1 phase will be increased. Therefore, Br can well be maintained in the Nd6Co13Cu restructured magnets in addition to the promoted densification. However, Hcj starts to decrease in 3 wt% Nd6Co13Cu restructured magnet due to the decline of magnetic anisotropy field HA caused by the partial substitution of Co for Fe in the 2:14:1 matrix phase.
Corrosion resistance of sintered Nd–Fe–B magnets in hot/humid atmosphere, acidic and saline environments is significantly improved by Nd6Co13Cu grain boundary restructuring. The improved corrosion resistance by slight introducing Nd6Co13Cu into near-stoichiometric Nd2Fe14B magnets can be attributed to the reduced rare earth-rich phase volume fraction and the formation of more stable intergranular phase. Besides, magnetic properties are well sustained owing to the formation of continuous and smooth grain boundary phase as well as the partial replacement of Fe by Co in the matrix phase. Therefore, grain boundary restructuring is a promising way to develop corrosion-resistant, high magnetic performance Nd–Fe–B magnets with reduced rare earth consumption.
Acknowledgments This work was supported by the National Natural Science Foundations of China (51171169 and 51401180) and the China Postdoctoral Science Foundation funded project (2014M561755).
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Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Improved corrosion resistance of low rare-earth Nd–Fe–B sintered magnets by Nd6Co13Cu grain boundary restructuring Pei Zhang, Tianyu Ma n, Liping Liang, Xiaolian Liu, Xuejiao Wang, Jiaying Jin, Yujing Zhang, Mi Yan n Department of Materials Science and Engineering, State Key Laboratory of Silicon Materials, Key Laboratory of Novel Materials for Information Technology of Zhejiang Province, Zhejiang University, Hangzhou 310027, China
art ic l e i nf o
a b s t r a c t
Article history: Received 21 July 2014 Received in revised form 10 November 2014 Accepted 16 December 2014 Available online 17 December 2014
Aiming to improve the corrosion resistance of Nd–Fe–B sintered magnets, Nd6Co13Cu powders are introduced into a near-stoichiometric Nd2Fe14B magnet by grain boundary restructuring. The restructured magnet containing only 28.46 wt% (Pr, Nd) shows much better corrosion resistance against hot/humid, acidic and saline environments than the un-restructured magnet with equivalent rare earth content (REC). It results mainly from the formation of higher electrode potential Co-rich phase and Cu-rich interface layers in the intergranular regions. Besides, magnetic properties of the restructured magnet are well retained with coercivity Hcj above 12.5 kOe, remanence Br above 14.0 kGs and maximum magnetic energy product (BH)max above 48.5 MGOe respectively. It can be ascribed to the formation of more continuous grain boundary phase that isolates well the neighboring Nd2Fe14B grains. & 2014 Published by Elsevier B.V.
Keywords: Permanent magnet Microstructure Magnetic property Corrosion
1. Introduction Since discovered in 1984, Nd–Fe–B sintered magnets have drawn extensive research interests due to their outstanding magnetic performance [1–3]. However, they exhibit high susceptibility to corrosion attack, limiting their applications in corrosive environments. The poor corrosion resistance is originated from the multiple-phase morphology, where the standard electrode potential of Nd-rich intergranular phase is much lower than the Nd2Fe14B matrix, being the main driving force for electrochemical corrosion [4–8]. Therefore, many efforts have been taken to improve the corrosion resistance of sintered Nd–Fe–B magnets. Normally, reducing the volume fraction of Nd–rich phase by decreasing the rare earth content (REC) is effective to improve the corrosion resistance [9–12]. Unfortunately, the near-stoichiometric magnets cannot reach full densification because of the insufficient liquid-phase during sintering, thus leading to poor magnetic performance. Alternatively, minimizing the difference of electrode potential between the matrix phase and Nd-rich intergranular phase by either intergranular addition or grain boundary restructuring is proved to be more feasible for improving corrosion resistance without declining magnetic properties [5,13–17]. Through introducing extra elements, they facilitate the formation n
Corresponding authors. Fax: þ 86 571 87952366. E-mail addresses: [email protected] (T. Ma), [email protected] (M. Yan).
