- Email: [email protected]
Study of microstructure and magnetic properties of AlNiCo(CuFe) high entropy alloy
Accepted Manuscript Study of microstructure and magnetic properties of AlNiCo(CuFe) high entropy alloy Raghavendra Kulkarni, B.S. Murty, V. Srinivas PII:
S0925-8388(18)30770-9
DOI:
10.1016/j.jallcom.2018.02.275
Reference:
JALCOM 45151
To appear in:
Journal of Alloys and Compounds
Received Date: 11 September 2017 Revised Date:
19 February 2018
Accepted Date: 22 February 2018
Please cite this article as: R. Kulkarni, B.S. Murty, V. Srinivas, Study of microstructure and magnetic properties of AlNiCo(CuFe) high entropy alloy, Journal of Alloys and Compounds (2018), doi: 10.1016/ j.jallcom.2018.02.275. 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
Study of Microstructure and Magnetic Properties of AlNiCo(CuFe) High Entropy Alloy Raghavendra Kulkarni1,3, B.S. Murty2 and V. Srinivas1 1
Department of Physics, 2Department of Metallurgical and Materials Engineering Indian Institute of Technology Madras, Chennai-600036, India Department of H & S, CVR College of Engineering, Hyderabad Email: [email protected]
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3
Abstract: Microstructural and magnetic properties of AlNiCo based high entropy alloys have
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been investigated. Equiatomic AlNiCo alloy exhibits superparamagnetic like behavior but addition of elements like Cu and Fe significantly alter the microstructure and magnetic
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properties. From the present study, it is clear that the addition of Cu leads to phase separation and significant increase in coercivity. On the other hand, addition of Fe leads to enhancement in ferromagnetic exchange interaction that in turn results in the development of soft magnetic behavior in AlNiCoCuFe alloy.
1. Introduction:
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Recent studies have shown some interesting electronic properties in equiatomic transition metal based alloys. Binary equiatomic alloys (such as FeAl, FeSi), so called Kondo insulators, exhibit high thermoelectric properties due to their unusual electronic structure, i.e., presence of pseudo gap in density of states near the Fermi energy [1]. On the other hand,
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ternary full and half Heusler (equiatomic) alloys exhibit magnetoresistance and magnetocaloric properties as a result of cooperative phenomenon [2]. Recently, equiatomic
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ternary, quaternary Heusler compounds such as CoFeMnZ (Al, Ga, Si, Ge), have been designed to tailor the center of the band gap close to the Fermi level [3, 4]. This new class of materials is termed as spin gapless semiconductors. It is also predicted that such compounds can have half metallic band structure with high spin polarization [5] and are subject of great interest worldwide. In this context investigation of physical properties of equiatomic multicomponent alloy compositions has attracted attention of scientific community. Multicomponent equiatomic alloys come under the category of high entropy alloys (HEAs) by virtue of their large configurational entropy, when they contain at least five elements (>1.5R) [6-8]. They are found to form solid solutions with simple structures, viz., BCC or FCC [7]. The physical properties of HEAs greatly depend on the crystal structure and 1
ACCEPTED MANUSCRIPT chemical composition of the alloys [9]. For example, adding Al to CuCoNiCrFe (Alx) promotes the formation of BCC structure [10]. Most of the alloy systems that contain more than 50 at. % of magnetic elements (Fe, Co, and Ni) are usually expected to exhibit ferromagnetic characteristics [11, 12]. Although conventional hard magnetic alloys such as Fe based AlNiCo have higher coercivity values,
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equiatomic multicomponent compositions (HEAs), particularly with the presence of nonmagnetic elements, haven’t been investigated systematically to identify the role of nonmagnetic elements on the magnetic properties. One of the advantages of HEAs is their higher thermal stability, unlike metallic glasses and hence they can be used for high-temperature
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applications [13-15]. Most of the HEAs have simple crystalline structures and some of these with FCC structure have reasonable malleability [16]. Besides this, Kao et al. [17] observed an enhancement in magnetic behavior when the crystal structure transformed from FCC to
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BCC in AlxCoCrFeNi HEAs. Although extensive work has been done on conventional AlNiCo alloys by varying the composition of the alloy [18, 19], a lot of work needs to be done in direction of understanding the magnetic properties of equiatomic AlNiCo based HEAs. Studies are going on among the researchers to understand the microstructure and magnetic properties of equiatomic AlNiCo based HEAs. Among various HEA compositions
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the as-cast equiatomic AlCoCrCuFeNi alloy is widely studied because of its interesting mechanical properties like high hardness, resistance to softening at high temperatures and good compressive strength [20-22]. From these studies it was found that as-cast alloy has mainly Al–Ni and Cr–Fe rich phases with bcc structures [22]. In view of recent interest in
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HEAs, we have attempted to synthesize and study the microstructure and magnetic properties
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of equiatomic AlNiCo based HEAs.
