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Article

The BCC/B2 Morphologies in AlxNiCoFeCr High-Entropy Alloys

by
Yue Ma
1,
Beibei Jiang
1,
Chunling Li
1,
Qing Wang
1,*,
Chuang Dong
1,
Peter K. Liaw
2,
Fen Xu
3 and
Lixian Sun
3
1
Key Laboratory of Materials Modification by Laser, Ion and Electron Beams (Ministry of Education), School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, China
2
Department of Materials Science and Engineering, The University of Tennessee, Knoxville, TN 37996, USA
3
Guangxi Key Laboratory of Information Materials, Guangxi Collaborative Innovation Center of Structure and Property for New Energy and Materials, School of Material Science and Engineering, Guilin University of Electronic Technology, Guilin 541004, China
*
Author to whom correspondence should be addressed.
Metals 2017, 7(2), 57; https://doi.org/10.3390/met7020057
Submission received: 31 December 2016 / Revised: 7 February 2017 / Accepted: 9 February 2017 / Published: 15 February 2017
(This article belongs to the Special Issue High-Entropy Alloys (HEAs))
Browse Figures
Versions Notes

Abstract

:
The present work primarily investigates the morphological evolution of the body-centered-cubic (BCC)/B2 phases in AlxNiCoFeCr high-entropy alloys (HEAs) with increasing Al content. It is found that the BCC/B2 coherent morphology is closely related to the lattice misfit between these two phases, which is sensitive to Al. There are two types of microscopic BCC/B2 morphologies in this HEA series: one is the weave-like morphology induced by the spinodal decomposition, and the other is the microstructure of a spherical disordered BCC precipitation on the ordered B2 matrix that appears in HEAs with a much higher Al content. The mechanical properties, including the compressive yielding strength and microhardness of the AlxNiCoFeCr HEAs, are also discussed in light of the concept of the valence electron concentration (VEC).

1. Introduction

Unlike conventional alloys based on one or two main elements, high-entropy alloys (HEAs) are generally composed of multiple principal elements in equimolar or near-equimolar proportions, also named compositionally-complex alloys (CCAs) [1,2,3,4,5,6]. This new type of alloys has attracted much attention due to the formation of simple crystalline structures, such as face-centered cubic (FCC)-, body-centered cubic (BCC)-, and close-packed hexagonal (HCP)-solid solutions, as well as their derivatives, instead of complex intermetallic compounds [6,7,8]. Until now, many HEAs have been reported in diverse systems, e.g., CoCrFeNi-based HEAs [9,10,11,12,13,14], Al-TMs HEAs (TMs: transition metals) [15,16,17,18,19,20,21,22,23], and refractory HEAs [24,25,26,27]. The special structures render HEAs with the unique performance of excellent mechanical properties and superior corrosion-/oxidation-resistances, as examplified by the fact that the single FCC CoCrFeMnNi HEA shows a remarkably high fracture toughness that outperforms all conventional alloys at cryogenic temperatures [3,6,11].
Typical HEAs were generally formed in Al-containing transition metal (Al-TM) systems, such as AlxNiCoFeCr [19,20,21,22] and AlxNiCoFeCrCu [15,16,17,18], in which the crystalline structures change with the increase of Al content. This variation tendency could also be characterized by the valence electron concentration (VEC) that is closely related to the phase stability [28]. For the AlxNiCoFeCr HEA series [21], the HEAs with x ≤ 0.45 (VEC ≥ 7.72) exhibit a single FCC phase, HEAs with 0.45 < x < 0.88 (7.30 < VEC < 7.72), a dual-phase structure of (FCC + BCC), and those with x ≥ 0.88 (VEC ≤ 7.30), a single BCC/B2 phase (B2: an ordered BCC phase with a CsCl-type structure). Consequently, the mechanical properties of this series of HEAs would vary with the crystalline structure. The FCC HEAs possess good ductility but lower strength, while the BCC/B2 HEAs have higher strength at the expense of ductility, as demonstrated by the fact that the equimolar AlNiCoFeCr HEA is very brittle with a nearly zero tensile ductility at room temperature due to a weave-like microstructure caused by spinodal decomposition of BCC and B2 phases [19]. However, changing the combination of TMs instead of Al could also make Al0.7NiCoFe2Cr HEA with a low Al content exhibit a BCC/B2-based structure [29]. More importantly, the unique morphology of the cuboidal B2 phase coherently-embedded into the disordered BCC matrix renders this HEA with both a higher tensile strength (σUTS = 1223 MPa) and a good ductility of 8% at room temperature. Although the BCC/B2 morphologies were reported in some HEAs sporadically [20,23,30], the elaborate morphology variations of the disordered BCC and its ordered B2 phases have not been studied systematically by far since BCC and B2 phases are always coherent, which is of crucial importance for improving the mechanical property.
Therefore, the present work will aim at the microscopic morphology evolution of the disordered BCC and its ordered B2 phases with increasing the Al content in the AlxNiCoFeCr HEA series, in which the coherent morphology of the BCC/B2 phases is closely related to the lattice misfit between them. The compressive mechanical property and Vickers hardness of these HEAs will be measured to show the property variation with the microstructure. The present work will contribute to a better understanding of the B2/BCC coherent morphology in HEAs.

