Fe70−xNd7B21Zr2Nbx (x = 0–3.0) Permanent Magnets Produced by Crystallizing Amorphous Precursors

The phase evolution, magnetic properties and microstructure of rod-shaped permanent magnets prepared by annealing the amorphous precursor Fe70−xNd7B21Zr2Nbx (x = 0–3.0) were systematically studied. X-ray diffraction analysis, magnetometer, microstructure and δM-plots studies show that the good magnetic properties of the magnet are attributed to the uniform microstructure composed of exchange-coupled α-Fe and Nd2Fe14B phases. Nb addition to Fe67.5Nd7B21Zr2Nb2.5 alloy led to an increase in the volume fraction of the soft magnetic phase, reinforced exchange coupling and improved magnetic properties. The magnetic properties of the optimized annealed Fe67.5Nd7B21Zr2Nb2.5 rod are: coercivity (Hci) = 513.92 kA/m, remanence (Br) = 0.58 T, squareness (Hk/Hci) = 0.24 and magnetic energy product ((BH)max) = 37.59 kJ/m3.


Introduction
Fe-based bulk amorphous alloys (BAA) have aroused widespread interest due to their excellent magnetic properties (MPs), mechanical properties and low raw material costs [1,2].Usually, Fe-based BAAs exhibit soft magnetic properties (SMPs) in the as-cast state.In 2002, Zhang et al. [3] studied the crystallization process of a rod-shaped Fe 67 Co 9.4 Nd 3.1 Dy 0.5 B 20 BAA with a diameter of 0.5 mm and found that after crystallization, the MPs of the alloy changed from soft to hard magnetic, that is, an Fe-based bulk permanent magnet alloy is obtained.This rare earth-containing Fe BAA is called "Re-Fe-based BAA" [4].The crystallization of such BAAs can obtain permanent magnet materials, which not only provides a new direction for the application of BAAs, but also provides a new method for preparing high-density permanent magnets.
Previous studies [5] have found that Fe 61 Nd 10 B 25 Nb 4 alloys exhibit excellent permanent MPs after composition adjustment and optimal heat treatment, with coercivity (H ci ) as high as 1191kA/m, remanence (B r ) of 0.42 T, and a maximum magnetic energy product ((BH) max ) of 31.72 kJ/m 3 .Zhang et al. [6] developed a rod-shaped Fe 64.32 Nd 9.6 B 22.08 Nb 4 BAA with H ci as high as 1100 kA/m; however, B r and (BH) max are 0.44 T and 32.96 kJ/m 3 after annealing for 5 min at 983 K. Subsequently, Cui et al. [7] and Man et al. [5] also investigated the Fe-Nd-B-Nb system alloys, and they successfully prepared bar-shaped Fe 71.5 Nd 9 B 15.5 Nb 4 and flake-shaped Fe 61 Nd 10 B 25 Nb 4 BAAs, and the alloys were crystallized with 1154 kA/m and 1191.1 kA/m H ci , and 0.59 T and 0.42 T B r .Yan et al. [8][9][10] studied the Fe-Nd-B-Mo alloy system and prepared a rod-shaped Fe 67 Nd 7 Mo 3 B 22 Zr 1 BAA.After annealing at 1013 K for 10 min, its B r and (BH) max were 0.53 T and 49.52 kJ/m 3 , respectively.However, although these types of alloys can obtain high coercivity, they generally have the disadvantage of low remanence, so how can the remanence and comprehensive MPs of the alloys be improved?
This study focuses on developing high-MP NdFeB permanent magnets through suction casting and one-step annealing technology.The Fe 70−x Nd 7 B 21 Zr 2 Nb x (x = 0-3.0)alloys are selected based on our previous works [11].The reasons for choosing Nb elements are: (1) The addition of appropriate Nb element [12][13][14] can enhance the amorphous formation ability of certain Fe-based alloys.Because the addition of the element Nb conforms to the three empirical laws proposed by Inoue for the formation of BAAs [15,16]: (a) the alloy consists of three or more group elements; (b) the difference between the atomic sizes of Nb and the major elements in the alloys are large (Nb-Fe: 15.27%, Nb-B: 43.62%), the addition of Nb induces a significant change in the topological parameter, leading to a more chaotic arrangement of atoms, thus increasing the viscosity and lowering the diffusion rate of the liquid alloy, which is conducive to the formation of an amorphous structure; (c) Nb and the major elements in the alloy have a relatively large negative heat of mixing (Nb-Fe: −16 kJ/mol, Nb-B: −54 kJ/mol).( 2) The incorporation of suitable Nb element can contribute to the improvement of the remanence and squareness of permanent magnets [17][18][19][20][21]. (3) Additionally, some studies indicated that Nb element acts as an effective additive for refining the grain size of NdFeB magnets [22][23][24].The effect of Nb content on phase evolution as well as MPs and microstructural properties is studied.
The remainder of this paper is structured as follows: Section 2 summarizes the experimental procedure for sample preparation and standardization of the alloy systems.Section 3 presents the experimental results of the alloy samples.Finally, Section 4 draws the conclusions of this paper based on the experimental results.

