Solidification microstructure and phase transition of La-Nd-Fe alloys

The solidification microstructure and phase transition of ten La-Nd-Fe alloys were studied experimentally by scanning electron microscopy (SEM) with energy dispersive x-ray spectroscopy (EDS) and differential thermal analysis (DTA). Phase compositions and phase transition temperatures of La-Nd-Fe alloys were measured and the formed phases were identified. The solidification behavior of La-Nd-Fe alloys was analyzed based on the experimental results of both solidification microstructure and phase transitions with the reported Nd-Fe, La-Fe and La-Nd sub-binary phase diagrams. The results indicated that the solidification processes of all La-Nd-Fe alloys begin with the precipitation of primary phase fcc(γ-Fe) and then follow by the formation of bcc(α-Fe) and/or Fe17Nd2 phases through different peritectic reactions. The solidification microstructure of three Fe65La29Nd6, Fe65La25Nd10 and Fe65La22Nd13 alloys presents three-phase microstructure with bcc(α-Fe), Fe17Nd2 and fcc(La,Nd) phases, while that of three Fe65La19Nd16, Fe65La9Nd26 and Fe65La15.5Nd19.5 alloys shows three-phase microstructure with bcc(α-Fe), Fe17Nd2 and dhcp(La,Nd) phases. The two-phase micorstructure with Fe17Nd2 and dhcp(La,Nd) ohases was formed in the solidification microstructure of four Fe65La12Nd23, Fe65La6.5Nd28.5, Fe65La4Nd31 and Fe65La1.5Nd33.5 alloys. Moreover, no stable ternary intermetallic compound was found in the present experiments. The solidification microstructure and phase transition of La-Nd-Fe alloys would provide a basis for the design of La-Nd-Fe-B magnetic alloys.


Introduction
Nd-Fe-B permanent magnets have excellent magnetic performances and have been widely used in the windpower, electric vehicles and other fields [1][2][3][4][5][6][7][8]. The superiority of these magnets arises from the large saturation magnetization and high anisotropy field of Nd 2 Fe 14 B main phase [9][10][11][12]. During the solidification process, the primary phase fcc(γ-Fe) (transformed to bcc(α-Fe) phase at low temperature) precipitates from Nd-Fe-B-based alloy melts at high temperature, and then Nd 2 Fe 14 B main phase is formed by the peritectic reaction, L+γ-Fe→Nd 2 Fe 14 B [13,14]. As the results, Nd-Fe-B-based alloys contain normally Nd 2 Fe 14 B main phase and α-Fe minor phase. In order to improve greatly the volume fraction of Nd 2 Fe 14 B main phase in the magnets, it is crucial to reduce the amounts of the deteriorated α-Fe minor phase during the solidification process. Strip-Casting (SC) technology as one of rapid solidification technologies has been used widely in the production of Nd 2 Fe 14 B-bassed permanent magnets [14][15][16][17][18][19]. The high cooling rates in the solidification process would restrain the precipitation of the primary phase γ-Fe from under-cooled melts, which is the most effective method to control the formation of α-Fe phase in the Nd-Fe-B-based magnets. Therefore, the microstructure evolution of Nd-Fe-B-based alloys during the solidification process is significant effect on their magnetic properties [11,18,19].
On the other hand, Nd-Fe-B permanent magnets were needed to the low-abundant and expensive heavy rare-earth metals Dy and Tb to achieve higher coercivity and better thermal stability [20,21]. It has limited the development of Nd-Fe-B magnets. In contrast, the high-abundant and cheap light rare-earth metals La, Ce and Y are overstocked. To balance the use of rare-earth metals, the application of the high-abundant rare-earth metals La, Ce and Y in Nd-Fe-B magnets to replace the part of heave rare-earth metals is an effective and promise way to develop novel Nd-Fe-B permanent magnets [22][23][24][25][26][27][28]. Recently, it has been reported that Nd-Fe-B magnets with La would exhibit a good magnetic performance [23,25,28]. For example, (Nd 0.4 La 0.6 ) 15 Fe 77.5 B 7.5 melt-spun ribbon prepared with the wheel speed of 26 m s −1 shows better magnetic properties (H ci =7.27 kOe, M r =90.94 emu g −1 , (BH) max =12.10 MGOe) [28]. In order to better understand the effect of La on phase formation, microstructure, phase transition and magnetic properties of Nd-Fe-B-based permanent magnet, the solidification behavior of La-Nd-Fe-B alloys is fundamental. Therefore, as a key ternary system in La-Nd-Fe-B alloys, the solidification microstructure and phase transition of La-Nd-Fe alloys were studied experimentally in this work.
