Extremely high B_s (Fe_1-xCox)₈₆Ni₁B₁₃ amorphous soft magnetic alloys with good bending ductility

Highlights

• Extremely high B_s was obtained through substitution of appropriate Co for Fe in (Fe_1-xCox)₈₆Ni₁B₁₃ amorphous alloys.

• The amorphous alloy with x = 0.1 at. % exhibit good bending ductility in the annealed state.

• The T_c of the alloy system was remarkable increase by adding Co.

Abstract

The effects of Co content on the thermal stability, soft-magnetic properties, corrosion resistance and mechanical properties of (Fe_1-xCox)₈₆Ni₁B₁₃ (x = 0–0.25) amorphous alloys were systematically investigated. Adequate Co addition can significantly increase saturation magnetic flux density (B_s). Extremely high B_s of 1.78–1.92 T was obtained when Co content was added in the range of x = 0.05–0.25. Besides, the Curie temperature (T_c) increases markedly with the introduction of Co, exhibiting the high thermal stability of soft magnetic properties. Compared with x = 0 (Co-free) alloys, the T_c of x = 0.1 (0.1Co) alloy increases more than 100 K, far exceeding the first crystallization temperature. There is also an obvious increase in temperature intervals between the two exothermic peaks of crystallization with Co content, implying high thermal stability of the Co-containing alloys. In addition, the corrosion resistance of 0.1Co alloy was also improved compared with the Co-free alloy. Among the (Fe_1-xCo_x)₈₆Ni₁B₁₃ amorphous alloys, the one with 0.1Co simultaneously exhibits high B_s of 1.87 T and low H_c of 10 A/m, as well as good bending ductility at annealed state. Moreover, the underlying mechanisms of the good ductility of the 0.1Co alloy annealed under the optimal condition was also discussed from the perspective of atom bonding nature. The combination of extremely high B_s and low H_c in conjunction with good manufacturability makes the (Fe_0.9Co_0.1)₈₆Ni₁B₁₃ alloys promising soft-magnetic materials for potential applications.

Introduction

Soft-magnetic materials play a key role in the conversion of energy throughout our world, which includes the efficient operation of the electronic-magnetic devices and electrical machines. The transition of human society to both a more electrified world and sustainable sources of energy, there is an urgent need for further improved soft magnetic magnets capable of efficient operation. Amorphous soft magnetic alloys stand out from numerous soft magnetic materials. They exert significant energy saving effects because their unique structure in favor of magnetic isotropy and extremely thin laminations work together to greatly attenuate eddy current losses even at high frequencies in comparison with silicon [1,2] However, fabricating challenges of brittle laminations and low B_s relative to silicon steel have greatly limited their application in large-scale transformers and electrical machines, so silicon steels still make up a majority of the global market for soft magnetic materials of choice [3,4]. Therefore, new amorphous soft magnetic alloys with high B_s and good ductility are greatly required.

Usually, there are three effective methods for achieving a high B_s in Fe based amorphous alloys, including increasing Fe content [5], nanocrystallization by subsequent annealing treatment [6], and substitution of Fe with Co [7]. Great effort has focused on developing high Fe content amorphous or nanocrystalline alloys with the purpose of increasing B_s toward the value of FeSi steel while maintaining good magnetic softness. Consequently, B_s over 1.7 T were obtained in Fe₈₄P₃B_8.5Si_4.5 and Fe₈₄B_8.5Si_4.1P_3.25C_0.15 [2,8], while remarkably high B_s value of 1.94 T was revealed in (within the Fe–Cu–Si–B–P system) nanocrystalline Fe_84.3Si₄B₈P₃Cu_0.7 ribbon [6]. Recent studies have shown that proper addition of metalloid C can not only effectively improve the AFA and inhibit surface crystallization, but also favor a uniform microstructure in high Fe-content NANOMET-type nanocrystalline alloys [9,10]. However, the effect of increasing Fe content alone on improving B_s value is limited while ensuring good soft magnetic properties. Besides, it has been found that the nanocrystallization of high B_s amorphous alloys tend to suffer harsh annealing process, non-uniform microstructure and poor manufacturability [6]. Recently, the composition design strategy of adding cobalt element in Fe-based amorphous alloys was adopted. Pauling described the effect of Co atoms on the magnetic moment of the alloy from the perspective of atomic orbitals [11]. Later Díaz-Ortiz et al. further explained the effect of different states of ordering in Fe–Co alloys on magnetic moment by using the method of variational cluster expansion method based on first-principles data [12]. Based on the idea, the maximum magnetic moment is obtained in Fe/Co (8:2) alloys, and several Co-containing Fe-based amorphous alloys around 0.2Co without decreasing AFA have been developed, of which the prominent ones with high B_s are (Fe_0.8Co_0.2)₈₃B₁₆Si₁ with 1.86 T and (Fe_0.8Co_0.2)₈₅B₁₄Si₁ with 1.92 T [13,14]. It should be noted that the composition point at 0.2Co for the maximum magnetic moment were derived from binary Fe–Co alloys, but it may not be suitable for multicomponent alloys. The effect of multiple metalloids and other metal elements on the magnetic nature of multi-component amorphous alloys including magnetic moment is very complicated and thus not known in detail (i.e. B, C, P and Si) [15]. Therefore, it is necessary to make a more extensive investigation of the multi-component Fe-based amorphous alloy system doped with Co. The addition of adequate quantities of metalloids is invariably necessary for the formation of the amorphous structure, but excessive metalloids tend to form strong transition metal-metalloid bonds, which is not conducive to plasticity [16]. Recently, some ferromagnetic amorphous alloys with excellent soft magnetic properties and good bending ductility after annealing have been reported [9]. However, the nature of the ductile-brittle transition for Fe-based amorphous alloys are seldom interpreted from the perspective of electronic structure.

The detailed composition can be widely varied, which allows coverage of a large spectrum of soft magnetic properties according to the demands of the application. Very recently, K. Suzuki found the dramatic grain refinement and magnetic softening induced by Ni addition in Fe–Ni–B nanocrystalline soft magnetic alloys [17]. However, they are unavailable on the current conditions of the laboratory due to the high Fe content and the harsh nanocrystallization technique [17,18]. In this paper, Co was intentionally added to the Fe–Ni–B alloy with the anticipation that the alloy would have excellent soft magnetic properties including exceptionally high B_s and low H_c values in the amorphous state. Simultaneously, extremely the high Fe content and relatively low metal-metalloid content can also be expected to toughen the amorphous alloy ribbon.