2. Experimental Details:
AlNiCo, AlNiCoCu, AlNiCoFe and AlNiCoCuFe alloy ingots were prepared by arc
melting the constituent high purity elements (99.5% purity) under inert gas atmosphere. The ingots were remelted several times to ensure homogeneity.
Subsequently, the alloy ingots
were annealed at 1000oC for 48 hours in vacuum sealed quartz ampules. The as-cast and annealed
alloy ingots
were sectioned
and
polished
to perform
microstructural
characterization. Structural characterization of the as-cast and annealed samples was done by X-ray diffraction (XRD, PANalytical) using Cu Kα radiation with λ=0.15406 nm, Field emission scanning electron microscopy (FESEM) along with energy dispersive spectroscopy (EDS) 2
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3. Results and Discussion:
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3.1 Structure and Microstructure: Figure 1 shows XRD patterns of as-cast and annealed AlNiCo based alloys. The phases identified and respective lattice constants are shown in Table 1. The lattice constants were determined by using Nelson-Riley function. The as-cast and annealed AlNiCo alloys show
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B2 phase with a marginal increase in lattice constant for the annealed alloy. As-cast AlNiCoFe showed B2 phase but upon annealing it changed to disordered BCC phase. From Fig. 1 it is clear that the FCC phase emerges out on the addition of Cu to AlNiCo alloy, but
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the BCC phase is stabilized on the addition of Fe, which suggests that the phase separation occurs due to the presence of Cu in AlNiCo alloy.
From SEM BSE images of the alloys (Fig. 2) it can be observed that copper containing alloys show two phases, while those without copper show a single phase in agreement with XRD pattern in Fig.1. The overall composition of as-cast and annealed
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AlNiCoCuFe HEA was determined by EDS and is depicted in Table 2. Though the microstructure of the alloy shows dendritic and interdendritic regions (Fig. 2), the overall composition of the alloy is close to the nominal equiatomic quinary composition. As shown in Fig. 3, annealing has resulted in an increase in the fraction of
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interdendritic region. From the EDS elemental mapping (Fig. 4) and the chemical composition of the two phases (Table 3) for AlNiCoCuFe alloy, it is clear that both in the as-
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cast and annealed alloys, the interdendritic region is rich in Cu and it is depleted within the dendrite. Al, Ni, Co and Fe are equally distributed in both the dendritic and interdendritic regions, while their concentration is slightly lower in interdendritic region. Earlier studies have shown that Cu, Co, Ni favor the formation of FCC phase, while Al promotes the formation of BCC phase [23] and this is consistent with present studies.
3.2 Magnetic Properties: Studies of magnetization (M) as a function of magnetic field (H) at room temperature were performed for all annealed alloys of the present study. It is observed that the equiatomic AlNiCo alloy shows low magnetization and coercivity (HC) values and non-saturation behavior in fields up to 20 kOe (Fig. 5). These features indicate, either the presence of 3
ACCEPTED MANUSCRIPT magnetic clusters or a superparamagnetic behavior. In order to identify the magnetic nature, the M-H data was fitted to Langevin function to determine the magnetic moment.
() = coth( ) − -- (1)
Fit to Langevin function, represented by equation (1) is carried out for the data above
and n is the number of magnetic
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10 kOe. Here MS is saturation magnetization, =
moments per particle in Bohr magnetons. The solid line through the data points in Fig. 5 is a fit to Langevin function. From the fit, the magnetic moment per particle was found to be 11,166µB. Absence of coercivity (HC) and remanence (MR) in M-H curve along with high value of magnetic moment per particle indicate the presence of unstable single domain and
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suggests the super paramagnetic behavior.
