High performance Fe-based nanocrystalline alloys with excellent thermal stability

Accepted Manuscript High performance Fe-based nanocrystalline alloys with excellent thermal stability Tao Liu, Fucheng Li, Anding Wang, Lei Xie, QuanFeng He, Junhua Luan, Aina He, Xinmin Wang, C.T. Liu, Yong Yang PII:

S0925-8388(18)34011-8

DOI:

https://doi.org/10.1016/j.jallcom.2018.10.319

Reference:

JALCOM 48137

To appear in:

Journal of Alloys and Compounds

Received Date: 5 September 2018 Revised Date:

10 October 2018

Accepted Date: 24 October 2018

Please cite this article as: T. Liu, F. Li, A. Wang, L. Xie, Q. He, J. Luan, A. He, X. Wang, C.T. Liu, Y. Yang, High performance Fe-based nanocrystalline alloys with excellent thermal stability, Journal of Alloys and Compounds (2018), doi: https://doi.org/10.1016/j.jallcom.2018.10.319. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT High performance Fe-based nanocrystalline alloys with excellent thermal stability

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Tao Liua,b,c, Fucheng Lib, Anding Wanga,b*, Lei Xiea, QuanFeng Heb, Junhua Luanb, Aina Hea,d, Xinmin Wanga,d, C. T. Liub, Yong Yang b* a Key Laboratory of Magnetic Materials and Devices, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, Zhejiang 315201, China b Center for Advanced Structural Materials, Department of Mechanical and Biomedical Engineering, College of Science and Engineering, City University of Hong Kong, Kowloon, Hong Kong SAR, China c University of Chinese Academy of Sciences, 19 A Yuquan Rd, Shijingshan District, Beijing, P.R.China 100049 d Zhejiang Province Key Laboratory of Magnetic Materials and Application Technology, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China Corresponding authors: AW ([email protected]) and YY ([email protected]) Abstract

Fe-based alloys are an important soft magnetic material that plays a pivotal role in a variety of energy-related industrial applications, such as electric transformers, motors

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and converters. However, the trade-off between magnetic softness and saturation magnetization (Bs) has been hindering the development of next-generation Fe-based soft magnetic materials. In this work, we demonstrate a facile route to obtain nanostructured Fe-based alloy out of a marginal glass former, which exhibits a

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core-shell like nanostructure and a synergetic increase of Bs, magnetic permeability (µ e) and hardness upon thermal annealing, giving rise to a unique combination of Bs >

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1.83 T, µ e ~25000 and ultrahigh hardness of ~15 GPa. In terms of the combined magnetic/mechanical properties, the nanostructured Fe-based alloy outperforms the variety of Fe-based soft magnetic alloys hitherto reported. Furthermore, compared to the existing high performance soft magnetic Fe-based nanocrystalline alloys, the core-shell like nanostructure in our Fe-based nanocrystalline alloy displays an excellent thermal stability, which offers a large time window which would facilitate materials processing for future large-scale industrial production.

Keywords: Soft magnetic material; Nanostructure; Hardness; Core-shell structure.

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ACCEPTED MANUSCRIPT 1. Introduction Soft magnetic alloys are an important engineering material for human civilization, which have been playing a crucial role in various critical engineering applications,

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such as power generation and conversion [1]. Recently, Fe-based soft magnetic amorphous alloys or metallic glasses (MGs) have attracted tremendous interest in both academia and industry due to their high efficiency and low core loss when used

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as core materials in transformers and motors. However, the saturation magnetization

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(Bs) of Fe-based MGs is much lower than that of Si-steels, the widely used soft magnetic alloy with Bs of 1.8~2.0 T [2]. To enhance Bs of Fe-based MGs, a number of strategies were proposed and tested. It was found that the increase of Fe content and/or the addition of Co element is effective to improve the Bs of Fe-bases MGs.

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Unfortunately, the Fe-based soft magnetic alloys so obtained often suffer from degradation in their magnetic softness. This trade-off was noted in the literature [3], which can be rationalized as a result of the increase in magnetic anisotropy due to the

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stabilization of magnetic domains with the increase of ferromagnetic elements [4].

