Effect of Titanium Substitution on Magnetic Properties and Microstructure of Nanocrystalline Monophase Nd-Fe-B Magnets

Abstract

Nd_12.3Fe_81.7−xTi_xB_6.0  (𝑥=0.5−3.0) ribbons have been prepared by rapid quenching and subsequent annealing treatment. Effect of Ti substitution and annealing treatment on the microstructure, magnetic properties, and crystallization behavior of the ribbons was systematically investigated by the methods of differential scanning calorimeter (DSC), X-ray diffraction (XRD), transmission electron microscopy (TEM), and vibrating sample magnetometer (VSM). It is found that Ti addition may increase the crystallization temperature and stabilize the amorphous phase. Ti element inhibits the grain growth during crystallization process and finally refines the microstructure. The exchange coupling interactions and magnetic properties of the ribbons increase with increasing 𝑥 from 0.5 to 1 and then decrease with further increasing 𝑥≥1.5. Optimum magnetic properties with (𝐵𝐻)_max=151.6 kJ/m³, 𝐻_ci=809.2 kA/m, 𝐽_𝑟=1.02 T are achieved in the Nd_12.3Fe_80.7Ti₁B_6.0 ribbons annealed at 600°C for 10 min.

1. Introduction

Nanocomposite Nd-Fe-B permanent magnets have attracted considerable attention as a probable new generation of permanent magnets because of their high remanence, high maximum energy product, and low cost [1, 2]. The presence of the soft phases such as α-Fe and Fe₃B increases the remanence but the coercive field is decreased, so that their application is limited [3]. The alloys with Nd content higher than 11 at  %, that is, close to the stoichiometry composition of Nd₂Fe₁₄B and usually believed to be single phase structure, are being studied to further improve coercivity. Since the discovery of the nanocrystalline Nd-Fe-B magnets, extensive efforts have been made to improve magnetic properties of Nd₂Fe₁₄B-based magnets [4–6]. However, the obtainable (𝐵𝐻)_max is still significantly lower than that predicted by theory. This phenomenon can be practically attributed to the difference between practical microstructure and theoretical model, which is of fine mixture of aligned hard magnetic grains and two ferromagnetic phases suitably dispersed, crystallographically, coherent, and mutually coupled. Elements such as Zr and Nb have been found to improve overall hard magnetic properties effectively by refining the microstructure [7–10]. Chang et al. [11] have reported that Zr and Ti addition can modify phase constitution and refine the grain size of the Nd-Fe-Ti-Zr-B rod fabricated by direct casting method, and to lead to the enhancement of the magnetic properties of the rods. The effect of Ti&C addition on the phase composition, microstructure, magnetic properties, temperature characteristics, and corrosion behavior of rapid-quenched Nd-Fe-B alloy has been investigated [9, 10, 12]. The combined addition of Ti and other elements such as C and Zr has been discussed a lot. However, the report of the single addition of pure Ti to Nd-Fe-B alloy is seldom. In this work, we present our research about the crystallization behavior, microstructure, and magnetic properties of single-phase Nd_12.3Fe_81.7−xTi_xB_6.0 ribbons.

2. Experimental

The ingots with nominal composition of Nd_12.3Fe_81.7−xTi_xB_6.0 (𝑥=0,0.5,1.0,1.5,2.0,2.5,3.0) were prepared by arc melting pure constituent elements under high-purity Ar atmosphere. The ingots were remelted four times to ensure homogeneity. Ribbons with a width of 2–4 mm and thickness of 30–50 μm were obtained by ejecting the molten alloys from a quartz tube with an orifice diameter about 0.6 mm onto a copper wheel at a surface speed (Vs) of 22 m/s. The melt spun ribbons were then sealed in a quartz tube under a vacuum of 4 × 10⁻³Pa and annealed at 550–800°C for 10 min to crystallize and develop desired fine nanoscale microstructure. Thermal analysis of the ribbons was carried out using NETZSCHSTA449 differential scanning calorimeter at a heating rate of 10°C/min to determine the crystallization temperature. Phases’ analysis of the samples was characterized by D/max-rB X-ray diffractometer (Cu Kα radiation). The microstructure of the ribbons was performed using with H-800 transmission electron microscopy (TEM). The thin foils for TEM observation were made by Ar-ion beam polishing. Hysteresis loops of the ribbons were measured using an LDJ 9600 vibrating sample magnetometer (VSM) with an applied field of up to 2 T (1600 kA/m). The length direction of the ribbons was parallel to the applied field in order to minimize the demagnetization effect. Wohlfarth’s remanence analysis was employed to determine the strength of exchange-coupling interactions of the materials obtained.

