Achieving high ductility in the 1.7 GPa grade

Materials Science and Engineering A 740-741 (2019) 336-341,

https://doi.org/10.1016/j.msea.2018.10.094

Achieving high ductility in the 1.7 GPa grade CoCrFeMnNi high-entropy alloy at 77 K S.J. Sun a,b, Y.Z. Tian a*, H.R. Lin a,b, H.J. Yang a, X.G. Dong c, Y.H. Wang d, Z.F. Zhang a,b* a Materials

Fatigue and Fracture Division, Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China

b

School of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, China c

d

School of Materials Science and Engineering, Shenyang Ligong University, Shenyang 110159, China

National Engineering Research Center for Equipment and Technology of Cold Strip Rolling, Yanshan University, Qinhuangdao 066004, China

Abstract CoCrFeMnNi high-entropy alloys (HEAs) with partially recrystallized (PR) structure were fabricated by cold rolling and annealing. The microstructures were characterized and the tensile properties were tested at 77 K and 293 K, respectively. In contrast to the early necking at 293 K, an ultrahigh yield strength of 1692 MPa and a considerable uniform elongation of 10.3% were obtained at 77 K. The notable uniform elongation at 77 K can be attributed to the enhanced strain-hardening capability via introducing multiple deformation mechanisms in the recrystallized grains. This work provides a strategy to design high-strength high-ductility HEAs for applications at cryogenic environments.

Keywords: High-entropy alloy (HEA); Recrystallization; Deformation twins; Yield strength; Elongation

* Corresponding author. [email protected] (Y.Z. Tian), [email protected] (Z.F. Zhang). 1



1. Introduction High-entropy alloys (HEAs) are a new type of equiatomic or near equiatomic multicomponent alloys, which open a new exciting research area in developing alloys [1-4]. In recent years, HEAs have been the object of much interest due to their outstanding properties [5-11]. Among various HEAs, the HEAs with FCC structure were extensively studied due to the superior cryogenic properties [12-14]. The mechanical nanotwinning was induced during the deformation at cryogenic temperature, which resulted in the continuous steady strain hardening and postponed necking instability [12,15]. However, a drawback of the CoCrFeMnNi HEAs is its relatively low yield strength due to the FCC structural characteristics of the alloy [15]. Many researchers make efforts to enhance the yield strength of the HEAs with FCC structure [16-20]. Among the methods, the severe plastic deformation (SPD) is attractive, which can reduce grain sizes down to the ultrafine-grained (UFG) and even nanocrystalline (NC) regimes [16,21]. The ultrahigh strength could be obtained in the nanostructured HEAs owing to the specific microstructural features such as nanoscale and ultrafine grains as well as a large volume fraction of grain boundaries. However, superstrong nanostructured metals typically have low ductility at ambient temperatures, which significantly limits their applications [22]. In recent years, many efforts have been reported to optimize microstructure to achieve a good balance of strength and ductility [23-28]. For example, the gradient structure was introduced in coarsegrained (CG) Cu, which exhibited a yield strength twice that of the CG counterpart and a comparable uniform elongation to that of the CG tensile sample [23]. A heterogeneous lamella structure with soft micrograined lamellae embedded in hard ultrafine-grained lamella matrix was introduced in CG Ti, and unusual high strength was obtained with the assistance of high back stress, whereas high ductility was attributed to back-stress hardening and dislocation hardening [25]. Dini et al. [27] have suggested that partial recrystallization by utilization of large cold rolling reduction and subsequently annealing treatment was an effective method to obtain twinning-induced plasticity (TWIP) steel with an excellent combination of strength and ductility. In fact, the mechanical properties of metallic materials are not only determined by their internal microstructures but also external environmental conditions. Nanostructured metals had a remarkable yield strength and high ductility at cryogenic temperature [29]. Because the nanostructured metals regain the strain-hardening ability due to the suppressed dynamic recovery at cryogenic temperature [29]. The objective of the present work is to explore strategies for strengthening CoCrFeMnNi HEAs by using partially recrystallized (PR) microstructures. The samples with PR microstructures were prepared by means of conventional cold rolling and subsequent annealing process. The microstructural evolution and mechanical properties of the PR HEA at 77 and 293 K were investigated, and mechanisms for the enhanced strength-ductility synergy at 77 K were discussed.