2. Experimental Section

The AlxNiCoFeCr serial alloy ingots were synthesized by arc-melting the raw metal materials under argon atmosphere, in which the purities of raw elemental metals are 99.99% for Al, Co, Fe, Ni, and 99.9% for Cr, respectively. These ingots with a weight of about 15 g were re-melted at least four times to ensure chemical homogeneity, and then copper-mold suction-cast into cylindrical rods with a diameter of 6 mm. The mass loss during the preparation process is less than 0.1%.
The crystalline structures of these suction-cast HEAs were analyzed by a Bruker D8 X-ray diffractometer (XRD, Ettlingen, Germany) with the Cu Kα radiation (λ = 0.15406 nm). The morphologies of HEAs were observed by both Olympus optical microscopy (OM, Tokyo, Japan) and Zeiss Supra55 scanning-electron microscopy (SEM, Oberkochen, Germany) with an etching solution of 5 g FeCl3·6H2O + 25 mL HCl + 25 mL C2H5OH. The further detailed microstructures of HEAs were characterized by the Philips Tecnai G2 transmission-electron microscopy (TEM, Amsterdam, the Netherlands) with a selected-area electron diffraction (SAED) analysis, in which the TEM samples were prepared by twin-jet electro-polishing in a solution of 10% HClO4 + 90% C2H5OH (volume fraction) at about −30 °C. The elemental distributions and compositional analyses were measured by the SHIMADZU electron-probe microanalysis (EPMA, Kyoto, Japan). The uniaxial compressive test was performed on an 810 Material Test System (MTS, Cary, NC, USA) with a strain rate of 2 × 10−4 /s at room temperature, in which a strain gage was used for the strain measurement. Two specimens for each alloy were tested with a size of φ3 × 6 mm, which were machined from the φ6 mm alloys rods. Microhardness of HEAs was measured with an HVS-1000 Vickers hardness tester (Laizhou, China) under a load of 500 g for 20 s, in which at least 15 indents were tested to obtain an average value.