Experimental Procedure
The WK-II (Beijing WuKe-II) vacuum arc-melting furnace was utilized to produce a master alloy with a nominal composition of Fe 70−x Nd 7 B 21 Zr 2 Nb x (x = 0-3.0)(atomic percentage) under a high-purity argon atmosphere.The metals Fe, Nd, Zr and Nb are all high-purity (≥99.99%)metals, while B is added in form of an Fe-B alloy.To ensure the uniformity of composition of the master alloys, each ingot underwent four repeated smelting processes.The copper mold suction casting technology was employed to remelt the alloys under argon gas protection, resulting in the production of rods with a diameter of 2 mm.The density of the alloy ingots was determined using the Archimedes drainage method.Subsequently, the alloy rods were heat-treated in a quartz tube furnace with a vacuum level of 3 × 10 −3 Pa, followed by rapid cooling after a 10 min heat preservation period.The heat treatment temperature ranged from 973 to 1023 K.The X-ray diffraction patterns (XRD-Ps) (XRD, Rigaku Corporation, Akishima-Shi, Tokyo, Japan) of the samples were measured using a D/max-2200 X-ray diffractometer manufactured, with a scanning rate of 1 • /min.Thermal analysis was performed on the sample using a NETZSCH DSC 404C (Diamond DSC, Perkin-Elmer, New Rochelle, NY, USA) high-temperature differential scanning calorimeter.The MPs of the sample were evaluated using a Lake Shore 7407 vibrating sample magnetometer (VSM, LakeShore Cryotronics, Westerville, OH, USA), and the magnetic interaction curve of the alloy (i.e., δM-H plots) was measured using the Quantum Design PPMS-9 (PPMS-9T, Quantum Design, San Diego, CA, USA) multifunctional physical property measurement system.Microstructure was examined using a transmission electron microscope (TEM) (JEM-2100F, JEOL Ltd., Tokyo, Japan).

