Author’s Accepted Manuscript Effects of AL addition on microstructure and mechanical properties of Al xCoCrFeNi Highentropy alloy Tengfei Yang, Songqin Xia, Shi Liu, Chenxu Wang, Shaoshuai Liu, Yong Zhang, Jianming Xue, Sha Yan, Yugang Wang www.elsevier.com
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S0921-5093(15)30374-9 http://dx.doi.org/10.1016/j.msea.2015.09.034 MSA32760
To appear in: Materials Science & Engineering A Received date: 8 July 2015 Revised date: 5 September 2015 Accepted date: 8 September 2015 Cite this article as: Tengfei Yang, Songqin Xia, Shi Liu, Chenxu Wang, Shaoshuai Liu, Yong Zhang, Jianming Xue, Sha Yan and Yugang Wang, Effects of AL addition on microstructure and mechanical properties of Al xCoCrFeNi High-entropy alloy, Materials Science & Engineering A, http://dx.doi.org/10.1016/j.msea.2015.09.034 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effects of Al addition on microstructure and mechanical properties of AlxCoCrFeNi high-entropy alloy Tengfei Yang a, Songqin Xia b, Shi Liu b, Chenxu Wang a, Shaoshuai Liu a, Yong Zhang b, Jianming Xue a, Sha Yan a, Yugang Wang a,* a
State Key Laboratory of Nuclear Physics and Technology, Center for Applied Physics
and Technology, Peking University, Beijing 100871, People’s Republic of China b
State Key Laboratory for Advanced Metals and Materials, University of Science and
Technology Beijing, Beijing 100083, China Abstract The effects of Al on microstructure and mechanical properties of AlxCoCrFeNi (x=0.1, 0.75 and 1.5) high-entropy alloys were systematically studied by using various characterization methods. It was found that the crystalline structure of AlxCoCrFeNi high-entropy alloy varies markedly with Al content, which changes from the initial single face-centered cubic (fcc) to fcc plus ordered body-centered cubic (bcc) structure (B2) and then to a duplex bcc structure (A2+B2) as the Al content is increased. The chemical composition analysis reveals that Al primarily partitions to B2 phase, suggesting Al is a stabilizer of B2 structure. With increasing Al content, more Ni and Al partition to the B2 phase due to the very negative mixing enthalpy of Ni and Al, and another phase enriched in Cr and Fe transforms from fcc to disordered bcc. Nano indentation measurements show that the hardness of AlxCoCrFeNi high-entropy alloy increases with Al content, accompanied by the decrease of ductility. The stability of single-phase solid solution in AlxCoCrFeNi HEAs is deduced from 1
various criteria. Combined with the experiment results of other similar HEA systems, such as AlxCoCrFeNiCu, the effects of Al addition on the microstructure of AlxCoCrFeNi HEAs are discussed based on the Gibbs free energy of all competing phases and the fundamental properties of constituent elements. The aim of current study is to provide experimental evidence to establish a correlation between the microstructure and mechanical properties to search for high-entropy alloys with higher performances.
