Recent progress in high B_s and low H_c Fe-based nanocrystalline alloys

2.1 Enhancing the saturated magnetic flux density of Fe-based alloys

It is known that the amplitude of the local magnetic moment of Fe atoms is strongly affected by local environments. A higher coordination number and a lower interatomic distance bring about a reduction in the amplitude of the local magnetic moment. Thus, the amplitude of the local magnetic moment rapidly decreases with increasing Fe content. On the other hand, the total B_s decreases with decreasing Fe content [6]. Accordingly, the maximum value of B_s is at approximately 82 at% in the Fe_xB_100–x amorphous alloy system. The substitution of Co causes an increase in the number density at the Fermi level, and B_s increases to 1.8 T. However, Co is an expensive element, and hence, Co-free alloys are required for commercial use [7, 8].

On the other hand, it is considered that Nb and Zr suppress the growth of nanocrystals in conventional Fe-Cu-Nb-Si-B and Fe-Zr-B nanocrystalline alloys, respectively. However, these nanocrystalline alloys include more than 5 mass% Nb or Zr and <90 mass% of Fe, and hence, B_s is at most 1.7 T. Therefore, to obtain higher-B_s alloys, it is necessary to reduce the content of these heavy nonmagnetic elements [9].

Fang et al. [10] reported on the present study of the Fe-based nanocrystalline Fe_84-xB₁₀C₆Cu_x soft magnetic alloys with high magnetic flux density. The Cu content dependence of magnetic properties and crystalline behavior for annealed alloy ribbons fabricated by melt spinning were discussed. It is found that the Fe_84-xB₁₀C₆Cu_x alloy system exhibits excellent magnetic properties after the appropriate heat treatment with a high B_s of 1.78 T, low H_c of 5.1 A/m (see Figure 2), and low core loss <4.3 W/kg at 1.0 T and 400 Hz that is about 55% that of oriented Si steel.

Figure 2

Cu content dependence of (A) coercive force and (B) saturation magnetic flux density (Fang et al. [10], Copyright AIP 2011).

Ohta and Yoshizawa [11] reported that a nanocrystalline phase with an average grain size of about 20 nm is obtained by annealing Cu-substituted and/or Cu and Si complex-substituted Fe-B amorphous alloys. The alloy exhibits a high B_s of more than 1.8 T. The effects of Cu substitution and Cu and Si complex substitution in Fe-B amorphous alloys on the soft magnetic properties were studied from the viewpoint of nanocrystallization, as shown in Figure 3.

Figure 3

Schematic crystallization process of (A) conventional Fe-based nanocrystalline alloy (Fe-Cu-Nb-Si-B), (B) Fe-(Si)-B amorphous alloy, and (C) Fe-Cu-(Si)-B (Ohta and Yoshizawa [11] Copyright IOP 2011).

In detail, the crystallization processes for a conventional Fe-based nanocrystalline alloy, an Fe-based amorphous alloy, and the present alloy are shown schematically. As shown in (1), the amorphous phase is obtained as the AS-Q in the conventional Fe-based nanocrystalline alloy. In the annealing process, Cu atoms aggregate, and Cu clusters are formed. These Cu clusters act as nucleation sites in the primary crystals. At a higher temperature, grain growth occurs, and a nanocrystalline phase appears.