3. Experimental Results

3.1. Microstructural Characterization

Alloy compositions in atomic percent (at. %) of this AlxNiCoFeCr HEA series are listed in Table 1, and all alloys are marked with Alx hereafter. The XRD results of these suction-cast alloys are shown in Figure 1, from which the Al0.41 alloy (S1) with a lower Al content of 9.38 at. % exhibits a single FCC solid solution structure, and the BCC and its ordered B2 phases will appear with increasing the Al content. There exist obvious diffraction peaks of the BCC/B2 phases in the Al0.57 alloy (S2, 12.5 at. % Al), although its matrix is still FCC. Then, the BCC and B2 phases are dominant in high-Al HEAs, as demonstrated by the Al0.74 alloy (S3, 15.63 at. % Al), and the FCC phase will disappear in the alloys with the Al content larger than 18 at. %, as seen in the XRD patterns of Al0.92 (S4), Al1.12 (S5), and Al1.33 (S6). This trend will be verified further by the OM observations.
The main matrix of the Al0.41 alloy (S1) shows bright FCC dendrites, as shown in Figure 2a, in which a small amount of BCC/B2 phases is distributed in the inter-dendritic regions (dark region) that could not be detected by XRD. The amount of the inter-dendritic BCC/B2 phases increases with Al, as evidenced by the Al0.57 alloy (S2, Figure 2b). In the Al0.74 alloy (S3, Figure 2c), the BCC/B2 phases show a microstructure of coarse columnar dendrites, and a minor FCC phase is in the region of inter-dendrites. Further increasing Al to Al1.12, the flower-like dendrites are distributed in the matrix (Figure 2e–f), which results from the composition segregation. While in the Al0.92 alloy (S4, Figure 2d), the BCC/B2 microstructure exhibits a mixed morphology of major columnar dendrites and minor flower-like ones, in which the latter appears in the central part only (Figure 2d).
Figure 3 gives the elemental distributions of the Al1.12 alloy (S5) measured by EMPA, from which it could be found that much greater amounts of Al and Ni are aggregated in the flower-like dendrites, and the other inter-dendrites contain much greater contents of Fe and Cr. The measured compositions of the flower-like dendrites and the inter-dendrites are Al26.01Ni23.80Co20.23Fe16.22Cr13.74 and Al17.21Ni18.73Co19.77Fe21.70Cr22.59 (at. %), respectively, different obviously from the nominal composition of Al21.88Ni19.53Co19.53Fe19.53Cr19.53 (at. %). In addition, it is noted that both the flower-like dendrites and the inter-dendrites have BCC and B2 structures without any FCC phase, as evidenced by XRD and the following SEM and TEM studies.
The specific microscopic morphologies of BCC/B2 phases in this HEA series are revealed by highly-magnified SEM images (Figure 4), in which the disordered BCC and its ordered B2 phases always exist coherently. Even in the FCC-based HEAs of Al0.41 (S1, Figure 4a) and Al0.57 (S2, Figure 4b), the BCC and B2 phases are coherent in the inter-dendrites with a weave-like morphology caused by spinodal decomposition. Such morphology still exists in the BCC Al0.74 (S3, Figure 4c) and Al0.92 alloys (S4, Figure 4d). When two typical BCC/B2 microstructures induced by composition segregation are obviously different, the microscopic morphologies of BCC/B2 phases would be changed. Take the Al1.12 alloy (S5, Figure 4g–i) for instance. In the flower-like dendrites (dark regions in Figure 4g), it is found that spherical nanoparticles are embedded into the matrix (Figure 4i), while the inter-dendrites (bright region in Figure 4g) still show a weave-like morphology (Figure 4h), similar to those in S1–S4 HEAs.
With the further TEM results (Figure 5), the spherical particles with a diameter of about 50 nm are identified as a BCC phase by the dark-field image and SAED pattern (Figure 5b). That is to say, it is the spherical BCC nanoparticles that are coherently precipitated on the ordered B2 matrix in the flower-like dendrites. While the weave-like spinodal decomposition of BCC and B2 phases appears in the inter-dendrites (Figure 5a,c). Similar morphologies of BCC and B2 phases also appear in the Al1.33 alloy (S6, Figure 4j–l) with spherical particles in the flower-like dendrites and weave-like spinodal morphology in the other part, in which the size of spherical particles is much smaller, being approximately 20 nm (Figure 5d).

3.2. Mechanical Properties

Figure 6 shows engineering compressive stress-strain curves of this AlxCoCrFeNi HEA series (except the Al0.92 alloy (S4)), from which the compressive yielding strength σY and plasticity δ are measured and listed in Table 1. The FCC-based alloy, S1-Al0.41, exhibits a good compressive plasticity but a relatively-lower strength with σY = 255 MPa. With the amount of the inter-dendritic BCC/B2 phase increases, the strength is enhanced up to σY = 607 MPa (S2-Al0.57). When the BCC/B2 phases are dominant in HEAs, the compressive σY could be enhanced drastically, and keeps constant at a high strength level of 1350–1400 MPa (S3–S6 HEAs). For example, the compressive yielding strength of the Al0.74 alloy (S3) is σY = 1394 MPa, much higher than the FCC-based HEAs, even about five times than that of the FCC-based Al0.41 alloy. In addition, Table 1 also lists the measured Vickers hardness (HV) of these HEAs, in which the tendency of HV with the crystalline structure is similar than that of the yielding strength.

