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A ductile high entropy alloy with attractive magnetic properties
Journal of Alloys and Compounds 694 (2017) 55e60
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
A ductile high entropy alloy with attractive magnetic properties Panpan Li, Anding Wang, C.T. Liu* Center for Advanced Structural Materials, Department of Mechanical and Biomedical Engineering, College of Science and Engineering, City University of Hong Kong, Hong Kong
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 June 2016 Received in revised form 24 August 2016 Accepted 19 September 2016
FeCoNiMn0.25Al0.25 high entropy alloy was investigated thoroughly in this paper from crystal structure to magnetic and mechanical properties. We found that this alloy formed simple face centered cubic structure which was very stable to deformation and heat treatment. Moreover, this alloy showed high saturated magnetization, low coercivity, high Curie temperature as well as good tensile ductility. This high entropy alloy shows soft magnetic properties much better than most high entropy alloys reported previously. This study suggests that the FeCoNiMn0.25Al0.25 high entropy alloy has the potential for use as soft magnets in the future. © 2016 Elsevier B.V. All rights reserved.
Keywords: High entropy alloyso Mechanical propertieso Magnetic propertieso
1. Introduction Alloy design has been a hot topic for metallurgists and material scientists for a long time. From ancient to very recently, alloy design are all based on only one or two principle elements without exception, while other elements in small amounts are usually added to enhance some specific properties of certain alloys. As is well known, steels are based on element iron, while chromium is used to enhance their corrosion resistance properties. Superalloy Ni3Al are based on elements nickel and aluminum, while boron is added to enhance the mechanical properties. Until the year of 2004, high entropy alloys (HEAs) with at least five principle elements with each element's concentration between 5 at% to 35 at%, which are not limited to one or two principle elements any more, have attracted intensive attentions by their extraordinary characteristics due to the increased mixing entropy, inciting a fascinating new area in metallurgy [1e6]. Due to their characteristics such as high entropy effect, sluggish diffusion and significant lattice distortion, they are reported to be good candidates as structural materials as they have high strength and hardness, excellent corrosion and wear resistance, as well as great fatigue resistance [1e6]. Usually, simple solid solution structures such as body centered cubic or face centered cubic are formed in high entropy alloys instead of ordered crystalline intermetallic phases due to the high entropy effect.
* Corresponding author. E-mail address: [email protected] (C.T. Liu). http://dx.doi.org/10.1016/j.jallcom.2016.09.186 0925-8388/© 2016 Elsevier B.V. All rights reserved.
Moreover, some high entropy alloys are even reported to have stable structures at high temperatures without precipitation [7e10]. Until now, many studies are focused on the microstructures and mechanical properties of HEAs, while few reports are available for functional properties (magnetization, magneto caloric effect, electric resistance, etc.) of HEAs. As is well known, soft magnetic materials play a fundamental role in many of the electrical systems such as electrical power generation and transmission, electric motors, magnetic shielding, electromagnets, etc. However, each kind of commercial soft magnets has its shortcomings, such as brittleness and low magnetization of soft ferrites, complex and time-consuming producing process of silicon steels; annealing brittleness and limited size of metallic glass; stress-sensitivity and low electrical resistivity of Fe-Ni alloys. Recently, some investigations have shown relatively high saturated magnetization in FeCoNiAlSi HEAs [11] and FeCoNiCrAl HEAs [12], which open up new applications of FeCoNi-based HEAs as novel soft magnets. However, studies on the magnetic properties of HEAs are still limited and more progress on this research is needed. For example, compared with most soft magnets, the saturation magnetization of 64 emu/g and coercivity of 4100 A/m found in FeCoNiCrAl HEAs are not good enough [12]. Many work needs to be done in order to improve the magnetic properties of HEAs. In this report, a new high entropy alloy FeCoNiMn0.25Al0.25 is studied thoroughly from structures to magnetic, electrical and
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mechanical properties. High saturated magnetization (>100 emu/ g), low coercivity (<1000 A/m) and high Curie temperature (~800 C) are found, indicating its potential application as soft magnet. Meanwhile, good structure stability and tensile ductility of this alloy indicate its excellent mechanical properties for industrial use. This study gives a new sight of investigating the fascinating high entropy alloys and offers a new application area of HEAs. 2. Experiments FeCoNiMn0.25Al0.25 alloys were fabricated by vacuum arc melting in a water-cooled Cu crucible using high purity raw elements. The Fe, Co, Ni, Al raw elements are ingots with the purity of 99.9%, while the Mn raw element is in small pieces with the purity of 99.7%. All the starting materials are polished