http://dx.doi.org/10.1016/j.jmmm.2014.12.044 0304-8853/& 2014 Published by Elsevier B.V.
of more stable intergranular phase with higher electrode potentials. Generally, intergranular adding rare-earth-free additives, such as Cu, Ta, MgO, SiO2 etc., always requires a high REC above 30 wt% in the starting magnet [13–16], providing sufficient Ndrich phase to ensure magnet densification. Differently, utilizing the recently developed grain boundary restructuring approach, adding high potential Nd6Fe13Cu into a slightly off-stoichiometric Nd2Fe14B-type (2:14:1 phase) alloy can improve corrosion resistance in low-rare-earth Nd–Fe–B magnets [17]. This approach is capable of improving wettability with the 2:14:1 phase grains, changing morphologies and chemical composition of the intergranular regions, and enhancing the Nd-rich phase stability, resulting in better corrosion resistance and reduction in REC simultaneously. In our previous work [17], the restructured magnets by introducing 3–12 wt% Nd6Fe13Cu into (Pr, Nd)28.30FebalB1.03 (wt%), still contain a relatively high volume fraction of rare-earthrich (RE-rich) intergranular phase. Therefore, here we investigated the slight introducing effects on the corrosion resistance and magnetic performance by a similar alloy Nd6Co13Cu (1–3 wt%) to further reduce the volume fraction of RE-rich phase as well as the REC. Besides, since the standard electrode potential of Co ( 0.277 V) is higher than both Fe (0.440 V) and Nd ( 2.431 V) [5], further decrease in the electrode potential difference is also expected. More comprehensive corrosion resistance against hot/humid, acidic and saline environments is achieved by employing only 2 wt% Nd6Co13Cu, where the rare-earth consumption is 28.46 wt%. Well retained
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magnetic properties with Br ¼14.03 kGs, Hcj ¼ 12.62 kOe, and (BH)max ¼48.55 MGOe are obtained simultaneously.
2. Experiments Starting magnet with near-stoichiometric composition of (Pr, Nd)28.00FebalB1.03 (wt%) was prepared by vacuum induction melting of Fe (purity 99.95%), Pr–Nd alloy containing 20 at% Pr and Fe– B alloy containing 20 at% B under the protection of high-purity argon atmosphere. The ingots were subsequently subjected to strip casting with a copper wheel. The flakes were pulverized into 4 μm powders by hydrogen decrepitated (HD) and jet-milling processes. The Nd6Co13Cu alloy was prepared by arc melting for 5 times to ensure homogenization in a vacuum furnace. The alloy ingots were crushed and ball-milled into fine powders with an average particle size of 1.7 μm. The two kinds of powders were then mixed under the protection of nitrogen. Afterward, the mixed powders were aligned in a magnetic field of 1.8 T under 6 MPa, followed by isostatic compressing under 200 MPa. The green compacts were sintered for 4 h at 1075 °C, followed by a two-step annealing treatment, which was performed for 2 h at 890 °C and for 3.5 h at 500 °C, respectively. For comparison, a magnet (Pr, Nd)28.46FebalB1.03 with equivalent REC (thereafter, defined as the un-restructured magnet) to the 2 wt% Nd6Co13Cu added magnet was prepared by the same liquid-phase sintering processing. Magnet density was determined according to the Archimedes principal. Thermal analysis of the Nd6Co13Cu alloy and the restructured magnets was conducted by the differential scanning calorimetry (DSC) upon heating at 10 K/min. The chemical compositions of final magnets were determined by the inductively coupled plasma (ICP) analysis. Accelerated corrosion test was performed by placing cubic samples (10 10 10 mm3) in 120 °C, 2 bar and 100% relative humid atmosphere. Polarization curves were measured with a CHI604B electrochemistry analyzer to characterize the electrochemical stability. All experiments were polished with 1000# sandpapers and performed in a standard three electrode cell consisting of Nd–Fe–B working electrode (10 10 mm2), saturated calomel reference electrode and Pt counter electrode. Each measurement was conducted at 25 70.1 °C in a 3.5% NaCl aqueous solution. Sulfuric acid corrosion test was performed to characterize the chemical stability of intergranular phase. The samples (10 10 10 mm3) were grinded by 400# sandpaper and immersed in 0.1 M H2SO4 aqueous solution for 1800 s. The amount of released hydrogen was measured by a universal gas flowmeter (ADM 2000) under one bar pressure. Magnetic properties were measured using an AMT-4 magnetic measurement device. Microstructures were observed under a field-emission scanning electron microscopy (FE-SEM SIRION-100) equipped with an energy dispersive X-ray spectroscopy (EDS).