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Alternatively, the formation of nanosized α-Fe grains in an amorphous matrix provides another route to improve the Bs of the original MG [5], since Bs is generally proportional to the volume fraction of α-Fe grains [6]. Following this strategy, tremendous nanocrystalline soft magnetic alloys with enhanced Bs were successfully developed, such as Fe73.5Si9B13.5Nb3Cu1 (Finemet®) [5] and Fe88Zr6B6 (Nanoperm®) [7] etc., which also exhibit good magnetic softness. These efforts were supplemented recently by further increasing the Fe content and reducing the use of transition metal

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ACCEPTED MANUSCRIPT elements (TM = Nb, Zr and Mo etc.) for further property enhancement. Nevertheless, it was found that the nanostructure of these high performance Fe-based soft magnetic alloys possesses a rather poor thermal stability. Consequently, the time window

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available for materials processing, such as isothermal annealing, is very limited for good magnetic softness. For instance, the annealing time (tA) should be controlled within 3 min in order to avoid any structural coarsening and property degradation in

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the high performance soft magnetic FeBCCu [8] and FePCCu [9] alloy systems. For

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the same reason, a very high heating rate is also demanded sometime to retain good magnetic softness in these high performance Fe-rich soft magnetic alloys [10,11]. These harsh and stringent requirements significantly raise the cost in making these high performance soft magnetic alloys and, thereby, hinders their large-scale

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industrial production.

Aside from the magnetic properties, the mechanical properties of soft magnetic materials, such as hardness (H), are also important because higher hardness gives rise

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to better wear resistance, therefore an improved mechanical durability [12]. In general,

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Fe-based MGs possess much higher H (~10 GPa) than crystalline soft magnetic materials (no more than 8 GPa) in a bulk form [13]. However, the data for the mechanical properties of ribbon samples are still very limited, especially for nanostructured Fe-based alloys. It is possible that the formation of nanosized grains can enhance alloy hardness, as reported in Al-based alloys [12]. However, the systematic studies are still rare. In this study, a nanostructured alloy with the chemical composition Fe84.75Si2B9P3C0.5Cu0.75 (in atomic percentage) was developed through

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ACCEPTED MANUSCRIPT the isothermal annealing of an amorphous precursor. It is attractive that the nanocrystalline alloy exhibits a synergetic increase of Bs, µ e and H upon thermal annealing, which is unparalleled by the existing commercial soft magnetic materials.

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Moreover, unlike the recently developed high performance nanostructured soft magnetic Fe-based alloys, the nanostructure we obtained possesses an excellent thermal stability, which greatly expands the time window for materials processing and,

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therefore, paves the way for a large-scale production of high performance soft

2. Material and methods

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magnetic alloys for a variety of industrial applications.

Multicomponent alloy with nominal atomic composition of Fe84.75Si2B9P3C0.5Cu0.75 was designed and melt by induction melting under Ar atmosphere after high vacuum

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of about 1×10-2 Pa with pure elements of Fe (99.99 wt.%), Si (99.99 wt.%), B (99.9 wt.%), Cu (99.99 wt.%) and pre-alloy of Fe3P and Fe-3.6%C. The master Fe84.75Si2B9P3C0.5Cu0.75 alloy was then prepared into ribbon samples with width of

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about 1 mm and thickness of about 20 µm through single roller melt-spinning method.

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These ribbon samples were placed in a quartz tube, which can be pumped to a high vacuum of ~5×10-3 Pa. Then the quartz tube was inserted into a preheated tube furnace to be annealed. In this study, isothermal annealing was carried out at 420 ºC for different time followed by water-quenching to synthesize the nanocrystalline alloys. Isothermal annealing was carried out in a high vacuum tube furnace at 420 ºC for different time followed by water-quenching to synthesize the nanocrystalline alloys.

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ACCEPTED MANUSCRIPT The magnetic properties, including saturation magnetic flux density (Bs), coercivity (Hc) and effective permeability (µ e) were measured with vibrating sample magnetometer (VSM, Lake Shore 7410) under the field of 800 kA/m, B-H loop tracer

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(EXPH-100) under the field of 800 A/m and impedance analyzer (Agilent 4294 A) under different applied magnetic field, respectively. Hardness (H) and reduced modulus (Er) were measured with nanoindentation (TI950, Hysitron, Minneapolis,

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MN), and the density of the master alloys was obtained by Archimedes method. The

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properties of these samples were averaged by multiple tests in order to ensure the accuracy.

Atomic- and nanostructure of the ribbon samples before and after annealing were identified by X-ray diffraction (XRD, Bruker D8 Advance) with Cu-Kα radiation and

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high-resolution transmission electron microscopy (TEM, TECNAI F20). Thermal performance of the as-quenched and annealed samples were analyzed with differential scanning calorimetry (DSC, NETZSCH 404C) at a heating rate of 40 ºC/min.

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Needle-shaped specimens required for atom probe tomography (APT) were fabricated

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by lift-outs and annular milled in a FEI Scios focused ion beam/scanning electron microscope (FIB/SEM). The APT characterizations were performed in a local electrode atom probe (CAMEACA LEAP 5000 R). The specimens were analyzed at 50 K in voltage mode, a pulse repetition rate of 200 kHz, a pulse fraction of 20%, and an evaporation detection rate of 0.5% atom per pulse. Imago Visualization and Analysis Software (IVAS) version 3.8 was used for creating the 3D reconstructions and data analysis.

