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Microstructures and mechanical properties of Nb-alloyed CoCrCuFeNi high-entropy alloys
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Microstructures and mechanical properties of Nb-alloyed CoCrCuFeNi high-entropy alloys Gang Qin, Shu Wang, Ruirun Chen ∗ , Xue Gong, Liang Wang, Yanqing Su, Jingjie Guo, Hengzhi Fu School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 9 September 2017 Received in revised form 12 October 2017 Accepted 27 October 2017 Available online xxx Keywords: High-entropy alloys Nb alloying Phase transition Mechanical properties Laves phase
a b s t r a c t Nb has a positive effect on improving the mechanical properties of metal materials, and it is expected to strengthen CoCrCuFeNi high-entropy alloys (HEAs) with outstanding ductility and relatively weak strength. In this paper, the alloying effects of Nb on the microstructural evolution and the mechanical properties of the (CoCrCuFeNi)100-x Nbx HEA were investigated systematically. The result shows that Nb promotes the phase transition from FCC (face-centered cubic) to Laves phase, and the volume fractions of Laves phase increase from 0% to 58.2% as the Nb content increases. Compressive testing shows that the addition of Nb has a positive effect on improving the strength of CoCrCuFeNi HEA. The compressive yield strength of (CoCrCuFeNi)100-x Nbx HEAs increases from 338 MPa to 1322 MPa and the fracture strain gradually reduces from 60.0% (no fracture) to 8.1% as the Nb content increases from 0 to 16 at.%. The volume fraction increase of hard Laves phase is the key factor for the strength increase, and the reduction of the VEC (valence electron concentration) value induced by the addition of Nb is beneficial for the increase of the Laves phase content in these alloys. © 2017 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.
1. Introduction A new alloy concept named high-entropy alloy (HEA) or multicomponent alloy was proposed independently by Yeh et al. and Cantor et al. in 2004 [1,2]. HEAs have been becoming a new research hotspot mainly due to their remarkable mechanical properties and simple solid solutions structure. They have become candidates with good engineering application prospects [3–11]. Indisputably, the appearance of HEAs accelerates the development of metallic materials and opens up a new path for exploring new alloys with outstanding properties. The traditional alloy design method is to select one or two elements as the major components based on the primary property requirement and to confer secondary properties by adding other alloying elements. Many similar methods have also been applied in the HEA field. For example, Lu et al. studied the ductile CoCrFeNiMox high-entropy alloys strengthened by hard intermetallic phases and found that the precipitation of hard and intermetallic compounds tremendously strengthened the
∗ Corresponding author. E-mail address: [email protected] (R. Chen).
CoCrFeNiMo0.3 HEA without any brittleness [12]. Liu et al. reported the alloying effects of Nb on the microstructure and tensile properties of the CoCrFeNi HEA system, and found that, with increasing Nb content, the microstructure changes from the initial single facecentered cubic (FCC) to duplex-phase of FCC plus Laves phase with a hexagonal close-packed (HCP) structure. The Nb-enriched Laves phase with HCP structure played an important role in improving the yield strength and fracture strength [13]. Based on surveying the existing binary phase diagrams with eutectic points and the computer-aided thermodynamic calculations, He et al. found that the formation of the eutectic alloys was a ductile face-centered cubic (FCC) phase and a hard Laves phase with excellent integrated mechanical properties of ductility and strength [14]. Stepanov et al. investigated the crystal structure, microstructure, microhardness and compression properties of CoCrFeMnNiVx (x = 0, 0.25, 0.5, 0. 75, 1). With the addition of the V element, the sigma-phase volume fraction increased, and in CoCrFeMnNiV HEA, the sigma phase became the matrix phase. The continuous strengthening and loss of ductility are caused by the increase of the sigmaphase volume fraction [15]. Moreover, there have also been studies of the effects of some elements and some physical parameters on the microstructure and mechanical properties in these papers [16–27]. Nb has a positive effect on improving the mechanical