4. Discussion

4.1. Microscopic Morphologies of BCC/B2 Phases in HEAs

It could be found obviously that whenever the FCC-based HEAs with a low Al content or the BCC/B2 HEAs with a high Al content in the AlxNiCoFeCr series are present, the ordered B2 phase is always coherent with the disordered BCC phase in the BCC-related regions, as shown in Figure 5 and Figure 6. However, their microscopic morphologies might be different. For instance, in the inter-dendritic regions of the FCC-based Al0.41 alloy with a lower Al content of 9.38 at. %, the BCC and B2 phases show a weave-like morphology caused by spinodal decomposition (Figure 4a), which is a typical microstructure in many BCC-based HEAs [20,21]. In the Al1.12 alloy (S5) with a high Al content of 21.88 at. %, besides this kind of weave-like morphology, the other kind of morphology of spherical BCC nanoparticles coherently embedded into B2 matrix appears in the flower-like dendrites (Figure 4i and Figure 5b). Such morphology of spherical precipitation of the BCC phase also appears in the Al1.33 alloy (S6, Figure 4l and Figure 5d). These two kinds of BCC/B2 microstructures are resulted from the composition segregation, which could be also verified by the XRD results that the diffraction peaks of BCC and B2 phases are widened clearly (Figure 1). Thereof, in the BCC/B2 regions with a very high Al content, the ordered B2 phase becomes the matrix, and the disorder BCC is the precipitated phase with a spherical morphology, which is contrary to the common image that the ordered phase is always precipitated on the disordered BCC matrix [30,31,32,33,34,35,36].
It is known that the size and the shape of precipitates are crucial for the mechanical properties of conventional alloys. Especially if the shape of the precipitates is closely related to the lattice misfit, ε, between the precipitated phase and the matrix phase [29,31,32,33,34,35,36,37,38]. Generally speaking, a smaller lattice misfit, ε, leads to a spherical shape, and when the misfit becomes larger, it is difficult to control the morphology of the precipitates, and the coherency between them will disappear gradually, resulting in coarse second particles finally. Only a moderate ε, neither large nor small, can produce cuboidal coherent precipitates, leading to excellent mechanical properties. Such a case is common in Ni-based superalloys [33,34], in which the cuboidal ordered nanoparticles with a L12 structure (Ni3Al-type) are coherently embedded into the disordered FCC matrix, exhibiting an incomparable mechanical property at high temperatures. While in sharp contrast, the weave-like spinodal morphology of ordered and disordered coherent phases with a relatively-larger ε will deteriorate alloy properties, as exemplified by the brittle AlNiCoFeCr HEA [19].
Thus, the lattice misfit between the disordered BCC and the ordered B2 phases in this present HEA series are calculated with the formula of ε = 2 × (αB2 − αBCC)/(αB2 + αBCC), where αB2 and αBCC are the lattice constants of B2 and BCC phases, respectively. Here, the lattice constants of BCC and B2 phases in Al0.57 (S2), Al0.74 (S3), and Al0.92 (S4) HEAs were calculated from XRD, and those in Al1.12 (S5) and Al1.33 (S6) HEAs were achieved from TEM. Table 2 lists these lattice constants (αB2 and αBCC), lattice misfits ε, and their corresponding microscopic morphologies of BCC and B2 phases. It could be concluded that a larger absolute value of ε (ε > 0.53%) corresponds to a weave-like morphology caused by the spinodal decomposition of BCC and B2 phases. A smaller ε (ε < 0.10%) can produce another morphology of spherical particles coherently precipitated into the matrix. Only a moderate ε can produce cuboidal precipitates, as demonstrated by the fact that the cuboidal B2 nanoprecipitates are coherently embedded into the BCC matrix in the BCC Al0.57NiCoFe2Cr HEA with a ε = −0.38% [29].
In fact, the weave-like morphology of BCC and B2 phases is common in the Al-containing BCC HEAs, which is due to that Al affects the lattice constants of BCC and B2 strongly, leading to a large lattice misfit ε. Moreover, the ε could increase with the Al content, as a result of a serious spinodal decomposition. Similar to Al, the elements Ti and Zr also can produce a larger misfit, as exemplified by the fact that the lattice misfit of ε = 2.38% between BCC and B2 phases in the refractory Al0.3NbTaTi1.4Zr1.3 BCC HEA is so high that a consequent spinodal decomposition seriously deteriorates the plasticity [24]. Thereof, it is difficult to obtain a moderate lattice misfit of BCC and B2 phases for an expected morphology through tuning Al (Ti, Zr) in BCC-based HEAs since they obviously affect the lattice constants.