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ACCEPTED MANUSCRIPT 3. Results and Discussion 3.1. Structure characterization The amorphous precursor with the chemical composition Fe84.75Si2B9P3C0.5Cu0.75

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was first fabricated directly into ribbons by induction melting and single-roller melt-spinning techniques. These ribbon samples were then annealed at 420 ºC for different time to form a nanosized structure. According to the X-ray diffraction (XRD)

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results shown in Fig. 1(a), the ribbon sample initially kept to its amorphous structure

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after 1-minute annealing. As the annealing time (tA) increased, crystallization took place, as identified by the sharp crystalline peaks corresponding to α-Fe. According to the Scherrer equation [14], we calculated the average size of the α-Fe nano-grains, which initially undergoes a steady increase and then levels to a plateau value of ~25

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nm for a prolonged annealing time of 96 min. This behavior indicates an excellent thermal stability of the nanostructured Fe-based alloys. For further structural examination, transmission electron microscopy (TEM) were

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conducted. The as-quenched sample shows no distinction in Fig. 1(b) with diffused

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rings in the selected area electron diffraction (SAED) pattern [Fig. 1(c)], which indicates the amorphous structure of the as-quenched ribbons. Interestingly, high density local structural ordering at the size of 2~5 nm can be easily observed in the high resolution TEM image, as shown in Fig. 1(d). These local ordered structures can be identified as bcc-like, according to the Fast Fourier Transformation (FFT) pattern conducted within the marked area in Fig. 1(e). In principle, one may infer that heterogeneous nucleation may be triggered near these local ordering sites, as reported

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ACCEPTED MANUSCRIPT in Al-based [15] and some high Fe content [16] nanocrystalline alloys. Fig. 1(f)-(h) show the bright field TEM images of the samples after annealing for 4, 48 and 96 min, respectively. Evidently, grains with an average size of ~20 nm can be observed and

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these nano-grains can be further identified to have crystallography identical to α-Fe from the selected area electron diffraction (SAED) pattern [Fig. 1(i)]. All these TEM

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observations are consistent with our XRD results.

Fig. 1. Atomic- and nanostructure of the Fe84.75Si2B9P3C0.5Cu0.75 alloy before and 7

ACCEPTED MANUSCRIPT after annealing at 420 ºC for different time; (a) XRD patterns with the grain size calculated according to Scherrer’s equation; (b) TEM image of the as-quenched sample with the (c) SAED pattern; (d) High resolution TEM image of the

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as-quenched samples with the (e) FFT pattern conducted within the marked area; (f)-(h) TEM bright field images of the samples after annealing for 4 min, 48 min and 96 min, respectively; (i) The SAED pattern of the annealed samples.

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3.2. Magnetic and mechanical properties

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The magnetic properties of the samples, including Bs and coercivity (Hc), after annealing for different time were shown in Fig. 2(a). The alloy’s Bs keeps around 1.61 T for the samples after annealing less than 2 min, and increases sharply to ~1.84 T after annealing for more than 4 min. Meanwhile, it is important to note that the Hc of

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these nanostructured alloys remain a low value, ranging from 2.9 to 9.8 A/m. Furthermore, the hardness H (reduced modulus Er) is around 9.5 GPa (160 GPa) for tA ≤ 2 min then increases to 15.0 GPa (210 GPa) for tA ≥ 4 min [Fig. 2(b)]. The magnetic

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and mechanical properties, including Bs, µe (at 1 KHz) and hardness, of the typical

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soft magnetic materials are summarized in Table 1. Note that, the µe values were measured with the ribbon samples under different applied magnetic field, ranging from 12000 to 25000 at 1 KHz. In sharp contrast to the various soft magnetic materials, our nanostructured alloys exhibit both high Bs and µe, as shown in Fig. 2(c), as well as very high H [Fig. 2(d)], almost five times that of silicon steels. This unique combination of mechanical and magnetic properties put our nanostructured Fe-based alloys on top of a variety of soft magnetic Fe-based alloys.

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Fig. 2. Magnetic and mechanical properties of the Fe84.75Si2B9P3C0.5Cu0.75 alloy

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before and after annealing at 420 ºC for different time; (a) Hysteresis loops and inset the Bs and Hc values of these alloys; (b) The histogram of Er and H; (c)-(d) The statistics of Bs, µe (at 1 kHz) and E, H for the soft magnetic materials, respectively.

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Table 1 Magnetic and mechanical properties of the soft magnetic materials µe (at 1 KHz)

H (GPa)

Ref.

