Microstructure and mechanical properties of Al0 ...

Intermetallics 93 (2018) 93–100

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Intermetallics journal homepage: www.elsevier.com/locate/intermet

Microstructure and mechanical properties of Al0.7CoCrFeNi high-entropyalloy prepared by directional solidification

T

Gang Liua,∗, Lin Liub, Xinwang Liuc, Zhijun Wangb, Zhenhua Hana, Guojun Zhanga, A. Kostkad a

School of Material Science and Engineering, Xi'an University of Technology, Xi'an 710048, China State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an 710072, China c School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China d Materials Research Department and Center for Interface Dominated Materials (ZGH), Ruhr-University Bochum, Germany b

A R T I C L E I N F O

A B S T R A C T

Keywords: High entropy alloy Alloy design Crystal growth Microstructure Transmission electron microscopy (TEM)

The high-entropy-alloy Al0.7CoCrFeNi (molar ratio) was prepared by vacuum arc melting followed by directional solidification (DS) with < 001 > oriented seed. The unique lamellar-dendrite microstructure was obtained over a wide cooling rate range. During solidification, Fe and Co are prone to segregate to the dendrite, while Cr and Al segregate to interdendrite. The solute pile-up of Cr and Al at the solid/liquid interface leads to the dendritic solidification. During the following cooling process, the BCC phase precipitates from the FCC dendrite to form the lamellar structure, while the ordered B2 phase precipitates from the interdendrite. Moreover, the lamellar spacing is significantly refined with increasing cooling rate, resulting in the higher hardness and compressive yield strength. Directional solidification is proved to be an efficient way to improve the mechanical properties of multi-phases high-entropy alloys.

1. Introduction The conventional alloys were based on one or two principle elements with other minor elements to modify properties. In 2004, a new alloy-design concept of high entropy alloy (HEAs) was proposed [1,2]. HEAs commonly contain more than five elements with equal or near equal molar percents, but to crystallize as solid solution, rather than intermetallics [3]. HEAs exhibit some unique properties, such as outstanding ductility at cryogenic temperature [4,5], good wear [6], corrosion and oxidation resistance [7,8] et al. In the past decade, a series of HEAs have been developed, including the representative CoCrFeMnNi [4,5] and MoNbTaVW alloys [9] et al. High configuration entropy was originally regarded as the crucial factor to stabilize solid-solution HEAs. However, recent researches indicates that it is hard to get the absolute single-phase solid-solution HEAs [10–12]. Even for the widely accepted Cantor alloy, CoCrFeMnNi, also have the risk to precipitate intermetallics during long-time thermal exposure [11]. Actually, the solid-solution HEAs are difficult to get a good balance between strength and ductility, thus restricting its practical application. To solve this problem, some multi-phases alloys based on HEAs have been developed, including the Fe80-xMnxCo10Cr10 [13], FeCoCrNiMox [14], AlCoCrFeNi2.1 [15] et al. For these multi-phases alloys, the HCP (hexagonal close-packed)/TCP (topological close-

∗

packed)/BCC (body-centered cubic) phases are used to strengthen HEA matrix, but do not cause severe brittlement. The quaternary equiatomic CoCrFeNi alloy has been widely accepted as a model HEA with simple FCC solid-solution structure and high toughness [16,17]. The addition of Al has been regarded as a promising way to promote the mechanical properties of CoCrFeNi HEA by forming the second phases. Previous research [18] has indicated that the crystal structure of AlxCoCrFeNi (x: molar ratio) gradually changed from FCC (x ≤ 0.45) to FCC + BCC (0.45 < x < 0.88), and finally to BCC phase (x ≥ 0.88) at room temperature. With increasing Al addition, AlxCoCrFeNi alloy exhibits the higher strength and hardness, but lower ductility [19]. To optimize the strength and ductility, AlxCoCrFeNi with a mixture of both ductile FCC and hard BCC phases has been suggested. Therefore, Al0.7CoCrFeNi alloy, which composition located at the FCC + BCC dual-phases zone, was chose for this investigation. It should be noted that the crystal structures and mechanical properties of HEAs strongly depend on the alloy composition and the processing procedures. A series of processing techniques, such as annealed cold-rolling [20], spark plasma sintering (SPS) [21] and high pressure torsion (HPT) [22] et al., have been used to develop the high performance HEAs. As an important processing technique, directional solidification (DS) has achieved striking success in manufacturing

Corresponding author. E-mail address: [email protected] (G. Liu).

https://doi.org/10.1016/j.intermet.2017.11.019 Received 11 June 2017; Received in revised form 30 October 2017; Accepted 27 November 2017 Available online 01 December 2017 0966-9795/ © 2017 Elsevier Ltd. All rights reserved.


