Effects of Ag on the magnetic and mechanical properties of sintered NdFeB permanent magnets

Effects of Ag on the magnetic and mechanical properties of sintered NdFeB permanent magnets

Journal of Magnetism and Magnetic Materials 485 (2019) 49–53 Contents lists available at ScienceDirect Journal of Magnetism and Magnetic Materials j...

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Journal of Magnetism and Magnetic Materials 485 (2019) 49–53

Contents lists available at ScienceDirect

Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm

Research articles

Effects of Ag on the magnetic and mechanical properties of sintered NdFeB permanent magnets

T



H. Chena, , X. Yanga, L. Sunb, P. Yub, X. Zhangb, L. Luob a b

Department of Mechanical, Materials and Manufacturing Engineering, Faculty of Science and Engineering, University of Nottingham Ningbo China, Ningbo 315100, China Ningbo Hesheng Magnetics Co., Ltd, Ningbo 315300, China

A R T I C LE I N FO

A B S T R A C T

Keywords: NdFeB permanent magnets Magnetic properties Mechanical properties Alloy elements Impact toughness

This paper investigates the effects of Ag on the magnetic and mechanical properties of sintered NdFeB permanent magnets. The microstructure of the NdFeB permanent magnets was characterised using scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS) analysis. 0.1 wt% of Ag was added to the overall NdFeB composition to study the effects of Ag on the microstructural changes. The NdFeB and NdFeB + Ag magnets exhibit a two-phase structure with the main Nd2Fe14B phase and the Nd-rich grain boundary phase. It is found that Ag is enriched in the Nd-rich grain boundary phase and tends to alloy with the minor element, Cu. An enhanced coercivity (Hcj) is obtained in the NdFeB + Ag whilst the remanence (Br) and maximum magnetic energy product ((BH)max) decrease due to the addition of 0.1 wt% Ag. Impact resistance tests were carried out to examine the impact toughness of NdFeB and NdFeB + Ag magnets. It is shown that the magnets still exhibit a brittle fracture behaviour and no significant increase in the impact resistance of the NdFeB + Ag magnet is presented.

1. Introduction Sintered NdFeB permanent magnets are widely used in aerospace, wind turbine and automotive industry due to their excellent magnetic properties [1–3]. In order to meet the demands of various applications, considerable efforts have been made to improve the magnetic properties of NdFeB magnets [4–7]. Work concerned on this aspect has been extensively reported, ranging from composition control to process manipulation [8–11]. Among the reported studies, one of the common and effective methods to increase the magnetic properties of NdFeB magnets is by adding the rare earth and non-rare earth elements to the NdFeB base alloy. The effects of rare earth and non-rare earth elements, such as Dy, Tb, Ce, Gd, Pr, Co, Ti, Al, Nb, etc., on the microstructure and magnetic properties have been reported [12–19]. The diffusion of the minor elements, like Dy, Tb, Ce and Gd, can facilitate the formation of the core-shell structure, enhancing the magnetic properties of NdFeB magnets [19–21], whilst the addition of other elements can also influence the magnetic and microstructural properties [22,23]. However, despite the improvements that have been made in the magnetic properties of NdFeB magnets, one of the major drawbacks which restrict their broad applications is the inherent brittleness of NdFeB magnets [24]. Generally, the NdFeB magnets exhibit a two-phase structure, ⁎

consisting of the main Nd2Fe14B phase and the Nd-rich grain boundary phase. It was reported that the Nd-rich phase exhibits lower strength and higher plasticity than the main Nd2Fe14B phase [24]. Since the Nd2Fe14B grains are large and brittle in nature, the failure mechanism of the NdFeB magnets usually follows the intergranular fracture mode. Fast fracture of NdFeB magnets may occur during the industrial applications, due to their low toughness. Hitherto, various attempts have been made to modify the mechanical properties of NdFeB magnets to expand their potential applications [23–26]. It is found by Liu et al. that the fracture toughness of sintered NdFeB magnets can be improved by increasing the volume fraction of the Nd-rich phase through adding small amounts of Al, Ga, Cu and Nb [27]. It is also noted by Wang et al. that the impact resistance of NdFeB magnets increases with Nd and Dy content but decreases with Pr content [9]. The above elements have been demonstrated to be helpful in modifying the mechanical properties of NdFeB magnets. Furthermore, it was proposed by Zhang et al., that the addition of Ag can also enhance the toughness of NdFeB magnets [28]. But the detailed effects of Ag on the magnetic and mechanical properties of NdFeB magnets do not appear to have been previously reported. Since the failure of the NdFeB magnets is usually caused by impact, it is worth investigating the effects of Ag on the microstructure, magnetic properties and impact stability of sintered NdFeB magnets, in an