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properties of many alloys based on the literature reported previously [13,14,16]. However, the effect of the addition of Nb on the phase formation, microstructure and mechanical properties of the (CoCrCuFeNi)100-x Nbx HEA has not been investigated in the existing reports. In this study, we selected the CoCrCuFeNi HEA (all elements in equal atomic proportions) with a single FCC phase solid solution as the matrix. The alloying effect of Nb on the phase evolution, microstructure, and the mechanical properties of the (CoCrCuFeNi)100-x Nbx HEA was investigated systematically, and the relationship between the mechanical properties and phase structure was revealed. 2. Experimental A series of HEAs (CoCrCuFeNi)100-x Nbx (x = 0–16 at.%, atomic percent, hereafter in at.%) were synthesized by arc-melting in a copper mold under high-purity argon atmosphere. Metal powders of Co, Cr, Cu, Fe, Ni, and Nb with a purity of more than 99.9% were selected as raw materials. The ingots were all smelted seven times to ensure the chemical homogeneity. We cut the ingots into several shapes using electric discharging machining. The crystal structures of these samples were characterized by X-ray diffraction (XRD) using CuK˛ radiation (MXP21VAHF) scanning from 20◦ to 100◦ . The microstructure and morphology of the specimens were observed by scanning electron microscopy (SEM) and the chemical compositions distribution was investigated by energy dispersive spectrometry (EDS). The SEM samples were grounded and polished and then electro-polished in a liquid mixture (90% acetic and 10% perchloric acid) at room temperature and an applied voltage of 27 V for 15 s. The diameter and height of the compression samples were respectively 4 mm and 6 mm. From each ingot at least three samples were taken for the compression tests to ensure the reliability of the result, and these samples were tested on an AG-Xplus 250 kN electronic universal material testing machine with a strain rate of 0.5 × 10−3 min−1 at room temperature. 3. Results and discussion 3.1. Crystal structure As shown in Fig. 1, the XRD patterns show that the crystal structure transforms from FCC phase to Laves phase structure with increasing Nb content. This transformation enables the alloy system to attain a duplex-phase structure, and the solid-solution strengthening effect is much more evident than single-phase [28]. As shown in Fig. 1, the blue balls represent the FCC phase and the yellow squares indicate Laves phase. When the Nb content is within the range of (0–4)%, the alloys show a single FCC phase structure. Only a weak reflection peak correct to the Laves phase appears when the Nb content is 8%. When the Nb content is within the range of (8–16)%, these alloys show a duplex-phase structure of FCC plus Laves phase. So Nb element has the capacity to induce a phase transformation from FCC phase to Laves phase in the (CoCrCuFeNi)100-x Nbx alloy system. 3.2. Microstructure Microstructures of (CoCrCuFeNi)100-x Nbx alloys were attained by scanning electron microscopy. The micrographs of as-cast (CoCrCuFeNi)100-x Nbx alloys showed a variety of microstructures, as seen in Fig. 2. The micrographs of Nb-0 alloy (Fig. 2a and b) show single FCC phase structure, and some Cu-enriched FCC phase is also found in this alloy (see Fig. 2b), which is consistent with the previous reports [29]. A small amount of Laves phase particles is found (in
Fig. 1. XRD patterns of (CoCrCuFeNi)100-x Nbx (x = 0,4,8,12,16) HEAs. Nb content prompts the transformation of the crystal structure from FCC phase structure to Laves phase structure. Table 1 Contents of different elements in regions I–III from EDS. Region
Co (%)
Cr (%)
Cu (%)
Fe (%)
Ni (%)
Nb (%)
I II III
16.61 20.76 20.69
15.75 20.63 22.50
3.51 12.15 15.13
8.90 18.41 20.83
11.47 20.60 18.91
43.76 7.44 1.94
Fig. 2c and d) when the content of Nb reaches 4%. The EDS analysis result shows that the Laves phase is Nb-rich. However, the Nb-rich Laves phase could not been found by X-ray diffraction (XRD) due to the smaller volume fraction. At this moment, the matrix mainly contains FCC phase and a small amount of Laves phase. When the content of Nb is 8%, the Laves phase grows gradually (in Fig. 2e and f). Fig. 2f displays three kinds of phase structure in the C region. Table 1 shows the content of this alloy in regions I, II, and III. For region I, the content of Nb is 43.76 at.%, so the Nb is enriched in the Laves phase. As shown in Fig. 2g and h, the Laves phase grows still further when the Nb content reaches 12%. The Laves phase grows gradually with the increase of Nb; a large amount of the Laves phase is distributed in the matrix when the Nb content reaches 16%, and the Cu-rich phase decreases (see Fig. 2i and j). This has a great effect in improving the strength of HEA as the volume fraction of Laves phase increases. 3.3. Mechanical properties The compressive mechanical properties of (CoCrCuFeNi)100-x Nbx HEA are displayed in Fig. 3. The compressive yield strength of (CoCrCuFeNi)100-x Nbx HEAs increases from 338 MPa to 1322 MPa (3.9 times) as the Nb content increases from 0 to 16 at.% (Fig. 4). Fig. 3 shows the compressive stress–strain curves of as-cast (CoCrCuFeNi)100-x Nbx (x = 0–16 at.%) HEAs. It shows that the compressive yield strength increases as the Nb content increases, and this suggests that the improvement of the compressive yield strength has a correlation with the volume