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Fig. 1. Longitudinal microstructure of Al0.7CoCrFeNi alloy directionally solidified with < 001 > orientated seed at the withdrawal rate of 50 μm/s and thermal gradient approximately 250 K/cm: (a) directionally solidified sample; (b) directionally solidified Al0.7CoCrFeNi; (c) transition zone from seed to Al0.7CoCrFeNi; (d) < 001 > orientated Nibased single-crystal seed.

Fig. 2. Schematic of the lamellar-dendrite microstructure.

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Fig. 3. Cross-sectional microstructure of directionally solidified Al0.7CoCrFeNi alloy at different withdrawal rate: (a) 50 μm/s; (b) 100 μm/s; (c) 500 μm/s; (d) high magnified SEM image marked in (a).

arc-melted at least 5 times in Ti-gettered Ar atmosphere to guarantee the homogeneity. The Φ4 mm rods were cut from the button ingots by wire electro-discharged machine and placed in the high purity alumina crucible (Φ4 × 100 mm) for directional solidification in a modified Bridgman apparatus described elsewhere [29]. Ga-In-Sn liquid metal was used as the coolant to get the high thermal gradient approximately 250 K/cm. Ni-based single-crystal superalloy was placed at the bottom of sample, and then partially re-melted followed by DS at different withdrawal rates (50, 100 and 500 μm/s). The phase constitutions was analyzed by the X-Ray diffraction measurement (Shimadzu XRD-7000), using Cu-Kα radiation scanning from 20° to 100° at a scanning rate of 4°/min. For comparison, the calculation of equilibrium phase was carried out by JMatPro 7.0 software. The DS microstructure was analyzed by optical microscope (Leica, DM-4000M) and scanning electron microscope (JOEL, JSM6700F). Compositional mapping analysis was performed by the energy dispersive spectrometry (EDS) affiliated to SEM. Thin TEM foils were prepared by twin-ject electropolishing. Then, the foils were observed by transmission electron microscopy (FEI, Tecnai G2 F20). Microhardness was measured with a Vickers hardness tester (KB Prüftechnik GmbH, KB30 BVZ) using a load of 200 g. Compressive property test were carried out on Φ4 × 6 mm samples at room temperature using the materials testing machine (Zwick/Roell, Z100) with a strain rate of 1.0 × 10−3 s−1.

Fig. 4. XRD Patterns of the arc-melted Al0.1CoCrFeNi and Al0.7CoCrFeNi alloys.

turbine blade for aero-engines [23]. It also can be used to improve the mechanical properties of in-situ composites, including Ni-Al-Cr (Mo) [24], Al2O3/YAG [25], Nb-Si [26] et al. Although there have been some attempts to cast HEAs with DS technique [27,28], few researches focused on HEAs with multi-phases microstructure. In the present work, Al0.7CoCrFeNi alloy were prepared by arc melting followed by directional solidification. The microstructural evolution and mechanical properties of Al0.7CoCrFeNi alloy were investigated.

3. Results Fig. 1 shows the longitudinal microstructure of DS Al0.7CoCrFeNi at the withdrawal rate of 50 μm/s. Ni-based single-crystal superalloy was placed at the bottom of the sample as the seed, which exhibits the representative < 001 > orientated dendritic structure at the initial stage of solidification (Fig. 1d). As DS proceed, the dendritic structure became more irregular in the transition zone from seed to Al0.7CoCrFeNi alloy (Fig. 1c). Finally, the < 001 > oriented dendrite changes to lamellar-dendrite and approximately has an angle of 60° with the growth direction (Fig. 1a). To better understand the solidification sequence of

2. Experimental procedure Button samples with nominal composition of Al0.7CoCrFeNi (molar ratio) were prepared by the arc melting. Approximately 150 g raw materials of high purity Al, Co, Cr, Fe and Ni (purity > 99.95%) were 95


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Fig. 5. Bright field TEM images of directionally solidified Al0.7CoCrFeNi and corresponding selected area electron diffraction: (a) bright field TEM image; (b) bright field TEM image with high magnification; (c–d) selected area electron diffraction marked in (b).