Keywords: High-entropy alloy; Transmission electron microscopy; Mechanical properties; Microstructure
* Corresponding author: a)
Email: [email protected]
Tel: + 86-10-62755406 2
1. Introduction High-entropy alloys (HEAs) are multicomponent mixtures of elements in equal or near-equal concentrations, where the high entropy of mixing benefits the formation of solid-solution phases with simple structures like a body-centered cubic (bcc) or a face-centered cubic (fcc)  and suppresses the formation of numerous intermetallic phases, avoiding the disadvantages of conventional multicomponent alloys. In order to achieve a high entropy of mixing, the HEAs must be composed of at least five major elements in similar concentrations, ranging from 5 to 35 at. % for each element. For HEAs, the severe lattice distortions caused by different atom sizes may lead to a high strength induced by solid solution hardening . Moreover, HEAs may also possess a good ductility due to the simple crystal structures. Since both the strength and ductility are significant for structural materials, HEAs have been intensively studied and many fascinating properties were discovered. Single phase HEAs are usually chosen as a model system to investigate the fundamental physical mechanisms and mechanical behavior due to the simple structure. Otto et al.  studied the influences of temperature on the tensile properties of a CoCrFeMnNi HEA, which crystallizes in a single fcc crystal structure. It was found that the yield strength, ultimate tensile strength and elongation to fracture all increase with decreasing temperature from 1073 to 77 K and nanoscale deformation twins were observed at 77 K, which results in the enhanced ductility at low temperatures. Recently, the CoCrFeMnNi HEA was found to exhibit an excellent fracture-resistant at low temperature (77 K), which is also due to the occurrence of 3
mechanical nanotwinning . Furthermore, Zhu et al.  studied the incipient plasticity and dislocation nucleation in the CoCrFeMnNi HEA at different temperatures by nano indentation and found the vacancy-mediated dislocation nucleation is triggered by the cooperative motion of several atoms instead of simple atom–vacancy exchange found in conventional fcc metals. Although the single phase HEAs are of attractive mechanical properties at low temperatures, many HEAs with good mechanical properties at room temperature or high temperatures are multiphase alloys [6-12], which suggests the significance of multiphase HEAs. Recently, Santodonato et al.  studied the elemental distribution and the evolution of the configuration entropy in a Al1.3CoCrCuFeNi multiphase alloy and found that a significant amount of disorder exists in the phases which were previously taken as ordered structure, due to the distribution of multiple elements. This suggests that we should not only focus on the single-phase HEAs, the HEA-design strategy can be also applied to other complex multiphase materials with the benefit of the entropy-enhanced stability . There have been significant efforts in the various mechanical properties of HEAs with multiphase structures, however, detailed investigations of microstructure and composition, which play an important role in the mechanical behavior, are very few due to the multiple major elements and its induced complicated microstructure and elemental distribution. Pradeep et al.  studied the atomic-scale composition in a nanocrystalline AlCrCuFeNiZn HEA by atom probe tomography (APT) and found the microstructure changes at the atomic scale upon annealing, which is in 4
contradiction with the HEA design principles. Therefore, it was proposed that the Gibbs free energy rather the mixing entropy should be taken as the dominated factor for determining the equilibrium state of HEAs. Recently, Xu et al.  found a nanoscale phase separation in the Al0.5CoCrCuFeNi HEA, which was believed to be a single fcc structure in the previous work . The coherent nano precipitates with L12 ordering are enriched in Fe, Co and Cr, while the fcc matrix is enriched in Cu. The nano-scale separation may lead to the unusual mechanical behavior and phase stability of HEA with fcc structure. Therefore, the detailed microstructure and composition characterization is helpful not only for evaluating their phase stabilities but also understanding the mechanical properties of HEAs. In the current study, the microstructure of AlxCoCrFeNi HEAs (x=0.1, 0.75 and 1.5) was systematically characterized using X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscopy (TEM). The chemical compositions of different phases in the three studied HEAs were measured by energy-dispersion X-ray analysis (EDX) under nano-probe condition. The mechanical properties were analyzed by nano indentation. Then the microstructure and composition dependences of mechanical properties and phase stabilities are discussed based on the experimental results of AlxCoCrFeNi and some other similar HEA systems. The aim of current study is to investigate the correlation between phase stabilities, microstructure, and mechanical properties for HEAs.
2. Experimental Samples with nominal compositions of AlxCoCrFeNi (x=0.1, 0.75 and 1.5) were prepared by arc melting in a Ti-gettered high-purity argon atmosphere. The pure elements have a purity higher than 99.6 weight percent. The alloys were remelted at least four times to improve homogeneity. The ingots were then remelted under high vacuum (10-3Pa) and injection cast into a water cooled copper mould to obtain cylindrical rods of Φ 5×70 mm3. Microstructure and compositions were first analyzed with a SEM (FEI, Nano430) attached with a X-ray energy dispersive spectrometer (SEM-EDX). Crystal structure was examined with a XRD (Philips X Pert Pro) at 40 kV and 40 mA with a scanning rate of 4˚/min from 20˚ to 90˚. TEM observations were carried out with a 300 keV Tecnai F30 microscope attached with a X-ray energy dispersive spectrometer (TEM-EDX) at the Electron Microscopy Laboratory of Peking University. TEM-EDX characterizations were performed under STEM nano-probe condition with a beam size of about 1 nm. TEM samples were prepared by mechanical polishing to approximately 100 μm thickness, followed by dual jet polishing in an ethanol solution containing 5 % HClO4. Nano indentation tests were performed using an Agilent G200 (Agilent Technologies Inc. U.S) nanoindenter with a Berkovich tip. A 5×5 indent matrix (~ 100 μm × 100 μm) was performed for each sample to reduce the statistical error.