Corresponding author. E-mail address: [email protected] (H. Chen).

https://doi.org/10.1016/j.jmmm.2019.04.071 Received 24 February 2019; Received in revised form 5 April 2019; Accepted 18 April 2019 Available online 19 April 2019 0304-8853/ © 2019 Elsevier B.V. All rights reserved.

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Table 1 Composition of the reference NdFeB magnet, commercially known as N45, and the NdFeB + Ag magnet. Elements (wt%)

NdFeB (N45) NdFeB + Ag

Fe

NdPr (Nd:Pr = 80:20)

B

Gd

Zr

Al

Cu

Co

Ag

Bal. Bal.

29.7% 29.7%

0.99% 0.99%

1.50% 1.50%

0.10% 0.10%

0.35% 0.35%

0.15% 0.15%

0.30% 0.30%

— 0.10%

attempt to improve the impact resistance and machinability of the NdFeB magnets. Therefore, the aim of this paper is to investigate the effects of Ag on the magnetic and mechanical properties of NdFeB magnets. 0.1 wt% Ag is selected here as this composition was reported to be effective in improving the toughness of NdFeB magnets. It is also of interest to see how the Ag element is distributed within the structure of NdFeB magnet. Thus, a relatively high Ag content (0.1 wt%) is investigated in this work. A commercially available sintered NdFeB composition (N45) is employed as the reference material. The work reported here aims at revealing the role of Ag in the magnetic and mechanical properties of sintered NdFeB magnets.

αk =

W A

(1)

where W is the energy absorbed by the impact in kJ, A is the crosssectional area of the specimen in m2 and αk is the impact toughness in kJ/m2. At least three samples were tested for each material and an average absorbed energy was used to calculate the impact toughness. Vickers hardness testing was performed in a Struers micro-hardness tester (DuraVision) using 50 kgf to measure the hardness of the magnets.

3. Results and discussion 2. Experimental procedures

3.1. Microstructural characterisation

The commercially available NdFeB magnet (known as N45, 45 represents the maximum magnetic energy product, (BH)max, of the NdFeB magnet is around 45 MGOe) was obtained from Ningbo Hesheng Magnetics Co., Ltd as the reference material. The reference NdFeB magnet was manufactured by the conventional powder metallurgy process with a nominal composition summarised in Table 1. A double alloy of NdPr was used instead of pure Nd since the NdPr has been approved to be a cost effective approach to achieve enhanced magnetic properties through industrial practice. To elucidate the effects of Ag on the magnetic and mechanical properties of the above NdFeB, 0.1 wt% Ag was added as the raw ingot to the overall composition prior to the magnet manufacturing process. The NdFeB with addition of Ag was named as NdFeB + Ag onwards to compare with the reference NdFeB magnet. Both alloys underwent exactly the same manufacturing processes. The casted alloy ribbons were produced by vacuum melt spinning process, followed by hydrogen decrepitation to obtain the coarse powders at a particle size range of 1–3 mm. The coarse powders were jet-milled in nitrogen gas at a pressure of 0.5–0.6 MPa to achieve an average powder particle size of 5.0–5.3 µm. Jet-milled powders were pressed and simultaneously aligned in a magnetic field of 1.5–2 T, then isostatically compacted in a hydraulic press machine under a pressure of 150–240 MPa for 50 s. The green compacts were sintered at 1000–1100 °C for 4–6 h, followed by annealing at 700–1000 °C for 2–3 h and 400–600 °C for 3–5 h. The magnetic properties of sintered magnets were measured by a hysteresis loop tracer (Electrctro-Magnet 264Y). The microstructures of sintered NdFeB and NdFeB + Ag were examined in a Zeiss field emission scanning electron microscope (SEM) operated at 20 kV using secondary electron (SE) and backscattered electron (BSE) imaging. Semi-quantitative energy dispersive X-ray spectroscopy (EDS) was employed to qualitatively analyse the phase compositions. Image analysis was performed in ImageJ [29] to measure the size of the Nd2Fe14B grains and the volume fraction of the Nd-rich phase. The impact resistance of the NdFeB and NdFeB + Ag magnets was measured by the TecQuipment impact test machine (model no. TE15). The impact specimens, 3.2 mm in diameter and 38 mm in length, were electro-discharge machined from the sintered magnet blocks. The specimens were loaded in the above pendulum impact test apparatus and a pendulum with a weight of 10 kg was released from the rest position. The magnet specimens were impacted to fracture and the energy absorbed by the magnets was recorded. The impact toughness can be represented by Eq. (1),

