Arab J Sci Eng (2015) 40:3657–3663 DOI 10.1007/s13369-015-1784-9
RESEARCH ARTICLE - MECHANICAL ENGINEERING
Effects of Fe Content on Microstructures and Properties of AlCoCrFe x Ni High-Entropy Alloys Qiushi Chen1 · Kaiyao Zhou1 · Li Jiang1 · Yiping Lu1 · Tingju Li1
Received: 5 January 2015 / Accepted: 13 July 2015 / Published online: 28 July 2015 © King Fahd University of Petroleum & Minerals 2015
Abstract The influences of added Fe element on the microstructures and properties of AlCoCrFex Ni high-entropy alloys (x denoted the atomic fraction of Fe element at 0.2, 0.4, 0.6, 0.8, 1.2, 1.4, 1.6, 1.8, and 2.0) were investigated. When x = 0.2, the alloy exhibited dendrites morphology, but as the Fe content increased, the AlCoCrFex Ni HEAs transformed to equiaxed grains morphology. Inside the equiaxed grains and dendritic grains, spinoidal decomposition microstructure could be clearly observed. The microstructures changed from Cr3 Ni2 + B2 + BCC structures to B2 + BCC mixed structures as x exceeded 0.6, the hardness declined from HV637.2 to HV460.2, and the compressive fracture strength showed a slight decrease, whereas the plastic property showed a distinct improvement with the addition of Fe element. The maximum compression strength was 2335 MPa when x = 0.2, and the maximum compression ratio was 36 % when x = 2.0. The alloys transformed from paramagnetic to ferromagnetic as the content of Fe element increased, and all of the alloys exhibited soft magnetic behaviors. Keywords High-entropy alloys · Microstructures · Properties · Morphology
1 Introduction Conventional alloy design strategy is mainly based on one or two elements as the principal components, while other elements are regarded as minor components to improve properties [1–3]. Differing from conventional alloys, high-
B 1
Yiping Lu
[email protected] School of Materials Science and Engineering, Dalian University of Technology, Dalian, 116024, China
entropy alloys (HEAs) or multi-principal element alloys proposed by Cantor et al. [4] and Yeh et al. [5] constitute a new type of metallic alloy. The alloying strategy in HEAs is a breakthrough in the history of physical metallurgy, and it has become a new research hot spot in the field of materials science [6–11]. The effects of elemental additions in AlCoCrFeNi alloy systems have been studied in many previous research projects; for examples, AlCoCrFeNiTix [12], AlCoCr FeNiMox [13], AlCoCrFeNiSix [14], AlCoCrFeNiNbx [15], AlCoCrFeNiCx [16], and AlCoCrFeNiVx [17] alloys and so on. However, in the AlCoCrFeNi alloy system, Fe element additions have been rarely studied [18]. In this study, Fe was selected for two reasons. First, AlCoCrFeNi system alloys exhibit high compressive strengths, although their plastic properties are not remarkable [19,20]. The addition of Fe element can reduce atomic size differences and decrease lattice distortion, which is beneficial for plastic property improvement. The second reason for choosing this alloy was due to the fact that some alloys containing Fe elements possess good ferromagnetic properties [21]. Therefore, the addition of ferromagnetic element, Fe element, could be a way to change the magnetic behavior of AlCoCrFex Ni HEAs. Therefore, in this paper, the effects of adding Fe element in amounts of 0.2–2.0 in molar fraction on the microstructures and properties of AlCoCrFex Ni HEAs were investigated in detail.