Characteristics of As-Cast Rods
Figure 1 illustrates the magnetic hysteresis loops for the as-cast Fe 70−x Nd 7 B 21 Zr 2 Nb x (x = 0-3.0)alloys.It can be observed that all the loops exhibit a bee waist shape.The saturation magnetization (M s ) gradually decreases from 105.95 to 92.85 Am 2 /kg with the addition of the element Nb.It suggests that the inclusion of a small amount of Nb element results in a reduction in M s of the alloys.In addition, the density (ϱ) also increases from 7.40 to 7.48 g/cm 3 as the Nb content increases.The soft magnetic parameters of the as-cast samples are documented in Table 1.
Materials 2024, 17, x FOR PEER REVIEW 3 of 9 addition of the element Nb.It suggests that the inclusion of a small amount of Nb element results in a reduction in Ms of the alloys.In addition, the density (ρ) also increases from 7.40 to 7.48 g/cm 3 as the Nb content increases.The soft magnetic parameters of the as-cast samples are documented in Table 1.  Figure 2a presents the XRD-Ps of the as-cast Fe70−xNd7B21Zr2Nbx (x = 0-3.0)samples.For x = 0, a single broad peak is observed along with some additional peaks, suggesting the alloy contains a significant amount of amorphous phases and a small amount of crystallization phases.The XRD-Ps for x = 1.5, 2.0, and 2.5 alloys only has large steamed bun peaks, indicating that the alloy is basically amorphous.When x is further increased to 3.0, the XRD-P shows additional diffraction peaks, indicating the formation of the Nd2Fe14B phase.Figure 2b presents the surface appearance of the x = 2.5 rod with a diameter of 2 mm.The image illustrates a metallic cluster without any signs of surface degradation or rupture, which is a typical characteristic of a BAA.x (x = 0-3.0)samples.For x = 0, a single broad peak is observed along with some additional peaks, suggesting the alloy contains a significant amount of amorphous phases and a small amount of crystallization phases.The XRD-Ps for x = 1.5, 2.0, and 2.5 alloys only has large steamed bun peaks, indicating that the alloy is basically amorphous.When x is further increased to 3.0, the XRD-P shows additional diffraction peaks, indicating the formation of the Nd 2 Fe 14 B phase. Figure 2b presents the surface appearance of the x = 2.5 rod with a diameter of 2 mm.The image illustrates a metallic cluster without any signs of surface degradation or rupture, which is a typical characteristic of a BAA.
Materials 2024, 17, x FOR PEER REVIEW 3 of 9 addition of the element Nb.It suggests that the inclusion of a small amount of Nb element results in a reduction in Ms of the alloys.In addition, the density (ρ) also increases from 7.40 to 7.48 g/cm 3 as the Nb content increases.The soft magnetic parameters of the as-cast samples are documented in Table 1.  Figure 2a presents the XRD-Ps of the as-cast Fe70−xNd7B21Zr2Nbx (x = 0-3.0)samples.For x = 0, a single broad peak is observed along with some additional peaks, suggesting the alloy contains a significant amount of amorphous phases and a small amount of crystallization phases.The XRD-Ps for x = 1.5, 2.0, and 2.5 alloys only has large steamed bun peaks, indicating that the alloy is basically amorphous.When x is further increased to 3.0, the XRD-P shows additional diffraction peaks, indicating the formation of the Nd2Fe14B phase.Figure 2b presents the surface appearance of the x = 2.5 rod with a diameter of 2 mm.The image illustrates a metallic cluster without any signs of surface degradation or rupture, which is a typical characteristic of a BAA.

Magnetic Properties
According to the DSC results (See Supplementary Figure S1), the as-cast alloys were annealed at various temperatures (973-1023 K) for 10 min.The values of the hard magnetic properties (HMPs) for the annealed samples are recorded in Table 2.The optimum annealing temperature (T a ) was determined as the temperature at which the (BH) max was achieved.Figure 3 shows the demagnetization curves for the alloys annealed at T a , Figure 4 illustrates the variations in B r , H ci , and squareness((H k /H ci ), where H k is knee-point coercivity) and (BH) max is a function of x for the annealed Fe 70−x Nd 7 B 21 Zr 2 Nb x (x = 0-3.0)alloys at T a .It can be observed that as x increases, B r gradually decreases, while H ci , H k /H ci and (BH) max initially increase and then decrease.The maximum value of H ci is obtained when x = 1.5.Whereas the maximum values of H k /H ci and (BH) max are achieved when x = 2.5.These results indicate that the addition of Nb element improves H ci , H k /H ci and (BH) max in Fe 70−x Nd 7 B 21 Zr 2 Nb x (x = 0-3.0)alloys.The optimal HMPs of B r = 0.58 T, H ci = 513.92kA/m, H k /H ci = 0.24, and (BH) max = 37.59 kJ/m 3 were achieved for the x = 2.5 alloy.