Addition of Cu increases the HC and MR with no significant change in magnetization
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value at 20 kOe. However, approach to saturation at higher magnetic fields is modified. This suggests that the addition of Cu leads microstructural changes (seen in SEM) and pin the domains or magnetic clusters. Quantification of the anisotropy or pinning effects can also be examined by analyzing the approach to saturation of M-H curves. In general magnetization as a function of field can be expressed as [24]:
() = (1 − −
− ⋯ ) -- (2)
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The coefficient ‘a’ is due to inclusions and/or micro stress in the crystal lattice and it is related to non-magnetic regions present within the sample. The coefficient ‘b’ represents the magneto crystalline anisotropy of the sample and if the cubic anisotropy axes is oriented in
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random directions, then ‘b’ is related to anisotropy constant as:
= " !$ -- (3) #
%
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Here K1 represents the first cubic anisotropic constant, µ 0 is permeability of free space and MS is saturation magnetization. The M(H) data obtained for all annealed alloys (Fig. 6) was fitted to eqn.(2) in the suitable range of fields by varying the parameters a, b and K1. The best fit parameter values were determined and are shown in Table 4. The anisotropy values obtained from the fits are consistent with the coercivity values reported in Table 1. Addition of Fe to AlNiCo alloy increases the value of magnetization significantly and reduces the HC value, which suggests that the soft magnetic properties of AlNiCo have improved with the addition of Fe. Addition of Cu to AlNiCoFe alloy results in development of finite HC and remanence (MR). The enhancement in magnetization values in Fe containing samples is due to the improvement in the Fe-Co exchange interaction. 4
ACCEPTED MANUSCRIPT Table 5 [25] compares the previously reported magnetization and coercivity values of different HEAs with the alloy of the current study.
The alloy of the current study,
AlNiCoCuFe, has reasonably good magnetization and highest coercivity value in comparison to those HEAs reported earlier. Hence this alloy can be considered of as first semi hard
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magnetic HEA synthesized by arc melting.
4. Conclusions:
Microstructural and magnetic properties of equiatomic AlNiCo, AlNiCoCu, AlNiCoFe and AlNiCoCuFe alloys have been investigated. The base alloy AlNiCo exhibits
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superparamagnetic like behavior. Present study indicates that addition of Cu to AlNiCo alloy increases the HC and MR due to pinning effect indicating the development of hard magnetic behavior. In contrast, Fe addition to AlNiCo alloy enhances magnetization due to
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improvement in Fe-Co exchange interaction. These results suggest that the AlNiCo alloy can be tuned to obtain soft or hard magnetic phase by the addition of Fe or Cu.
Acknowledgments: Raghavendra Kulkarni would like to thank the management and Head
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Dept. of H & S of CVR College of Engineering for their support
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Table 1: Structure and magnetic properties of HEAs
Magnetic Properties (300K) of Annealed Samples
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Structure Lattice constant (nm) Composition As-cast Annealed As-cast Annealed Phases observed
B2
B2
0.282
0.285
AlNiCoFe
B2 BCC+ FCC BCC+ FCC
BCC FCC+ BCC FCC+ BCC
0.287 0.286 0.360 0.287 0362