2 Experimental Ingots with nominal compositions of AlCoCrFex Ni (x = molar ratio, x = 0.2, 0.4, . . ., 1.8, and 2.0, denoted by Fe0.2, Fe0.4, …, Fe1.8, and Fe2.0, respectively) were prepared by arc melting of the mixture of the constituent
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The crystalline structures of the ingots were identified via X-ray diffractometer (XRD, Shimadzu XRD-6000) with Cu Kα radiation, and the scanning angle ranged from 20◦ to 100◦ at a speed of 4 deg/min. Microstructures and phase compositions were observed using a scanning electron microscope with energy dispersive spectrometry (SEM, Zeiss supra55). The size of the samples for the tests of compressive properties was 5 × 10 mm, as shown in Fig. 2. The tests were conducted at room temperature with a strain rate of 1 × 10−3 /s. The hardness values of the alloys were measured with a Vickers hardness tester (MH-50) under a load of 1000 g and a holding time of 15 s. Five measurements were taken of each sample to obtain the averaged experimental data. The magnetization curves were measured with a JDM-13T vibrating sample magnetometer.
3 Results and Discussion Fig. 1 The schematic diagram of vacuum arc furnace melting
3.1 X-Ray Diffraction Analysis Figure 3 shows the XRD patterns of the AlCoCrFex Ni HEAs. When x = 0.2−0.6, the alloys exhibited the mixed structures of B2 + BCC + Cr3 Ni2 . As the Fe contents increased, the intensity of the Cr3 Ni2 phase diffraction peaks decreased, and finally disappeared when x = 0.8. When x ≥ 0.8, the AlCoCrFex Ni HEAs transformed to B2 + BCC mixed structures. The transformation mentioned above was attributed to the Fe element addition, which decreased the lattice distortion (as shown in Table 1), and was beneficial for the formation of a solid solution [22]. A decreased difference in atomic size ratios decreased the sluggish diffusion of atoms in the matrix, which enhanced the phase transformation rate from compounds to solid solutions [22]. Therefore, the addition of Fe element reduced the Cr–Ni segregation and restrained the formation of Cr3 Ni2 compounds, and more solid solution phases formed instead. 3.2 Microstructures and Characterization
Fig. 2 The image of compressive test sample
elements, with purities higher than 99.9 wt%, in a watercooled copper heart under a Ti-gettered high-purity argon atmosphere, as shown in Fig. 1. When the vacuum was 3 × 10−3 Pa, melting and casting were performed in a vacuum of 0.01 atm by purging with argon (Ar). The purity of all the raw materials (Al, Co, Cr, Fe, Ni) was above 99.9 wt%. The alloys were flipped and re-melted at least five times. The button ingots ∼30 g were immediately solidified in a watercooled cold copper hearth.
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Figures 4 and 5 show the metallographic and SEM images of the microstructures of the AlCoCrFex Ni HEAs (x = 0.2, 0.4, 1.2, and 1.8). The Fe0.2 alloy exhibited typical dendritic morphology, as shown in Figs. 4a and 5a. The dendritic and interdendritic areas were denoted as DA and DB, respectively. The typical spinoidal decomposition microstructure was observed in both dendrites and interdendrites. According to the EDS results, the DA section and DB section were composed of (Al, Ni)-rich element and (Cr, Co, Ni)-rich element, whereas section DA had a higher amount of Ni than section DB, which was also mentioned in Ref. [23].