300-2800

7.0

[17]

1000-2800

7.1-7.5

[18]

3000-30000

6.4

[19]

Ni-Zn

FG-series

Ferrite

ZN-series

Mn-Zn

HPF-series

Bs (T) 0.23-0.38 0.34-0.35 0.32-0.49

Ferrite

Mn1-xZnxFe2O4

0.39-0.47

1700-5900

7.2

[20]

Permalloy

4-79 Permalloy

053-0.88

8800-100000

1.2-2.4

[21]

Sendust

Fe-Si-Al-(Ni)

0.87-1.36

14000-35700

3.9-6.7

[22,23]

Fe-TM c)-M

Fe-Nb-B

0.64-1.23

15100-19800

11.2-12.0

[24]

Fe-Al-Ga-BPC

1.14-1.25

19000

8.5-11.2

[25]

Metglas® 2605

1.56-1.64

~9000

8.1-11.2

[26]

Fe80P11C9

1.49

11000

8.8-9.7

[27]

Fe78Si9-xB13Px

1.53-1.58

3800-11900

~10.0

[28]

Composition (at. %)

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Type

(BMG)

a)

Fe-M b) (MG)

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1.12-1.25

62000-81000

9.2-13.2

[5]

Fe72-77Si3-17B6-13TM1-5Cu1-2Al0-7

1.05-1.43

8000-150000

8.5-11.1

[29]

Nanoperm

Fe84-91TM7B2-9Cu0-2

1.42-1.7

10000-50000

7.4-13.8

[30]

Hitperm

(Fe0.5Co0.5)93-xTMxB6Cu1

1.70-1.92

1800-10000

Fe-M-Cu

Fe83-xSi4B13Cux

1.73-1.77

Nano-struct

Fe83.3Si4B12-xPxCu0.7

Finemet

ured

[30] [31]

1.70-1.83

4000-12000

[32]

1.83

16000-24000

[33]

Nanomet

Fe81.2Co4Si0.5B9.5P4Cu0.8®

Fe-Si 3 wt. % or 6.5 wt. %

1.78-2.01

2100-6100

3.1-4.3

This study

Fe84.75Si2B9P3C0.5Cu0.75

1.84

12000-25000

15.6

[34,35]

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Si-steel

a) BMG: Bulk metallic glass

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b) M: Metallic elements (Si, B, P, C etc.) c) TM: Transitional metal elements (Nb, Zr, Mo etc.)

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2.3. Mechanism for thermal stability of the nanocrystalline alloys

To understand the formation of nano-sized grains with the excellent thermal stability in Fe84.75Si2B9P3C0.5Cu0.75 alloy, differential scanning calorimetry (DSC) was

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performed on the as-quenched amorphous precursor and nanostructured alloys. As seen in Fig. 3(a), two separated exothermic peaks can be easily observed for the as-quenched amorphous sample. The first peak occurs at a temperature slightly above

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400 ºC, which is very close to the typical crystallization temperature of α-Fe [36]. The

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second one occurs at a rather high temperature close to 500 ºC, which agrees with the typical crystallization temperature of Fe-containing metallic compounds, such as boride, phosphide and carbide [37]. After the sample was annealed at 420 ºC for tA ≥ 4 min, the first crystallization peak disappears, which suggests that the precipitation of α-Fe grains levels to a plateau value after annealing for 4 min. This is also consistent with our XRD and TEM results (Fig. 1). Interestingly, the second crystallization peak shifts to a lower temperature as the first crystallization peak disappears. This delivers

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ACCEPTED MANUSCRIPT a strong message that the precipitation of the low temperature α-Fe like phase leads to the enrichment of metalloid elements and facilitates the precipitation of the second or the high temperature phase, which will be studied by APT in the following part.

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Based on our DSC results, now we can propose a mechanism of “dual phase co-growth” to understand the kinetics of nano-grain growth. In the 3D space, this co-growth mechanism may be pictured as follows. As illustrated by the inset of Fig. the α-Fe like grains initially precipitate out of the amorphous matrix, which

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3(b),

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have a Fe concentration higher than the average composition and therefore absorbs Fe atoms from their surroundings. As a result, this creates a Fe-lean inter-layer or interphase wrapping around the α-Fe like grains. As the interphase is rich in metalloid elements, Fe-containing compounds are likely to form. Consequently, it would

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become difficult for the α-Fe like grains to grow further if the diffusion of Fe from the amorphous matrix into the grains can be effectively shielded by the interphases. To validate the above thinking, we first develop a simple kinetics model to account for

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the co-growth of the α-Fe like grains with their inter-phases. For a steady state flux

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during grain growth of spherical grains [38], the time rate of change of the grain radius R(t) can be derived as

dR(t ) CI − CM (t ) D = dt CI − CP R(t )

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where D is the volume diffusion coefficient or diffusivity; CP and CI stand for the metalloid elements (B, P and C) concentration in the grain and interphase, 11

ACCEPTED MANUSCRIPT respectively; and CM(t) is the average concentration in the matrix at time t. Furthermore, the law of conservation for solutes leads to