0.277 0.359 0.287 0.361 0.287
AlNiCoCu
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AlNiCoCuFe
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AlNiCo
8
Ms (emu/g)
Nonsaturating 97
Hc MR µB/f.u. (Oe) (emu/g) ̴0
0.17
̴0
̴0
3.76
̴0
5.9
650
0.25
2.6
84
162
3.59
5.3
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Alloy Composition Al Ni Co Cu Fe Nominal 33.33 33.33 33.33 As-cast AlNiCo EDS 29.74±0.42 34.65±0.32 35.20±0.68 Nominal 25 25 25 25 As-cast AlNiCoCu EDS 25.93±1.81 23.46±1.16 26.93±1.87 23.67±2.60 Nominal 25 25 25 25 As-cast AlNiCoFe EDS 23.91±0.37 24.19±0.15 24.89±0.25 26.99±0.36 Nominal 20 20 20 20 20 As-cast AlNiCoCuFe EDS 21.29±0.35 19.34±0.15 20.00±0.14 17.90±0.59 21.44±0.14 Nominal 33.33 33.33 33.33 Annealed AlNiCo EDS 31.48±0.06 33.08±0.03 35.42±0.03 Nominal 25 25 25 25 Annealed AlNiCoCu EDS 26.19±1.34 24.65±0.53 27.59±1.06 22.62±0.59 Nominal 25 25 25 25 Annealed AlNiCoFe EDS 22.98±1.91 24.29±0.37 25.31±0.85 26.82±0.99 Nominal 20 20 20 20 20 Annealed AlNiCoCuFe EDS 21.62±1.06 19.33±0.26 19.94±0.23 17.49±0.36 21.60±0.27 Table 3: Elemental distribution in the phases of as-cast and annealed AlNiCoCuFe HEA by EDS analysis (at.%, n = 7) Al
Ni
Co
Cu
13.64±1.17 18.49±0.47 24.05±0.34 43.83±0.65
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Composition (at. %) Interdendritic (FCC) As-cast AlNiCoCu Dendritic (BCC) Interdendritic (FCC) As-cast AlNiCoCuFe Dendritic (BCC) Interdendritic (FCC) Annealed AlNiCoCu Dendritic (BCC) Interdendritic (FCC) Annealed AlNiCoCuFe Dendritic (BCC)
28.64±1.98 27.55±0.84 30.15±0.80 13.65±1.00
-
18.26±2.39 16.03±1.29 13.88±1.59 36.28±6.34 15.43±1.70 21.26±0.31 20.00±1.00 20.94±0.37 15.76±0.60 22.46±0.32
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Fe
13.79±1.60 18.73±0.64 24.43±0.78 43.05±1.14
-
29.87±1.47
-
28.37±2.6
26.98±2.05 14.79±3.65
11.74±0.81 16.37±0.10 18.76±0.19 29.48±1.45 24.17±0.12 25.88±1.39 21.86±0.28 21.62±0.56
9.33±0.96
21.29±0.76
Table 4: Saturation magnetization and anisotropy values of different alloys studied in the present work
AlNiCo AlNiCoCu AlNiCoFe AlNiCoCuFe
Ms (emu/g) 10 8 105 84
a -8763 -1898 -247 -759 9
b 2.9 x 107 2.1 x 106 -2.8 x 104 2.2 x 106
K1 (erg/cm3) 1.9 x 105 1.1 x 105 6.3 x 104 4.6 x 105
ACCEPTED MANUSCRIPT Table 5: Comparison of magnetic properties of HEAs MS-Saturation magnetization, HC-Coercivity HC(A/m) 3582 268 1273-4100 1416 629 915 3435 10804 315-1508 12892
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Reference [22,26,27] [25] [27] [27] [28,29] [17] [30]
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Ms(emu/g) 38-46 101 33-34 13-18 15-64 43.05 148 80 80 1.39 78-93 95-131 84
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Structure BCC+FCC FCC FCC BCC BCC BCC B2+BCC BCC+FCC L21+BCC FCC FCC+BCC FCC BCC+FCC
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Alloy FeCoNiCrCuAl FeCoNiMn0.25Al0.25 FeCoNiCrPd1-2 FeCoNiCrAl2 FeCoNiCrAl FeCoNiCrAl1.25 FeCoNiMnAl FeCoNiMnGa FeCoNiMnSn FeCoNiMnCr FeCoNi(CuAl)x FeCoNiAl0.2Si0.2 FeCoNiCuAl
[31] [16,32,33] Current study
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Fig. 1: XRD patterns of (a) as-cast and (b) annealed equiatomic AlNiCo, AlNiCoFe, AlNiCoCu, AlNiCoCuFe alloys.
Fig. 2: BSE images of as-cast and annealed equiatomic alloys
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Fig. 3: BSE images of as-cast and annealed Cu containing alloys at high magnification.
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Fig. 4: Elemental mapping of (a) as-cast (b) annealed AlNiCoCuFe HEA.
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Fig. 5: Langevin fit for annealed AlNiCo alloy
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Fig. 6: M-H loops for annealed alloys at room temperature.