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Fig. 3 XRD patterns of the AlCoCrFex Ni HEAs
Table 1 Chemical compositions of AlCoCrFex Ni alloys in atomic percentage Alloys
Al
Co
Cr
Fe
Ni
Normal
23.8
23.8
23.8
4.8
23.8
DA
31.4
16.5
3.6
21.5
27.0
DB
18.1
32.1
5.5
24.5
19.8
Normal
22.7
22.7
22.7
9.2
22.7
DA
29.7
18.3
7.9
22.3
21.8
DB
16.4
31.6
11.5
25.0
15.5
Normal
19.2
19.2
19.2
23.2
19.2
DA
22.2
17.8
21.2
18.8
20.0
DC
21.1
15.9
26.7
17.1
19.2
Normal
17.2
17.2
17.2
31.2
17.2
DA
20.1
16.6
29.1
15.9
18.3
DC
18.6
17.0
33.3
15.7
15.4
Fe0.2
Fe0.4
Fe1.2
Fe1.8
fore, the interdendritic regions were deduced to be Cr3 Ni2 phase. When x ≥ 0.8, the crystal boundaries of equiaxed grains were rich in (Al, Fe) elements, donated as DC, as seen in Table 2. And combined with the XRD results, the DC regions were inferred to be (Al, Fe)-rich B2 phases. The transformation of crystal morphology was probably also due to the addition of Fe element. The interdendritic regions in Fe0.2 were mainly of Cr3 Ni2 phase. The formation of Cr3 Ni2 phase could be attributed to two reasons: the large atomic size difference, δ (as shown in Table 1) could cause serious lattice distortion in the alloy, and the corresponding strain energy showed the largest value when x = 0.2. The large atomic size differences could result in the increase in free energy in the alloy, which is not beneficial to the formation of a solid solution. Here, in order to describe the effect of the atomic size difference in n-element alloy, the parameter δ is expressed as follows: n ri 2 ci 1 − δ= r
(1)
i=1
When x = 0.4−2.0, all of the alloys exhibited equiaxed grain morphology. The interior of the grains was composed of (Al, Ni)-rich elements, and combined with previous research [24] and XRD results, this region was deduced to be the BCC-structural phase. The interdendritic regions in Fe0.2 tended to become narrow as the Fe element was added. And when x = 0.4, the (Cr, Co, Ni)-rich interdendritic regions totally transformed to crystal boundary of equiaxed grains, but the molar fraction between Cr and Ni stayed at 3:2 until x = 0.6. Correspondingly, the intensity of the Cr3 Ni2 phase diffraction peaks disappeared as x exceeded 0.6. There-
where ci is the atomic percentage of the ith component, r¯ is the average atomic radius, and ri is the atomic radius, which could be obtained in Ref. [23]. Conversely, significant differences in atomic size ratios can lead to the sluggish diffusion of atoms in the matrix, which lowers the phase transformation rate and makes the Cr–Ni atoms segregate in the interdendritic regions. However, large amounts of Fe element can decrease the atomic size differences (as seen in Table 1) and weaken the lattice distortion. Therefore, the Fe element addition can accelerate the formation of solid solution and break the orig-
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Fig. 4 SEM images of AlCoCrFex Ni HEAs: a Fe0.2, b Fe0.4, c Fe1.2, d Fe1.8
inal composition of the interdendritic region. And as a result, the morphology is changed indirectly. 3.3 Compressive Properties Figure 6 shows the compressive stress–strain curves of AlCoCrFex Ni alloys with different Fe contents at the strain rate of 1.0 × 10−3 mm/s. The curve in Fig. 6 shows a trend of slight decrease. Before x reached 0.8, the compressive strength showed similar values, about 2335 MPa. From Fe0.8 to Fe2.0, the alloys exhibited a slight decrease to about 2100 MPa. However, as for the compression ratio, the designed alloys revealed a distinct increase from 9 to 36 %. As Fe contents increased, the compressive strength exhibited a decreasing trend, but the compressive ratio showed an increase. Both of the changes could be attributed to the decreasing Cr3 Ni2 phases and atomic size differences. Effects of precipitation strength on the alloys were weak-
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ened as Fe element was added, but the disappearance of the Cr3 Ni2 phase precipitated the improvement of the plastic property, as shown in Fig. 6. When x exceeded 0.6, the plastic property of AlCoCrFex Ni alloys showed a continual increase. This could have been due to the decreasing atomic size differences, which can distinctly reduce the lattice distortion. During the plastic deformation, the resistance of the dislocation slip is a key factor for compressive strength and compressive ratio. The decreased lattice distortion caused by the Fe addition reduced the dislocation resistance, which can increase the compressive ratio and decrease the compressive strength. 3.4 Microhardness of AlCoCrFe x Ni HEAs The microhardness of AlCoCrFex Ni alloys is shown in Fig. 7. All AlCoCrFex Ni HEAs possess high hardness levels. As the Fe contents increased from 0.2 to 2.0, the hardness decreased from HV637.2 to HV460.2, which corresponded
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Fig. 5 Metallographic images of AlCoCrFex Ni HEAs: a Fe0.2, b Fe0.4, c Fe1.2, d Fe1.8
Table 2 Atomic size differences among AlCoCrFex Ni alloys
Samples
δ (%)
Fe0.2
5.8
Fe0.4
5.7
Fe0.6
5.6
Fe0.8
5.5
Fe1.4
5.3
Fe2.0
5.1
to the compressive test. And the decreasing lattice distortion caused by the Fe element addition, as shown in Table 1, played a dominating role in the decrease in hardness. This change was consistent with the XRD and SEM analyses.