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Fig. 2. SEM micrographs of as-cast (CoCrCuFeNi)100-x Nbx alloys. (a, c, e, g, i. Backscattered images of Nb-0 and Nb-4, Nb-8, Nb-12, Nb-16 alloys, respectively; b, d, f, h, j. Partial magnification backscattered images of Nb-0 and Nb-4, Nb-8, Nb-12, Nb-16 alloys, respectively).
fraction of the Laves phase. The yield strength increases from 338 MPa to 488 MPa when the Nb content increases from 0% to 4%, and the improvement of yield strength is mainly caused by
the solid solution strengthening effect at this moment. When the Nb content increases further from 4% to 16%, the yield strength is greatly enhanced from 488 MPa to 1322 MPa and the fracture
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Fig. 3. Compressive stress-strain curves of as-cast (CoCrCuFeNi)100-x Nbx (x = 0–16) HEAs.
Fig. 4. Relationship between the yield strength of (CoCrCuFeNi)100-x Nbx and volume fraction of Laves phase with Nb content increases.
strain reduces from 60.0% (without fracture) to 8.1%. The improvement of yield strength caused by the Laves phase generated in the matrix with Nb content further increases based on the second phase reinforcement mechanism at that moment.
Fig. 4 shows the relationship between the yield strength of (CoCrCuFeNi)100-x Nbx and the volume fraction of Laves phase as the Nb content increases. It clearly shows that the Nb element plays a positive role in the phase transition from FCC to Laves phase. The volume fraction of Laves phase increases from 0% to 58.2%, and the compressive yield strength also increases from 338 MPa to 1322 MPa. This indicates that Nb element plays a positive role in the phase translation from FCC to Laves and in improving the strength of this alloy. Fig. 5 shows the atomic diffusion model, with a hypothesis set up for describing the formation process of Laves phase. The alloy has a single FCC structure when the Nb content is 0% (see Fig. 5a), but with the increasing Nb content, more and more atoms are incorporated into the matrix and replace the matrix element (see Fig. 5b and c), which leads to a large lattice distortion and encourages the strength improvement. When the Nb content exceeds the maximal ability of Nb to dissolve in the matrix, some Nb atoms will be separated out and form a new phase (Laves phase) with some matrix atoms dissolved in it (see Fig. 5d). This enables each phase in this matrix to achieve the maximum degree of solid solubility. As the Nb content increases, more and more atoms in the matrix attach themselves onto the new phase (Laves phase), and promote the growth of the new phase (see Fig. 5e). HEAs with FCC structure have better plasticity and relatively weak strength [30], while the Laves phase is relatively hard. Two phases (FCC phase and Laves phase) are contained simultaneously in this HEAs system of (CoCrCuFeNi)100-x Nbx . The strength and plasticity can be estimated roughly based on this empirical formula [31], ı = ıfcc Vfcc + ılaves Vlaves , where ıfcc and ıLaves represent the strengths of the FCC and the Laves phase, and Vfcc and Vlaves represent the volume fraction of the two phases, respectively. In this kind of HEA, the volume fractions of the Laves phase increase gradually and the volume fraction of the FCC phase reduces gradually as the Nb content increases. Intrinsically, the strength increases and the plasticity reduces gradually with increasing Nb content. Guo et al. summarized the correlation between the VEC and phase evolution rule, from which it was found that when the value of VEC is higher than 8 (VEC ≥ 8), the FCC phase tends to be formed in the alloy matrix. And the BCC phase is more stable when VEC is lower than 6.87 (VEC ≤ 6.87), while both BCC and FCC are formed when VEC is higher than 6.87 and lower than 8
Fig. 5. Atomic diffusion model for (CoCrCuFeNi)100-x Nbx : (a) The matrix of Nb-0, FCC; (b, c) The atomic replacement and solid solution of Nb-4 and Nb-8, respectively; (d) The matrix, FCC phase and the Laves phase; (e) Crystal grows.