The Fe content in the remaining amorphous phase decreases, and the Nb or Zr content increases with the progression of crystallization [11]. Therefore, the thermal stability of the remaining amorphous state increases, and the growth of nanocrystalline grains is suppressed. However, because Nb or Zr content exceeding 5 mass% is required to obtain such amorphous and nanocrystalline phases, B_s is at most 1.7 T in conventional Fe-based nanocrystalline alloy systems. Accordingly, to obtain higher-B_s alloys, it is necessary to reduce the content of these heavy nonmagnetic elements. However, as shown in (2), grain coarsening occurs upon the annealing of a high-Fe content amorphous alloy without Cu due to the lack of nucleation sites of the primary crystals. There are two important factors in obtaining the nanocrystalline phase. One is the existence of a high number density of nucleation sites in the alloy before crystallization. The other is the suppression of the thermal growth of nanocrystals. As the heat of mixing between Fe and Cu is positive, an AS-Q amorphous alloy with a high number density of Cu clusters is obtained by melt quenching a liquid-phase alloy with an oversaturated Cu content, as shown in (3). Moreover, boron (B) hardly solidifies in bcc Fe, and hence, the B content in the remaining amorphous phase increases with the progression of nanocrystal growth. The B-rich amorphous phase inhibits grain growth, similarly to the Nb- or Zr-rich amorphous phase in conventional nanocrystalline [10, 11].

2.2 Effect of grain size on the soft magnetic properties

As can be found in the previous section, alloying elements play a huge role on the final soft magnetic properties of the Fe-based amorphous/nanocrystalline alloys. The role of these alloying elements in Fe-based amorphous/nanocrystalline system can be summarized in the following categories:

• having purifying effect to eliminate heterogeneous nucleation sites during solidification,

• acting as nucleation sites during crystallization,

• improving the glass-forming ability of the alloy.

In a nutshell, the aim of this alloying design is to achieve nanograins (preferably <20 nm) embedded in amorphous matrix. Figure 4 clearly addresses the significance of grain size on the coercivity of the Fe-based amorphous nanocrystalline alloy.

Figure 4

Coercivity (H_c) vs. grain size (D), for various soft magnetic metallic alloys (Herzer [12], Copyright 1997).

As can be seen above, the 1/D relation between grain size and coercivity refers to the conventional rule that soft magnetic properties of the materials require coarse grain size. But, new Fe-based amorphous/nanocrystalline alloys fill the gap between Fe-based amorphous and conventional alloys. The D⁶ dependence of coercivity and grain size indicates how the grain size of the α-Fe particles can affect the final soft magnetic properties of Fe-based amorphous/nanocrystalline alloys and how the boundary between soft and hard magnetic properties is delicate. This issue verifies the importance of grain size and achieving nanosized grains in Fe-based amorphous/nanocrystalline alloys [12].

Random anisotropy model (RAM) can be used to throw light on the effect of grain size on the soft magnetic behavior.

According to this model, if the grain size (D) is reduced below minimum exchange length (L₀), the magnetic anisotropy is reduced significantly, and subsequently, soft magnetic properties are improved. The magnetic anisotropy basically implies that the magnetic properties strongly depend on the direction in which they are measured. Three important kinds of magnetic anisotropy are (1) crystal anisotropy, (2) shape anisotropy, and (3) stress anisotropy [13].

It has been shown that the typical value of L₀ for Fe-base alloys is between 20 and 40 nm, and it is expected that Fe-based amorphous/nanocrystalline alloys with grain size of 5–20 nm drop into the regime of L₀<<D [13].

The following equations evidently show the connection between grain sizes and magnetic properties based on the random anisotropy model [13]:

Lex=A<K>  

<K>≈υcrK1N=υcrK1(DLex)32.

If there are no other anisotropies, coercivity and initial permeability are closely related to <K>, as given below:

where <K> is the magnetic anisotropy, A is the exchange stiffness constant, L₀ is the exchange length, D is the grain size, H_c is the coercivity, μ_i is the permeability, and P_c and P_μ are the dimensionless pre-factors.

Regarding equations (1) and (2), the smaller grain sizes, the lower magnetic anisotropy and, consequently, higher soft magnetic properties (i.e., lower H_c and higher μ_i) can be obtained.

Briefly speaking, alloying elements and microstructural features such as grain size and the presence of secondary phases can tremendously alter the final magnetic properties of the alloy, and this problem will be discussed in Section 2.2.1.

2.2.1 Minimizing the coercive force of Fe-based amorphous/nanocrystalline alloys

Although Fe-based nanocrystalline alloys with a B_s higher than 1.8 T have been developed, the coercive force (H_c) of these materials are still relatively high (5.7∼10.0 A/m). Hence, it is significant for applications to further improve the soft magnetic properties of these alloys.