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S0925-8388(18)30770-9
DOI:
10.1016/j.jallcom.2018.02.275
Reference:
JALCOM 45151
To appear in:
Journal of Alloys and Compounds
Received Date: 11 September 2017 Revised Date:
19 February 2018
Accepted Date: 22 February 2018
Please cite this article as: R. Kulkarni, B.S. Murty, V. Srinivas, Study of microstructure and magnetic properties of AlNiCo(CuFe) high entropy alloy, Journal of Alloys and Compounds (2018), doi: 10.1016/ j.jallcom.2018.02.275. 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
Study of Microstructure and Magnetic Properties of AlNiCo(CuFe) High Entropy Alloy Raghavendra Kulkarni1,3, B.S. Murty2 and V. Srinivas1 1
Department of Physics, 2Department of Metallurgical and Materials Engineering Indian Institute of Technology Madras, Chennai-600036, India Department of H & S, CVR College of Engineering, Hyderabad Email: [email protected]
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3
Abstract: Microstructural and magnetic properties of AlNiCo based high entropy alloys have
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been investigated. Equiatomic AlNiCo alloy exhibits superparamagnetic like behavior but addition of elements like Cu and Fe significantly alter the microstructure and magnetic
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properties. From the present study, it is clear that the addition of Cu leads to phase separation and significant increase in coercivity. On the other hand, addition of Fe leads to enhancement in ferromagnetic exchange interaction that in turn results in the development of soft magnetic behavior in AlNiCoCuFe alloy.
1. Introduction:
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Recent studies have shown some interesting electronic properties in equiatomic transition metal based alloys. Binary equiatomic alloys (such as FeAl, FeSi), so called Kondo insulators, exhibit high thermoelectric properties due to their unusual electronic structure, i.e., presence of pseudo gap in density of states near the Fermi energy [1]. On the other hand,
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ternary full and half Heusler (equiatomic) alloys exhibit magnetoresistance and magnetocaloric properties as a result of cooperative phenomenon [2]. Recently, equiatomic
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ternary, quaternary Heusler compounds such as CoFeMnZ (Al, Ga, Si, Ge), have been designed to tailor the center of the band gap close to the Fermi level [3, 4]. This new class of materials is termed as spin gapless semiconductors. It is also predicted that such compounds can have half metallic band structure with high spin polarization [5] and are subject of great interest worldwide. In this context investigation of physical properties of equiatomic multicomponent alloy compositions has attracted attention of scientific community. Multicomponent equiatomic alloys come under the category of high entropy alloys (HEAs) by virtue of their large configurational entropy, when they contain at least five elements (>1.5R) [6-8]. They are found to form solid solutions with simple structures, viz., BCC or FCC [7]. The physical properties of HEAs greatly depend on the crystal structure and 1
ACCEPTED MANUSCRIPT chemical composition of the alloys [9]. For example, adding Al to CuCoNiCrFe (Alx) promotes the formation of BCC structure [10]. Most of the alloy systems that contain more than 50 at. % of magnetic elements (Fe, Co, and Ni) are usually expected to exhibit ferromagnetic characteristics [11, 12]. Although conventional hard magnetic alloys such as Fe based AlNiCo have higher coercivity values,
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equiatomic multicomponent compositions (HEAs), particularly with the presence of nonmagnetic elements, haven’t been investigated systematically to identify the role of nonmagnetic elements on the magnetic properties. One of the advantages of HEAs is their higher thermal stability, unlike metallic glasses and hence they can be used for high-temperature
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applications [13-15]. Most of the HEAs have simple crystalline structures and some of these with FCC structure have reasonable malleability [16]. Besides this, Kao et al. [17] observed an enhancement in magnetic behavior when the crystal structure transformed from FCC to
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BCC in AlxCoCrFeNi HEAs. Although extensive work has been done on conventional AlNiCo alloys by varying the composition of the alloy [18, 19], a lot of work needs to be done in direction of understanding the magnetic properties of equiatomic AlNiCo based HEAs. Studies are going on among the researchers to understand the microstructure and magnetic properties of equiatomic AlNiCo based HEAs. Among various HEA compositions
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the as-cast equiatomic AlCoCrCuFeNi alloy is widely studied because of its interesting mechanical properties like high hardness, resistance to softening at high temperatures and good compressive strength [20-22]. From these studies it was found that as-cast alloy has mainly Al–Ni and Cr–Fe rich phases with bcc structures [22]. In view of recent interest in
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HEAs, we have attempted to synthesize and study the microstructure and magnetic properties
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of equiatomic AlNiCo based HEAs.