and prepared well for use. 2 at% more of Mn was added to balance its evaporation during the melting process. The ingots were re-melted five times to ensure homogeneity. Then they were suction casted into a 3 mm 12 mm 80 mm copper mold (as-cast samples). The plates were cold rolled of 70% thickness reduction (cold rolled samples) followed by heat treatment at 900 C for 1 h to obtain full recrystallization then quenched in water (annealed samples). The compositions of the three different states are identified by EDS, as shown in Table 1. It is clear that the actual compositions are quite near the nominal one, indicating there is no obvious composition change during the process. The ingots were sectioned, polished and etched with nitric acid and alcohol solutions for observation and photographed by Leica optical microscope. A Rigaku Smartlab X-ray diffractometer was utilized for identifying of the crystal structure, using Cu Ka radiation. Magnetization hysteresis loops of the samples were measured in a Lakeshore vibrating sample magnetometer (VSM) up to 2.0 T applied field. Differential scanning calorimetry (Netzsch DSC 204) and thermomagnetic measurements in the heating/cooling speed of 20 K/min were taken to search for the phase transformation and Curie transition. The electric resistance is measured by a four-probe method. Hardness measurements of the alloys were conducted using a Vickers hardness tester at a load of 1.5 N for a loading time of 20 s. Several indentations were performed and averaged for each specimen. Tensile samples with a cross-section of 3.2 1.4 mm and a gauge length of 12.5 mm were cut by electro-discharge machining, and polished carefully on each side with SiC paper through 2000 grit. Tensile tests were conducted on an MTS tensile testing machine at a strain rate of 103 s1 at room temperature and tensile properties were determined from the stress-strain curves. Fracture surfaces were examined by JEOL JSM-5600 scanning electron microscope (SEM). 3. Results and discussion Fig. 1 shows the X-ray diffraction patterns and related morphology images of as-cast, cold rolled and annealed
Table 1 Chemical compositions of as-cast, cold-rolled and annealed FeCoNiMn0.25Al0.25 alloys by EDS analysis. Compositions
Fe (at.%)
Co (at.%)
Ni (at.%)
Mn (at.%)
Al (at.%)
Nominal As-cast Cold-rolled Annealed
28.57 28.98 28.94 29.09
28.57 28.08 28.81 28.93
28.57 27.77 27.44 27.38
7.14 7.25 7.18 7.13
7.14 7.92 7.63 7.47
Fig. 1. X-ray diffraction spectrum and morphology (insets) of as-cast, cold-rolled and annealed FeCoNiMn0.25Al0.25 alloys.
FeCoNiMn0.25Al0.25 alloys. It is clearly shown that for these samples, simple FCC structures were formed. It is hard to understand why this alloy shows no any indication of line broadening after cold work. From the dashed line in this figure, it is also observed that the diffraction peaks have no obvious shift, suggesting the lattice parameters of this alloy are very stable to variations of stress and temperature. This feature is very different from some traditional soft magnets. For example, stresses can cause significant decrease of magnetization of FeeNi alloys [13] while the annealing at 900 C can induce structural variation in Fe-based metallic glasses as soft magnets [14]. For the annealed sample, a superlattice peak symbolized by “D” was found in the XRD spectrum, indicating that a minor ordered phase might be formed. This ordered phase was suggested as a B2 structure phase, as reported previously [15e17]. Inset images in Fig. 1 reveal the morphology evolution of FeCoNiMn0.25Al0.25 alloys. Typical dendrite and secondary dendrite structure can be observed from the as-cast sample, with the secondary dendrite arm spacing less than 10 mm. Stretched grains along the rolling direction are observed in the cold rolled sample. From the morphology of the annealed sample, it can be found that a full crystallization occurred after heat treatment at 900 C for 1 h. The crystal size after annealing is about 20e30 mm. To further identify the thermal stability of the as-cast, coldrolled and annealed FeCoNiMn0.25Al0.25 alloy, DSC measurement was taken and the corresponding curves were shown in Fig. 2(a). No obvious changes were observed up to 1000 C, indicating no phase transformation occurred until 1000 C. We have already known from Fig. 1 that annealing as-cast FeCoNiMn0.25Al0.25 alloy at 900 C can induce the formation of a minor B2 phase. However, from Fig. 2 (a), the DSC curve shows no detection of the phase, indicating a very small quantity of this phase. In addition, thermomagnetic curves from room temperature to 950 C were tested for the as-cast, cold-rolled and annealed FeCoNiMn0.25Al0.25 alloy. The Curie temperature is determined to be around 800 C, suggesting its potential use in high temperatures. The Curie temperature has not changed obviously in the three states as the structure and composition have not altered apparently. From Figs. 1 and 2, we can draw a conclusion that the crystal structure of FeCoNiMn0.25Al0.25 alloy is quite stable to the variations of stress and temperature. Fig. 3 shows the magnetization curves of as-cast, cold-rolled and annealed FeCoNiMn0.25Al0.25 alloys at room temperature. All of the
P. Li et al. / Journal of Alloys and Compounds 694 (2017) 55e60
Fig. 2. DSC curves (a) and thermomagnetic curves (b) of as-cast, cold-rolled and annealed FeCoNiMn0.25Al0.25 alloys.