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G. Qin et al. / Journal of Materials Science & Technology xxx (2017) xxx–xxx Table 2 VEC values of some metallic elements. Element
Co
Cr
Cu
Fe
Ni
Nb
VEC value
9
6
11
8
10
5
5
and the fracture strain reduces gradually from 60.0% (no fracture) to 8.1% when the Nb content increases from 0% to 16 at.%. The addition of Nb has a positive effect on improving the strength of CoCrCuFeNi HEA. The low VEC induced by the addition of Nb promotes the formation of Laves phase in this alloy system. The increased volume fraction of Laves phase is the key factor for the strength increase of this alloy. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 51331005) and the National Science Fund for Distinguished Young Scholars (No. 51425402). References
Fig. 6. Valence electron concentration (VEC) of (CoCrCuFeNi)100-x Nbx (x = 0, 4, 8, 12, 16) HEAs.
(6.87 < VEC ≤ 8) [22,25]. The VEC be calculated based on the formula (VEC =
n
i=1
Ci (VEC)i ), where Ci is the atomic percentage,
and (VEC)i is the VEC for the ith element respectively [22]. The VEC values of Co, Cr, Cu, Fe, Ni, Nb are listed in Table 2 [25]. The VEC of (CoCrCuFeNi)100-x Nbx can be attained based on the linear function VEC = 8.8–0.038X. When the Nb content increases, the VEC value of (CoCrCuFeNi)100-x Nbx HEAs decreases gradually. Fig. 6 shows the VEC of (CoCrCuFeNi)100-x Nbx HEA. They can be divided into three parts based on the value of VEC, which are respectively the BCC area (VEC < 6.87), FCC area (VEC ≥ 8) and BCC + FCC area (6.87 < VEC ≤ 8). The HEA series of (CoCrCuFeNi)100-x Nbx (x = 0–16) are in the area of the BCC, however the alloys did not show the single BCC phase structure, and the Laves phase was generated in this system differently. This implies that there are correlations between VEC and the FCC/Laves phase volume fraction in HEA, whereby the smaller the VEC is, the greater the volume fraction of Laves phase is. A higher VEC leads to a greater atomic bonding force, which promotes the formation of an atom arrangement with a larger atomic packing density (FCC phase structure). If the VEC is smaller, atoms tend to be arranged with a lower atomic packing density (Laves phase structure). The VEC of Nb (VEC = 5) is lower than the average VEC of CoCrCuFeNi HEA (VEC = 8.8), which is an important factor for phase structure translation in this alloy system. 4. Conclusions Nb element promotes the phase transition from FCC to Laves phase, where the volume fractions of Laves phase increase from 0% to 58.2%. Compressive testing shows that the yield strength of (CoCrCuFeNi)100-x Nbx HEA increases from 338 MPa to 1322 MPa
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ARTICLE IN PRESS
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Journal of Materials Science & Technology xxx (2017) xxx–xxx
Contents lists available at ScienceDirect
Journal of Materials Science & Technology journal homepage: www.jmst.org
Microstructures and mechanical properties of Nb-alloyed CoCrCuFeNi high-entropy alloys Gang Qin, Shu Wang, Ruirun Chen ∗ , Xue Gong, Liang Wang, Yanqing Su, Jingjie Guo, Hengzhi Fu School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 9 September 2017 Received in revised form 12 October 2017 Accepted 27 October 2017 Available online xxx Keywords: High-entropy alloys Nb alloying Phase transition Mechanical properties Laves phase
