A novel FeCoNiCr0.2Si0.2 high entropy alloy with an excellent balance of mechanical and soft magnetic properties

doi:10.1016/j.jmmm.2019.01.096

Journal of Magnetism and Magnetic Materials

Volume 478, 15 May 2019, Pages 116-121

https://doi.org/10.1016/j.jmmm.2019.01.096 Get rights and content

Highlights

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A good balance between magnetic and mechanical properties is achieved in the HEA.
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The H_c of the HEA is much lower than most of the previously reported HEAs.
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After rolling and annealing, the plastic deformation of the HEA is near to 60%.
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The UTS × EL of the annealed HEA is one of the highest values in HEAs.
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The present HEA is comprised of a simple and stable FCC solid solution structure.

Abstract

A novel FeCoNiCr_0.2Si_0.2 (at. %, thereafter, all mean atomic ratios) high-entropy alloy (HEA) was synthesized. The as-cast HEA exhibits a combination of excellent mechanical and magnetic properties with a large plastic deformation of about 60% and low coercivity (H_c) of about 187.9 A/m, which are prominent in the HEAs reported so far. Based on the large plasticity, rolling and annealing were adopted as a strategy for improving magnetic and mechanical properties of the HEA. The process of rolling followed by annealing leads to the significant enhancement of the yield strength (YS) and ultimate tensile strength (UTS) of alloy rolled at 773 K, increasing to 320 and 920 MPa respectively. Meanwhile the large plasticity and good soft magnetic properties remain. The enhancement mechanism of annealed after rolling was analyzed. Consequently, the optimal balance of magnetic and mechanical properties is achieved. The present work suggests a promising way to develop HEAs with a combination of excellent magnetic and mechanical properties.

Introduction

The emergence of high entropy alloys (HEAs) provided a new alloy design concept [1], [2]. Generally, HEAs contain at least five principal elements with concentration between 5 and 35 at. %, which is different from traditional alloys containing one or two principal elements, and is beneficial for broadening the compositional range of alloys. Recent studies further extended the boundary of HEAs by categorizing quaternary and ternary alloy system with moderate component and solid solution structure into HEAs [3], [4], [5]. HEAs tend to form single-phase solid solutions rather than intermetallics, which is commonly regarded to be resulted from high entropy effect [6], [7]. The properties of HEAs such as their high strength, excellent room-temperature ductility, good thermal stability and high electrical resistivity have been investigated extensively [4], [8], [9], [10], [11], [12], [13], [14]. From the view of tuning coupled structural-magnetic transition, Huang et al. put forward a systematic study of the Curie temperature (T_c) for a number of equiatomic medium- and high-entropy alloys using first-principle theory, which are helpful to identify promising magnetic compositions [15].

As soft magnetic materials, a good balance of magnetic and mechanical properties is vitally important. Among current magnetic materials, silicon steel and Fe-based amorphous alloy have been treated as two kinds of desired core materials. However, owing to the intrinsically limited ductility and formability of 6.5 wt% Si electrical steel [16], it is extremely hard to be manufactured by conventional thick-slab casting and rolling process. And Fe-based amorphous alloy has not the ability of rolling deformation due to their lack of ductility. A series of FeCoNi-containing HEAs with excellent mechanical properties and promising ferromagnetic properties have been developed in recent years [17], [18], [19], [20], among which FeCoNiSiB [18], [21], FeCoNi(AlSi)_0.2 [22] and FeCoNiCrAl [23] HEAs show relatively high saturated magnetization (M_s) and malleability. Moreover, the FeCoNi-containing HEAs usually exhibit high thermal stability at elevated temperatures, which provides a great opportunity to be utilized as high-temperature magnetic material [26], [27], [28]. High ductility of HEAs can greatly overcome the formability limit of silicon steel. However, the H_c of FeCoNi-containing HEAs currently developed is obviously higher than those of Fe-based amorphous alloy and even silicon steel, which remains a challenge for future practical applications. Therefore, more investigations deserve to be conducted on developing new HEAs with both excellent malleability and low H_c.

In the present study, a novel FeCoNiCr_0.2Si_0.2 HEA is developed with the aim of obtaining a good balance of improved soft magnetic properties, excellent mechanical properties and high structural stability. Rolling and annealing, widely used as a formability route, were introduced as a strategy for improving magnetic and mechanical properties in the present work.

Section snippets

Experimental procedure

Alloy ingots with nominal composition of FeCoNiCr_0.2Si_0.2 (at. %) were prepared by arc melting the mixtures of Fe, Co, Ni, Cr and Si (purity above 99.9 wt%) in a water-cooled Cu crucible under Ti-gettered argon atmosphere. The ingots were re-melted 5 times to ensure chemical homogeneity. The plates with 75 mm × 30 mm × 6 mm were produced by the casted method of tilt-pour steel mold casting. The plates were deformed by multi-pass hot rolling at 773 K to thickness strain of 59%. Then the rolled

Results

Fig. 1 shows the XRD patterns of as-cast, as-rolled and annealed after rolling FeCoNiCr_0.2Si_0.2 HEA. Only FCC phase could be detected in the three statuses, indicating that no phase transformation occurred during hot rolling and annealing. Generally, the introduction of deformation and stress during repeatedly rolling can lead to broadening and shifting of diffraction peak. But it is interesting and abnormal that the present alloy does not exhibit this behavior, which is similar to that

Discussion

Previous work indicates that the T_c of HEAs is sensitive to their base composition, additional alloying elements, and crystal structure [15], [31], [34]. For the present HEA, T_c becomes slightly higher after rolling, which is similar to the result achieved by Lucas et al. in FeCoCrNi HEA [25]. And the possible reason is that both the as-cast and annealed after rolling samples have some degree of short-range chemical ordering, but the rolled sample is chemically disordered during the mechanical

Conclusion

In summary, FeCoNiCr_0.2Si_0.2 HEA developed in this work was prepared by casting and processed by rolling and recrystallization annealing route. Structural, thermal, magnetic and mechanical properties of the alloys were investigated and compared. Major conclusions of this work are drawn as follows:

1.
The good balance of improved soft magnetic properties and excellent mechanical properties is obtained in novel FeCoNiCr_0.2Si_0.2 HEA. The plastic deformation of the HEA is near to 60%. The H_c of the

Acknowledgements

The work was supported by National Natural Science Foundation of China [grant numbers U1704159, 51701183]; China Postdoctoral Science Foundation funded project [grant numbers 2017M622368, 2018M630834]; and Henan Technical Innovation Guidance projects [grant number 182107000050].

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Research articlesA novel FeCoNiCr0.2Si0.2 high entropy alloy with an excellent balance of mechanical and soft magnetic properties

Highlights

Abstract

Introduction

Section snippets

Experimental procedure

Results

Discussion

Conclusion

Acknowledgements

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Prog. Mater. Sci.

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Acta Mater.

Acta Mater.

Scripta Mater.

Intermetallics

J. Magn. Magn. Mater.

J. Magn. Magn. Mater.

J. Magn. Magn. Mater.

Acta Mater.

Mater. Lett.

Mater. Sci. Eng. A-Struct.

J. Alloy. Compd.

Mater. Sci. Eng. A-Struct.

J. Alloy. Compd.

Intermetallics

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Research articles
A novel FeCoNiCr_0.2Si_0.2 high entropy alloy with an excellent balance of mechanical and soft magnetic properties