3. Results 3.1 XRD and SEM Figure 1 shows the XRD patterns of the AlxCoCrFeNi HEA. For x=0.1 (Fig. 1(a)), all the diffraction peaks can be identified to a single phase with the fcc crystal structure, indicating only a single fcc structure is formed. As the Al content is increased to x=0.75 (Fig. 1(b)), besides the fcc structure, a new structure identified as ordered bcc (B2) can be observed. It should be noted that disordered bcc (A2) probably also exists in Al0.75CoCrFeNi HEA, however TEM observation (as will be shown in the Section 3.3) confirms that Al0.75CoCrFeNi HEA only consists of fcc and B2 phases. For x=1.5, a duplex bcc structure, including A2 and B2, is identified. Table 1 summaries the microstructures and lattice parameters of the three investigated HEAs. As the Al content is increased, the lattice parameter of fcc phases increases and the crystal structure transforms from fcc to bcc. Furthermore, the Al1.5CoCrFeNi alloy consists of two bcc phases (A2 and B2) with identical lattice parameter, suggesting the coherent nature of both bcc phases. Representative SEM backscatter images of AlxCoCrFeNi HEAs are shown in Fig. 2. The Al0.1CoCrFeNi alloy exhibits a coarse equiaxed grain structure with size ranging from several tens to several hundreds μm, as shown in Fig. 2(a), and no other microstructural character was found. The XRD and SEM results verify that Al0.1CoCrFeNi alloy has a single fcc structure. The Al0.75CoCrFeNi (Fig. 2(b)) presents a more complicated microstructure. A great number of Widmanstätten side plates can be observed , and the rest region (referred to inter-sideplate regions 7
hereafter) exhibits a nano-scale basket-weave-like structure accompanied by some irregular-shaped grains (as shown in inset of Fig. 2(b)). The Widmanstätten side plates, irregular-shaped grains and part of basket-weave-like structure present a brighter contrast in the Fig. 2(b), indicating that these regions may contain a higher content of heavy elements or a lower Al content. Figure 2(c) shows the microstructure of Al1.5CoCrFeNi, which is very similar to that of Al0.1CoCrFeNi. However, it was found that numerous nano-scale spherical precipitates disperse in the equiaxed grains, as shown in inset of Fig. 2(c). It should be noted that these nano-scale spherical precipitates are not uniformly distributed in the matrix, some interconnected regions with a higher concentration of precipitates can be found, as indicated by the red arrow in Fig. 2(c). SEM-EDX was used to qualitatively analyze the composition distribution, it was found that the regions containing a high concentration of precipitates are enriched with Fe and Cr, while the other regions are enriched with Al and Ni. This indicates that these precipitates may be enriched in Fe, Cr, while the matrix is enriched in Al and Ni.