3. Results and discussion Fig. 1 reveals the different corrosion properties of the starting magnet, restructured magnets and un-restructured magnet in hot/ humid atmosphere, acidic and saline environments. Due to the low electrode potential and active chemical property, the intergranular Nd-rich phase can be preferentially corroded in corrosive environments, leading to the degradation of the matrix grains finally. Consequently, the magnets exhibit mass loss in hot/humid atmosphere and release hydrogen in acidic solutions [14]. In saline solution, the intergranular Nd-rich phase acts as the anode and Nd2Fe14B phase acts as the cathode. From the polarization curves in Fig. 1c, it can be seen that the initial anode side of the polarization curves is contributed by active dissolution reactions and
Fig. 1. Corrosion behaviors of the starting magnet, un-restructured magnet, and restructured magnets in different corrosive environments. Mass loss in 120 °C, 2 bar and 100% relative humid atmosphere (a), volume of released hydrogen in 0.1 M H2SO4 aqueous solution for 1800 s under one bar pressure (b), and potentiokinetic polarization curves in 3.5% NaCl aqueous solutions (c).
the anodic current density increases quickly with the enhanced anode potential. Both the corrosion potential Ecorr and current density icorr can be calculated by Tafel slope extrapolation method [18]. More positive Ecorr and lower icorr indicate much better electrochemical stability of the magnets [14]. The REC of our starting magnet is 28.00 wt%, just 1.33 wt% higher than 26.67 wt% for the stoichiometric Nd2Fe14B. Although it exhibits good corrosion resistance due to the low REC, the
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Fig. 2. Demagnetization curves of the starting magnet, un-restructured magnet and restructured magnets with 1 wt%, 2 wt% and 3 wt% Nd6Co13Cu additions.
magnetic properties are quite poor with Br ¼13.97 kGs, Hcj ¼10.62 kOe and (BH)max ¼43.12 MGOe, respectively (Fig. 2). It is because such low rare earth content cannot supply sufficient liquid phase during sintering, leading to low magnet density (7.42 g/cm3) and discontinuous Nd-rich intergranular phase (Fig. 3a). Directly increasing REC to 28.46 wt% (the un-restructured magnet), promoted density of 7.54 g/cm3 and improved magnetic properties with Br ¼13.85 kGs, Hcj ¼ 11.24 kOe and (BH)max ¼ 46.38 MGOe can be achieved accordingly. However, corrosion resistance deteriorates severely due to the enlarged volume fraction of low electrode potential RE-rich phase (Fig. 3b). The mass loss after exposing for 96 h in hot/humid atmosphere increases drastically from 0.23 to 1.06 mg/cm2, volume of released hydrogen in 0.1 M H2SO4 solution for 1800 s rises from 12.14 to 29.42 ml, and Ecorr in 3.5% NaCl solution decreases from 0.691 to 0.711 V, respectively. Meanwhile, for the 2 wt% Nd6Co13Cu restructured magnet with equivalent REC, much better corrosion resistance in the hot/humid atmosphere, acidic and saline environments is obtained, where the corresponding mass loss, volume of released hydrogen and Ecorr are 0.28 mg/cm2, 18.52 ml and 0.705 V, respectively. Supposing that they contain equivalent RE-rich phase volume fraction, it is inferred that the remarkable improvement of corrosion resistance could be ascribed to the electrode potential change of the RE-rich phase. Besides, magnetic properties of the restructured magnet are also superior to the un-restructured one with Br ¼14.03 kGs, Hcj ¼12.62 kOe, and (BH)max ¼ 48.55 MGOe (Fig. 2). Among the restructured magnets, although there is a slight deterioration in corrosion resistance when 3 wt% Nd6Co13Cu is added, it still shows much better corrosion resistance in different environments and superior magnetic properties than the un-restructured one. In comparison with our previously restructured magnets with 3 wt% Nd6Fe13Cu addition, where the mass loss is 0.60 mg/cm2 [17], the Nd6Co13Cu restructured magnet possesses much lower mass loss of only 0.30 mg/cm2, further