2. Experimental procedure 2.1. Sample preparation La-Nd-Fe alloy samples were prepared from the pure metals of La (purity, 99.9%), Nd (purity, 99.9%) and Fe (purity, 99.9%). The alloys samples were melted four times by non-consumable tungsten electrode under an argon atmosphere protection to ensure homogeneity of the composition. The weight loss during the arc-melting was less than 1% and thus their compositions were considered to approach their nominal ones. In order to achieve composition homogeneity and prevent the samples oxidation, La-Nd-Fe alloy samples were sealed in evacuated quartz tubes under vacuum (<10 −3 Pa) to be annealed at 873 K for 1440 h in a high-precision diffusion furnace, and then quickly quench into ice water to maintain microstructure at certain temperature.

Microstructure characterization
For solidification microstructure examination, considering the easy oxidization of rare-earth metals, La-Nd-Fe alloy samples were prepared under the condition of ethyl alcohol absolute. The alloy samples were prepared with the standard metallographic procedure. The alloy samples were first ground using silicon carbide paper and then polished with diamond with approximately 0.05 μm particle sizes. The alloy samples were ultrasonically cleaned in ethyl alcohol absolute for 300 s after each step of grinding and polishing. The microstructure of the alloy samples was examined using scanning electron microscopy (SEM) using back scatted electron (BSE) mode and the phase compositions of alloy samples were measured using energy-dispersive x-ray spectra (EDS).

Thermal analysis
To determine the temperatures of phase transitions in La-Nd-Fe alloy samples, thermal analysis measurements were carried out using Al 2 O 3 crucibles under a flow of pure N 2 atmosphere. The instrument calibration was carried out using calibration metals In, Sn, Bi, Zn, Al, Ag, Au and Ni as standard samples to reduce the random and systematic errors. The high-purity Al 2 O 3 crucibles were employed in the thermal analysis experiments. The alloy samples about 15-20 mg were measured by heating up to 1673 K and cooling down to 373 K at both heating and cooling rates of 20 K min −1 . The accuracy of the present measurements is evaluated to be within ±1 K in the measured temperature range.

Results and discussion
The microstructure characterization and thermal analysis measurements of La-Nd-Fe alloy samples were carried out in this work. The phase compositions of La-Nd-Fe alloy samples measured by EDS were summarized in table 1. Based on the thermal analysis curves of the alloy samples, the onset temperature of the peak was determined as the reactions, while the last peak temperature was selected to be the liquidus temperature.   Figure 1 is the BSE micrograph of as-cast and annealed Fe 65 La 29 Nd 6 alloy. As shown in figure 1(a), the gray dark phase is the primary phase fcc(γ-Fe) (Fe99.37-La0.04-Nd0.79), which would transform to bcc(α-Fe) phase at low temperature, while the white phase is fcc(La,Nd) (Fe1.71-La83.08-Nd15.21) phase according to the EDS results in table 1. The reported Fe-Nd binary phase diagram [29] shows that Fe 17 Nd 2 phase is formed by the peritectic reaction, L+fcc(γ-Fe)→Fe 17 Nd 2 at 1490.2 K, while the La-Nd binary phase diagram shows fcc(La,Nd) phase in the rich-La part and dhcp(La,Nd) phase in the rich-Nd part [30]. Figure 1(   observed clearly, which was formed through the reaction, L→fcc(La,Nd)+Fe 