Abstract
Nd12.3Fe81.7βxTixB6.0ββ 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 from 0.5 to 1 and then decrease with further increasing . Optimum magnetic properties with βkJ/m3, βkA/m, βT are achieved in the Nd12.3Fe80.7Ti1B6.0 ribbons annealed at 600Β°C for 10βmin.
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 Ξ±-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 11βatββ%, 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 [4β6]. However, the obtainable 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 [7β10]. 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 Nd12.3Fe81.7βxTixB6.0 ribbons.
2. Experimental
The ingots with nominal composition of Nd12.3Fe81.7βxTixB6.0 () 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 2β4βmm and thickness of 30β50βΞΌm were obtained by ejecting the molten alloys from a quartz tube with an orifice diameter about 0.6βmm onto a copper wheel at a surface speed (Vs) of 22βm/s. The melt spun ribbons were then sealed in a quartz tube under a vacuum of 4 Γ 10β3βPa and annealed at 550β800Β°C for 10βmin 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 10Β°C/min to determine the crystallization temperature. Phasesβ analysis of the samples was characterized by D/max-rB X-ray diffractometer (Cu KΞ± 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 2βT (1600βkA/m). The length direction of the ribbons was parallel to the applied field in order to minimize the demagnetization effect. Wohlfarthβs 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 Nd12.3Fe81.7-xTixB6.0 (, ) ribbons with Vs of 22βm/s and 30βm/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 2β:β14β:β1 decreases with increasing Vs from 22βm/s to 30βm/s and the addition Ti element, suggesting that the amount of 2β:β14β:β1 phase decreases and amorphous phase increases with increasing Vs and Ti content. The ribbons with melt-spun by 30βm/s consist of hard magnetic 2β:β14β:β1 phase and some amorphous phase. The XRD patterns of the ribbons with melt-spun by 20βm/s and 30βm/s present primarily amorphous phase. This indicates that Ti addition improves significantly the amorphization tendency of Nd12.3Fe81.7-xTixB6.0 alloys. Figure 2 shows XRD patterns of as-cast Nd12.3Fe80.7Ti1B6 alloy annealed at 1050Β°C for 10βh. It can be seen that amorphous phase transforms completely to 2β:β14β:β1 phase.
3.2. Effect of Ti on Crystallization Process and Microstructure
Figure 3 shows the differential scanning calorimetry (DSC) curves of the melt-spun Nd12.3Fe81.7βxTixB6.0 ribbons at a heating rate of 10βK/min from room temperature to 1073βK. The ribbons with and 1 show one exothermic peak corresponding to transformation from amorphous to 2β:β14β:β1 structure. Compared to the ribbon with , the crystallization temperature peak for the ribbon with increases by approximately 20βK, 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 Nd12.3Fe81.7B6 magnets.
Figure 4 shows TEM bright field micrographs of (a) Nd12.3Fe80.7Ti1B6 and (b) Nd12.3Fe81.7B6 ribbons annealed at 600Β°C for 10βmin. The Nd12.3Fe81.7B6 alloy shows an average grain size of around 80βnm and the grain size distribution is nonuniform. Large grains up to about 260βnm 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 35βnm for the Nd12.3Fe80.7Ti1B6 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 βkJ/m3 is obtained for the sample.
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3.3. Effect of Ti Content on Magnetic Properties of Ribbons
The , , and of the Nd12.3Fe81.7βxTixB6.0 melt-spun ribbons by 22βm/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 , and then decrease with further increasing Ti content. The sample with shows a very low coercivity and remanence.
The intrinsic coercivity , remanence polarization , and maximum energy product 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 2β:β14β:β1 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 and Ti content on , , and of Nd12.3Fe81.7βxTixB6.0 ribbons. It can be seen that , , and increase first with increasing and Ti content , until reaching the maximum values, then decrease with further increasing and Ti content . Optimum magnetic properties with βkJ/m3, βkA/m, T are achieved by annealing the melt-spun ribbons with at 600Β°C for 10βmin, which are much higher than the magnetic properties of Nd6Pr1Fe80B13 ribbons with Ti&C addition reported by Ohkubo et al. [13].
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Table 1 shows the variation of the optimum ,, and with Ti content of the Nd12.3Fe81.7βxTixB6.0 () ribbons. It can be seen that Ti addition significantly affects the magnetic properties of the samples. The ,, , and of optimally processed Nd12.3Fe81.7βxTixB6.0 ribbons initially increase with increasing Ti content from to 1.0, but all of them decrease with further increasing Ti content. The and increase from 0.79βT and 616.1βkA/m for the sample with to 1.02βT and 809.2βkA/m for the sample with , respectively. The of the 1.0βat.% Ti-doped sample increases from 76.2βkJ/m3 to 151.6βkJ/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.
3.4. Effect of Ti on Exchange Coupling
The exchange coupling interaction is often evaluated using plots, which can be defined as ; where is demagnetization remanence and is isothermal magnetization remanence [14]. Both of these values are normalized by the saturation remanence. According to Wohlfarthβs analysis, higher positive peaks indicate stronger exchange coupling interactions. Figure 7 shows the plots with respect to the applied magnetic field of the Nd12.3Fe81.7βxTixB6.0 annealed ribbons, respectively. The positive peak in the 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 (1.0βat.% 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 and , in the samples with higher Ti substitution.
4. Conclusion
The effect of Ti addition and annealing treatment on the magnetic properties and microstructure of nanocrystalline Nd12.3Fe81.7βxTixB6.0 () alloys has been systematically investigated. The remanence polarization , intrinsic coercivity , and maximum energy product of optimally processed Nd12.3Fe81.7βxTixB6.0 ribbons increased first with an increase in Ti content, reached the maximum values at , then decreased with further increasing Ti content. The excellent magnetic properties with βkJ/m3, βkA/m, βT were obtained by annealing a melt-spun amorphous Nd12.3Fe80.7Ti1.0B6.0 alloy at 600Β°C for 10βmin. 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.