3.2 TEM A representative TEM bright-field (BF) image of Al0.1CoCrFeNi is shown in Fig. 3 and the corresponding selected-area electron diffraction (SAED) pattern in (100) zone axis is given in inset. No precipitate can be observed in Fig. 3 and the Al0.1CoCrFeNi alloy exhibits a single-phase solid solution with fcc structure, demonstrating the XRD and SEM results. The actual composition determined by the 8
measurements of TEM-EDX from ten different grains is given in Table 1. Figure 4(a) shows the BF TEM image of Al0.75CoCrFeNi, a duplex phase structure can be observed, which is very similar to the basket-weave-like structure presented in SEM image (Fig. 2(b)). SAED patterns suggest that in Fig. 4(a) the bright region is a B2 phase and the dark region is an fcc phase. Some other alternating duplex microstructures consisting of B2 phase and fcc phase, can be also observed (not presented here). No other structure, such as A2, is found. High resolution TEM (HRTEM) images combined with Fast Fourier Transformation (FFT), as shown in the Fig. 4(b), reveals that the corresponding crystallographic-orientation relationship of both phases is bcc//fcc and bcc//[01̅ 1]fcc. The TEM characterization shown above confirms the Al0.75CoCrFeNi consists of B2 and fcc phases, and reveals the crystallographic-orientation relationship of both phases. In order to determine the chemical compositions of both phases in Al0.75CoCrFeNi alloy, EDX line scanning under nano-probe condition was performed, as shown in Fig. 5. The scan line across a grain with fcc structure is marked in both Fig. 4(a) and Fig. 5(a). EDX results show that the fcc phase is enriched in Fe, Cr and Co, while the B2 phases are enriched in Al and Ni. Cr and Al exhibit the most significant segregation to fcc and B2 phases, respectively. The measured concentrations of constituent elements in both phases are given in Table 1. For Al1.5CoCrFeNi alloy, a representative TEM BF image is shown in Fig. 6(a), and the corresponding selected-area electron diffraction (SAED) pattern in (100) zone axis is given in inset. It can be observed that numerous spherical precipitates with 9
average diameter of ~ 80 nm distribute throughout the matrix, the SAED shows the super-lattice diffraction of B2 structure, which is consistent with XRD result and reveals the A2 and B2 structures are coherent. Figure 6(b) shows the dark-field (DF) image taken with the super-lattice diffraction (001), as indicated by the red circle in the inset of Fig. 6(a), it suggests that the matrix is B2 structure and the precipitates are A2 structure. The coherent A2/B2 phase mixture is a characteristic feature of Spinodal decomposition and should be formed by periodic composition modulations [13, 17]. Figure 7 shows the STEM HAADF image of Al1.5CoCrFeNi and EDX line scanning results. It reveals that Al, Ni and Co predominantly partition to the B2 phase (matrix), and Fe and Cr partition to A2 phases (precipitates). Al, Ni and Cr have strong segregation while the segregations of Fe and Co are relatively weak, which is similar with Al0.75CoCrFeNi and results in the formation of the nearly Cr-free matrix and Ni-free precipitates. The compositions of matrix and precipitates are shown in Table 1.
3.3 Nanoindentation Nanoindentation was employed in current study to investigate the mechanical properties of AlxCoCrFeNi (x=0.1, 0.75 and 1.5) alloys. The typical load-displacement curves under depth-controlled mode for the three HEAs are given in Fig.8 (a), the maximum penetration depth was set as 2 μm. The calculated nanohardness and elastic moduli are given in Table 2 and the details of the calculation are described elsewhere . It can be found that the hardness increases with increasing Al content, but the 10
Al0.75CoCrFeNi has the highest elastic modulus. The mechanical properties are further analyzed by comparing the reversible work Wu (elastic work) and irreversible work Wp (plastic work), which are the area under unloading curve and the area enclosed by the loading and unloading curves, respectively , as illustrated in the inset of Fig. 8(a). Generally, the ratio of Wp to the total work-Wp+Wu can be used to qualitatively characterize the material’s ductility, the better ductility the higher ratio of plastic work-Wp/(Wp+Wu). The relationship between H/E* (E* is the reduced modulus) and Wp/(Wp+Wu) is plotted in Fig. 8(b), some representative materials, such as Cu, W, fused silica, sapphire are also included for comparison . An approximately linear correlation between both can be observed, the calculated Wp/(Wp+Wu) for Al0.1CoCrFeNi is ~0.93, which is close to Cu (~0.95), suggesting that Al0.1CoCrFeNi is of a good ductility. In contrast, the Wp/(Wp+Wu) of Al1.5CoCrFeNi is considerably smaller than W, Cu and Al, suggesting that Al1.5CoCrFeNi probably exhibits an apparent brittleness. The indent morphologies, as shown in Fig. 8(a), also reveal the different ductility of AlxCoCrFeNi. Fine discrete slip bands can be observed in the indent of Al0.1CoCrFeNi, demonstrating its good ductility. For the indent of Al1.5CoCrFeNi, an obvious surface rupture is present in the pile-up region, verifying the poor ductility of Al1.5CoCrFeNi. Although nanoindentation can be used to qualitatively characterize the mechanical properties of AlxCoCrFeNi alloy, the detailed and quantitative results must depend on the uniaxial tensile and compression tests, which are underway, and are not covered herein.