Fig. 3. Magnetization curves of as-cast, cold-rolled and annealed FeCoNiMn0.25Al0.25 alloys.
alloys can be very easily magnetized to the saturated state with coercivity lower than 1000 A/m, suggesting their soft magnetic properties. The saturated magnetization Ms and coercivity Hc are
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changed differently by cold rolling and annealing. The values are summarized in Table 2 and compared with other high entropy alloys reported previously in Table 3. The magnetic properties in the present study are much better than most of the previously reported values of high entropy alloys. Further discussion is detailed in the discussion part of Tables 2 and 3. Fig. 4 shows the room-temperature tensile testing results of ascast, cold-rolled and annealed FeCoNiMn0.25Al0.25 samples. It is easy to understand that the as-cast sample shows the best ductility and lowest strength, while the cold-rolled sample shows the highest strength and worst ductility due to work hardening. It is worth noticing that the annealed FeCoNiMn0.25Al0.25 alloy has a balanced ductility (47.1% total elongation) and yield strength (331.4 MPa). The ductility of this alloy is much better than that of grain oriented silicon steel (elongation-to-failure df ¼ 10e20%), one of the most popular commercial soft magnets [18]. The mechanical parameters are also included in Table 2. The balanced performance of strength and ductility might be considered due to the B2 structure found in Fig. 1, as B2 phase has been recognized as an effective way to control strength and ductility in Ni3Al superalloy [19] as well as in bulk metallic glasses [20]. To further identify the underlying mechanism of the ductility variation in FeCoNiMn0.25Al0.25 alloys, their fracture surfaces after tensile tests were carefully examined by SEM, as shown in Fig. 5 (a) as-cast, (b) cold-rolled and (c) annealed, respectively. As is shown, dimples with features of typical ductile fracture were observed for all the three samples. In general, the average diameter or depth of the dimples for as-cast sample is the largest. And the dimples become much shallower and finer for cold-rolled sample. The annealed sample with the moderate ductility shows the medium sized dimples. The structure, magnetic, electrical and mechanical properties of FeCoNiMn0.25Al0.25 alloys are summarized in Table 2. It should be noted that the FeCoNiMn0.25Al0.25 alloys have several special properties. Firstly, the as-cast FeCoNiMn0.25Al0.25 alloy forms FCC simple solid solution. The crystal structure has not changed by cold rolling and annealing. By annealing, a small quantity of the ordered B2 phase was detected by XRD. However, this phase might be very tiny that the saturated magnetization has barely been effected. Secondly, the saturated magnetization was slightly increased by cold rolling and annealing while the coercivity and permeability were changed significantly. This is due to the magnetization is primarily determined by composition and crystal structure while the coercivity is affected by impurity, deformation, grain size, stress and heat treatment. Therefore, the coercivity increased from 268 A/ m (as-cast) to 625 A/m (cold rolled) as a result of internal stress and defects induced by cold rolling. Then, the coercivity decreased from 625 A/m (cold rolled) to 230 A/m (annealed) because of the released internal stress and recrystallization. The coercivity of annealed sample (230 A/m) is slightly lower than that of as-cast sample (268 A/m), suggesting the average crystal size of the annealed sample here is a little larger than that of the as-cast sample according to the well-known coercivity-crystal size relationship reported by Herzer [21]. As is well known, the initial permeability and coercivity behave in a contrary manner [22]. Therefore, the annealed alloy has the largest permeability in this research. Thirdly, the high Curie temperature around 800 C has been observed in the annealed FeCoNiMn0.25Al0.25 alloy, suggesting its potential applications in high temperature area, such as electric cars, aeronautics and astronautics [23,24]. For example, in the area of high temperature aerospace power generation, both mechanical and magnetic properties are demanded due to the higher speed and higher operating temperature in these devices [23,24]. Also,
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Table 2 Crystal structures, saturated magnetization (Ms), coercivity (Hc), permeability (me) at 1 kHz, Curie temperature (Tc), electric resistivity (r), yield strength (s0.2), tensile strength (sul), elongation-to-failure (df) and hardness of FeCoNiMn0.25Al0.25 alloys. Alloy state
Crystal structure
Ms (emu/g)
me
Hc (A/m)
Tc ( C)
r (U$m)
s0.2 (MPa)
sul (MPa)
df (%)
Hardness (HV)
As-cast Cold-rolled Annealed
FCC FCC FCC
101.0 104.1 104.2
297.2 71.5 479.6
268 625 230
805 810 812
1.0 106 1.1 106 0.91 106
138.1 662.7 331.4
483.9 1029.6 651.0
58.1 7.9 47.9
150.6 357.2 175.8
Table 3 Comparison of magnetic properties of high entropy alloys. Ms denotes saturated magnetization, Hc for coercivity and Tc for the Curie temperature. Alloy FeCoNiAl0.2Si0.2 FeCoNiCrCuAl FeCoNiCrPd1-2 FeCoNiCrAl FeCoNiCrAl1.25 FeCoNiCrAl2 FeCoNiMn0.25Al0.25
Structure FCC BCC þ FCC FCC BCC BCC BCC FCC
Ms (emu/g) 95e131 38e46 33e34 64 43.05a 13e18 101
a
Hc (A/m) 315e1508 3582 4100 1416 268
Tc ( C) 25 167e230 165 157 ~800
Reference [11,26,27] [28e30] [29] [12] [31] [29] Current study
a
For the convenience of comparison, the magnetic units are unified. The Ms of FeCoNiAl0.2Si0.2 alloy is reported as 0.838e1.151 T by reference 11 and 26. According to the magnetic unit conversion table, 1 T ¼ 104/4p emu/cm3 and 1 emu/cm3 ÷r (g/cm3) ¼ 1/r emu/g. normally the alloy density r is between 6.5 g/cm3 to 7.5 g/cm3. Here, we use r ¼ 7, so 0.838e1.151 T ¼ 667e916 emu/cm3 ¼ 95e131 emu/g. The magnetization of FeCoNiCrAl1.25 is also calculated according to the above equation.