17 Nd 2 . The microstructure of as-cast Fe 65 La 22 Nd 13 alloy in figure 2(b) contains fcc(γ-Fe), Fe 17 Nd 2 , fcc(La,Nd) three phases, and shows the two-phase microstructure with fcc(La,Nd) and Fe 17 Nd 2 phases, which is similar to the microstructure of as-cast Fe 65 La 25 Nd 10 alloy. Figure 3 displays the BSE micrograph of as-cast Fe 65 La 19 Nd 16 and Fe 65 La 9 Nd 26 alloys. In figure 3(a), the rareearth phase in Fe 65 La 19 Nd 16 alloy is dhcp(La,Nd) phase rather than fcc(La,Nd) phase. It was explained that there is a phase transition from fcc(La,Nd) phase to dhcp(La,Nd) phase with the increase of Nd content according to the La-Nd binary phase diagram [30]. According to the EDS results in table 1, three different phases, e.g. bcc(α-Fe), Fe 17 Nd 2 and dhcp(La,Nd), were identified in the BSE image as shown in figure 3(a). Figure 3(b) is the BSE micrograph of as-cast Fe 65 La 9 Nd 26 alloy. As can be seen, the solidification microstructure of as-cast Fe 65 La 9 Nd 26 alloy is similar with that of Fe 65 La 19 Nd 16 alloy, which consists of fcc(γ-Fe), Fe 17 Nd 2 , dhcp(La,Nd) phases and the two-phase microstructure with Fe 17 Nd 2 and dhcp(La,Nd) phases. Figure 4 presents the BSE micrograph of as-cast and annealed Fe 65 La 15.5 Nd 19.5 alloy. Based on the EDS results in table 1, three phases, e.g. fcc(γ-Fe), Fe 17 Nd 2 and dhcp(La,Nd) phases were identified in the microstructure as shown in figure 4(a), and the two-phase microstructure with Fe 17 Nd 2 and dhcp(La,Nd) phases is displayed. However, the microstructure of annealed alloy in figure 4(b) shows three different phases. The dark gray with lamellar structure is Fe 17 Nd 2 phase, while the gray phase and light gray phase are Nd-rich dhcp(La,Nd) (Fe1.54-La13.08-Nd85.38) phase and La-rich dhcp(La,Nd) (Fe0.28-La60.86-Nd38.86) phase, respectively. Compared with the microstructure of as-cast alloy, fcc(γ-Fe) phase disappears because fcc(γ-Fe) phase transforms to form Fe 17 Nd 2 phase through the peritectic reaction. Therefore, Fe 65 La 15.5 Nd 19.5 alloy undergoes the phase transition, L→Fe 17 Nd 2 +dhcp(La,Nd) and three-phase field fcc(γ-Fe)+Fe 17 Nd 2 +dhcp(La,Nd) during the solidification process. On the basis of the reported results [6,32], the La-Nd-Fe ternary system shows a three-phase eutectic transition and equilibrium three-phase region for the four-phase U-type reaction, L+fcc(γ-Fe)→Fe 17 Nd 2 +dhcp(La,Nd).   19 Nd 16 alloy, the microstructure of these four as-cast alloys indicate that the formation of dhcp(La,Nd) phase, although the phase transition from fcc(La,Nd) phase to dhcp(La,Nd) phase was not occurred according to the La-Nd binary phase diagram [30]. The microstructure of these four alloys displays similar solidification characteristics. On basis of the EDS results in table 1, these four alloys consist of angular strip structure and the continuous Fe 17 Nd 2 phase. The two-phase microstructure with dhcp(La,Nd) and Fe 17 Nd 2 phases was found, which indicated that the phase transition, L→dhcp(La,Nd)+Fe 17 Nd 2 , occurred during the solidification process. Figure 6 is the thermal analysis curves of as-cast Fe 65 La 29 Nd 6 , Fe 65 La 25 Nd 10 and Fe 65 La 22 Nd 13 alloys. From the heating curve in figure 6(a), two exothermic peaks were observed clearly. Based on the microstructure analysis as shown in figure 1 and the Nd-Fe binary phase diagram [29], the high temperature peak at 1214.0 K is corresponding to the