4. Discussion 4.1 Phase stability There are many investigations on the effects of Al on the microstructure and mechanical properties of AlxCoCrFeNi HEA system [17, 21-23]. It has been revealed that the AlxCoCrFeNi alloy exhibits a single fcc structure when the content of Al, x, is smaller than 0.5, and only bcc phases can be found when x is larger than 0.9. The mixture of fcc and bcc phases is formed when the content of Al is in the intermediate range 0.5 < x < 0.9. The microstructure evolution of AlxCoCrFeNi HEA with Al can be interpreted by the phase transformation induced by lattice distortion . The lattice strain ε can be expressed as ε=|𝑎 − 𝑎0 |/a0
where a and a0 are the lattice constant of actual alloy and perfect crystal without distortion. Due to the large atomic radius of Al, the lattice strain ε significantly increases with Al content . Since bcc is not a close-packed structure, it can be expected that the structural transformation from the initial fcc to bcc occurs to reduce the lattice distortion induced by the atomic size mismatch of constituent elements. Furthermore, Guo et al.  proposed an empirical criterion of phase stability for HEA based on the valance electron concentration (VEC). It predicts that sole fcc phase exists for VEC≧8.0 and sole bcc phase exists for VEC < 6.87, fcc and bcc will co-exist in the intermediate range. According to VEC criterion, the ranges of Al content, x, for sole fcc and bcc phases are x < 0.2 and x > 1.4, respectively. The phase stability of AlxCoCrFeNi HEA predicted from the VEC criterion shows an apparent 12
difference from the experiment results, however, both are consistent with current experimental results. Recently, Poletti et al.  studied the electronic and thermodynamic effects on the phase stability of multicomponent solid solutions and proposed a criterion based on the atomic radius mismatch δ, Eq. (2), and the difference in electronegativity between elements Δχ, Eq. (3): r
δ=∑ni=1 ci ∙ |1- r i | ) a
Δχ=∑ni=1 ci ∙(1- χ 𝑖 ) 𝑎
Where ci , ri and χi are the atom fraction, radius and electronegativity of i-th element, respectively, ra and χa are the average radius and electronegativity of the elements in the alloy. It was found that multicomponent solid solutions (single phase) locate at the region of 1% < δ < 6% and 3% < Δχ < 6%. This is attributed to that the small atomic radius mismatch δ results in a relatively small lattice distortion, stabilizing the multicomponent solid solutions. The small electronegativity difference in general suggests a small negative enthalpy of mixing between constituent elements, which can retard the formation of intermetallics. The atomic radius mismatch and electronegativity difference for AlxCoCrFeNi are plotted in Fig. 9, the corresponding threshold Al content for single phase solid solution is x=0.65, which is essentially consistent with the experimental results. In the present study, it was found that an ordered bcc phase (B2) enriched in Ni and Al is firstly formed as increasing Al content. Otto et al.  studied various factors that aﬀect phase stability in HEAs. It was found that the stability of single 13
phase HEA (random solid solution phase) depends on the competition between conﬁgurational entropy and enthalpy and non-conﬁgurational entropy. When the contribution of conﬁgurational entropy to the total Gibbs free energy becomes dominant, the stable single phase HEA can form. In general, however, the enthalpy and non-conﬁgurational entropy have greater influences on phase stability, resulting in the formation of multiple phases and intermetallic phases. Moreover, if any constituent elements have strong tendency to form compounds with each other, it may reflect in the microstructure of the formed HEA. Based on these conclusions, it can be expected that as the Al content is increased the segregations of Ni, Al and Co will occur, forming a B2 phase due to the strongly negative formation enthalpy of B2 structure for Al-Ni and Al-Co, which are -66 kJ·mol-1 and -58 kJ·mol-1, respectively . Furthermore, more Ni and Co segregate to