a b s t r a c t Nb has a positive effect on improving the mechanical properties of metal materials, and it is expected to strengthen CoCrCuFeNi high-entropy alloys (HEAs) with outstanding ductility and relatively weak strength. In this paper, the alloying effects of Nb on the microstructural evolution and the mechanical properties of the (CoCrCuFeNi)100-x Nbx HEA were investigated systematically. The result shows that Nb promotes the phase transition from FCC (face-centered cubic) to Laves phase, and the volume fractions of Laves phase increase from 0% to 58.2% as the Nb content increases. Compressive testing shows that the addition of Nb has a positive effect on improving the strength of CoCrCuFeNi HEA. The compressive yield strength of (CoCrCuFeNi)100-x Nbx HEAs increases from 338 MPa to 1322 MPa and the fracture strain gradually reduces from 60.0% (no fracture) to 8.1% as the Nb content increases from 0 to 16 at.%. The volume fraction increase of hard Laves phase is the key factor for the strength increase, and the reduction of the VEC (valence electron concentration) value induced by the addition of Nb is beneficial for the increase of the Laves phase content in these alloys. © 2017 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.
1. Introduction A new alloy concept named high-entropy alloy (HEA) or multicomponent alloy was proposed independently by Yeh et al. and Cantor et al. in 2004 [1,2]. HEAs have been becoming a new research hotspot mainly due to their remarkable mechanical properties and simple solid solutions structure. They have become candidates with good engineering application prospects [3–11]. Indisputably, the appearance of HEAs accelerates the development of metallic materials and opens up a new path for exploring new alloys with outstanding properties. The traditional alloy design method is to select one or two elements as the major components based on the primary property requirement and to confer secondary properties by adding other alloying elements. Many similar methods have also been applied in the HEA field. For example, Lu et al. studied the ductile CoCrFeNiMox high-entropy alloys strengthened by hard intermetallic phases and found that the precipitation of hard and intermetallic compounds tremendously strengthened the
∗ Corresponding author. E-mail address: [email protected] (R. Chen).
CoCrFeNiMo0.3 HEA without any brittleness [12]. Liu et al. reported the alloying effects of Nb on the microstructure and tensile properties of the CoCrFeNi HEA system, and found that, with increasing Nb content, the microstructure changes from the initial single facecentered cubic (FCC) to duplex-phase of FCC plus Laves phase with a hexagonal close-packed (HCP) structure. The Nb-enriched Laves phase with HCP structure played an important role in improving the yield strength and fracture strength [13]. Based on surveying the existing binary phase diagrams with eutectic points and the computer-aided thermodynamic calculations, He et al. found that the formation of the eutectic alloys was a ductile face-centered cubic (FCC) phase and a hard Laves phase with excellent integrated mechanical properties of ductility and strength [14]. Stepanov et al. investigated the crystal structure, microstructure, microhardness and compression properties of CoCrFeMnNiVx (x = 0, 0.25, 0.5, 0. 75, 1). With the addition of the V element, the sigma-phase volume fraction increased, and in CoCrFeMnNiV HEA, the sigma phase became the matrix phase. The continuous strengthening and loss of ductility are caused by the increase of the sigmaphase volume fraction [15]. Moreover, there have also been studies of the effects of some elements and some physical parameters on the microstructure and mechanical properties in these papers [16–27]. Nb has a positive effect on improving the mechanical
https://doi.org/10.1016/j.jmst.2017.11.007 1005-0302/© 2017 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.