3. Results and Discussion

3.1. Phase Analysis

Figure 1 shows XRD patterns of the as-spun Nd_12.3Fe_81.7-xTi_xB_6.0 (𝑥=0, 𝑥=1) ribbons with Vs of 22 m/s and 30 m/s. It can be seen that the ribbons are composed of amorphous phase and some Nd₂Fe₁₄B phase, and the quantity and the relative intensity of diffraction peak for 2 : 14 : 1 decreases with increasing Vs from 22 m/s to 30 m/s and the addition Ti element, suggesting that the amount of 2 : 14 : 1 phase decreases and amorphous phase increases with increasing Vs and Ti content. The ribbons with 𝑥=0 melt-spun by 30 m/s consist of hard magnetic 2 : 14 : 1 phase and some amorphous phase. The XRD patterns of the ribbons with 𝑥=1.0 melt-spun by 20 m/s and 30 m/s present primarily amorphous phase. This indicates that Ti addition improves significantly the amorphization tendency of Nd_12.3Fe_81.7-xTi_xB_6.0 alloys. Figure 2 shows XRD patterns of as-cast Nd_12.3Fe_80.7Ti₁B₆ alloy annealed at 1050°C for 10 h. It can be seen that amorphous phase transforms completely to 2 : 14 : 1 phase.

Figure 1 

XRD patterns of Nd12.3Fe81.7−xTixB6.0 ribbons quenched at various wheel speeds.

Figure 2 

XRD patterns of as-cast Nd12.3Fe80.7Ti1B6 alloy annealed at 1050°C for 10 h.

3.2. Effect of Ti on Crystallization Process and Microstructure

Figure 3 shows the differential scanning calorimetry (DSC) curves of the melt-spun Nd_12.3Fe_81.7−xTi_xB_6.0 ribbons at a heating rate of 10 K/min from room temperature to 1073 K. The ribbons with 𝑥=0 and 1 show one exothermic peak corresponding to transformation from amorphous to 2 : 14 : 1 structure. Compared to the ribbon with 𝑥=0, the crystallization temperature peak 𝑇_𝑝 for the ribbon with 𝑥=1 increases by approximately 20 K, indicating that Ti element enhances the thermal stability of amorphous phase in Nd-Fe-B alloy. Enrichment of Ti atoms along grain boundary stabilizes amorphous phase in the melt-spun and annealing process of the ribbons, which is believed to be a main reason that thermal stability is enhanced by Ti addition in the studied nanocrystalline Nd_12.3Fe_81.7B₆ magnets.

Figure 3 

DSC curves of as-spun Nd12.3Fe81.7−xTixB6.0 ribbons at 30 m/s.

Figure 4 shows TEM bright field micrographs of (a) Nd_12.3Fe_80.7Ti₁B₆ and (b) Nd_12.3Fe_81.7B₆ ribbons annealed at 600°C for 10 min. The Nd_12.3Fe_81.7B₆ alloy shows an average grain size of around 80 nm and the grain size distribution is nonuniform. Large grains up to about 260 nm are present in the microstructure of the sample, which decreases exchange coupling interactions between neighbor grains. The average grain size is estimated to be 25 to 35 nm for the Nd_12.3Fe_80.7Ti₁B₆ alloy and the grains become more uniformly distributed. Thus, it can be concluded that the addition of a small amount of Ti to the ternary alloy is effective for reducing the grain size of the magnets, which enhances the coercive field and remanence. So the best magnetic property with (𝐵𝐻)_max=151.6 kJ/m³ is obtained for the sample.

(a)

(b)

Figure 4 

TEM images of Nd12.3Fe80.7Ti1B6 (a) and Nd12.3Fe81.7B6 (b) ribbons annealed at 600°C for 10 min.

3.3. Effect of Ti Content on Magnetic Properties of Ribbons

The 𝐽_𝑟, 𝐻_ci, and (𝐵𝐻)_max of the Nd_12.3Fe_81.7−xTi_xB_6.0 melt-spun ribbons by 22 m/s are shown in Figure 5. It is clear that the magnetic properties of melt-spun ribbons increase first with increasing Ti content, reaching the maximum value at 𝑥=1.5, and then decrease with further increasing Ti content. The sample with 𝑥=3.0 shows a very low coercivity and remanence.

Figure 5 

Variation of the 𝐽𝑟, 𝐻ci and (𝐵𝐻)max with Ti content of as-spun Nd12.3Fe81.7−xTixB6.0 ribbons.

The intrinsic coercivity 𝐻_ci, remanence polarization 𝐽_𝑟, and maximum energy product (𝐵𝐻)_max of the melt-spun ribbons are relatively low, which can be ascribed to incomplete crystallization of the ribbons, as evidenced by the superposition of broad peaks of amorphous precursor alloy and the characteristic of 2 : 14 : 1 peaks shown in Figure 1. In order to achieve the best magnetic properties for each alloy, a thermal treatment was employed individually to quenched ribbons to induce a fine grain structure crystallizing from the amorphous state. Figure 6 summarizes the effect of annealing temperature 𝑇_𝑎 and Ti content 𝑥 on 𝐻_ci, 𝐽_𝑟, and (𝐵𝐻)_max of Nd_12.3Fe_81.7−xTi_xB_6.0 ribbons. It can be seen that 𝐻_ci, 𝐽_𝑟, and (𝐵𝐻)_max increase first with increasing 𝑇_𝑎 and Ti content 𝑥, until reaching the maximum values, then decrease with further increasing 𝑇_𝑎 and Ti content 𝑥. Optimum magnetic properties with (𝐵𝐻)_max=151.6 kJ/m³, 𝐻_ci=809.2 kA/m, 𝐽_𝑟=1.02T are achieved by annealing the melt-spun ribbons with 𝑥=1.0 at 600°C for 10 min, which are much higher than the magnetic properties of Nd₆Pr₁Fe₈₀B₁₃ ribbons with Ti&C addition reported by Ohkubo et al. [13].