phase and the ordered BCC phase (such as B2 phase). Therefore, the detailed phase identification was performed by TEM with select area electron diffraction (SAD). Fig. 5 is bright-field (BF) TEM images of DS Al0.7CoCrFeNi and the corresponding SAD patterns. As shown in Fig. 5a, DS Al0.7CoCrFeNi exhibits the lamellar-dendrite and interdendritic structure. Fig. 5b is BF image of lamellar-dendrite (marked in Fig. 5a). The diffraction patterns shown in Fig. 5c (SAD1) and Fig. 5d (SAD2) indicate the FCC phase and disordered BCC phases in the lamellar-dendrite region, respectively. In addition, EDS attached to the TEM was utilized to measure the chemical composition of the lamellar phases. As shown in Table 1, Al and Ni strongly segregate to BCC phase, while Cr and Fe apparently segregate to FCC phase. There is no obvious segregation tendency of Co between FCC and BCC phases. Fig. 6 is the TEM images of DS Al0.7CoCrFeNi alloy at interdendritic region and the corresponding SAD patterns. Fig. 6b is the SAD taken from interdendritic region, which indicates the ordered BCC crystal structure. By comparing the dark-field (DF) images corresponding to (110) spot (Fig. 6c) and (100) superlattice spot (Fig. 6d), it indicates the small cubic B2 (ordered BCC) phase in the disordered BCC matrix. The previous investigation has been proved that the B2 phase precipitates from the BCC matrix during the following cooling process [30]. Therefore, the final crystal structure of DS Al0.7CoCrFeNi alloy consists of the lamellar-dendrite (disordered FCC and BCC phases) and interdendrite (B2 phase and disordered BCC matrix). The dependence of micro-hardness and lamellar spacing of DS Al0.7CoCrFeNi on the withdrawal rate was shown in Fig. 7a. Apparently, the microhardness gradually increases from 314HV (V = 50 μm/ s) to 354HV (V = 100 μm/s), and finally to 417HV (V = 500 μm/s). The engineering compressive stress-strain curves are shown in Fig. 7b. All the samples exhibit the good compressive ductility (engineering

Table 1 Chemical compositions of the lamellar-dendrite and interdendrite, with the corresponding segregation ratio (segregation ratio is defined as the composition in dendrite over that in the interdendrite region). Region

Lamellar-dendrite BCC phase in lamellar-dendrite FCC phase in lamellar-dendrite Interdendrite Segregation ratio

Elements (at. %) Al

Cr

Ni

Fe

Co

13.97 24.84 7.56 16.80 0.83

21.40 6.18 24.46 24.36 0.88

21.77 34.30 21.09 19.27 1.13

21.63 13.52 23.24 20.19 1.07

21.23 21.15 23.63 19.38 1.09

Al0.7CoCrFeNi alloy, a schematic plot is provided in Fig. 2. DS Al0.7CoCrFeNi alloy exhibits the lamellar-dendrite structure (Fig. 2). The formation mechanism of lamellar-dendrite will be discussed later. Fig. 3 shows the cross-sectional microstructure of DS Al0.7CoCrFeNi alloys with increasing withdrawal rates. All the samples exhibit the similar lamellar structure (Fig. 3a–c). However, the lamellar spacing, λ, was significantly refined with the increasing withdrawal rate. The λ decreased from 1.6 μm (V = 50 μm/s) to 0.5 μm (V = 100 μm/s), and finally to 0.3 μm (V = 500 μm/s). In addition, SEM image with high magnification (marked in Fig. 3a) was shown in Fig. 3d. Some small cubic phases were clearly seen in the inter-lamellar region. Fig. 4 shows the XRD pattern of arc-melted Al0.1CoCrFeNi and Al0.7CoCrFeNi alloys. The XRD pattern of Al0.1CoCrFeNi alloy indicates the single FCC phase with a lattice parameter of 0.3588 nm. With increasing Al addition, Al0.7CoCrFeNi alloy exhibits the FCC + BCC dualphases structure with the additional (110)BCC, (200) BCC and (211)BCC XRD peaks. However, XRD testing is difficult to distinguish the co-existing phases with similar crystal structures, such as the disordered BCC 96


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Fig. 6. TEM images of directionally solidified Al0.7CoCrFeNi alloys: (a) bright field image; (b) selected area electron diffraction of the circle region in (a); (c) darkfield image corresponding to (100) spot in (b); (d) dark-field image corresponding to (110) superlattice spot in (b).