The cross-sectional microstructure of the NdFeB and NdFeB + Ag is shown in Fig. 1. It can be seen that both materials exhibit a dual phase structure, comprising the dark contrast main phase, presumably the Nd2Fe14B phase, and the bright contrast Nd-rich phase. It is seen from Fig. 1c that there are many pits in the NdFeB + Ag magnet, and from the image it can be seen that these pits are most likely Nd-rich phase that were shed during the polishing of the sample. Thus, these pits are included in the image analysis to measure the volume fraction of the Nd-rich phase in the NdFeB + Ag magnet. It is found that the volume fraction of the Nd-rich phase is around 10.6% and 8.8% for the NdFeB and NdFeB + Ag respectively. This indicates that the addition of Ag in the NdFeB would not result in signification difference in the volume fraction of the Nd-rich phase. It is evident from Fig. 1 that no significant changes occurred in the morphology of the Nd-rich phase in the NdFeB + Ag magnet, except that the size of some discrete Nd-rich phase appears to be larger in Fig. 1d. The compositions of normal and coarse Nd-rich phase in the NdFeB + Ag are summarised in Table 2. The major elements in the normal Nd-rich phase are Nd and Pr since a double alloy NdPr was used as the raw material. It is known that the Nd2Fe14B phase is the main phase that exhibits the magnetic properties. The unique atomic structure of the Nd2Fe14B phase makes the Ag difficult to be enriched in it. Due to the low viscosity and high flowability of Ag in the molten stage, it is believed that the Ag is mainly enriched in the Nd-rich phase at the grain boundaries. It is noted from Table 2 that the coarse Nd-rich phase is enriched with Cu and Ag, which means that Ag tends to segregate with Cu in the Nd-rich grain boundary phase. It is reported that Ag has good ductility and wettability with many materials and the AgCu alloys are often used as eutectic filler metals to wet and join ceramic materials [30–32]. During the casting stage of the NdFeB + Ag magnet, it is possible that the formation of eutectic AgCu occurs within the grain boundary Nd-rich phase, due to its low melting point. This allows the coarse Nd-rich phase to form in the NdFeB + Ag magnet, as seen in Fig. 1d. It is seen that the addition of Ag has no effects on the structure of Nd2Fe14B phase and since the amount of elements used to prepare NdFeB and NdFeB + Ag magnets is almost the same except Ag, the volume fraction of Nd-rich phase should also be similar, only the distribution of Nd-rich phase might be different. The measured volume fraction of 10.6% and 8.8% for the NdFeB and NdFeB + Ag, which exhibits no significant difference, further supports this view.

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Fig. 1. Cross-sectional microstructural of sintered NdFeB (a and b) and NdFeB + Ag (c and d). Table 2 The EDS composition of normal and coarse Nd-rich phase in the NdFeB + Ag magnet, all in wt%.