(a)

(b)

(c)

Figure 6 

Magnetic properties of Nd12.3Fe81.7−xTixB6.0 ribbons annealed at various temperatures for 10 min.

Table 1 shows the variation of the optimum 𝐽_𝑟/𝐽_𝑠,𝐽_𝑟,𝐻_ci, and (𝐵𝐻)_max with Ti content of the Nd_12.3Fe_81.7−xTi_xB_6.0 (𝑥=0.5−3.0) ribbons. It can be seen that Ti addition significantly affects the magnetic properties of the samples. The 𝐽_𝑟/𝐽_𝑠,𝐽_𝑟, 𝐻_ci, and (𝐵𝐻)_max of optimally processed Nd_12.3Fe_81.7−xTi_xB_6.0 ribbons initially increase with increasing Ti content from 𝑥=0.5 to 1.0, but all of them decrease with further increasing Ti content. The 𝐽_𝑟 and 𝐻_ci increase from 0.79 T and 616.1 kA/m for the sample with 𝑥=0.5 to 1.02 T and 809.2 kA/m for the sample with 𝑥=1.0, respectively. The (𝐵𝐻)_max of the 1.0 at.% Ti-doped sample increases from 76.2 kJ/m³ to 151.6 kJ/m³. Ti elements are benefit to refine grain sizes and enhance exchange coupling interactions between the neighbor grains. But the strength of exchange coupling interactions in the ribbons is determined by two opposite factors: (1) grain refinement and (2) magnetic dilution and grain isolation especially at higher Ti content.

Table 1 

Magnetic properties of Nd_12.3Fe_81.7−xTi_xB₆ (𝑥=0.5,1.0,2.0,3.0) ribbons annealed at 600°C for 10 min.

3.4. Effect of Ti on Exchange Coupling

The exchange coupling interaction is often evaluated using 𝛿𝑀 plots, which can be defined as 𝑚_𝑑(𝐻)-(1-2𝑚_𝑟(𝐻)); where 𝑚_𝑑 is demagnetization remanence and 𝑚_𝑟 is isothermal magnetization remanence [14]. Both of these values are normalized by the saturation remanence. According to Wohlfarth’s analysis, higher positive 𝛿𝑀 peaks indicate stronger exchange coupling interactions. Figure 7 shows the 𝛿𝑀 plots with respect to the applied magnetic field of the Nd_12.3Fe_81.7−xTi_xB_6.0 annealed ribbons, respectively. The positive peak in the 𝛿𝑀 plots suggests the existence of exchange coupling interaction between Nd₂Fe₁₄B and Nd₂Fe₁₄B phases. It can be seen that the strength of exchange coupling interaction is significantly enhanced at a dilute Ti-substitution (1.0 at.% Ti) and then weakened when Ti content is further increased. This trend agrees well with the variation of magnetic properties with Ti content previously shown in Figure 6. Hence, the changes in magnetic properties of the samples on increasing Ti content may mainly arise from the variation of the exchange-coupling interaction in the samples. Meanwhile, it is worth noting that the decrease of saturation magnetization with increasing Ti content also contributes to the reduction of 𝐽_𝑟 and (𝐵𝐻)_max, in the samples with higher Ti substitution.

Figure 7 

The 𝛿𝑀(𝐻) curves of Nd12.3Fe81.7−xTixB6 (𝑥=0.5,1.0,2.0,3.0) ribbons annealed at 600°C for 10 min.

4. Conclusion

The effect of Ti addition and annealing treatment on the magnetic properties and microstructure of nanocrystalline Nd_12.3Fe_81.7−xTi_xB_6.0 (𝑥=0.5−3.0) alloys has been systematically investigated. The remanence polarization 𝐽_𝑟, intrinsic coercivity 𝐻_ci, and maximum energy product (𝐵𝐻)_max of optimally processed Nd_12.3Fe_81.7−xTi_xB_6.0 ribbons increased first with an increase in Ti content, reached the maximum values at 𝑥=1.0, then decreased with further increasing Ti content. The excellent magnetic properties with (𝐵𝐻)_max=151.6 kJ/m³, 𝐻_ci=809.2 kA/m, 𝐽_𝑟=1.02 T were obtained by annealing a melt-spun amorphous Nd_12.3Fe_80.7Ti_1.0B_6.0 alloy at 600°C for 10 min. The variation of magnetic properties with increasing Ti content can be attributed to both the variation of the exchange coupling interactions and the decrease in saturation magnetization in the ribbons.

Acknowledgment

This work has been supported by the National Natural Science Foundation of China under Grant no. 51174030.