Fig. 7. (a) Hardness and lamellar spacing of directionally solidified Al0.7CoCrFeNi alloy at different withdrawal rates; (b) compressive stress-strain curves of DS Al0.7CoCrFeNi alloy at different withdrawal rates.

strain, ε > 10%). With the increasing withdrawal rate, the yield strength increased from 583 MPa (V = 50 μm/s) to 717 MPa (V = 100 μm/s), and then to 1022 MPa (V = 500 μm/s).

microstructure of Al0.5CoCrFeNi alloy. The crystal structure of Al0.5CoCrFeNi alloy gradually transform from disordered FCC structure to BCC-based structure at higher temperature. To clarify the formation mechanism of lamellar-dendrite structure of Al0.7CoCrFeNi alloy. The equilibrium phase fraction as a function of temperature is calculated by the thermodynamic software JMatPro. As shown in Fig. 8, the FCC phase firstly solidified from the melt as the primary phase. At the end of solidification, the BCC and FCC phases coexist, which indicates the formation of FCC/BCC eutectics. During the following cooling process, the higher BCC phase fraction with lower FCC phase fraction indicates the precipitation of BCC phase from the FCC matrix. Below 890 °C, the σ

4. Discussion 4.1. Formation mechanism of lamellar-dendrite It should be noted that the as-cast microstructure is observed at room temperature, which is different to the microstructure at high temperature. For example, Lin et al. [7] studied the annealed 97


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Fig. 10. Schametic of the crystal structure of Al0.7CoCrFeNi alloy during solidification and the following cooling process.

Al0.7CoCrFeNi alloy. It clearly shows that Fe, Co and Ni partition to the lamellar-dendrite, while Al and Cr segregate to the interdendritic region. Moreover, the chemical compositions of lamellar-dendrite and interdendritic regions are shown in Table 1. The segregation ratios, k, is introduced to demonstrate the degree of segregation between dendrite and interdendrite, which is defined as follows:

Fig. 8. Equilibrium mole fraction of various phases as a function of temperature for Al0.7CoCrFeNi alloy.

k′ = CDC /CID

phase precipitate from the FCC matrix. Zhang et al. [31] calculated the equilibrium phase diagram of AlxCoCrFeNi alloys by using the CALPHAD (Calculation of phase diagrams) approach, which also shows the similar results. Fig. 9 is the EDS mapping of the cross-sectional microstructure of DS

where CDC is element concentration in the lamellar-dendrite, CID is that in the interdendritic region. It also indicates the severe segregation of Al and Cr (kAl < kCr < 1). During directional solidification, Al and Cr are accumulated in front of the Solid/Liquid (S/L) interface. Considering the sluggish diffusion effects of HEA [3], the accumulated Al and Cr are difficult to be removed by diffusion, thus leading to the constitutional

(1)

Fig. 9. EDS mapping of directionally solidified Al0.7CoCrFeNi alloy:(a) Backscatter electron image; (b) through (f) elemental mapping for Al, Cr, Fe, Co and Ni, respectively.

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1. Directionally solidified Al0.7CoCrFeNi alloys exhibit the lamellardendrite structures. The lamellar-dendrite is composed by the disordered FCC and BCC phases, while the interdendritic structure is composed by the B2 phases and disordered BCC phase. 2. During solidification, Ni, Fe and Co segregate to the lamellar-dendrite region, while Al and Cr segregate to the interdendritic region. The excessive enrichment of Al and Cr ahead of the solid/liquid interface promotes the formation of dendrite. 3. Lamellar spacing of DS Al0.7CoCrFeNi decreases from 1.6 μm to 0.3 μm with the increasing withdrawal rate ranging from 50 μm/s to 500 μm/s. DS Al0.7CoCrFeNi at the withdrawal rate of 500 μm/s exhibits the highest yield strength of 1022 MPa.