B2 phases with increasing Al content, which results in the formation of a Al- and Ni-free and Cr-rich second phase and its phase transformation from fcc to A2. For the B2 phases in Al1.5CoCrFeNi, the concentration of Al is ~ 40 at. %, it is therefore assumed that the Al atoms preferentially occupy the center sites of unit cell, Ni and Co randomly occupy the corner sites, forming a B2 structure similar with AlNi. Based on the above analyses, it can be found that the phase formation in the AlxCoCrFeNi is consistent with the conclusion that the strong interactions between constituent elements will be reflected in the microstructure of HEA. Furthermore, the nature of the influence of Al can be attributed to the large atomic radius and its strong tendency to form B2 phase with Ni and Co according to thermodynamics and kinetics 14
analyses, respectively. In fact, the similar effects of Al addition were also found in other HEAs, such as AlxFeCoNiCrMn , AlxCoCrFeNiTi , AlCoCrCuFeNi . The addition of Al always results in the structural transformation from initial fcc to a duplex fcc plus bcc structure and then to the bcc structure as the increasing Al content. Furthermore, the identical microstructure, including the A2 precipitates enriched in Fe, Cr and B2 matrix enriched in Al, Ni, was also observed in AlxFeCoNiCrMn HEA , verifying the similar influence mechanisms of Al. However, it has been pointed out that the current criterion for the phase stability of HEA can be only used to rationalize the formation of the single phase HEA , a more universal and accurate criterion that can effectively predict the phase formation in the multi-component alloy systems is still needed.
4.2 Mechanical properties Since the hardness of B2 and A2 phases should be much higher than that of fcc phase, the structural transformations with increasing Al content result in the increasing hardness, as revealed by nanoindentation. However, due to the poor ductilities of the B2 phases enriched in Al and Ni and the A2 phase enriched in Cr, the ductility of AlxCoCrFeNi significantly decreases, as shown by Fig. 8. Furthermore, the nano-scale alternating two phase microstructure formed in x=0.75 and 1.5 alloys promotes the hardening and embrittlement due to the introduction of numerous grain boundaries . A similar microstructure and mechanical properties evolution with Al content was also found in a six-component (FeCoNiCrMn)100-xAlx HEAs , 15
suggesting the effects of Al on the mechanical properties of HEAs with different compositions may be universal. Tian et al. studied the structural stability of AlxCoCrFeNi and calculated the elastic moduli by adopting state-of-the art ab initio density functional theory . It was found that the AlxCoCrFeNi exhibits a single fcc structure for x < 0.6 and a single bcc structure for x > 1.23. An fcc+bcc duplex structure exists at the intermediate region (0.6 < x < 1.23). The calculated elastic moduli of AlxCoCrFeNi first increases with Al content, and reaches a maximum at x = 1.0. As the content of Al further increases, the elastic moduli decreases for x > 1.0. The calculated elastic modulus of polycrystalline Al0.1CoCrFeNi is ~203 GPa, which is well consistent with current experiment result, as shown in Table 2. However, for Al0.75CoCrFeNi and Al1.5CoCrFeNi, the calculated elastic moduli are 187 and 167 GPa, respectively, which significantly deviate from the nano indentation results. This is attributed to that the AlxCoCrFeNi HEAs were all taken as fcc or bcc single phase solid solutions, and the phase separations and elemental segregations are not considered in the calculations. Therefore, only the Al0.1CoCrFeNi which is an fcc single phase solid solution, matches the assumption in the calculation and the calculation result can coincide well with the measured value. The difference in the experiment and calculation results suggests that the microstructure must be considered in the prediction of mechanical properties of HEA due to the fact that the single phase HEA is only a minor part in multi-component alloy systems.