4.2. Relationship among σY, HV, and Structure with VEC of HEAs

It is well known that the phase stability of an alloy is related to multiple factors simultaneously, such as the atom size, electron concentration, and enthalpy of mixing [39,40,41,42]. Here the valence electron concentration (VEC) is introduced to characterize the strength evolution of the designed HEAs due to its simplicity. It can be calculated with the formula of VEC = Σci × (VEC)i, where ci is the atomic percentage and (VEC)i is the VEC value for the i element, being 3 for Al, 6 for Cr, 8 for Fe, 9 for Co, and 10 for Ni, respectively. Figure 7 shows the compressive yielding strength, σY, and Vickers hardness, HV, as a function of VEC of this HEA series, in which the VEC for the characterization of the BCC/B2 region is VEC < 7.4. The σY and HV of HEAs with a dual-phase of BCC and FCC HEAs would be obviously enhanced by increasing the amount of the BCC/B2 phases. While the σY and HV of the BCC HEAs keep almost constant at a relatively-higher strength level (σY = 1350–1400 MPa, HV = 520–530) at the expense of plasticity with decreasing VEC (Table 1). In comparison with the BCC-based Al0.7NiCoFe2Cr HEA (Al12.5Ni17.5Co17.5Fe35.0Cr17.5 at. %, VEC = 7.55) with cuboidal B2 nanoprecipitates coherent with the BCC matrix [28], exhibiting a compressive strength of σY = 1166 MPa (HV = 486) and a plasticity of δ = 30.2%, the Al0.74 alloy (S2) with the same Al content exhibits a FCC matrix with a relatively-lower strength (σY = 607 MPa). While the plasticity of this BCC AlxNiCoFeCr series with high Al contents are decreased obviously although they have a higher strength (Table 1 and Figure 6). Therefore, changing the combination of transition metals to substitute for the Al could achieve a moderate lattice misfit for the cuboidal morphology of B2 precipitates in the BCC matrix, which renders the HEAs with high strength and good ductility.

5. Conclusions

The evolutions of microstructures and mechanical properties of the AlxNiCoFeCr HEA series with the changing Al content have been studied, in which the microscopic morphologies of disordered BCC and ordered B2 phases were investigated particularly. It is found that the B2 phase is always coherently present with the BCC phase in the BCC-related regions of both FCC-based and BCC HEAs. There are two typical morphologies of BCC/B2 phases in this alloy series, one being the weave-like spinodal morphology and the other the morphology of spherical BCC nanoprecipitates in the B2 matrix. The shapes of coherent precipitates are closely related to the absolute value of the lattice misfit, ε, between BCC and B2 phases, in which a small ε (ε < 0.10%) could produce a spherical precipitation and a larger ε (ε > 0.53%) corresponds to a weave-like spinodal decomposition that is sensitive to Al. The AlxNiCoFeCr BCC HEAs possess a higher strength (compressive σY = 1350–1400 MPa) at the expense of plasticity. That tuning the combination of transition metals instead of Al is expected to obtain a moderate ε for cuboidal precipitation, which will lead to prominent mechanical properties.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (No. 51171035), the International Science & Technology Cooperation Program of China (No. 2015DFR60370), the Natural Science Foundation of Liaoning Province of China (No. 2015020202), the Foundation of Guangxi Key Laboratory of Information Materials [No. 161002-K], and the Fundamental Research Funds for the Central Universities (No. DUT16ZD212). P.K. Liaw would like to acknowledge the Department of Energy (DOE), Office of Fossil Energy, National Energy Technology Laboratory (DE-FE-0008855, DE-FE-0024054, and DE-FE-0011194), the U.S. Army Research Office project (W911NF-13-1-0438), the National Science Foundation (DMR-1611180), and the QuesTek Innovations LLC (No. DE-SC0013220).