undercooling. Consequently, the S/L interface of DS Al0.7CoCrFeNi exhibits the dendrite morphology in this work (Fig. 1). Base on the thermodynamic calculation, Al0.7CoCrFeNi alloy is far away from the composition of eutectic (Fig. 8). The supersaturated FCC dendrite, enriched with Fe, Co and Ni, firstly solidified from the melt as the primary phase. At the end of solidification, the local enrichment of Al and Cr leads to the formation of FCC/BCC eutectic. Due to the low eutectic fraction, the FCC phase preferentially affiliates with the primary dendrite phase, while the BCC phase distributes along the boundary of dendrite (Fig. 3a), which is termed as the divorced eutectic. The mixing enthalpy (△Hmix) [32,33] is commonly used to characterize the tendency of constituent elements to forming stable intermetallic compounds. Al-(Ni,Co,Cr,Fe) have a much higher negative mixing enthalpy than the other atom pairs in Al-Co-Cr-Fe-Ni alloy system. Due to the severe segregation of Al, the Al-Ni rich phase (B2 phase) has a strong tendency to precipitate from the divorced BCC eutectic phase in the interdendritic region. Simultaneously, the Al(Ni,Co,Cr,Fe) rich phase (BCC phase) tend to precipitates from the supersaturated FCC dendrite, thus leading to the lamellar-dendrite structure at room temperature. Because of the non-equilibrium condition, the σ phase is not observed in the as-cast sample. In conclusion, the phase transformation of Al0.7CoCrFeNi alloy is summarized as the following sequence (Fig. 10): Liquid→primary-dendrite (FCC phase) + divorced eutectic (BCC phase)→lamellar-dendrite (FCC + BCC) + interdendrite (B2 phase precipitates from the BCC matrix).

Acknowledgements The work was supported by National Natural Science Foundation of China (Grants Nos. 51604222, 51401160, 51504191), China Postdoctoral Science Foundation (No.152064) and Fund of the State Key Laboratory of Solidification Processing in NWPU (No.SKLSP201310). References [1] J.W. Yeh, S.K. Chen, S.J. Lin, J.Y. Gan, T.S. Chin, T.T. Shun, C.H. Tsau, S.Y. Chang, Nanostructured high-entropy alloys with multiple principal elements: novel alloy design concepts and outcomes, Adv. Eng. Mater. 6 (2004) 299–303. [2] B. Cantor, I.T.H. Chang, P. Knight, A.J.B. Vincent, Microstructure development in equiatomic multicomponent alloys, Mater. Sci. Eng. A 375–377 (2004) 213–218. [3] Y. Zhang, T.T. Zuo, Z. Tang, M.C. Gao, K.A. Dahmen, P.K. Liaw, Z.P. Lu, Microstructure and properties of high-entropy alloys, Prog. Mater. Sci. 61 (2014) 1–93. [4] F. Otto, A. Dlouhý, Ch Somsen, H. Bei, G. Eggeler, E.P. George, The influences of temperature and microstructure on the tensile properties of a CoCrFeMnNi highentropy alloy, Acta Mater. 61 (2013) 5743–5755. [5] B. Gludovatz, A. Hohenwarter, D. Catoor, E.H. Chang, E.P. George, R.O. Ritchie, A fracture-resistant high-entropy alloy for cryogenic applications, Science 345 (2014) 1153–1158. [6] M.H. Chuang, M.H. Tsai, W.R. Wang, S.J. Lin, J.W. Yeh, Microstructure and wear behavior of AlxCo1.5CrFeNi1.5Tiy high-entropy alloys, Acta Mater. 59 (2011) 6308–6317. [7] C.M. Lin, H.L. Tsai, Evolution of microstructure, hardness, and corrosion properties of high-entropy Al0.5CoCrFeNi alloy, Intermetallics 19 (2011) 288–294. [8] G.H. Meng, T.M. Yue, X. Lin, H.O. Yang, H. Xie, X. Ding, Laser surface forming of AlCoCrCuFeNi particle reinforced AZ91D matrix composites, Opt. Laser Technol. 70 (2015) 119–127. [9] O.N. Senkov, G.B. Wilks, D.B. Miracle, C.P. Chuang, P.K. Liaw, Refractory highentropy alloys, Intermetallics 18 (2010) 1758–1765. [10] F. He, Z.J. Wang, Q.F. Wu, J.J. Li, J.C. Wang, C.T. Liu, Phase separation of metastable CoCrFeNi high entropy alloy at intermediate temperatures, Scr. Mater. 126 (2017) 15–19. [11] F. Otto, A. Dlouhý, K.G. Pradeep, M. Kuběnová, D. Raabe, G. Eggeler, E.P. George, Decomposition of the single-phase high-entropy alloy CrMnFeCoNi after prolonged anneals at intermediate temperatures, Acta Mater. 112 (2016) 40–52. [12] F. Otto, Y. Yang, H. Bei, E.P. George, Relative effects of enthalpy and entropy on the phase stability of equiatomic high-entropy alloys, Acta Mater. 61 (2013) 2628–2638. [13] Z.M. Li, K.G. Pradeep, Y. Deng, D. Raabe, C.C. Tasan, Metastable high-entropy dualphase alloys overcome the strength-ductility trade-off, Nature 534 (2016) 227–232. [14] W.H. Liu, Z.P. Lu, J.Y. He, J.H. Luan, Z.J. Wang, B. Liu, Y. Liu, M.W. Chen, C.T. Liu, Ductile CoCrFeNiMox high entropy alloys strengthened by hard intermetallic phases, Acta Mater. 116 (2016) 332–342. [15] Y.P. Lu, Y. Dong, S. Guo, L. Jiang, H.J. Kang, T.M. Wang, B. Wen, Z.J. Wang, J.C. Jie, Z.Q. Cao, H.H. Ruan, T.J. Li, A promising new class of high-temperature alloys: eutectic high-entropy alloys, Sci. Rep. 4 (2014) 1–5. [16] M. Vaidya, S. Trubel, B.S. Murty, G. Wilde, S.V. Divinski, Ni tracer diffusion in CoCrFeNi and CoCrFeMnNi high entropy alloys, J. Alloy Compd. 688 (2016) 994–1001. [17] Z. Wu, H. Bei, G.M. Pharr, E.P. George, Temperature dependence of the mechanical properties of equiatomic solid solution alloys with face-centered cubic crystal structures, Acta Mater. 81 (2014) 428–441. [18] Y.F. Kao, T.J. Chen, S.K. Chen, J.W. Yeh, Microstructure and mechanical property of as-cast, -homogenized, and –deformed AlxCoCrFeNi (0 ≤ x ≤ 2) high-entropy alloys, J. Alloy Compd. 488 (2009) 57–64. [19] J.Y. He, W.H. Liu, H. Wang, Y. Wu, X.J. Liu, T.G. Nieh, Z.P. Lu, Effects of Al addition on structural evolution and tensile properties of the FeCoNiCrMn high-entropy alloy system, Acta Mater. 62 (2014) 105–113. [20] G. Laplanche, A. Kostka, O.M. Horst, G. Eggeler, E.P. George, Microstructure evolution and critical stress for twinning in the CrMnFeCoNi high entropy alloy, Acta Mater. 118 (2016) 152–163.