(2)

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4π 4π 3 (CI − CP )[ R 3 (t ) − R 3 (0)] = RS (t )[CM (0) − CM (t )] 3 3

where 2Rs stands for an average inter-granular spacing. At t = 0, R(0) = 0, because our

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alloy is still amorphous and crystallization has not started yet. If we approximate CM(t)

which yields:

  3λ Dt   2 λH Rs 1 − exp  − H 2   3 Rs    

1 2

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R(t ) =

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≈ CI, we can obtain an analytical solution to the above equations (see Appendix A),

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 C (0) − CI  3 where λH =  M  . Next, we can fit the model to our experimental data,  CP − CI  i.e. the grain size obtained at 420 and 460 ºC for different times [Fig. 1(a)], by

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treating λH and Rs as the two fitting parameters. Note that, during data fitting, D is taken to be in the order of ~10-18, which is a typical value for the Fe-based MGs in the similar compositions [39]. As seen in Fig. 3(b), the general trends of our experimental data for the two annealing temperatures (420 and 460 ºC) can be captured remarkably well by our model for the same set of fitting parameters

λH = 0.8, Rs = 19 nm. This finding readily supports our theory about the mechanism for the excellent thermal stability of our nanostructured alloys and is 12

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magnetic alloys.

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Fig. 3. (a) DSC curves for the Fe84.75Si2B9P3C0.5Cu0.75 alloy before and after annealing at 420 ºC for different time; (b) Inset the proposed “dual phase co-growth model” for the grain growth during the isothermal annealing; The symbols are experimental data from XRD results and the lines are the modeling curves. 2.4. Atom Probe Tomography (APT) analysis To further substantiate our analyses, we performed the atom probe tomography (APT) on the sample after annealing at 420 ºC for 48 min. As shown in Fig. 4(a)-(b),

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ACCEPTED MANUSCRIPT chemical segregation can be clearly observed. The high Fe concentration regions exhibit a granular shape and have an average size of ~20 nm, which is very close to the α-Fe like grains in TEM image [Fig. 1(e)]. The distribution of Si is similar to that

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of Fe, as dissolved in α-Fe like grains. In that sense, the Fe-rich grains might be viewed as nano-sized silicon steels. High density Cu-clusters with the size of 2~4 nm were also observed, which are distributed around the Fe-rich grains. In principle,

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these Cu-clusters could serve as heterogeneous nucleation sites, as previously

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reported for Finemet alloys [40,41]. Evidently, the metalloid elements (B, P, and C) are seemingly excluded from the Fe-rich regions, like an interphase shielding the α-Fe like grains. Fig. 4(c) shows the elemental line scan spectra extracted from a selected area as illustrated in Fig. 4(a), from which the average size of the Fe-rich grains and

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Cu-clusters could be estimated, as well as the site dependent compositional variation. In addition, the number density of α-Fe like grains was estimated to be NFe ≈ 2 × 1023 from the elemental map delineated by 90 at.% iso-concentration surfaces, as shown in

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Fig. 4(d). Notably, the number density of Cu-clusters, as obtained in Fig. 4(e), is

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around 6 times higher than NFe. A similar finding was reported for the Finemet alloys [40], in which Cu-clusters facilitate grain formation by serving as the heterogeneous nucleation sites. In other words, this similar finding on our alloy hints that Cu clusters may also trigger heterogeneous nucleation of α-Fe grains.

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Fig. 4. (a) APT elemental map (60 × 60 × 110 nm) of the Fe84.75Si2B9P3C0.5Cu0.75

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alloy after annealing at 420 ºC for 48 min; (b) Elemental maps of Fe, Si, Cu, B, P and C, respectively; (c) Concentration depth profile from the selected area (3 × 3 × 60 nm) in (a); (d)-(e) The elemental maps of Fe and Cu delineated by 90 at.% and 4 at.%

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iso-concentration surfaces, respectively.

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Based on the elemental concentration analyses in Fig. 5, the proposed “dual phase co-growth model” was further verified quantitatively. It is obvious that Fe element is enrich in the α-Fe(Si) grains with the concentration of 94.41 at.% and, the concentration of B, P and C elements in the interphase are 14.34 at.%, 5.88 at.% and 1.24 at.%, respectively. Consequently, λH is estimated to be ~0.79, which is very close to our fitting result of λH = 0.8. In theory, the inter-granular spacing can be approximated as Rs = (1/NFe)1/3, where NFe is the number density of the grains. With

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ACCEPTED MANUSCRIPT the aid of APT, we obtain NFe ≈ 2.46 × 1023 from the elemental map as shown in Fig. 4(d). Then, we obtain Rs ≈ 16 nm, which agrees quite well with our fitting result. In other words, with the key parameters (λH, Rs) obtained from APT, we can use our

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model to predict the grain growth during isothermal annealing.