Please cite this article in press as: G. Qin, et al., J. Mater. Sci. Technol. (2017), https://doi.org/10.1016/j.jmst.2017.11.007
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properties of many alloys based on the literature reported previously [13,14,16]. However, the effect of the addition of Nb on the phase formation, microstructure and mechanical properties of the (CoCrCuFeNi)100-x Nbx HEA has not been investigated in the existing reports. In this study, we selected the CoCrCuFeNi HEA (all elements in equal atomic proportions) with a single FCC phase solid solution as the matrix. The alloying effect of Nb on the phase evolution, microstructure, and the mechanical properties of the (CoCrCuFeNi)100-x Nbx HEA was investigated systematically, and the relationship between the mechanical properties and phase structure was revealed. 2. Experimental A series of HEAs (CoCrCuFeNi)100-x Nbx (x = 0–16 at.%, atomic percent, hereafter in at.%) were synthesized by arc-melting in a copper mold under high-purity argon atmosphere. Metal powders of Co, Cr, Cu, Fe, Ni, and Nb with a purity of more than 99.9% were selected as raw materials. The ingots were all smelted seven times to ensure the chemical homogeneity. We cut the ingots into several shapes using electric discharging machining. The crystal structures of these samples were characterized by X-ray diffraction (XRD) using CuK˛ radiation (MXP21VAHF) scanning from 20◦ to 100◦ . The microstructure and morphology of the specimens were observed by scanning electron microscopy (SEM) and the chemical compositions distribution was investigated by energy dispersive spectrometry (EDS). The SEM samples were grounded and polished and then electro-polished in a liquid mixture (90% acetic and 10% perchloric acid) at room temperature and an applied voltage of 27 V for 15 s. The diameter and height of the compression samples were respectively 4 mm and 6 mm. From each ingot at least three samples were taken for the compression tests to ensure the reliability of the result, and these samples were tested on an AG-Xplus 250 kN electronic universal material testing machine with a strain rate of 0.5 × 10−3 min−1 at room temperature. 3. Results and discussion 3.1. Crystal structure As shown in Fig. 1, the XRD patterns show that the crystal structure transforms from FCC phase to Laves phase structure with increasing Nb content. This transformation enables the alloy system to attain a duplex-phase structure, and the solid-solution strengthening effect is much more evident than single-phase [28]. As shown in Fig. 1, the blue balls represent the FCC phase and the yellow squares indicate Laves phase. When the Nb content is within the range of (0–4)%, the alloys show a single FCC phase structure. Only a weak reflection peak correct to the Laves phase appears when the Nb content is 8%. When the Nb content is within the range of (8–16)%, these alloys show a duplex-phase structure of FCC plus Laves phase. So Nb element has the capacity to induce a phase transformation from FCC phase to Laves phase in the (CoCrCuFeNi)100-x Nbx alloy system. 3.2. Microstructure Microstructures of (CoCrCuFeNi)100-x Nbx alloys were attained by scanning electron microscopy. The micrographs of as-cast (CoCrCuFeNi)100-x Nbx alloys showed a variety of microstructures, as seen in Fig. 2. The micrographs of Nb-0 alloy (Fig. 2a and b) show single FCC phase structure, and some Cu-enriched FCC phase is also found in this alloy (see Fig. 2b), which is consistent with the previous reports [29]. A small amount of Laves phase particles is found (in
Fig. 1. XRD patterns of (CoCrCuFeNi)100-x Nbx (x = 0,4,8,12,16) HEAs. Nb content prompts the transformation of the crystal structure from FCC phase structure to Laves phase structure. Table 1 Contents of different elements in regions I–III from EDS. Region
Co (%)
Cr (%)
Cu (%)
Fe (%)
Ni (%)
Nb (%)
I II III
16.61 20.76 20.69
15.75 20.63 22.50
3.51 12.15 15.13
8.90 18.41 20.83
11.47 20.60 18.91
43.76 7.44 1.94