Fig. 6 The compressive properties of the AlCoCrFex Ni HEAs: x = 0.2, 0.8, 1.4 and 2.0
3.5 Magnetic Properties The magnetization curves of AlCoCrFex Ni HEAs are shown in Fig. 8a, b. Obviously, the Fe0.8–2.0 alloys showed typical
ferromagnetic behaviors, while the Fe0.2, Fe0.4, and Fe0.6 alloys showed typical paramagnetic behavior, as shown in Fig. 8a. The specific saturation magnetizations of the Fe0.8,
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where χ is the permeability, M is the magnetization, H is the magnetic field intensity, σ is the specific saturation magnetization, ρ is the density of alloys. When the content of Fe element increased, all alloys still exhibited ferromagnetic property, but the permeability showed a slight increase, from 4.19 × 10−4 to 4.35 × 10−4 , according to Eq. (2).
4 Conclusions
Fig. 7 The microhardness of the AlCoCrFex Ni HEAs
Fe1.4, and Fe2.0 alloys under the magnetic field of 4000 Oe were 50.70, 82.12, and 98.29 emu/g, as seen in Fig. 8b. The specific saturation magnetizations of Fe0.2–Fe0.6 could not be reached in such a high magnetic field intensity (15,000 Oe). With the increase in Fe contents, the transition from paramagnetic to ferromagnetic behaviors could have been due to the addition of the ferromagnetic Fe element into the alloy system. Moreover, the coercive forces were 23.3, 20.0, 25.2, 31.3, 64.1, and 59.8 Oe for Fe0.2, Fe0.4, Fe0.6, Fe0.8, Fe1.4, and Fe2.0, respectively, which revealed soft magnetic behavior. The following equation reveals the classification of magnetic properties: σ (emu/g) · ρ g/cm3 M (A/m) = χ= H (A/m) 4π · H (Oe)
Fig. 8 The magnetization curves of AlCoCrFex Ni HEAs
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(2)
The AlCoCrFex Ni (x = 0.2−2.0) HEAs were prepared by arc melting. The microstructures and properties were investigated. XRD and SEM results revealed that as the Fe content increased, the structure of the AlCoCrFex Ni HEAs transformed from B2 + BCC + Cr3 Ni2 structures to B2 + BCC structures. The addition of Fe element facilitated the morphology changes from dendrites to equiaxed grains when x exceeded 0.2. A rich-(Cr,Ni) precipitated phase appeared in the interdendritic regions, and gradually turned to a crystal boundary of equiaxed grains. Also, the addition of Fe element improved the plasticity, owing to the decreased lattice distortion. However, the strength of the alloys decreased with increased Fe elements. With the increase in Fe elements, the hardness decreased from HV637.2 to HV460.2. This was attributed to the disappearance of the Cr3 Ni2 phases and reduced lattice distortion. It was found that the specific saturation magnetizations decreased slightly and the HEAs designed in this paper transformed from paramagnetic to ferromagnetic as the contents of Fe element increased, and all of the alloys exhibited soft magnetic behaviors.
Arab J Sci Eng (2015) 40:3657–3663 Acknowledgments This work was supported by the National Natural Science Foundation of China Nos. (51104029, 51134013, 51471044) respectively, the fundamental research funds for the central universities, key laboratory of basic research Projects (LZ2014007) of Liaoning province department of education and the natural science foundation of Liaoning Province (2014028013).
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