Normal Nd-rich phase Coarse Nd-rich phase

Fe

Nd

Pr

Ag

Cu

26% 13%

57% 49%

17% 15%

— 13%

— 10%

3.2. Magnetic properties The magnetic properties of sintered NdFeB and NdFeB + Ag magnets are tabulated in Table 3 and the second quadrant demagnetisation curves are shown in Fig. 2. It should be noted that the same manufacturing process and parameters were used for both magnet samples. No efforts have been taken to optimise the processing parameter as it is one of the aims in this study to investigate the effects and feasibility of Ag through industrial practice. The NdFeB magnet without Ag is commercially known as N45 and its magnetic properties have been well established and characterised. As such, it is used as the reference sample to compare with NdFeB + Ag. It can be seen that the addition of Ag in the NdFeB + Ag causes the Br and (BH)max to decrease. Almost identical Hcb values in NdFeB and NdFeB + Ag are noted. But it is further noticed that the NdFeB + Ag exhibits a higher Hcj compared to the reference NdFeB. It is reported that the remanence (Br) and the maximum magnetic energy product ((BH)max) decrease significantly when the Ag content is above 0.1 wt% [28], which is consistent with Table 3 and Fig. 2. Since no detailed microstructures of NdFeB with Ag were reported previously, it is the first time in this work to report the microstructure of NdFeB + Ag. It was also reported that the small amounts of addition of Ag had nearly no effect on the coercivity of NdFeB magnets [28]. It is seen from Table 3 and Fig. 2 that the almost identical Hcb are obtained from NdFeB and NdFeB + Ag. However, an enhanced Hcj is achieved in the NdFeB + Ag. It is widely recognised

Fig. 2. Second quadrant demagnetisation plot of NdFeB and NdFeB + Ag.

that the core-shell structure around the Nd2Fe14B grains is very effective in enhancing the magnetic properties [33]. But no evidence of coreshell structure can be found in the NdFeB + Ag specimen in Fig. 1, indicating that the addition of Ag has limited effect on forming the coreshell structure. It is seen from Fig. 1 that the addition of Ag results in the formation of coarse Nd-rich phase with a small reduction in the volume fraction of the Nd-rich phase. Thus, an increase in the volume fraction of the Nd2Fe14B phase is expected, allowing an enhanced Hcj to be achieved since the magnetic properties are dominated by the Nd2Fe14B phase. Since Ag is a non-magnetic element, the addition of 0.1 wt% Ag is likely to cause a decrease in the remanence and maximum magnetic energy product. Similar findings have also been reported by others [16,17,22,23], in which the additions of non-magnetic elements like Zn, Nb and Al can increase the coercivity but may decrease the remanence and maximum magnetic energy product. In this work, to avoid significant loss in Br and (BH)max and to allow the Ag element to be traceable, 0.1 wt% Ag is examined as our first attempt to study the effects of Ag on the magnetic and subsequent impact properties of NdFeB magnets.

Table 3 Magnetic properties of sintered NdFeB and NdFeB + Ag. Specimen

Br/mT

Hcb/kA m−1

Hcj/kA m−1

(BH)max/kJ m−3

NdFeB NdFeB + Ag

1361 ± 9 1256 ± 16

980 ± 8 977 ± 26

1030 ± 9 1137 ± 9

359 ± 5 310 ± 15

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that the volume fraction of the Nd-rich phase slightly decreases due to the addition of Ag, indicating a possible deficiency in the ductility of NdFeB + Ag. The toughness of the NdFeB + Ag is believed to be smaller than the NdFeB. Since no significant difference is noted in the impact toughness between NdFeB and NdFeB + Ag in Fig. 3, it is proposed that the effect of Ag on improving the impact toughness is limited when Ag content is 0.1 wt%. 3.4. Fracture mechanism The fracture surfaces of NdFeB and NdFeB + Ag are shown in Fig. 4. The fracture surfaces are full of polygonal crystals that are presumably the Nd2Fe14B grains. It is seen that both magnets exhibit an intergranular fracture behaviour, in which the cracking occurs and propagates predominantly along the brittle Nd2Fe14B grain boundaries. This brittle fracture mechanism has been widely recognised in the NdFeB magnetic materials. From the fracture surfaces, it can be seen from Fig. 4a that the grain size of the NdFeB falls into a range of 7–8 µm, similar to that in NdFeB + Ag of Fig. 4b. No effects of Ag on the grain size refinement have been noticed. The evidence of the Nd-rich phase can be seen from the fracture surface as the bright contrast features in Fig. 4a and b. In the two-phase NdFeB magnets, the Nd2Fe14B matrix phase usually exhibit higher strength and more brittle than the Nd-rich phase. The less deformable Nd2Fe14B phase leads to the rapid brittle fracture of the NdFeB magnets. Even though the Nd-rich phase has better plasticity than the Nd2Fe14B phase, the plastic deformation that can be tolerated by the Nd-rich phase is quite limited due to the size of Nd2Fe14B phase is much larger than the Nd-rich phase. Therefore, the impact fracture occurs rapidly along the polygonal planes of the Nd2Fe14B phase, exhibiting a brittle cleavage fracture characteristic at the fracture surface. Since the volume fraction of the Nd-rich phase is almost the same between NdFeB and NdFeB + Ag magnets, a similar fracture mode is expected. It has also been demonstrated from Fig. 3 and Table 4 that the Ag has limited effects on the mechanical properties of the NdFeB magnet. Therefore, it is believed that the addition of 0.1 wt% Ag would not result in a significant difference on the fracture behaviour of NdFeB magnet.