2. Experimental Details:
AlNiCo, AlNiCoCu, AlNiCoFe and AlNiCoCuFe alloy ingots were prepared by arc
melting the constituent high purity elements (99.5% purity) under inert gas atmosphere. The ingots were remelted several times to ensure homogeneity.
Subsequently, the alloy ingots
were annealed at 1000oC for 48 hours in vacuum sealed quartz ampules. The as-cast and annealed
alloy ingots
were sectioned
and
polished
to perform
microstructural
characterization. Structural characterization of the as-cast and annealed samples was done by X-ray diffraction (XRD, PANalytical) using Cu Kα radiation with λ=0.15406 nm, Field emission scanning electron microscopy (FESEM) along with energy dispersive spectroscopy (EDS) 2
ACCEPTED MANUSCRIPT was used to analyze the chemical composition. The magnetic properties were determined through SQUID vibration sample magnetometer (VSM) at 300 K under external fields up to 2T.
3. Results and Discussion:
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3.1 Structure and Microstructure: Figure 1 shows XRD patterns of as-cast and annealed AlNiCo based alloys. The phases identified and respective lattice constants are shown in Table 1. The lattice constants were determined by using Nelson-Riley function. The as-cast and annealed AlNiCo alloys show
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B2 phase with a marginal increase in lattice constant for the annealed alloy. As-cast AlNiCoFe showed B2 phase but upon annealing it changed to disordered BCC phase. From Fig. 1 it is clear that the FCC phase emerges out on the addition of Cu to AlNiCo alloy, but
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the BCC phase is stabilized on the addition of Fe, which suggests that the phase separation occurs due to the presence of Cu in AlNiCo alloy.
From SEM BSE images of the alloys (Fig. 2) it can be observed that copper containing alloys show two phases, while those without copper show a single phase in agreement with XRD pattern in Fig.1. The overall composition of as-cast and annealed
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AlNiCoCuFe HEA was determined by EDS and is depicted in Table 2. Though the microstructure of the alloy shows dendritic and interdendritic regions (Fig. 2), the overall composition of the alloy is close to the nominal equiatomic quinary composition. As shown in Fig. 3, annealing has resulted in an increase in the fraction of
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interdendritic region. From the EDS elemental mapping (Fig. 4) and the chemical composition of the two phases (Table 3) for AlNiCoCuFe alloy, it is clear that both in the as-
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cast and annealed alloys, the interdendritic region is rich in Cu and it is depleted within the dendrite. Al, Ni, Co and Fe are equally distributed in both the dendritic and interdendritic regions, while their concentration is slightly lower in interdendritic region. Earlier studies have shown that Cu, Co, Ni favor the formation of FCC phase, while Al promotes the formation of BCC phase [23] and this is consistent with present studies.
3.2 Magnetic Properties: Studies of magnetization (M) as a function of magnetic field (H) at room temperature were performed for all annealed alloys of the present study. It is observed that the equiatomic AlNiCo alloy shows low magnetization and coercivity (HC) values and non-saturation behavior in fields up to 20 kOe (Fig. 5). These features indicate, either the presence of 3
ACCEPTED MANUSCRIPT magnetic clusters or a superparamagnetic behavior. In order to identify the magnetic nature, the M-H data was fitted to Langevin function to determine the magnetic moment.
() = coth( ) − -- (1)
Fit to Langevin function, represented by equation (1) is carried out for the data above
and n is the number of magnetic
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10 kOe. Here MS is saturation magnetization, =
moments per particle in Bohr magnetons. The solid line through the data points in Fig. 5 is a fit to Langevin function. From the fit, the magnetic moment per particle was found to be 11,166µB. Absence of coercivity (HC) and remanence (MR) in M-H curve along with high value of magnetic moment per particle indicate the presence of unstable single domain and
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suggests the super paramagnetic behavior.