4.2. Relationship between cooling rate and mechanical property Not only the alloy composition, but also the synthesis routes apparently affect the mechanical properties of HEAs [22,23]. By controlling the withdrawal rate and thermal gradient, Al0.7CoCrFeNi alloy can be directionally solidified at different cooling rate. It provides a convenient way to investigate the relationship between solidification patterns and mechanical properties. In this work, DS Al0.7CoCrFeNi at the withdrawal rate of 500 μm/s exhibits the highest yield strength of 1022 MPa, which can be mainly attributed to the greatly refined lamellar structure. Generally, the BCC phase is more brittle but stronger than FCC phase due to the less available slip system. For Al0.7CoCrFeNi alloy, the BCC phase is regarded as the strengthening agent. As describe above, the BCC phase precipitates from the supersaturated FCC dendrite, thus resulting in the lamellar structure. The precipitation of BCC phase is controlled by the solute mutual diffusion. The higher cooing rate hinders the diffusion of constituent elements such as Al and Ni, thus resulting in the finer lamellar-dendrite structure. Because the lamellar interface is regarded as the effective barriers to the motion of dislocations, dislocations are prone to pile up on slip planes at the lamellar interface. Due to the reduced mean free path of dislocation, DS Al0.7CoCrFeNi alloy with refined lamellar structure exhibits the higher yield strength and hardening rate. The similar results were also founded in other in-situ composite systems, such as TC4-DT titanium alloy [34], NiAl-Cr-Mo [35] and Ni-Zr-Al eutectic alloys [36] et al. The reduced lamellar thickness leads to the higher strength. By optimizing the size, shape and distribution of strengthening BCC phases, DS Al0.7CoCrFeNi alloy can get the better mechanical properties. The present study indicates that DS technique is an efficient way to improve the mechanical properties of multi-phases HEAs. 5. Conclusions The directionally solidified Al0.7CoCrFeNi alloys with different withdrawal rate were investigated with respect to the crystal structures, microstructure and mechanical properties. The following conclusion can be drawn from this work. 99


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