Author Contributions

Qing Wang conceived and designed the experiments; Yue Ma performed the experiments, analyzed the data, and wrote the paper; Beibei Jiang and Chunling Li provided assistance in the experiments; Chuang Dong and Peter K. Liaw discussed the results and modified the paper; Fen Xu and Lixian Sun contributed reagents/materials.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. XRD patterns of the suction-cast AlxNiCoFeCr HEAs.
Figure 1. XRD patterns of the suction-cast AlxNiCoFeCr HEAs.
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Figure 2. Optical microscopy (OM) graphs of the AlxNiCoFeCr HEAs. (a) S1-Al0.41; (b) S2-Al0.57; (c) S3-Al0.74; (d) S4-Al0.92; (e) S5-Al1.12; (f) S6-Al1.33.
Figure 2. Optical microscopy (OM) graphs of the AlxNiCoFeCr HEAs. (a) S1-Al0.41; (b) S2-Al0.57; (c) S3-Al0.74; (d) S4-Al0.92; (e) S5-Al1.12; (f) S6-Al1.33.
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Figure 3. Elemental distributions of the S5-Al1.12 HEA mapped by the electron-probe microanalysis (EPMA).
Figure 3. Elemental distributions of the S5-Al1.12 HEA mapped by the electron-probe microanalysis (EPMA).
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Figure 4. SEM morphologies of the AlxNiCoFeCr HEAs. (a) S1-Al0.41; (b) S2-Al0.57; (c) S3-Al0.74; (d–f) S4-Al0.92; (g–i) S5-Al1.12; (j–l) S6-Al1.33. Besides the weave-like morphology of BCC and B2 phases, there exists another type of microstructures of spherical BCC nanoparticles distributed in the B2 matrix of high-Al HEAs.
Figure 4. SEM morphologies of the AlxNiCoFeCr HEAs. (a) S1-Al0.41; (b) S2-Al0.57; (c) S3-Al0.74; (d–f) S4-Al0.92; (g–i) S5-Al1.12; (j–l) S6-Al1.33. Besides the weave-like morphology of BCC and B2 phases, there exists another type of microstructures of spherical BCC nanoparticles distributed in the B2 matrix of high-Al HEAs.
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Figure 5. TEM images of the S5-Al1.12 (a–c) and S6-Al1.33 (d) HEAs. (a) is the bright-field image of S5, (b) and (c) are the magnified dark-field images corresponding to regions in (a) marked with the red and yellow rectangles, respectively, showing two types of morphologies of BCC and B2 phases: a morphology of spherical BCC nanoparticles (the SAED pattern in the inset of (b)) coherently precipitated in the BCC matrix and a weave-like morphology by spinodal decomposition (c). (d) is the dark-field image of S6, also showing spherical BCC nanoprecipitates by the SAED pattern in the inset.
Figure 5. TEM images of the S5-Al1.12 (a–c) and S6-Al1.33 (d) HEAs. (a) is the bright-field image of S5, (b) and (c) are the magnified dark-field images corresponding to regions in (a) marked with the red and yellow rectangles, respectively, showing two types of morphologies of BCC and B2 phases: a morphology of spherical BCC nanoparticles (the SAED pattern in the inset of (b)) coherently precipitated in the BCC matrix and a weave-like morphology by spinodal decomposition (c). (d) is the dark-field image of S6, also showing spherical BCC nanoprecipitates by the SAED pattern in the inset.
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Figure 6. Engineering compressive stress-strain curves of the AlxNiCoFeCr HEAs.