Addition of Cu increases the HC and MR with no significant change in magnetization
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value at 20 kOe. However, approach to saturation at higher magnetic fields is modified. This suggests that the addition of Cu leads microstructural changes (seen in SEM) and pin the domains or magnetic clusters. Quantification of the anisotropy or pinning effects can also be examined by analyzing the approach to saturation of M-H curves. In general magnetization as a function of field can be expressed as [24]:
() = (1 − −
− ⋯ ) -- (2)
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The coefficient ‘a’ is due to inclusions and/or micro stress in the crystal lattice and it is related to non-magnetic regions present within the sample. The coefficient ‘b’ represents the magneto crystalline anisotropy of the sample and if the cubic anisotropy axes is oriented in
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random directions, then ‘b’ is related to anisotropy constant as:
= " !$ -- (3) #
%
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Here K1 represents the first cubic anisotropic constant, µ 0 is permeability of free space and MS is saturation magnetization. The M(H) data obtained for all annealed alloys (Fig. 6) was fitted to eqn.(2) in the suitable range of fields by varying the parameters a, b and K1. The best fit parameter values were determined and are shown in Table 4. The anisotropy values obtained from the fits are consistent with the coercivity values reported in Table 1. Addition of Fe to AlNiCo alloy increases the value of magnetization significantly and reduces the HC value, which suggests that the soft magnetic properties of AlNiCo have improved with the addition of Fe. Addition of Cu to AlNiCoFe alloy results in development of finite HC and remanence (MR). The enhancement in magnetization values in Fe containing samples is due to the improvement in the Fe-Co exchange interaction. 4
ACCEPTED MANUSCRIPT Table 5 [25] compares the previously reported magnetization and coercivity values of different HEAs with the alloy of the current study.
The alloy of the current study,
AlNiCoCuFe, has reasonably good magnetization and highest coercivity value in comparison to those HEAs reported earlier. Hence this alloy can be considered of as first semi hard
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magnetic HEA synthesized by arc melting.
4. Conclusions:
Microstructural and magnetic properties of equiatomic AlNiCo, AlNiCoCu, AlNiCoFe and AlNiCoCuFe alloys have been investigated. The base alloy AlNiCo exhibits
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superparamagnetic like behavior. Present study indicates that addition of Cu to AlNiCo alloy increases the HC and MR due to pinning effect indicating the development of hard magnetic behavior. In contrast, Fe addition to AlNiCo alloy enhances magnetization due to
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improvement in Fe-Co exchange interaction. These results suggest that the AlNiCo alloy can be tuned to obtain soft or hard magnetic phase by the addition of Fe or Cu.
Acknowledgments: Raghavendra Kulkarni would like to thank the management and Head
References: [1]
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Dept. of H & S of CVR College of Engineering for their support
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Table 1: Structure and magnetic properties of HEAs
Magnetic Properties (300K) of Annealed Samples
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Structure Lattice constant (nm) Composition As-cast Annealed As-cast Annealed Phases observed
B2
B2
0.282
0.285
AlNiCoFe
B2 BCC+ FCC BCC+ FCC
BCC FCC+ BCC FCC+ BCC
0.287 0.286 0.360 0.287 0362