Fig. 5. Proximity histograms of the interphase and α-Fe(Si) grain for the

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Fe84.75Si2B9P3C0.5Cu0.75 nanocrystalline alloy after annealing at 420 ºC for 48 min.

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2.5 The “nano” effect

Based on our above discussions and analyses, one can see that the synergetic

property enhancement results from the formation of a nano crystals in the amorphous matrix. Now, let us discuss this “nano” effect on the magnetic and mechanical properties of soft magnetic alloys. In principle, good magnetic softness comes from low magnetic anisotropy, which is closely related to the exchange interaction of the structural units [42]. For the Fe-based nanocrystalline alloys, the exchange interaction

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ACCEPTED MANUSCRIPT length is reported to be around 40 nm [37], which is larger than our grain size of ~20 nm. As a result, the effective magnetic anisotropy will be an average over several structural units and thus will be reduced in magnitude [43], which has been

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successfully addressed in the random anisotropy model (RAM). According to which the magnetic anisotropy is proportional to D6, where D represents the grain size [44]. This explains quite well why the uniformly small grain size (~20 nm) in our dual

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nano-phase alloy, as shown in Fig. 1, brings about the good magnetic softness, such as

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low Hc (inset of Fig. 2). Meanwhile, the precipitation of α-Fe(Si) grains also gives rise to the increase in Bs, since the Bs of α-Fe(Si) phase is much higher than that of amorphous phase [6]. Notably, there is a sharp increase of Bs before 4-miniute annealing, which could be attributed to the transient growth behavior of the α-Fe(Si)

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grains. With the further increase of the annealing time, the growth of the α-Fe(Si) grains slows down and their volume fraction (Vc) reaches a plateau. As a result, Bs levels off to a saturation value of 1.84 T, as observed in Fig. 2(a). Interestingly, the

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alloy hardness (H) [shown in Fig. 2(b)] exhibits a similar trend of annealing-induced

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increase as Bs. This suggests that the alloy hardness in general should be also proportional to Vc, in line with the theoretical model developed by McHenry for nanostructured alloys [45]. Finally, we would like to briefly discuss the implication of our current work. In

theory, the proposed “dual phase co-growth model” provides a quantitative insight into the synthesis of Fe-based nanocrystalline soft magnetic alloys with fine grain size determined by λH and Rs (or NFe). In general, a smaller λH and/or a higher NFe is

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ACCEPTED MANUSCRIPT conducive for a refined grain size, as simulated in Fig. 6, and hence, improved mechanical/magnetic performance. However, there should be a temporal limit above which the core-shell like structure may coarsen under isothermal annealing because of

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the break-down of the interphase. In other words, the thermal stability of our dual nano-phase structure may be ultimately governed by the thermal stability of the nano-scale interphase of metallic compounds. In our opinion, this is a very important

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issue that warrants further research.

Fig. 6. The theoretical prediction of our co-growth model for the grain growth curve during isothermal annealing with different values of λH and NFe. 4. Conclusions In summary, we report a facile and scalable metallurgy technique in this study to 18

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unique combination of properties, i.e. Bs ~1.84 T, µe ~25000 and H ~15 GPa, which outperforms the variety of soft magnetic materials hitherto reported. At the fundamental level, we proposed a “dual phase co-growth” mechanism to explain the

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thermal stability of the nano-grains during isothermal annealing, which is validated by

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the theoretical analyses and APT analysis. From the application viewpoint, our findings are important, which paves the way for a large-scale production of high performance Fe-based nanocrystalline soft magnetic alloys for a variety of industrial

Acknowledgement

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applications.

This work was supported by the National Key Research and Development Program

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of China (2017YFB0903902), the National Natural Science Foundation of China

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(Grant No. 51601206, 51771159). The research of YY is supported by the Research Grant Council, the Hong Kong government through the General Research Fund with the grant No. CityU11209317. Atom probe tomography research was conducted at the Inter-University 3D Atom Probe Tomography Unit of City University of Hong Kong, which is supported by the CityU grant 9360161 and CRF grant C1027-14E.