Fig. 4. Tensile curve of as-cast, cold-rolled and annealed FeCoNiMn0.25Al0.25 samples.
the electrical resistivity is an important parameter related to the energy loss and the eddy current effect in the application of soft magnets. The electrical resistivity of this alloy is larger than those of FeCoNi(AlSi)x x ¼ 0e0.5 high entropy alloys reported previously [11] and comparable to that of Fe-based metallic glass as soft magnets [25]. Also, the electric resistance is not sensitive to the metallurgical variations. Finally, the high strength and good ductility observed in this alloy indicate that this material is easy to process and can meet many mechanical requirements in industry. Usually, hardness is increased by work hardening, which is the reason why the cold rolled sample has the highest hardness. Table 3 compared the soft magnetic properties of previously reported research on HEAs with current study. It can be cleared seen that compared to most high entropy alloys showing soft magnetic properties, our samples have the lowest Hc, highest Ms and Tc. With a stable structure and much better soft magnetic properties, our FeCoNiMn0.25Al0.25 alloys show a great prospect as soft magnets. Fig. 6 compares the saturated magnetization (Ms) and coercivity (Hc) of different soft magnets. The FeCoNiMn0.25Al0.25 alloy (marked by a pentagram) reported in this paper presents better
magnetic properties than soft ferrites and comparable properties with FeeNi alloys. Besides, our alloy has several advantages including stable structure, high Curie temperature and good ductility. Moreover, many soft magnetic properties (coercivity, permeability) can be further optimized by controlling the grain size or alloying minor elements [21]. Soft magnetic materials have been developed for decades. As is well known, electric steels have been most widely used as the core material for transformers and motors as they have the highest Ms with relatively low price. However, the complex production process and giant energy consumption make electric steels less appealing. A new energy paradigm with increased concern for energy efficiency in the total energy life cycle has accelerated research into new materials such as amorphous and nanocrystalline soft magnetic materials. There is a persistent demand for core material which has an even higher energy efficiency. We do see the potential of this high entropy alloy as the process is simple and their structures are stable. High entropy alloys deserve much more attention in the area of soft magnets. 4. Conclusion In this paper, the high entropy alloy FeCoNiMn0.25Al0.25 was prepared by casting, cold rolling and recrystallization annealing. Structural, thermal, magnetic, electrical and mechanical properties of the alloys were investigated and compared. Structural and thermal analyses indicate the crystal structure is barely affected by variations of the stress and temperature. Magnetic tests reveal the alloys possess a high magnetization, high Curie temperature and low coercivity. Mechanical tests demonstrate the alloys have a good ductility. The good combination of mechanical and magnetic properties makes the FeCoNiMn0.25Al0.25 alloy potentially to be a new soft magnetic material. Major conclusions of this paper can be drawn as follows: (1) The as-cast FeCoNiMn0.25Al0.25 high entropy alloy forms a simple FCC solid solution structure, which is quite stable to deformation and temperature. By annealing, a small quantity of the ordered B2 phase was detected by XRD. (2) The annealed FeCoNiMn0.25Al0.25 high entropy alloy shows the highest Ms, lowest Hc, high Tc and electrical resistivity,
P. Li et al. / Journal of Alloys and Compounds 694 (2017) 55e60
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Fig. 6. The saturated magnetization and coercivity of FeCoNiMn0.25Al0.25 alloy (marked by pentagram) compared with other commercialized soft magnets [22].
significantly, further in-depth research in this area is expected in the future. Acknowledgments This research was supported by funding from the Research Grant Council (RGC), the Hong Kong Government through the General Research Fund (GRF) with the account number of CityU 11209314. References
Fig. 5. Fractographic SEM images of as-cast (a), cold-rolled (b) and annealed (c) FeCoNiMn0.25Al0.25 samples.
