426 Kezhi Ren et al.
Zr was used to visualize and identify the phase boundary, as indicated by perpendicular lines in Figure 3. The left-hand side of the concentration profile (negative distances) with a chemical composition near aNd:Fe:B stoichiometry of 2:14:1 is believed to be Nd2Fe14B phase. Two larger precipitates (marked as 1 in Fig. 3a and 2 in Fig. 3b) were chosen. The corresponding proxigrams in Figures 3c and 3d show that both Zr and Nd contents increase with an increasing distance from the phase boundary. B is found to enrich on the phase boundary. Furthermore, the precipitates are depleted in Fe and Co. Dy atoms have a uniform distribution in the Nd2Fe14B grain, and precipitates 1 and 2. Nd enrichment in the phase is caused by the rejection of Nd from the Dy-substituted Nd2Fe14B phase. It indicates that the (Nd, Dy)2Fe14B phase is formed resulting in an enhancement of the coercivity due to higher anisotropy field than that of the Nd2Fe14B phase (Herbst, 1991). The amount of precipitates increases with Nb addition.
Figure 1. X-ray powder diffraction patterns of the Nd9.5Fe76- Co5Zr3B6.5 (A-0), Nd8.5Fe76Co5Zr3B6.5Dy1 (A-Dy), Nd9.5Fe75Co5Zr3 B6.5Nb1 (A-Nb), and Nd9.5Fe75.4Co5Zr3B6.5Ga0.6 (A-Ga) alloys.
(Ametek Inc, Berwyn, PA, USA) local electrode atom probe. The specimens were analyzed in voltagemodewith a specimen temperature at 50K with a target evaporation rate of 0.5%, a pulse voltage fraction of 20% DC voltage, and an ultra-high vacuum to <10−8 Pa. Atom probe data reconstruction was conducted using IVASTM 3.6.8 software. Our analysis found that Nb, Dy, and Ga, in combination with Zr, showed fuzzy interfaces. Hence, an 8 at% Zr isoconcentration surface was used to visualize and identify the phase boundary. The iso- concentration surfaces and composition analyses of all APT data were done using an x–y–z voxel size of 1×1×1nmwith a delocalization of 3×3×1.5nm, respectively.
RESULTS
XRD patterns show that all samples consist of Nd2Fe14B phase and α-Fe phase (Fig. 1). The relative intensities of the diffraction peaks of α-Fe phase decreased significantly with the addition of Dy, Nb, and Ga elements. This suggests that Dy, Nb, and Ga additions inhibit the growth of α-Fe phase and increase the volume fraction of theNd2Fe14B phase.
The average grain sizes measured from bright-field
TEM images of the A-0, A-Dy, A-Nb, and A-Ga alloys are ~30, 18, 15, and 35nm, respectively (see Fig. 2 and Table 1). Hence, both Dy and Nb additions can reduce the grain size, whereas Ga addition marginally coarsens the microstructure. The partitioning of Dy, Nb, and Ga is seen from APT results (see Figs. 3–5). An isoconcentration surface of 8 at%
Two precipitates (marked as 3 and 4) were chosen in Figures 4a and 4b. The proxigrams in Figures 4c and 4d reveal that the enrichment of B is in the phase boundary, whereas the enrichment of Zr and Nb is in the phase. The precipitates are depleted in Fe, Nd, and Co. In the A-Ga sample, however, the amount of precipitates
decreases. The corresponding proxigram concentration pro- files of two precipitates marked as 5 and 6 are shown in
Figures 5c and 5d, respectively. The enrichment of Zr and Nb is found in the phase. B is found to enrich on the interface. Furthermore, Ga content exhibits a small increase, whereas Co and Fe are depleted in both precipitates. The major hysteresis loops of four alloys at room
temperature are shown in Figure 6. All samples reveal typical single-phase characteristics although they consist of soft magnetic α-Fe phase and hard magnetic Nd2Fe14B phase. Among the four samples, the A-Dy sample has a maximum intrinsic coercivity (Hci) with 872 kA/m, whereas the A-Ga sample shows the maximum remanence (Jr = 0.92T) and maximum energy product ((BH)max) with 121 kJ/m3. The values of key magnetic parameters are shown in Table 1. An effective method of understanding the phenomenon
of magnetic interaction is via the so called δM plot. On the basis of Wohlfarth’s theory, Kelly et al. (1989) have defined:
δM=Md H ½; ðÞ- 1 - 2Mr H ðÞ
where H is an applied magnetic field, Md the reduced mag- netization, and Mr the reduced magnetization remanence. The positive δM peak indicates the existence of exchange coupling interaction between the magnetic phases, and the maximum δM value reflects the strength of exchange coupling interaction. In Figure 7, for the A-Dy, A-Nb, and A-Ga samples, a positive δM is observed, confirming the existence of exchange coupling interaction between the magnetic phases. It is worth noting that the magnitude of the positive δM peak for the A-Nb sample is larger than that
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