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ACCEPTED MANUSCRIPT Appendix A

 C (0) − CM (t )  R (t ) =  Rs 3 M + R3 (0)  CP − CI  

1 3

RI PT

From Eq. (2) it follows that

(A.1)

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Differentiating Eq. (A.1) and substituting into Eq. (1) then leads to

 C (0) − CM (t )  dCM (t ) 3D = − 3 (CI − CM (t ))  Rs 3 M + R3 (0)  dt Rs CI − CP  

1 3

(A.2)

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Assuming that the solution of Eq. (A.2) takes on the following form

CM (t ) = CI − (CI − CM (0)) exp ( − f (t ) )

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(A.3)

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Substituting Eq. (A.3) into Eq. (A.2), we obtain

1

df (t ) 3D = − 2 λH dt Rs

 R3 (0) −3  3 1 − exp − f ( t ) + λH  ( )  Rs 3  

1

(A.4)

 C (0) − CI  3 where λH =  M  . In the case of exp[-f(t)]→0. i.e. CM(t) is close to CM(0).  CP − CI  The above equation can be simplified as

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df (t ) 3D ≈ − 2 λH dt Rs

 R 3 (0) −3  3 λH  1 + Rs 3  

(A.5)

RI PT

For our case, R(0) = 0 because our alloy is initially in an amorphous state without crystallization. As a result, we obtain

(A.6)

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 3λ Dt  CM (t ) = CI − (CI − CM (0)) exp  − H 2  Rs  

following solution:

M AN U

Substituting Eq. (A.6) into Eq. (1) and carrying out the integral from 0 to t yields the

1

[1]

Gutfleisch, O. et al. Magnetic Materials and Devices for the 21st Century: Stronger, Lighter, and More Energy Efficient. Adv. Mater. 23 (2011) 821-842. DeCristofaro, N. Amorphous metals in electric-power distribution applications. MRS Bull. 23 (1998) 50-56. Zhao, C. et al. Correlation between soft-magnetic properties and Tx1-Tc in high Bs FeCoSiBPC amorphous alloys. J. Alloys Compd. 659 (2016) 193-197. Jiles, D. Introduction to magnetism and magnetic materials. (CRC press, 2015). Yoshizawa, Y. et al. New Fe-based soft magnetic alloys composed of ultrafine grain structure. J. Appl. Phys. 64 (1988) 6044-6046. Herzer, G. Modern soft magnets: Amorphous and nanocrystalline materials. Acta Mater. 61 (2013) 718-734. Suzuki, K. et al. High saturation magnetization and soft magnetic properties of bcc Fe–Zr–B alloys with ultrafine grain structure. Mater. Trans., JIM 31 (1990) 743-746.

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[2]

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References

[3]

[4]

[5] [6] [7]

(A.7)

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R (t ) =

  3λ Dt   2 2 λH Rs 1 − exp  − H 2   3 Rs    

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[13]

[14]

[15] [16] [17]

[18] [19]

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[20]

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[12]

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[11]

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[10]

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Fan, X. D. et al. Soft magnetic properties in Fe84−xB10C6Cux nanocrystalline alloys. J. Magn. Magn. Mater. 326 (2013) 22-27. Xiang, R. et al. The excellent soft magnetic properties and corrosion behaviour of nanocrystalline FePCCu alloys. J. Mater. Sci-Mater. El. 25 (2014) 2979-2984. Ohta, M. & Yoshizawa, Y. Effect of Heating Rate on Soft Magnetic Properties in Nanocrystalline Fe80.5Cu1.5Si4B14 and Fe82Cu1Nb1Si4B12 Alloys. Appl. Phys. Express 2 (2009) 23005-23005. Sharma, P. et al. Competition driven nanocrystallization in high Bs and low coreloss Fe-Si-B-P-Cu soft magnetic alloys. Scripta Mater. 95 (2015) 3-6. Greer, A. et al. Wear resistance of amorphous alloys and related materials. Int. Mater. Rev. 47 (2002) 87-112. Inoue, A. et al. Ultra-high strength above 5000 MPa and soft magnetic properties of Co-Fe-Ta-B bulk glassy alloys. Acta Mater. 52 (2004) 1631-1637. Klug, H. P. & Alexander, L. E. X-Ray Diffraction Procedures: For Polycrystalline and Amorphous Materials, 2nd Edition, by Harold P. Klug, Leroy E. Alexander, pp. 992. ISBN 0-471-49369-4. Jacovkis, D. et al. Mechanisms driving primary crystallization of Al87Ni7Cu3Nd3 amorphous alloy. Acta Mater. 52 (2004) 2819-2826. Hirata, A. et al. Mechanism of nanocrystalline microstructure formation in amorphous Fe-Nb-B alloys. Phys. Rev. B 74 (2006) 184204. Kulikowski, J. & Leśniewski, A. Properties of Ni-Zn ferrites for magnetic heads: Technical possibilities and limitations. J. Magn. Magn. Mater.19 (1980) 117-119. Rezlescu, E. et al. The influence of additives on the properties of Ni-Zn ferrite used in magnetic heads. J. Magn. Magn. Mater. 117 (1992) 448-454. Hirota, E. et al. Hot-pressed Mn-Zn ferrite for magnetic recording heads. IEEE Trans. Magn. 7 (1971) 337-341. Shokrollahi, H. & Janghorban, K. Influence of additives on the magnetic properties, microstructure and densification of Mn-Zn soft ferrites. Mater. Sci. Eng.: B 141 (2007) 91-107. Miyazaki, T. et al. New magnetic alloys for magnetic recording heads. IEEE Trans. Magn. 8 (1972) 501-502. Helms Jr, H. & Adams, E. Sendust Sheet ‐ Processing Techniques and Magnetic Properties. J. Appl. Phys. 35 (1964) 871-872. Arai, K. et al. Magnetic properties of ribbon-form Sendust alloy. J. Magn. Magn. Mater. 19 (1980) 85-87. Inoue, A. et al. Super-high strength of over 4000 MPa for Fe-based bulk glassy alloys in [(Fe1-xCox)0.75B0.2Si0.05]96Nb4 system. Acta Mater. 52 (2004) 4093-4099. Makino, A. et al. Soft magnetic Fe-Si-B-P-C bulk metallic glasses without any glass-forming metal elements. J. Alloys Compd. 483 (2009) 616-619. Cantor, B. Novel nanocrystalline alloys and magnetic nanomaterials. (CRC