Fig. 3. The impact toughness of NdFeB and NdFeB + Ag. Table 4 Vickers hardness of sintered NdFeB and NdFeB + Ag magnets. Specimen

NdFeB

NdFeB + Ag

Hardness (HV)

587 ± 20

580 ± 19

3.3. Impact toughness The impact toughness of NdFeB and NdFeB + Ag calculated by Eq. (1) is depicted in Fig. 3. The values are presented as the average of at least five measurements. Since the ductility of the NdFeB magnets is very low, the two specimens failed instantly after the impact, causing the energy absorbed during the impact is very limited. Fig. 3 shows that the NdFeB and NdFeB + Ag exhibit similar impact toughness. Although a slightly lower impact toughness of NdFeB + Ag is noted, the values quoted are within the experimental uncertainties. This means that no significant improvements in the impact toughness have been achieved through the addition of 0.1 wt% Ag. It was previously reported that the impact toughness can be improved significantly when the amount of addition of Ag is no more than 0.05 wt% and the positive effect of Ag diminishes when the Ag content is beyond 0.1 wt% [28], which shows good consistency with Fig. 3 when Ag content is 0.1 wt%. The strength of the magnet is mainly controlled by the Nd2Fe14B phase and since little or even no changes occurred to the Nd2Fe14B phase, it is thus believed that the strength between NdFeB and NdFeB + Ag is almost identical. The results of micro-hardness testing for the NdFeB and NdFeB + Ag magnets are tabulated in Table 4 to represent the strength of the two materials. It can be seen that the both NdFeB and NdFeB + Ag exhibit almost identical hardness, indicating that the addition of 0.1 wt% Ag has little effects on the hardness and strength of the NdFeB magnet. Meanwhile, the ductility of the NdFeB magnets is dominated by the Nd-rich phase. The overall toughness can then be related to the volume fraction of the Nd-rich phase. It is seen from Fig. 1

4. Conclusions The effects of 0.1 wt% Ag on the microstructure, magnetic properties and impact stability of sintered NdFeB magnets (commercially known as N45) have been investigated. The NdFeB + Ag has been compared with the reference NdFeB magnet specimen to elucidate the role of Ag. The following conclusions are drawn in this work:

• The NdFeB and NdFeB + Ag exhibit a two-phase structure, con-

sisting of the main Nd2Fe14B phase and the Nd-rich grain boundary phase. No significant difference in the volume fraction of the Ndrich phase is noticed after the addition of Ag. It is found that the Ag tends to segregate in the Nd-rich phase and produce some discrete coarse Nd-rich phase.

Fig. 4. Fracture surfaces of NdFeB (a) and NdFeB + Ag (b). 52

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• The •



coercivity (Hcj) of the NdFeB + Ag increases while the remanence (Br) and maximum energy product ((BH)max) decrease due to the addition of 0.1 wt% Ag. The enhanced Hcj of the NdFeB + Ag is likely resulted from the slightly larger volume fraction of the Nd2Fe14B phase. No significant improvement in the impact toughness of the NdFeB + Ag is found, which is due to that the addition of Ag does not enhance the mechanical properties of NdFeB + Ag. The fractographic investigation shows that both magnet materials exhibit the brittle intergranular and cleavage fracture along the polygonal Nd2Fe14B grains. Future work will be necessary to investigate the addition of Ag in a broader composition range to study its effects on the magnetic and mechanical properties of sintered NdFeB permanent magnets.

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