Magnetic Properties
According to the DSC results (See Supplementary Figure S1), the as-cast alloys were annealed at various temperatures (973-1023 K) for 10 min.The values of the hard magnetic properties (HMPs) for the annealed samples are recorded in Table 2.The optimum annealing temperature (Ta) was determined as the temperature at which the (BH)max was achieved.Figure 3 shows the demagnetization curves for the alloys annealed at Ta, Figure 4 illustrates the variations in Br, Hci, and squareness((Hk/Hci), where Hk is knee-point coercivity) and (BH)max is a function of x for the annealed Fe70−xNd7B21Zr2Nbx (x = 0-3.0)alloys at Ta.It can be observed that as x increases, Br gradually decreases, while Hci, Hk/Hci and (BH)max initially increase and then decrease.The maximum value of Hci is obtained when x = 1.5.Whereas the maximum values of Hk/Hci and (BH)max are achieved when x = 2.5.These results indicate that the addition of Nb element improves Hci, Hk/Hci and (BH)max in Fe70−xNd7B21Zr2Nbx (x = 0-3.0)alloys.The optimal HMPs of Br = 0.58 T, Hci = 513.92kA/m, Hk/Hci = 0.24, and (BH)max = 37.59 kJ/m 3 were achieved for the x = 2.5 alloy.3, with the increase in Nb in the alloys, both the values of I (110)Fe /I (214)2:14:1 and I (110)Fe /I (310)1.1:4:4gradually increase and then decrease, and reach the maximum value when x = 2.5.It indicates that the relative content of α-Fe in the alloys first increases and then decreases, that is to say, the addition of appropriate Nb is favorable to the precipitation of α-Fe, which may be the main reason leading to the B r of the alloy increase.Therefore, the presence of the Nb element plays a crucial role in adjusting the precipitation phase and enhancing the MPs.The Fe 67.5 Nd 7 B 21 Zr 2 Nb 2.5 alloy, annealed at 993 K, demonstrated good HMPs, likely due to the strong exchange coupling interaction (ECI) between soft and hard magnetic phases (SHMPs).

XRD-Ps and Phase Compositions
Figure 5 shows the XRD-Ps of the Fe70-xNd7B21Zr2Nbx(x = 0-3.0)alloys after annealing at the optimum temperature, and the relative intensity ratios of the diffraction peaks of the phases are shown in Table 3.It can be seen with the addition of the element Nb to the Fe70−xNd7B21Zr2Nbx (x = 0-3.0)alloys, the diffraction peaks were all indexed to α-Fe, Nd2Fe14B and Nd1.1Fe4B4 phases.The intensities of (110) plane for α-Fe, (214) plane for Nd2Fe14B and (310) plane for Nd1.1Fe4B4 phase diffractions, which are used to estimate the relative volume fraction of α-Fe, Nd2Fe14B and Nd1.1Fe4B4 phases, are denoted as I(110)Fe, I(214)2:14:1 and I(310)1.1:4:4,respectively.As shown in Table 3, with the increase in Nb in the alloys, both the values of I(110)Fe/I(214)2:14:1 and I(110)Fe/I(310)1.1:4:4gradually increase and then decrease, and reach the maximum value when x = 2.5.It indicates that the relative content of α-Fe in the alloys first increases and then decreases, that is to say, the addition of appropriate Nb is favorable to the precipitation of α-Fe, which may be the main reason leading to the Br of the alloy increase.Therefore, the presence of the Nb element plays a crucial role in adjusting the precipitation phase and enhancing the MPs.The Fe67.5Nd7B21Zr2Nb2.5 alloy, annealed at 993 K, demonstrated good HMPs, likely due to the strong exchange coupling interaction (ECI) between soft and hard magnetic phases (SHMPs).