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Press, 2004). Wang, J. et al. Ternary Fe-P-C bulk metallic glass with good soft-magnetic and mechanical properties. Scripta Mater. 65 (2011) 536-539. Makino, A. et al. FeSiBP bulk metallic glasses with unusual combination of high magnetization and high glass-forming ability. Mater. Trans. 48 (2007) 3024-3027. Willard, M. A. & Harris, V. G. Soft magnetic materials: Nanocrystalline alloys from amorphous precursors. JOM 54 (2002) 44-46. McHenry, M. E. et al. Amorphous and nanocrystalline materials for applications as soft magnets. Prog. Mater. Sci. 44 (1999) 291-433. Li, Y. et al. Soft magnetic Fe-Si-B-Cu nanocrystalline alloys with high Cu concentrations. J. Alloys Compd. 722 (2017) 859-863. Wang, A. D. et al. Effect of P on crystallization behavior and soft-magnetic properties of Fe83.3Si4Cu0.7B12-xPx nanocrystalline soft-magnetic alloys. Thin Solid Films 519 (2011) 8283-8286. Setyawan, A. D. et al. Magnetic properties of 120-mm wide ribbons of high Bs and low core-loss NANOMET® alloy. J. Appl. Phys. 117 (2015) 17B715. Li, C.-S. et al. Ordered phases and microhardness of Fe-6.5% Si steel sheet after hot rolling and annealing. Mater. Sci. Eng.: A 650 (2016) 84-92. Takada, Y. et al. Commercial scale production of Fe-6.5 wt.% Si sheet and its magnetic properties. J. Appl. Phys. 64 (1988) 5367-5369. Liu, T. et al. Fe(Co)SiBPCCu nanocrystalline alloys with high Bs above 1.83 T. J. Magn. Magn. Mater. 441 (2017) 174-179. Herzer, G. Nanocrystalline soft magnetic alloys. Handbook of magnetic materials 10 (1997) 415-462. Jackson, K. A. Kinetic Processes: crystal growth, diffusion, and phase transformations in materials. (John Wiley & Sons, 2006). Köster, U. et al. Diffusion in some iron-based metallic glasses. J. Mater. Sci. 15 (1980) 2125-2128. Hono, K. et al. Cu clustering and Si partitioning in the early crystallization stage of an Fe73.5Si13.5B9Nb3Cu1 amorphous alloy. Acta Mater. 47 (1999) 997-1006. Pradeep, K. G. et al. Atom probe tomography study of ultrahigh nanocrystallization rates in FeSiNbBCu soft magnetic amorphous alloys on rapid annealing. Acta Mater. 68 (2014) 295-309. Kraus, L. et al. Magnetic anisotropy in as-quenched and stress-annealed amorphous and nanocrystalline Fe73.5Cu1Nb3Si13.5B9 alloys. J. Magn. Magn. Mater. 112 (1992) 275-277. Flohrer, S. et al. Interplay of uniform and random anisotropy in nanocrystalline soft magnetic alloys. Acta Mater. 53 (2005) 2937-2942. Bitoh, T. et al. Random anisotropy model for nanocrystalline soft magnetic alloys with grain-size distribution. Mater. Trans. 44 (2003) 2011-2019. Um, C. Y. et al. Effect of crystal fraction on hardness in FINEMET and NANOPERM nanocomposite alloys. J. Appl. Phys. 97 (2005) 10F504.

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[42]

[43] [44] [45]

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ACCEPTED MANUSCRIPT

Highlights: 1.

Nano-sized grains in FeSiBPCCu exhibit excellent stability upon thermal annealing Formation of nanocrystals gives a synergetic increase of Bs and hardness

3.

The growth of grains can be shielded effectively by the interphases

4.

High stability of the nanostructure was clarified by the proposed co-growth model

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2.