Fig. 2c and d) when the content of Nb reaches 4%. The EDS analysis result shows that the Laves phase is Nb-rich. However, the Nb-rich Laves phase could not been found by X-ray diffraction (XRD) due to the smaller volume fraction. At this moment, the matrix mainly contains FCC phase and a small amount of Laves phase. When the content of Nb is 8%, the Laves phase grows gradually (in Fig. 2e and f). Fig. 2f displays three kinds of phase structure in the C region. Table 1 shows the content of this alloy in regions I, II, and III. For region I, the content of Nb is 43.76 at.%, so the Nb is enriched in the Laves phase. As shown in Fig. 2g and h, the Laves phase grows still further when the Nb content reaches 12%. The Laves phase grows gradually with the increase of Nb; a large amount of the Laves phase is distributed in the matrix when the Nb content reaches 16%, and the Cu-rich phase decreases (see Fig. 2i and j). This has a great effect in improving the strength of HEA as the volume fraction of Laves phase increases. 3.3. Mechanical properties The compressive mechanical properties of (CoCrCuFeNi)100-x Nbx HEA are displayed in Fig. 3. The compressive yield strength of (CoCrCuFeNi)100-x Nbx HEAs increases from 338 MPa to 1322 MPa (3.9 times) as the Nb content increases from 0 to 16 at.% (Fig. 4). Fig. 3 shows the compressive stress–strain curves of as-cast (CoCrCuFeNi)100-x Nbx (x = 0–16 at.%) HEAs. It shows that the compressive yield strength increases as the Nb content increases, and this suggests that the improvement of the compressive yield strength has a correlation with the volume
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Fig. 2. SEM micrographs of as-cast (CoCrCuFeNi)100-x Nbx alloys. (a, c, e, g, i. Backscattered images of Nb-0 and Nb-4, Nb-8, Nb-12, Nb-16 alloys, respectively; b, d, f, h, j. Partial magnification backscattered images of Nb-0 and Nb-4, Nb-8, Nb-12, Nb-16 alloys, respectively).
fraction of the Laves phase. The yield strength increases from 338 MPa to 488 MPa when the Nb content increases from 0% to 4%, and the improvement of yield strength is mainly caused by
the solid solution strengthening effect at this moment. When the Nb content increases further from 4% to 16%, the yield strength is greatly enhanced from 488 MPa to 1322 MPa and the fracture
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Fig. 3. Compressive stress-strain curves of as-cast (CoCrCuFeNi)100-x Nbx (x = 0–16) HEAs.
Fig. 4. Relationship between the yield strength of (CoCrCuFeNi)100-x Nbx and volume fraction of Laves phase with Nb content increases.
strain reduces from 60.0% (without fracture) to 8.1%. The improvement of yield strength caused by the Laves phase generated in the matrix with Nb content further increases based on the second phase reinforcement mechanism at that moment.
Fig. 4 shows the relationship between the yield strength of (CoCrCuFeNi)100-x Nbx and the volume fraction of Laves phase as the Nb content increases. It clearly shows that the Nb element plays a positive role in the phase transition from FCC to Laves phase. The volume fraction of Laves phase increases from 0% to 58.2%, and the compressive yield strength also increases from 338 MPa to 1322 MPa. This indicates that Nb element plays a positive role in the phase translation from FCC to Laves and in improving the strength of this alloy. Fig. 5 shows the atomic diffusion model, with a hypothesis set up for describing the formation process of Laves phase. The alloy has a single FCC structure when the Nb content is 0% (see Fig. 5a), but with the increasing Nb content, more and more atoms are incorporated into the matrix and replace the matrix element (see Fig. 5b and c), which leads to a large lattice distortion and encourages the strength improvement. When the Nb content exceeds the maximal ability of Nb to dissolve in the matrix, some Nb atoms will be separated out and form a new phase (Laves phase) with some matrix atoms dissolved in it (see Fig. 5d). This enables each phase in this matrix to achieve the maximum degree of solid solubility. As the Nb content increases, more and more atoms in the matrix attach themselves onto the new phase (Laves phase), and promote the growth of the new phase (see