0.277 0.359 0.287 0.361 0.287
AlNiCoCu
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AlNiCoCuFe
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AlNiCo
8
Ms (emu/g)
Nonsaturating 97
Hc MR µB/f.u. (Oe) (emu/g) ̴0
0.17
̴0
̴0
3.76
̴0
5.9
650
0.25
2.6
84
162
3.59
5.3
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Alloy Composition Al Ni Co Cu Fe Nominal 33.33 33.33 33.33 As-cast AlNiCo EDS 29.74±0.42 34.65±0.32 35.20±0.68 Nominal 25 25 25 25 As-cast AlNiCoCu EDS 25.93±1.81 23.46±1.16 26.93±1.87 23.67±2.60 Nominal 25 25 25 25 As-cast AlNiCoFe EDS 23.91±0.37 24.19±0.15 24.89±0.25 26.99±0.36 Nominal 20 20 20 20 20 As-cast AlNiCoCuFe EDS 21.29±0.35 19.34±0.15 20.00±0.14 17.90±0.59 21.44±0.14 Nominal 33.33 33.33 33.33 Annealed AlNiCo EDS 31.48±0.06 33.08±0.03 35.42±0.03 Nominal 25 25 25 25 Annealed AlNiCoCu EDS 26.19±1.34 24.65±0.53 27.59±1.06 22.62±0.59 Nominal 25 25 25 25 Annealed AlNiCoFe EDS 22.98±1.91 24.29±0.37 25.31±0.85 26.82±0.99 Nominal 20 20 20 20 20 Annealed AlNiCoCuFe EDS 21.62±1.06 19.33±0.26 19.94±0.23 17.49±0.36 21.60±0.27 Table 3: Elemental distribution in the phases of as-cast and annealed AlNiCoCuFe HEA by EDS analysis (at.%, n = 7) Al
Ni
Co
Cu
13.64±1.17 18.49±0.47 24.05±0.34 43.83±0.65
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Composition (at. %) Interdendritic (FCC) As-cast AlNiCoCu Dendritic (BCC) Interdendritic (FCC) As-cast AlNiCoCuFe Dendritic (BCC) Interdendritic (FCC) Annealed AlNiCoCu Dendritic (BCC) Interdendritic (FCC) Annealed AlNiCoCuFe Dendritic (BCC)
28.64±1.98 27.55±0.84 30.15±0.80 13.65±1.00
-
18.26±2.39 16.03±1.29 13.88±1.59 36.28±6.34 15.43±1.70 21.26±0.31 20.00±1.00 20.94±0.37 15.76±0.60 22.46±0.32
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Fe
13.79±1.60 18.73±0.64 24.43±0.78 43.05±1.14
-
29.87±1.47
-
28.37±2.6
26.98±2.05 14.79±3.65
11.74±0.81 16.37±0.10 18.76±0.19 29.48±1.45 24.17±0.12 25.88±1.39 21.86±0.28 21.62±0.56
9.33±0.96
21.29±0.76
Table 4: Saturation magnetization and anisotropy values of different alloys studied in the present work
AlNiCo AlNiCoCu AlNiCoFe AlNiCoCuFe
Ms (emu/g) 10 8 105 84
a -8763 -1898 -247 -759 9
b 2.9 x 107 2.1 x 106 -2.8 x 104 2.2 x 106
K1 (erg/cm3) 1.9 x 105 1.1 x 105 6.3 x 104 4.6 x 105
ACCEPTED MANUSCRIPT Table 5: Comparison of magnetic properties of HEAs MS-Saturation magnetization, HC-Coercivity HC(A/m) 3582 268 1273-4100 1416 629 915 3435 10804 315-1508 12892
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Reference [22,26,27] [25] [27] [27] [28,29] [17] [30]
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Ms(emu/g) 38-46 101 33-34 13-18 15-64 43.05 148 80 80 1.39 78-93 95-131 84
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Structure BCC+FCC FCC FCC BCC BCC BCC B2+BCC BCC+FCC L21+BCC FCC FCC+BCC FCC BCC+FCC
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Alloy FeCoNiCrCuAl FeCoNiMn0.25Al0.25 FeCoNiCrPd1-2 FeCoNiCrAl2 FeCoNiCrAl FeCoNiCrAl1.25 FeCoNiMnAl FeCoNiMnGa FeCoNiMnSn FeCoNiMnCr FeCoNi(CuAl)x FeCoNiAl0.2Si0.2 FeCoNiCuAl
[31] [16,32,33] Current study
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Fig. 1: XRD patterns of (a) as-cast and (b) annealed equiatomic AlNiCo, AlNiCoFe, AlNiCoCu, AlNiCoCuFe alloys.
Fig. 2: BSE images of as-cast and annealed equiatomic alloys
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Fig. 3: BSE images of as-cast and annealed Cu containing alloys at high magnification.
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Fig. 4: Elemental mapping of (a) as-cast (b) annealed AlNiCoCuFe HEA.
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Fig. 5: Langevin fit for annealed AlNiCo alloy
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Fig. 6: M-H loops for annealed alloys at room temperature.
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