FeCo-based amorphous alloys with high ferromagnetic elements and large annealing processing window

Highlights

• Ultra-high ferromagnetic element content of 87 atomic% was achieved in (FeCo)_xBSiVCuY amorphous/nanocrystalline alloys.

• The highest saturation magnetization of these as-cast ribbons can reach 1.80 T.

• Significant different crystallization activation energies exist in the primary and secondary exothermic peaks.

• A large annealing temperature window about 200 K facilitates to obtain dual-phase with improved magnetic properties.

Abstract

A seven-component (FeCo)_xBSiVCuY amorphous/nanocrystalline alloy system with high ferromagnetic element contents (x = 85, 86 and 87, at atomic percent) was synthesized by rapid quenching method. The microstructure, crystal structure, magnetic property and crystallization behavior were systematically investigated. It is found that the highest saturation magnetization of these as-cast ribbons reaches 1.80 T. Upon heating, two different crystal precipitated phases bcc-Fe (Co) and Fe-metalloids appear successively with temperature intervals (ΔT_x) as large as 193 K. Comparing with typical Fe-based amorphous/nanocrystallization alloys, (Fe_0.8Co_0.2)₈₇B₁₀Si₁V_0.5Cu_0.5Y₁ alloy shows an ultra-low primary crystallization activation energy and a large secondary crystallization activation energy, indicating an extremely large annealing temperature window more than 200 K beneficial for their processing. Therefore, this series of amorphous alloys provide potential precursors with decreasing annealing temperature and time for producing ferromagnetic nanocrystalline alloys, which can significantly reduce the cost for industrial production.

Introduction

Since first discovered more than thirty years ago [1], Fe-based nanocrystalline alloys have attracted considerable interests owing to their soft magnetic properties and wide applications in many electromagnetic devices. This kind of alloys is often formed by annealing the amorphous precursor or other processes to induce nanocrystallization, and thus often have an amorphous-nanocrystalline dual-phase structure. As a result, the Fe-based nanocrystalline alloys could have a combination of superior magnetic properties even better than the soft-magnetic Fe-based amorphous alloys. During past decades, exceptional improvement of magnetic properties for Fe-based nanocrystalline alloys has been achieved based on the major demand of exploiting high performance materials with higher saturation magnetic polarization (J_s), lower coercivity (H_c), higher magnetic permeability (μ), and enhanced core loss performance (W) at high frequencies [[1], [2], [3], [4], [5]].

The properties significantly depend on the structure of nanocrystalline precipitated phase. As for Fe-based nanocrystalline alloys, the easiest magnetization direction is determined by statistical fluctuations of the grains within the exchange correlation length (this quantity is referred to as the effective anisotropy: <K>), which is significantly different from polycrystalline materials (magnetocrystalline anisotropy: K₁) [6]. Herzer [7] reported that there is a correlation between coercivity/permeability and the 6th power of nanocrystalline grain size. By proper controlling the morphology of the nanocrystals, the values of effective anisotropy <K> are far lower than expected from polycrystalline materials, resulting in a huge improvement in soft magnetic properties. Therefore, a thorough investigation of the nanocrystallization process of the amorphous precursors is the key for optimization the properties of Fe-based nanocrystalline alloys.

Owing to the thermodynamically metastable nature [8], amorphous alloys will go through the process of relaxation and finally crystallization when sufficient energy was imposed. The behavior of crystallization is quite complicated, which include both the nucleation and grain growth process. To initiate the nucleation and grain growth process of nanocrystallization in amorphous alloys, many methods including annealing [9], energizing [10], stressing [11] and pressuring [12] could be used. The crucial factor to govern this dynamic process of nanocrystallization is the activation energy, which represents the energy barrier need to be overcome. As for annealing, a common way to induce nanocrystallization in amorphous alloys, the activation energy of crystallization indicates the difficulty of the nucleation and grain growth process, thus ultimately reflects the annealing temperature or annealing time [13]. Over the past decades, there have been numerous works on the activation energy of crystallization in Fe-based amorphous/nanocrystalline alloys. This energy is usually between about 250 kJ/mol and 450 kJ/mol. For example, the average activation energy of the first crystallization event E_x1 of Fe_73.5Si_13.5B₉Nb₃Cu₁ (Finemet-type) [14], Fe₈₈Zr₇B₄Cu (Nanoperm-type) [15] and Fe_44.5Co_44.5Zr₇B₄ (Hitperm-type) [15] alloys are 378 kJ/mol, 298 kJ/mol and 366 kJ/mol, respectively. Borrego et al. [16] found that partial substitution of early transition metals in Fe–Si–B–Cu–Nb system alloys has a substantial effect on the activation energy, which reduce when Ta, V and Mo are substituted for Nb while increase for Zr substitution. Kim et al. [17,18] found the activation energy reduces from 396 kJ/mol to 248 kJ/mol in Fe_77.5Si_13.5B₉ by replacing 1 atomic% Fe with Cu, resulting from the Cu-rich regions (clusters) act as the easier nucleation site of α-Fe in the early stage of the crystallization process. In practice, a high value of activation energy is not favorable since it will greatly increase the annealing temperature and time, and thus increase the cost for industrial production of these alloys.

In general, the saturation magnetization of Fe-based nanocrystalline soft alloys is determined by the mass fraction rather than the atomic fraction of the ferromagnetic elements content [19]. Therefore, to develop such materials which would break through the limitation of ferromagnetic elements content has always been the unwearied pursuit of researchers [20]. What's more, the high contents of ferromagnetic elements also facilitate the precipitation of nanocrystalline phases. In this work, we reported a series of new FeCo-based amorphous/nanocrystalline alloys with the content of ferromagnetic elements as high as 87 atomic%. These alloys exhibit superior soft magnetic properties with the saturation magnetization of as-cast ribbons reaches 1.8 T. In addition, the activation energies of these alloys are relatively lower for primary precipitation and higher for secondary precipitation as compared to previous typical Fe-based amorphous alloys, making them quite processable in the subsequent annealing process. Therefore, these amorphous alloys may be potential precursors for producing ferromagnetic nanocrystalline alloys with high performance and low cost.