Figure 6. Engineering compressive stress-strain curves of the AlxNiCoFeCr HEAs.
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Figure 7. The tendencies of compressive yielding stress, σY, and microhardness, HV, with VEC of the AlxNiCoFeCr HEAs.
Figure 7. The tendencies of compressive yielding stress, σY, and microhardness, HV, with VEC of the AlxNiCoFeCr HEAs.
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Table
Table 1. The alloy compositions, valence electron concentration (VEC), Vickers hardness (HV), compressive mechanical property (yielding strength σY and plasticity δ), and phase constitution of the AlxNiCoFeCr HEA series.
Table 1. The alloy compositions, valence electron concentration (VEC), Vickers hardness (HV), compressive mechanical property (yielding strength σY and plasticity δ), and phase constitution of the AlxNiCoFeCr HEA series.
No.Alloy CompositionsVEC 1HVσY (MPa)δ (%)Phase Constitution
Molar ProportionAtomic Percent (at. %)
S1Al0.41NiCoFeCr (Al0.41)Al9.38Ni22.66Co22.66Fe22.66Cr22.667.76178 ± 7255without fractureFCC matrix + minor BCC
S2Al0.57NiCoFeCr (Al0.57)Al12.5Ni21.88Co21.88Fe21.88Cr21.887.59272 ± 960736FCC matrix + BCC/B2
S3Al0.74NiCoFeCr (Al0.74)Al15.63Ni21.09Co21.09Fe21.09Cr21.097.43529 ± 11139424BCC/B2 matrix + minor FCC
S4Al0.92NiCoFeCr (Al0.92)Al18.75Ni20.31Co20.31Fe20.31Cr20.317.27531 ± 9--BCC/B2
S5Al1.12NiCoFeCr (Al1.12)Al21.88Ni19.53Co19.53Fe19.53Cr19.537.10527 ± 5136617BCC/B2
S6Al1.33NiCoFeCr (Al1.33)Al25.00Ni18.75Co18.75Fe18.75Cr18.756.94517 ± 7134814BCC/B2
1 VEC = Σci* (VEC)i.
Table
Table 2. The lattice misfits ε and the corresponding morphologies of BCC and B2 phases in the AlxNiCoFeCr HEAs, in which the lattice constants (αB2 and αBCC) of BCC and B2 phases calculated from both XRD and TEM analysis are also included.
Table 2. The lattice misfits ε and the corresponding morphologies of BCC and B2 phases in the AlxNiCoFeCr HEAs, in which the lattice constants (αB2 and αBCC) of BCC and B2 phases calculated from both XRD and TEM analysis are also included.
No.αB2 (Å)αBCC (Å)ε 1 (%)Morphology
S2-Al0.572.8712.8540.59Weave-like spinodal decomposition
S3-Al0.742.8552.870−0.53Weave-like
S4-Al0.922.8782.897−0.69Weave-like
S5-Al1.122.8092.831−0.78Weave-like
2.8552.858−0.11Spherical particles
S6-Al1.332.9082.911−0.10Spherical particles
Al0.7NiCoFe2Cr [29]2.8512.862−0.38Cuboidal particles
1 ε = 2 × (αB2 − αBCC)/(αB2 + αBCC).

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Ma, Y.; Jiang, B.; Li, C.; Wang, Q.; Dong, C.; Liaw, P.K.; Xu, F.; Sun, L. The BCC/B2 Morphologies in AlxNiCoFeCr High-Entropy Alloys. Metals 2017, 7, 57. https://doi.org/10.3390/met7020057

AMA Style

Ma Y, Jiang B, Li C, Wang Q, Dong C, Liaw PK, Xu F, Sun L. The BCC/B2 Morphologies in AlxNiCoFeCr High-Entropy Alloys. Metals. 2017; 7(2):57. https://doi.org/10.3390/met7020057

Chicago/Turabian Style

Ma, Yue, Beibei Jiang, Chunling Li, Qing Wang, Chuang Dong, Peter K. Liaw, Fen Xu, and Lixian Sun. 2017. "The BCC/B2 Morphologies in AlxNiCoFeCr High-Entropy Alloys" Metals 7, no. 2: 57. https://doi.org/10.3390/met7020057

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