5. Conclusions In the current study, the microstructure and mechanical properties of AlxCoCrFeNi (x=0.1, 0.75 and 1.5) HEAs are systematic studied. When x = 0.1, only a single fcc phase was found. When the content of Al, x, is increased to 0.75, a fcc+B2 duplex structure is observed. The fcc phases are enriched in Fe and Cr, while the B2 phases are enriched in Al and Ni. Two bcc phases, including the precipitates with A2 structure and matrix with B2 structure, are found in the Al1.5CoCrFeNi alloy. The A2 phases are enriched in Fe and Cr, and B2 phases are enriched with Ni, Al and Co. This also demonstrates that if any constituent elements have strong tendency to form phases with each other, it can be reflected in the microstructure of multi-component alloys, which suggests that the conﬁgurational entropy is not the only factor that affects phase stability in HEAs and enthalpy and non-conﬁgurational entropy should also be considered in the design of HEAs. The measured bulk modulus of Al0.75CoCrFeNi and Al1.5CoCrFeNi HEA are higher than the calculated results using ab initio density functional theory, which is due to the duplex phase microstructure. Since the high density of grain boundaries and the poor ductilities of A2 and B2 phases, the Al1.5CoCrFeNi HEA exhibits an apparent embrittlement.
Acknowledgment This work was ﬁnancially supported by the ITER 973 Program 2015GB113000 in China and the National Natural Science Foundation of China (11335003, 91226202). YZ very much appreciates the financial support from the National Natural Science Foundation of China (Nos. 51471025 and 51210105006), 111 Project (B07003), and the Program for Changjiang Scholars and the Innovative Research Team of the University.
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Figures and Figure Captions:
XRD patterns of (a) Al0.1CoCrFeNi, (b) Al0.75CoCrFeNi and (c)
Fig. 2. Back-scattering SEM images of (a) Al0.1CoCrFeNi, (b) Al0.75CoCrFeNi and (c) Al1.5CoCrFeNi HEAs. The inset in (b) shows the nano-scale basket-weave-like structure and some irregular-shaped grains in inter-sideplate regions, and the inset in (c) reveals that numerous nano-scale precipitates distribute in the matrix. The red arrow in (c) indicates the region containing a higher concentration of nano-scale precipitates.
Fig. 3. BF TEM image of the microstructure of Al0.1CoCrFeNi alloy. The inset is the SAED pattern in  zone axis.
Fig. 4. (a) BF TEM image of the microstructure of Al0.75CoCrFeNi alloy and corresponding SAED patterns, showing the alloy consists of a B2 phase and a fcc phase. The red line is the EDX scan line. (b) HRTEM image of a grain boundary between both phases and corresponding FFT. The superlattice diffractions belonging to B2 structure are indicated by red circles.
Fig. 5. (a) STEM HAADF image of Al0.75CoCrFeNi alloy, showing the scan line of nanoprobe EDX and compositional proﬁles across a grain with fcc structure for (b) Al, (c) Co, (d) Cr, (e) Fe and (f) Ni.
Fig. 6. (a) BF TEM image of the microstructure of Al1.5CoCrFeNi alloy, showing numerous nano-scale precipitates disperse in the matrix. The inset shows the indexed SAED pattern in (100) zone axis, the super-lattice diffractions belonging to B2 structure are indicated by red color. (b) the corresponding DF TEM image taken with (001) super-lattice diffraction spot.
Fig. 7. (a) STEM HAADF image of Al1.5CoCrFeNi alloy, showing the scan line of nanoprobe EDX and compositional proﬁles across the matrix and precipitate for (b) Al, (c) Co, (d) Cr, (e) Fe and (f) Ni.
Fig. 8. (a) Representative load-displacement curves and indent morphologies for AlxCoCrFeNi (x=0.1, 0.75 and 1.5) HEAs, the inset illustrates the elastic work Wu and plastic work Wp during nanoindentation. (b) the H/E* dependence of Wp/(Wp+Wu) for AlxCoCrFeNi, some other materials are also included for comparison. 28
Fig. 9. Radius and electronegativity mismatches for AlxCoCrFeNi.
Table 1 Microstructures and chemical compositions (at.%) of AlxCoCrFeNi (x=0.1, 0.75 and 1.5) HEAs. Alloy
Table 2 Elastic moduli and hardness of AlxCoCrFeNi (x=0.1, 0.75 and 1.5). Al0.1CoCrFeNi