indicating its applications as soft magnets. Furthermore, this alloy exhibits excellent stability and reliability at high temperatures. (3) The annealed FeCoNiMn0.25Al0.25 high entropy alloy also show good ductility, advocating it can be easily deformed and meet many deformation requirements in industry. It is very likely that we can get much better soft magnetic properties as well as mechanical properties by controlling the grain size and the composition of the high entropy alloys. With the possibility of improving magnetic and mechanical properties
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
A ductile high entropy alloy with attractive magnetic properties Panpan Li, Anding Wang, C.T. Liu* Center for Advanced Structural Materials, Department of Mechanical and Biomedical Engineering, College of Science and Engineering, City University of Hong Kong, Hong Kong
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 June 2016 Received in revised form 24 August 2016 Accepted 19 September 2016
FeCoNiMn0.25Al0.25 high entropy alloy was investigated thoroughly in this paper from crystal structure to magnetic and mechanical properties. We found that this alloy formed simple face centered cubic structure which was very stable to deformation and heat treatment. Moreover, this alloy showed high saturated magnetization, low coercivity, high Curie temperature as well as good tensile ductility. This high entropy alloy shows soft magnetic properties much better than most high entropy alloys reported previously. This study suggests that the FeCoNiMn0.25Al0.25 high entropy alloy has the potential for use as soft magnets in the future. © 2016 Elsevier B.V. All rights reserved.
Keywords: High entropy alloyso Mechanical propertieso Magnetic propertieso
1. Introduction Alloy design has been a hot topic for metallurgists and material scientists for a long time. From ancient to very recently, alloy design are all based on only one or two principle elements without exception, while other elements in small amounts are usually added to enhance some specific properties of certain alloys. As is well known, steels are based on element iron, while chromium is used to enhance their corrosion resistance properties. Superalloy Ni3Al are based on elements nickel and aluminum, while boron is added to enhance the mechanical properties. Until the year of 2004, high entropy alloys (HEAs) with at least five principle elements with each element's concentration between 5 at% to 35 at%, which are not limited to one or two principle elements any more, have attracted intensive attentions by their extraordinary characteristics due to the increased mixing entropy, inciting a fascinating new area in metallurgy [1e6]. Due to their characteristics such as high entropy effect, sluggish diffusion and significant lattice distortion, they are reported to be good candidates as structural materials as they have high strength and hardness, excellent corrosion and wear resistance, as well as great fatigue resistance [1e6]. Usually, simple solid solution structures such as body centered cubic or face centered cubic are formed in high entropy alloys instead of ordered crystalline intermetallic phases due to the high entropy effect.
* Corresponding author. E-mail address: [email protected] (C.T. Liu). http://dx.doi.org/10.1016/j.jallcom.2016.09.186 0925-8388/© 2016 Elsevier B.V. All rights reserved.
Moreover, some high entropy alloys are even reported to have stable structures at high temperatures without precipitation [7e10]. Until now, many studies are focused on the microstructures and mechanical properties of HEAs, while few reports are available for functional properties (magnetization, magneto caloric effect, electric resistance, etc.) of HEAs. As is well known, soft magnetic materials play a fundamental role in many of the electrical systems such as electrical power generation and transmission, electric motors, magnetic shielding, electromagnets, etc. However, each kind of commercial soft magnets has its shortcomings, such as brittleness and low magnetization of soft ferrites, complex and time-consuming producing process of silicon steels; annealing brittleness and limited size of metallic glass; stress-sensitivity and low electrical resistivity of Fe-Ni alloys. Recently, some investigations have shown relatively high saturated magnetization in FeCoNiAlSi HEAs [11] and FeCoNiCrAl HEAs [12], which open up new applications of FeCoNi-based HEAs as novel soft magnets. However, studies on the magnetic properties of HEAs are still limited and more progress on this research is needed. For example, compared with most soft magnets, the saturation magnetization of 64 emu/g and coercivity of 4100 A/m found in FeCoNiCrAl HEAs are not good enough [12]. Many work needs to be done in order to improve the magnetic properties of HEAs. In this report, a new high entropy alloy FeCoNiMn0.25Al0.25 is studied thoroughly from structures to magnetic, electrical and