XRD-Ps and Phase Compositions
Figure 5 shows the XRD-Ps of the Fe70-xNd7B21Zr2Nbx(x = 0-3.0)alloys after annealing at the optimum temperature, and the relative intensity ratios of the diffraction peaks of the phases are shown in Table 3.It can be seen with the addition of the element Nb to the Fe70−xNd7B21Zr2Nbx (x = 0-3.0)alloys, the diffraction peaks were all indexed to α-Fe, Nd2Fe14B and Nd1.1Fe4B4 phases.The intensities of (110) plane for α-Fe, (214) plane for Nd2Fe14B and (310) plane for Nd1.1Fe4B4 phase diffractions, which are used to estimate the relative volume fraction of α-Fe, Nd2Fe14B and Nd1.1Fe4B4 phases, are denoted as I(110)Fe, I(214)2:14:1 and I(310)1.1:4:4,respectively.As shown in Table 3, with the increase in Nb in the alloys, both the values of I(110)Fe/I(214)2:14:1 and I(110)Fe/I(310)1.1:4:4gradually increase and then decrease, and reach the maximum value when x = 2.5.It indicates that the relative content of α-Fe in the alloys first increases and then decreases, that is to say, the addition of appropriate Nb is favorable to the precipitation of α-Fe, which may be the main reason leading to the Br of the alloy increase.Therefore, the presence of the Nb element plays a crucial role in adjusting the precipitation phase and enhancing the MPs.The Fe67.5Nd7B21Zr2Nb2.5 alloy, annealed at 993 K, demonstrated good HMPs, likely due to the strong exchange coupling interaction (ECI) between soft and hard magnetic phases (SHMPs).

ECI and Microstructure
To understand the behavior of ECI between the SHMPs for Fe 70 Nd 7 B 21 Zr 2 and Fe 67.5 Nd 7 B 21 Zr 2 Nb 2.5 magnets, the δM plot [25] was constructed.It is defined as δM = [m d (H) − {1−2m r (H)}], where M d (H) is the reduced demagnetization remanence and M r (H) is the reduced magnetization remanence.δM = 0, whereas nonzero δM indicates the presence of interactions.Figure 6