Fig. 5e). HEAs with FCC structure have better plasticity and relatively weak strength [30], while the Laves phase is relatively hard. Two phases (FCC phase and Laves phase) are contained simultaneously in this HEAs system of (CoCrCuFeNi)100-x Nbx . The strength and plasticity can be estimated roughly based on this empirical formula [31], ı = ıfcc Vfcc + ılaves Vlaves , where ıfcc and ıLaves represent the strengths of the FCC and the Laves phase, and Vfcc and Vlaves represent the volume fraction of the two phases, respectively. In this kind of HEA, the volume fractions of the Laves phase increase gradually and the volume fraction of the FCC phase reduces gradually as the Nb content increases. Intrinsically, the strength increases and the plasticity reduces gradually with increasing Nb content. Guo et al. summarized the correlation between the VEC and phase evolution rule, from which it was found that when the value of VEC is higher than 8 (VEC ≥ 8), the FCC phase tends to be formed in the alloy matrix. And the BCC phase is more stable when VEC is lower than 6.87 (VEC ≤ 6.87), while both BCC and FCC are formed when VEC is higher than 6.87 and lower than 8
Fig. 5. Atomic diffusion model for (CoCrCuFeNi)100-x Nbx : (a) The matrix of Nb-0, FCC; (b, c) The atomic replacement and solid solution of Nb-4 and Nb-8, respectively; (d) The matrix, FCC phase and the Laves phase; (e) Crystal grows.
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G. Qin et al. / Journal of Materials Science & Technology xxx (2017) xxx–xxx Table 2 VEC values of some metallic elements. Element
Co
Cr
Cu
Fe
Ni
Nb
VEC value
9
6
11
8
10
5
5
and the fracture strain reduces gradually from 60.0% (no fracture) to 8.1% when the Nb content increases from 0% to 16 at.%. The addition of Nb has a positive effect on improving the strength of CoCrCuFeNi HEA. The low VEC induced by the addition of Nb promotes the formation of Laves phase in this alloy system. The increased volume fraction of Laves phase is the key factor for the strength increase of this alloy. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 51331005) and the National Science Fund for Distinguished Young Scholars (No. 51425402). References
Fig. 6. Valence electron concentration (VEC) of (CoCrCuFeNi)100-x Nbx (x = 0, 4, 8, 12, 16) HEAs.
(6.87 < VEC ≤ 8) [22,25]. The VEC be calculated based on the formula (VEC =
n
i=1
Ci (VEC)i ), where Ci is the atomic percentage,
and (VEC)i is the VEC for the ith element respectively [22]. The VEC values of Co, Cr, Cu, Fe, Ni, Nb are listed in Table 2 [25]. The VEC of (CoCrCuFeNi)100-x Nbx can be attained based on the linear function VEC = 8.8–0.038X. When the Nb content increases, the VEC value of (CoCrCuFeNi)100-x Nbx HEAs decreases gradually. Fig. 6 shows the VEC of (CoCrCuFeNi)100-x Nbx HEA. They can be divided into three parts based on the value of VEC, which are respectively the BCC area (VEC < 6.87), FCC area (VEC ≥ 8) and BCC + FCC area (6.87 < VEC ≤ 8). The HEA series of (CoCrCuFeNi)100-x Nbx (x = 0–16) are in the area of the BCC, however the alloys did not show the single BCC phase structure, and the Laves phase was generated in this system differently. This implies that there are correlations between VEC and the FCC/Laves phase volume fraction in HEA, whereby the smaller the VEC is, the greater the volume fraction of Laves phase is. A higher VEC leads to a greater atomic bonding force, which promotes the formation of an atom arrangement with a larger atomic packing density (FCC phase structure). If the VEC is smaller, atoms tend to be arranged with a lower atomic packing density (Laves phase structure). The VEC of Nb (VEC = 5) is lower than the average VEC of CoCrCuFeNi HEA (VEC = 8.8), which is an important factor for phase structure translation in this alloy system. 4. Conclusions Nb element promotes the phase transition from FCC to Laves phase, where the volume fractions of Laves phase increase from 0% to 58.2%. Compressive testing shows that the yield strength of (CoCrCuFeNi)100-x Nbx HEA increases from 338 MPa to 1322 MPa
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