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mechanical properties. High saturated magnetization (>100 emu/ g), low coercivity (<1000 A/m) and high Curie temperature (~800 C) are found, indicating its potential application as soft magnet. Meanwhile, good structure stability and tensile ductility of this alloy indicate its excellent mechanical properties for industrial use. This study gives a new sight of investigating the fascinating high entropy alloys and offers a new application area of HEAs. 2. Experiments FeCoNiMn0.25Al0.25 alloys were fabricated by vacuum arc melting in a water-cooled Cu crucible using high purity raw elements. The Fe, Co, Ni, Al raw elements are ingots with the purity of 99.9%, while the Mn raw element is in small pieces with the purity of 99.7%. All the starting materials are polished and prepared well for use. 2 at% more of Mn was added to balance its evaporation during the melting process. The ingots were re-melted five times to ensure homogeneity. Then they were suction casted into a 3 mm 12 mm 80 mm copper mold (as-cast samples). The plates were cold rolled of 70% thickness reduction (cold rolled samples) followed by heat treatment at 900 C for 1 h to obtain full recrystallization then quenched in water (annealed samples). The compositions of the three different states are identified by EDS, as shown in Table 1. It is clear that the actual compositions are quite near the nominal one, indicating there is no obvious composition change during the process. The ingots were sectioned, polished and etched with nitric acid and alcohol solutions for observation and photographed by Leica optical microscope. A Rigaku Smartlab X-ray diffractometer was utilized for identifying of the crystal structure, using Cu Ka radiation. Magnetization hysteresis loops of the samples were measured in a Lakeshore vibrating sample magnetometer (VSM) up to 2.0 T applied field. Differential scanning calorimetry (Netzsch DSC 204) and thermomagnetic measurements in the heating/cooling speed of 20 K/min were taken to search for the phase transformation and Curie transition. The electric resistance is measured by a four-probe method. Hardness measurements of the alloys were conducted using a Vickers hardness tester at a load of 1.5 N for a loading time of 20 s. Several indentations were performed and averaged for each specimen. Tensile samples with a cross-section of 3.2 1.4 mm and a gauge length of 12.5 mm were cut by electro-discharge machining, and polished carefully on each side with SiC paper through 2000 grit. Tensile tests were conducted on an MTS tensile testing machine at a strain rate of 103 s1 at room temperature and tensile properties were determined from the stress-strain curves. Fracture surfaces were examined by JEOL JSM-5600 scanning electron microscope (SEM). 3. Results and discussion Fig. 1 shows the X-ray diffraction patterns and related morphology images of as-cast, cold rolled and annealed
Table 1 Chemical compositions of as-cast, cold-rolled and annealed FeCoNiMn0.25Al0.25 alloys by EDS analysis. Compositions
Fe (at.%)
Co (at.%)
Ni (at.%)
Mn (at.%)
Al (at.%)
Nominal As-cast Cold-rolled Annealed
28.57 28.98 28.94 29.09
28.57 28.08 28.81 28.93
28.57 27.77 27.44 27.38
7.14 7.25 7.18 7.13
7.14 7.92 7.63 7.47
Fig. 1. X-ray diffraction spectrum and morphology (insets) of as-cast, cold-rolled and annealed FeCoNiMn0.25Al0.25 alloys.
FeCoNiMn0.25Al0.25 alloys. It is clearly shown that for these samples, simple FCC structures were formed. It is hard to understand why this alloy shows no any indication of line broadening after cold work. From the dashed line in this figure, it is also observed that the diffraction peaks have no obvious shift, suggesting the lattice parameters of this alloy are very stable to variations of stress and temperature. This feature is very different from some traditional soft magnets. For example, stresses can cause significant decrease of magnetization of FeeNi alloys [13] while the annealing at 900 C can induce structural variation in Fe-based metallic glasses as soft magnets [14]. For the annealed sample, a superlattice peak symbolized by “D” was found in the XRD spectrum, indicating that a minor ordered phase might be formed. This ordered phase was suggested as a B2 structure phase, as reported previously [15e17]. Inset images in Fig. 1 reveal the morphology evolution of FeCoNiMn0.25Al0.25 alloys. Typical dendrite and secondary dendrite structure can be observed from the as-cast sample, with the secondary dendrite arm spacing less than 10 mm. Stretched grains along the rolling direction are observed in the cold rolled sample. From the morphology of the annealed sample, it can be found that a full crystallization occurred after heat treatment at 900 C for 1 h. The crystal size after annealing is about 20e30 mm. To further identify the thermal stability of the as-cast, coldrolled and annealed FeCoNiMn0.25Al0.25 alloy, DSC measurement was taken and the corresponding curves were shown in Fig. 2(a). No obvious changes were observed up to 1000 C, indicating no phase transformation occurred until 1000 C. We have already known from Fig. 1 that annealing as-cast FeCoNiMn0.25Al0.25 alloy at 900 C can induce the formation of a minor B2 phase. However, from Fig. 2 (a), the DSC curve shows no detection of the phase, indicating a very small quantity of this phase. In addition, thermomagnetic curves from room temperature to 950 C were tested for the as-cast, cold-rolled and annealed FeCoNiMn0.25Al0.25 alloy. The Curie temperature is determined to be around 800 C, suggesting its potential use in high temperatures. The Curie temperature has not changed obviously in the three states as the structure and composition have not altered apparently. From Figs. 1 and 2, we can draw a conclusion that the crystal structure of FeCoNiMn0.25Al0.25 alloy is quite stable to the variations of stress and temperature. Fig. 3 shows the magnetization curves of as-cast, cold-rolled and annealed FeCoNiMn0.25Al0.25 alloys at room temperature. All of the
P. Li et al. / Journal of Alloys and Compounds 694 (2017) 55e60
Fig. 2. DSC curves (a) and thermomagnetic curves (b) of as-cast, cold-rolled and annealed FeCoNiMn0.25Al0.25 alloys.