ECI and Microstructure
To understand the behavior of ECI between the SHMPs for Fe70Nd7B21Zr2 an Fe67.5Nd7B21Zr2Nb2.5 magnets, the δM plot [25] was constructed.It is defined as δM [md(H) − {1−2mr(H)}], where Md(H) is the reduced demagnetization remanence and Mr( is the reduced magnetization remanence.δM = 0, whereas nonzero δM indicates the pre ence of interactions.Figure 6 depicts the δM plot as a function of the applied magne field for the two samples.Comparing these alloys, the Fe67.5Nd7B21Zr2Nb2.5 alloy displa a higher positive δM peak, suggesting a stronger ECI between the phases in comparis to the Fe70Nd7B21Zr2 alloy.This indicates that the introduction of Nb has a beneficial im pact on establishing robust ECI within the magnetic phases of the Fe67.5Nd7B21Zr2Nb2.5 loy.The strong ECI phenomena in the Fe67.5Nd7B21Zr2Nb2.5 alloy can be attributed to t fine grain size, ideal volume fractions of SHMPs, as well as their homogeneous distrib tion in the microstructure.To clearly characterize the internal structure of the alloys, TEM bright field images Fe70Nd7B21Zr2 and Fe67.5Nd7B21Zr2Nb2.5 alloys after optimal heat treatment are shown Figures 7a and 7b, respectively.As shown in Figure7a, the annealed Fe70Nd7B21Zr2 samp mainly consists of α-Fe (see Figure 7c) phase and Nd2Fe14B phase (see Figure 7d).Appa ently, the Fe67.5Nd7B21Zr2Nb2.5 sample also consists of α-Fe phase (see Figure 7e) an Nd2Fe14B phase (see Figure 7f).It is evident that the annealed Fe70Nd7B21Zr2 alloy witho Nb exhibits a coarse and uneven distribution of grain sizes, with some individual grai exceeding 150 nm (see Figure 7g).Consequently, the MPs of this alloy are poor.On t other hand, the Fe67.5Nd7B21Zr2Nb2.5 alloy with 2.5at% Nb shows a refined and more even distributed grain size, with an average size of approximately 70 nm (see Figure 7h).Th optimized microstructure promotes enhanced ECI between the SHMPs, resulting in im proved MPs.The addition of Nb elements significantly refines the grain size of the allo which further enhances the ECI between the SHMPs, thereby improving the remanenc  7a and 7b, respectively.As shown in Figure 7a, the annealed Fe 70 Nd 7 B 21 Zr 2 sample mainly consists of α-Fe (see Figure 7c) phase and Nd 2 Fe 14 B phase (see Figure 7d).Apparently, the Fe 67.5 Nd 7 B 21 Zr 2 Nb 2.5 sample also consists of α-Fe phase (see Figure 7e) and Nd 2 Fe 14 B phase (see Figure 7f).It is evident that the annealed Fe 70 Nd 7 B 21 Zr 2 alloy without Nb exhibits a coarse and uneven distribution of grain sizes, with some individual grains exceeding 150 nm (see Figure 7g).Consequently, the MPs of this alloy are poor.On the other hand, the Fe 67.5 Nd 7 B 21 Zr 2 Nb 2.5 alloy with 2.5at% Nb shows a refined and more evenly distributed grain size, with an average size of approximately 70 nm (see Figure 7h).This optimized microstructure promotes enhanced ECI between the SHMPs, resulting in improved MPs.The addition of Nb elements significantly refines the grain size of the alloy, which further enhances the ECI between the SHMPs, thereby improving the remanence.
The better MPs of Fe 67.5 Nd 7 B 21 Zr 2 Nb 2.5 alloy compared to those of the reported α-Fe/Nd 2 Fe 14 B magnets are speculated to be due to the appropriate alloy composition, especially the Fe: Nd ratio as well as the existence of an ideal microstructure.That is to say, the higher HMPs in the alloy is ascribed to three factors: First, the formation of grains, which leads to strong magnetic exchange interactions between magnetically soft and hard phases.Second, the soft phase increment.Third, the existence of the fine grain boundary phase, which would be favorable to relate the magnetization reverse.

Table 1 .Figure
Figure 2a presents the XRD-Ps of the as-cast Fe 70−x Nd 7 B 21 Zr 2 Nb x (x = 0-3.0)samples.For x = 0, a single broad peak is observed along with some additional peaks, suggesting the alloy contains a significant amount of amorphous phases and a small amount of crystallization phases.The XRD-Ps for x = 1.5, 2.0, and 2.5 alloys only has large steamed bun peaks, indicating that the alloy is basically amorphous.When x is further increased to 3.0, the XRD-P shows additional diffraction peaks, indicating the formation of the Nd 2 Fe 14 B phase.Figure2bpresents the surface appearance of the x = 2.5 rod with a diameter of 2 mm.The image illustrates a metallic cluster without any signs of surface degradation or rupture, which is a typical characteristic of a BAA.