Kong et al. [14] fabricated P-doped Fe-based nanocrystalline alloys with extremely low coercive force of <5 A/m. In this alloy system, the appropriate addition of the Cu element promotes the precipitation of a-Fe(Si), as well as inhibits the precipitation of other phases. The saturation magnetic flux density B_s increases, and the coercivity H_c markedly decreases simultaneously with increasing Cu content. The Fe_82.75Si₄B₈P₄Cu_1.25 alloy annealed at 873 K for 1.8 ks shows a high B_s of 1.83 T and excellent soft magnetic properties such as a low H_c of 2.1 A/m and a high effect permeability of 31,600. It is found that a 1%–1.25% addition of Cu is effective for the improvement of soft magnetic properties for the present alloy system (Figure 5).

Figure 5

Changes in H_c for Fe_84-xSi₄B₉P₄Cu_x alloys as a function of annealing temperature for 1.8 ks (Kong et al. [14], Copyright AIP 2011).

The excellent magnetic properties for the present alloys can be attributed to the formation of the microstructure of nanoscale a-Fe(Si) embedded in the amorphous matrix. The Cu element performs the role of nucleation agents, and the proper addition of Cu causes a decrease in the average grain sizes, as well as an increase in the number density of nanograins, which results in better soft magnetic properties.

More recently, Kong et al. [14] found that the proper substitution of B with P was found to be effective in decreasing grain size in the alloys annealed at 793 K for 120 s. The coercive force H_c markedly decreases from 67.1 to 1.1 A/m, and the saturation magnetic flux density B_s shows a slightly decreasing trend with increasing P content from x=1 to 5 (as seen in Figure 6). The nanocrystalline Fe_82.65Si₂B₉P₅Cu_1.35 alloy, with an average grain size of 15 nm (see Figure 7), shows a combination of high B_s and excellent soft magnetic properties.

Figure 6

Changes of B_s and H_c for Fe_82.65Si₂B_14–xP_xCu_1.35 (x=1–6) alloys annealed at 793 K for 120 s (Kong et al. [14], Copyright AIP 2012).

Figure 7

Bright-field TEM images, selected area electron diffraction (SAED) patterns, and distributions of grain sizes of the nanocrystalline (A) Fe_82.65Si₂B₁₃P₁Cu_1.35 and (B) Fe_82.65Si₂B₉P₅Cu_1.35 alloys (Kong et al. [14], Copyright AIP 2012).

In order to understand the reason for grain refinement by substituting B with P, the mixing enthalpy among the constituent elements were considered. Earlier studies reported that Cu atoms tend to separate from Fe and form clusters prior to the onset of the crystallization reaction due to the positive mixing enthalpy (+13 kJ/mol) between Cu and Fe [11]. On the other hand, attractive interactions exist between P and Fe, B and Fe due to the negative mixing enthalpy (-39.5 and -26 kJ/mol, respectively). When α-Fe heterogeneously nucleated at the site of Cu clusters from the amorphous matrix, P and B are excluded out of the crystalline phase. Consequently, residual amorphous phase becomes richer in P and B, which increase the stability of the amorphous phase and suppress the growth of α-Fe grains [13]. The mixing enthalpy with negative value for the Fe-P atomic pair is larger than that of the Fe-B atomic pair. Therefore, the bonding nature among the constituent elements was increased by substituting B with P. Different P to B concentration ratios result in different sizes of α-Fe grains.

2.3 Applications of Fe-based nanocrystalline alloys

As to the applications of Fe-based nanocrystalline/amorphous alloys, most of the studies are focused on transformer applications, and only few companies are developing these materials for electric motor applications. For example, Hitachi Special Metals is developing a high-performance electric motor (efficiency=86%) using nanocrystalline alloys; however, these motors have not been commercialized.

2.4 Challenges in Fe-based nanocrystalline alloys