Fig. 3. Magnetization curves of as-cast, cold-rolled and annealed FeCoNiMn0.25Al0.25 alloys.
alloys can be very easily magnetized to the saturated state with coercivity lower than 1000 A/m, suggesting their soft magnetic properties. The saturated magnetization Ms and coercivity Hc are
57
changed differently by cold rolling and annealing. The values are summarized in Table 2 and compared with other high entropy alloys reported previously in Table 3. The magnetic properties in the present study are much better than most of the previously reported values of high entropy alloys. Further discussion is detailed in the discussion part of Tables 2 and 3. Fig. 4 shows the room-temperature tensile testing results of ascast, cold-rolled and annealed FeCoNiMn0.25Al0.25 samples. It is easy to understand that the as-cast sample shows the best ductility and lowest strength, while the cold-rolled sample shows the highest strength and worst ductility due to work hardening. It is worth noticing that the annealed FeCoNiMn0.25Al0.25 alloy has a balanced ductility (47.1% total elongation) and yield strength (331.4 MPa). The ductility of this alloy is much better than that of grain oriented silicon steel (elongation-to-failure df ¼ 10e20%), one of the most popular commercial soft magnets [18]. The mechanical parameters are also included in Table 2. The balanced performance of strength and ductility might be considered due to the B2 structure found in Fig. 1, as B2 phase has been recognized as an effective way to control strength and ductility in Ni3Al superalloy [19] as well as in bulk metallic glasses [20]. To further identify the underlying mechanism of the ductility variation in FeCoNiMn0.25Al0.25 alloys, their fracture surfaces after tensile tests were carefully examined by SEM, as shown in Fig. 5 (a) as-cast, (b) cold-rolled and (c) annealed, respectively. As is shown, dimples with features of typical ductile fracture were observed for all the three samples. In general, the average diameter or depth of the dimples for as-cast sample is the largest. And the dimples become much shallower and finer for cold-rolled sample. The annealed sample with the moderate ductility shows the medium sized dimples. The structure, magnetic, electrical and mechanical properties of FeCoNiMn0.25Al0.25 alloys are summarized in Table 2. It should be noted that the FeCoNiMn0.25Al0.25 alloys have several special properties. Firstly, the as-cast FeCoNiMn0.25Al0.25 alloy forms FCC simple solid solution. The crystal structure has not changed by cold rolling and annealing. By annealing, a small quantity of the ordered B2 phase was detected by XRD. However, this phase might be very tiny that the saturated magnetization has barely been effected. Secondly, the saturated magnetization was slightly increased by cold rolling and annealing while the coercivity and permeability were changed significantly. This is due to the magnetization is primarily determined by composition and crystal structure while the coercivity is affected by impurity, deformation, grain size, stress and heat treatment. Therefore, the coercivity increased from 268 A/ m (as-cast) to 625 A/m (cold rolled) as a result of internal stress and defects induced by cold rolling. Then, the coercivity decreased from 625 A/m (cold rolled) to 230 A/m (annealed) because of the released internal stress and recrystallization. The coercivity of annealed sample (230 A/m) is slightly lower than that of as-cast sample (268 A/m), suggesting the average crystal size of the annealed sample here is a little larger than that of the as-cast sample according to the well-known coercivity-crystal size relationship reported by Herzer [21]. As is well known, the initial permeability and coercivity behave in a contrary manner [22]. Therefore, the annealed alloy has the largest permeability in this research. Thirdly, the high Curie temperature around 800 C has been observed in the annealed FeCoNiMn0.25Al0.25 alloy, suggesting its potential applications in high temperature area, such as electric cars, aeronautics and astronautics [23,24]. For example, in the area of high temperature aerospace power generation, both mechanical and magnetic properties are demanded due to the higher speed and higher operating temperature in these devices [23,24]. Also,
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Table 2 Crystal structures, saturated magnetization (Ms), coercivity (Hc), permeability (me) at 1 kHz, Curie temperature (Tc), electric resistivity (r), yield strength (s0.2), tensile strength (sul), elongation-to-failure (df) and hardness of FeCoNiMn0.25Al0.25 alloys. Alloy state
Crystal structure
Ms (emu/g)
me
Hc (A/m)
Tc ( C)
r (U$m)
s0.2 (MPa)
sul (MPa)
df (%)
Hardness (HV)
As-cast Cold-rolled Annealed
FCC FCC FCC
101.0 104.1 104.2
297.2 71.5 479.6
268 625 230
805 810 812
1.0 106 1.1 106 0.91 106
138.1 662.7 331.4
483.9 1029.6 651.0
58.1 7.9 47.9
150.6 357.2 175.8
Table 3 Comparison of magnetic properties of high entropy alloys. Ms denotes saturated magnetization, Hc for coercivity and Tc for the Curie temperature. Alloy FeCoNiAl0.2Si0.2 FeCoNiCrCuAl FeCoNiCrPd1-2 FeCoNiCrAl FeCoNiCrAl1.25 FeCoNiCrAl2 FeCoNiMn0.25Al0.25
Structure FCC BCC þ FCC FCC BCC BCC BCC FCC
Ms (emu/g) 95e131 38e46 33e34 64 43.05a 13e18 101
a
Hc (A/m) 315e1508 3582 4100 1416 268
Tc ( C) 25 167e230 165 157 ~800
Reference [11,26,27] [28e30] [29] [12] [31] [29] Current study
a
For the convenience of comparison, the magnetic units are unified. The Ms of FeCoNiAl0.2Si0.2 alloy is reported as 0.838e1.151 T by reference 11 and 26. According to the magnetic unit conversion table, 1 T ¼ 104/4p emu/cm3 and 1 emu/cm3 ÷r (g/cm3) ¼ 1/r emu/g. normally the alloy density r is between 6.5 g/cm3 to 7.5 g/cm3. Here, we use r ¼ 7, so 0.838e1.151 T ¼ 667e916 emu/cm3 ¼ 95e131 emu/g. The magnetization of FeCoNiCrAl1.25 is also calculated according to the above equation.