Figure 5
Figure 5 shows the XRD-Ps of the Fe 70-x Nd 7 B 21 Zr 2 Nb x (x = 0-3.0)alloys after annealing at the optimum temperature, and the relative intensity ratios of the diffraction peaks of the phases are shown in Table 3.It can be seen with the addition of the element Nb to the Fe 70−x Nd 7 B 21 Zr 2 Nb x (x = 0-3.0)alloys, the diffraction peaks were all indexed to α-Fe, Nd 2 Fe 14 B and Nd 1.1 Fe 4 B 4 phases.The intensities of (110) plane for α-Fe, (214) plane for Nd 2 Fe 14 B and (310) plane for Nd 1.1 Fe 4 B 4 phase diffractions, which are used to estimate the relative volume fraction of α-Fe, Nd 2 Fe 14 B and Nd 1.1 Fe 4 B 4 phases, are denoted as I (110)Fe , I (214)2:14:1 and I (310)1.1:4:4, respectively.As shown in Table3, with the increase in Nb in the alloys, both the values of I (110)Fe /I (214)2:14:1 and I (110)Fe /I (310)1.1:4:4gradually increase and then decrease, and reach the maximum value when x = 2.5.It indicates that the relative content of α-Fe in the alloys first increases and then decreases, that is to say, the addition of appropriate Nb is favorable to the precipitation of α-Fe, which may be
depicts the δM plot as a function of the applied magnetic field for the two samples.Comparing these alloys, the Fe 67.5 Nd 7 B 21 Zr 2 Nb 2.5 alloy displays a higher positive δM peak, suggesting a stronger ECI between the phases in comparison to the Fe 70 Nd 7 B 21 Zr 2 alloy.This indicates that the introduction of Nb has a beneficial impact on establishing robust ECI within the magnetic phases of the Fe 67.5 Nd 7 B 21 Zr 2 Nb 2.5 alloy.The strong ECI phenomena in the Fe 67.5 Nd 7 B 21 Zr 2 Nb 2.5 alloy can be attributed to the fine grain size, ideal volume fractions of SHMPs, as well as their homogeneous distribution in the microstructure.

Figure 6 .
Figure 6.δM plots as a function of applied field for Fe70Nd7B21Zr2 and Fe67.5Nd7B21Zr2Nb2.5 alloys

Figure 6 .
Figure 6.δM plots as a function of applied field for Fe 70 Nd 7 B 21 Zr 2 and Fe 67.5 Nd 7 B 21 Zr 2 Nb 2.5 alloys.To clearly characterize the internal structure of the alloys, TEM bright field images of Fe 70 Nd 7 B 21 Zr 2 and Fe 67.5 Nd 7 B 21 Zr 2 Nb 2.5 alloys after optimal heat treatment are shown in Figures7a and 7b, respectively.As shown in Figure7a, the annealed Fe 70 Nd 7 B 21 Zr 2 sample mainly consists of α-Fe (see Figure7c) phase and Nd 2 Fe 14 B phase (see Figure7d).Apparently, the Fe 67.5 Nd 7 B 21 Zr 2 Nb 2.5 sample also consists of α-Fe phase (see Figure7e) and Nd 2 Fe 14 B phase (see Figure7f).It is evident that the annealed Fe 70 Nd 7 B 21 Zr 2 alloy without Nb exhibits a coarse and uneven distribution of grain sizes, with some individual grains exceeding 150 nm (see Figure7g).Consequently, the MPs of this alloy are poor.On the other hand, the Fe 67.5 Nd 7 B 21 Zr 2 Nb 2.5 alloy with 2.5at% Nb shows a refined and more evenly distributed grain size, with an average size of approximately 70 nm (see Figure7h).This optimized microstructure promotes enhanced ECI between the SHMPs, resulting in improved MPs.The addition of Nb elements significantly refines the grain size of the alloy, which further enhances the ECI between the SHMPs, thereby improving the remanence.The better MPs of Fe 67.5 Nd 7 B 21 Zr 2 Nb 2.5 alloy compared to those of the reported α-Fe/Nd 2 Fe 14 B magnets are speculated to be due to the appropriate alloy composition, especially the Fe: Nd ratio as well as the existence of an ideal microstructure.That is to say, the higher HMPs in the alloy is ascribed to three factors: First, the formation of grains, which leads to strong magnetic exchange interactions between magnetically soft and hard phases.Second, the soft phase increment.Third, the existence of the fine grain boundary phase, which would be favorable to relate the magnetization reverse.
a (K) H ci (kA/m) B r (T) H k /H