reducing by 50%. In addition, the corresponding magnetic properties are also superior, especially the coercivity reaches 11.35 kOe, 25.6% higher than 9.04 kOe for the Nd6Fe13Cu case. As proved by many previous literatures [4–8], the large potential difference between the Nd-rich phase and the matrix leads to the preferentially dissolution of Nd-rich phase in corrosive environments. The reduced REC in 1 wt% Nd6Co13Cu restructured magnet does bring continuously improvement in corrosion resistance, where the mass loss and volume of released hydrogen further reduces to 0.25 mg/cm2, and 16.52 ml, respectively. To understand the superior corrosion resistance of the restructured
Fig. 3. SEM-back scattered images of the starting magnet (a), un-restructured magnet (b), and magnet with 2 wt% Nd6Co13Cu addition (c). The dark contrast refers to the matrix 2:14:1 phase, the gray corresponds to the intergranular RE-rich phase.
magnets, we focus in the following on the electrode potential change of the intergranular phase. The chemical composition analysis by EDS in Fig. 4 show that Co and Cu, both of which have higher electrode potential than Nd and Fe, dissolve into the RErich phase. Co is detected in both the intergranular and the matrix regions. Comparably, Co is richer in the center of the intergranular region than the matrix phase due to the limited sintering and
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Fig. 4. Mapping of Nd, Fe and Co elements in the selected area for the magnet with 3 wt% Nd6Co13Cu (a). (Prþ Nd), Fe, Co, and Cu distributions across the Nd2Fe14B matrix and triple junction phase, as indicated by the red line in a (b). The dash lines in b distinguish the matrix from triple junction phase schematically. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
annealing time for diffusion. This is beneficial to enhance the electrode potential of the intergranular region. Different from Co, Cu is enriched in the interface between the matrix and intergranular phases but excluded in the matrix due to the low solubility of Cu in the 2:14:1 phase, which is in agreement with Sagawa and Hono's work [19,20]. The higher concentration of Cu at the interface is ascribed to the diffusion of Co into the matrix from the interface region. The high electrode potential Cu-rich interface layers can avoid the direct contact between the matrix and intergranular phase, thus decreasing the corrosion driving force and improving the corrosion resistance. The distributions of (Pr, Nd), Fe, Co and Cu across the matrix phase and the triple junction
phase are shown in Fig. 4b. In the center of the triple junction region, the composition is approximately to be 30 at% (Pr, Nd), 65 at% (Fe, Co) and 5 at% Cu, which is consistent with that of Nd6Co13Cu. Therefore, the existence of high electrode potential Nd6Co13Cu in the triple junctions companied with the Cu-rich layers at the interface can effectively decrease the driving force for electrochemical corrosion. The improved magnetic properties for the restructured magnets can be attributed mainly to the change in morphology. As proved, the continuous grain boundary phase isolating the ferromagnetic grains is a prerequisite for high coercivity of magnets due to the decoupling effect and the reduced defects [21,22]. In the
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To further understand the morphology change and Co distribution, the influences of Nd6Co13Cu addition on the sintering process should be stressed. It has been reported that during liquidphase sintering, a transition from partial wetting to complete wetting of solid Nd2Fe14B particles occurs with increasing temperature [27]. The Nd-rich liquid phase starts to wet the Nd2Fe14B phase grains at 690 710 °C, a little higher than the eutectic temperature 655 °C for the three-phases Nd2Fe14B, Nd1.1Fe4B4, and liquid Nd-rich, and reaches a complete wetting at 1150 710 °C [28]. In Fig. 5b, the DSC measurement reveals that Nd6Co13Cu alloy starts to melt at 605 °C, which is lower than the melting temperature of Nd-rich phase. Then, the addition of Nd6Co13Cu reduces the starting wetting temperature when compared to the unstructured magnet. Besides, addition of Nd6Co13Cu also enlarges the volume fraction of liquid phase during sintering, increasing the direct contact area between grain boundary phases and the matrix grains. Therefore, better wettability between the intergranular RErich and the 2:14:1 phases can be achieved, enabling the formation of continuous grain boundary phase and promoted densification. It is also noted that the wider temperature scope from the liquid phase precipitation to the sintering allows the diffusion of partial Co atoms towards the Nd2Fe14B phase. In our previous work [17,29], Nd–Fe–Cu and Dy–Ni, which start to melt at 506 °C and 693 °C respectively, have also been introduced into near-stoichiometric (Pr, Nd)2Fe14B magnets to improve the corrosion resistance and magnetic performance efficiently. This work further demonstrates that introducing RE-contained alloys with melting point below or close to the starting wetting temperature can be a distinct approach to prepare high performance magnets with low rare earth content.
4. Conclusions
Fig. 5. DSC curves for the restructured magnets with different Nd6Co13Cu additions (curves from the bottom to the top refer to the starting magnet, un-restructured magnet, 1 wt%, 2 wt% , and 3 wt% Nd6Co13Cu added magnets respectively) (a) and for the alloy Nd6Co13Cu (b). Each curve is measured upon heating at 10 °C/min. Curie transition temperature TC is indicated by arrows.
Nd6Co13Cu restructured magnets, continuous grain boundary phase are formed between the neighboring 2:14:1 phase grains in Fig. 3c. It is also noted that, in Fig. 4a, Co partially enters the matrix phase, which will change the intrinsic magnetic properties. It is evident that the Curie temperature TC increases gradually from 307.9 to 314.6 °C (Fig. 5a) when the addition amount increases from 0 to 3 wt%. This may be due to the strengthened interaction of the 3d sub-lattice by Co substituting for Fe [23–25]. It is also reported that moderate replacement of Co for Fe (below 10 at%) in the 2:14:1 phase can slightly increase its saturation magnetization Ms because of the electron transfer from Co to Fe atoms, filling primarily the 3d up-band and raising the total magnetic moment accordingly [26]. For the magnet with 3 wt% Nd6Co13Cu addition, the EDS results (Fig. 4) show that the content of Co at the interface between the matrix phase and grain boundary phase is about 7 at%, which is less than 10 at%, indicating that saturation magnetization of the 2:14:1 phase will be increased. Therefore, Br can well be maintained in the Nd6Co13Cu restructured magnets in addition to the promoted densification. However, Hcj starts to decrease in 3 wt% Nd6Co13Cu restructured magnet due to the decline of magnetic anisotropy field HA caused by the partial substitution of Co for Fe in the 2:14:1 matrix phase.
Corrosion resistance of sintered Nd–Fe–B magnets in hot/humid atmosphere, acidic and saline environments is significantly improved by Nd6Co13Cu grain boundary restructuring. The improved corrosion resistance by slight introducing Nd6Co13Cu into near-stoichiometric Nd2Fe14B magnets can be attributed to the reduced rare earth-rich phase volume fraction and the formation of more stable intergranular phase. Besides, magnetic properties are well sustained owing to the formation of continuous and smooth grain boundary phase as well as the partial replacement of Fe by Co in the matrix phase. Therefore, grain boundary restructuring is a promising way to develop corrosion-resistant, high magnetic performance Nd–Fe–B magnets with reduced rare earth consumption.
Acknowledgments This work was supported by the National Natural Science Foundations of China (51171169 and 51401180) and the China Postdoctoral Science Foundation funded project (2014M561755).
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