Fig. 4. Tensile curve of as-cast, cold-rolled and annealed FeCoNiMn0.25Al0.25 samples.
the electrical resistivity is an important parameter related to the energy loss and the eddy current effect in the application of soft magnets. The electrical resistivity of this alloy is larger than those of FeCoNi(AlSi)x x ¼ 0e0.5 high entropy alloys reported previously [11] and comparable to that of Fe-based metallic glass as soft magnets [25]. Also, the electric resistance is not sensitive to the metallurgical variations. Finally, the high strength and good ductility observed in this alloy indicate that this material is easy to process and can meet many mechanical requirements in industry. Usually, hardness is increased by work hardening, which is the reason why the cold rolled sample has the highest hardness. Table 3 compared the soft magnetic properties of previously reported research on HEAs with current study. It can be cleared seen that compared to most high entropy alloys showing soft magnetic properties, our samples have the lowest Hc, highest Ms and Tc. With a stable structure and much better soft magnetic properties, our FeCoNiMn0.25Al0.25 alloys show a great prospect as soft magnets. Fig. 6 compares the saturated magnetization (Ms) and coercivity (Hc) of different soft magnets. The FeCoNiMn0.25Al0.25 alloy (marked by a pentagram) reported in this paper presents better
magnetic properties than soft ferrites and comparable properties with FeeNi alloys. Besides, our alloy has several advantages including stable structure, high Curie temperature and good ductility. Moreover, many soft magnetic properties (coercivity, permeability) can be further optimized by controlling the grain size or alloying minor elements [21]. Soft magnetic materials have been developed for decades. As is well known, electric steels have been most widely used as the core material for transformers and motors as they have the highest Ms with relatively low price. However, the complex production process and giant energy consumption make electric steels less appealing. A new energy paradigm with increased concern for energy efficiency in the total energy life cycle has accelerated research into new materials such as amorphous and nanocrystalline soft magnetic materials. There is a persistent demand for core material which has an even higher energy efficiency. We do see the potential of this high entropy alloy as the process is simple and their structures are stable. High entropy alloys deserve much more attention in the area of soft magnets. 4. Conclusion In this paper, the high entropy alloy FeCoNiMn0.25Al0.25 was prepared by casting, cold rolling and recrystallization annealing. Structural, thermal, magnetic, electrical and mechanical properties of the alloys were investigated and compared. Structural and thermal analyses indicate the crystal structure is barely affected by variations of the stress and temperature. Magnetic tests reveal the alloys possess a high magnetization, high Curie temperature and low coercivity. Mechanical tests demonstrate the alloys have a good ductility. The good combination of mechanical and magnetic properties makes the FeCoNiMn0.25Al0.25 alloy potentially to be a new soft magnetic material. Major conclusions of this paper can be drawn as follows: (1) The as-cast FeCoNiMn0.25Al0.25 high entropy alloy forms a simple FCC solid solution structure, which is quite stable to deformation and temperature. By annealing, a small quantity of the ordered B2 phase was detected by XRD. (2) The annealed FeCoNiMn0.25Al0.25 high entropy alloy shows the highest Ms, lowest Hc, high Tc and electrical resistivity,
P. Li et al. / Journal of Alloys and Compounds 694 (2017) 55e60
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Fig. 6. The saturated magnetization and coercivity of FeCoNiMn0.25Al0.25 alloy (marked by pentagram) compared with other commercialized soft magnets [22].
significantly, further in-depth research in this area is expected in the future. Acknowledgments This research was supported by funding from the Research Grant Council (RGC), the Hong Kong Government through the General Research Fund (GRF) with the account number of CityU 11209314. References
Fig. 5. Fractographic SEM images of as-cast (a), cold-rolled (b) and annealed (c) FeCoNiMn0.25Al0.25 samples.
indicating its applications as soft magnets. Furthermore, this alloy exhibits excellent stability and reliability at high temperatures. (3) The annealed FeCoNiMn0.25Al0.25 high entropy alloy also show good ductility, advocating it can be easily deformed and meet many deformation requirements in industry. It is very likely that we can get much better soft magnetic properties as well as mechanical properties by controlling the grain size and the composition of the high entropy alloys. With the possibility of improving magnetic and mechanical properties
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