formation of Fe 17 Nd 2 phase through the reaction, L+fcc(γ-Fe)→Fe 17 Nd 2 , while the low temperature peak at 1047.2 K is corresponding to the formation of fcc(La, Nd) phase through the reaction, L→Fe 17 Nd 2 +fcc(La, Nd). As shown in the thermal analysis curves of Fe 65 La 25 Nd 10 alloy in figure 6(b), the first peak at 1061.1 K is corresponding to the phase transition, L→fcc(La,Nd)+Fe 17 Nd 2 , while another peak at 1210.9 K is the transition temperature of formation of Fe 17 Nd 2 phase during the solidification process in combination with the solidification microstructure analysis in figure 2(a). The solidification microstructure of Fe 65 La 25 Nd 10 alloy is composed of fcc(La,Nd), Fe 17 Nd 2 and bcc(α-Fe) (transformed from fcc(γ-Fe) phase) phases. As a result, the solidification process of Fe 65 La 25 Nd 10 alloy can be described as: L→L+fcc(γ-Fe)→L+Fe 17 Figure 7 presents the thermal analysis curves of as-cast Fe 65 La 19 Nd 16 , Fe 65 La 9 Nd 26 and Fe 65 La 15.5 Nd 19.5 alloys. As can be seen in figure 7(a), the thermal analysis curves of Fe 65 La 19 Nd 16 alloy shows that the high temperature peak at 1298.1 K is corresponding to the reaction, L+fcc(γ-Fe)→Fe 17 Nd 2 , while the low temperature peak at 1027.9 K belongs to the reaction, L→Fe 17 Nd 2 +dhcp(La,Nd) based on the similar analysis mentioned above. However, the transition temperature of fcc(La,Nd) phase to dhcp(La,Nd) phase was not measured in this work due to small thermal effect of solid phase transformation. It was noted that the high-temperature phase fcc(γ-Fe) transforms into bcc(α-Fe) phase at low temperature. It means that the thermal analysis results are in good consistent with the solidification microstructure in figure 3(a). It was concluded that the solidification behavior of Fe 65 La 19 Nd 16 alloy can be expressed as L→L+fcc(γ-Fe)→L+Fe 17 Nd 2 →L+fcc(La,Nd)+Fe 17 Nd 2 →bcc(α-Fe)+dhcp(La,Nd)+Fe 17 Nd 2 . Based on the thermal analysis curves in figure 7(b), the solidification behavior of Fe 65 La 9 Nd 26 alloy is similar with that of Fe 65 La 19 Nd 16 alloy. The peritectic reaction, L+fcc(γ-Fe)→Fe 17 Nd 2 occurs at 1423.4 K, while the formation of Fe 17 Nd 2 +dhcp (La,Nd) phase occurs at 1000.7 K. In figure 7(c), the thermal analysis curves of Fe 65 La 15.5 Nd 19.5 alloy show that the first peak at 1021.4 K is corresponding to the reaction, L→Fe 17 Nd 2 +dhcp(La,Nd), while the another peak at 1329.5 K is the temperature of the reaction, L+fcc(γ-Fe)→Fe 17 figure 8. The peritectic reaction, L+fcc(γ-Fe)→Fe 17 Nd 2 , occurred at high temperature. Compared with the experimental results of six alloys discussed above, fcc(γ-Fe) phase was not observed in the solidification microstructure, while the proportion of Fe 17 Nd 2 phase increases due to the transformation of fcc(γ-Fe) phase to form Fe 17 Nd 2 phase during the solidification process. The solidification behavior of these four alloys can be expressed as: L→L+fcc(γ-Fe)→L+Fe 17 Nd 2 →Fe 17 Nd 2 +dhcp(La,Nd).

Conclusions
In this work, the solidification process of La-Nd-Fe alloys was analyzed based on the experimental investigation of solidification microstructure and phase transitions with the reported Nd-Fe, La-Fe and La-Nd sub-binary phase diagrams. The results shows that the solidification processes of three Fe 65 La 29 Nd 6 , Fe 65 La 25 Nd 10 and Fe 65 La 22 Nd 13 alloys were described as L→L+fcc(γ-Fe)→L+Fe 17 Nd 2 →bcc(α-Fe)+Fe 17 Nd 2 +fcc(La,Nd), whereas