2. Experimental procedures Bulk CoCrFeMnNi HEAs were prepared by magnetic levitation melting technique, followed by hot forging and cold rolling into sheets 1 mm thick at room temperature. Then the coldrolled sheets were annealed at 600 °C for 30 minutes to obtain PR specimens. Details of the 2



processing procedures were reported in the previous work [30]. Tensile tests were conducted at an initial strain rate of 10-3 s-1 using an Instron 5982 testing machine at room temperature (293 K) and cryogenic temperature (77 K), and the loading direction was parallel to the rolling direction of the sheet. Tensile specimens with a gauge length of 10 mm, a width of 2 mm and a thickness of 1 mm were cut from annealed sheets by electrical discharge machine. Microscopic analyses before and after tensile tests were carried out using a LEO Supra 35 field emission scanning electron microscope (SEM) equipped with an electron backscattering diffraction (EBSD) system. The EBSD measurement was conducted with a step size of 20 nm for PR specimens to measure the recrystallization percentage. Deformation microstructures were also characterized by an FEI Tecnai F20 transmission electron microscope (FE-TEM). TEM foils were cut from the tensile specimens and the observation direction is parallel to the transverse direction of the tensile specimen. The foils were prepared using twin-jet electropolishing method by Tenupole-5 in a solution of 70% methanol, 20% glycerine and 10% perchloric acid with a voltage of 20 V at −20 °C. XRD measurements were conducted by using a Rigaku Smartlab X-ray diffractometer with a Cu-K target scanning 2θ from 40˚ to 100˚ and a step size of 0.02˚. The operating voltage and current are 45 kV and 200 mA, respectively.

3. Results and discussion 3.1 Microstructures

Fig.1. Microstructure of the PR sample. (a) X-ray diffraction pattern showing FCC peaks, (b) IPF map observed from RD-ND plane, (c) and (d) TEM micrographs observed from RD-ND plane.

An X-ray diffraction pattern of the PR HEA is shown in Fig. 1a. All diffraction peaks can be indexed assuming a single FCC phase. Typical microstructures of the PR HEA are exhibited in 3



Fig. 1b, which were characterized on the RD-ND plane of the sheets after cold rolling and annealing at 600 °C for 30 minutes. It is found that most of the regions are still in ‘mosaic’ colors, which are related to deformation microstructures where sound Kikuchi-lines could not be obtained; in comparison, the recrystallized areas can be well indexed and the grains are apparently larger than the unrecrystallized areas, as shown in Fig. 1b. Accordingly, the recrystallized fraction of the PR specimen is measured to be 26.6%. TEM observation indicates that the unrecrystallized area is characterized with nanocrystalline grains and deformation twins and high-density dislocations, as shown in Fig. 1c and d. In contrast, the recrystallized areas are nearly dislocation free and the average grain size is around 500 nm, as shown in Fig. 1c. The formation of profuse annealing twins (red dotted lines) further indicates the recrystallization process. In these recrystallized grains, some precipitates were detected, which has been reported in previous studies [30]. However, they are small in number possibly due to the short annealing time and are believed to impact negligible effect on the mechanical properties.

Fig. 2. Mechanical properties of the PR CoCrFeMnNi HEA. (a) Tensile engineering stress-strain curves of the PR HEAs at 77 K and 293 K, and the stress-strain curve of CG HEAs was provided for comparison [30]. (b) True stress-strain curves and strain-hardening curves of PR HEA at 77 K and 293 K.

Fig. 2a shows the tensile engineering stress-strain curves of the PR specimens at 77 K and 293 K. The yield strength and uniform elongation of the PR specimens are 1210 MPa and 1.1% at 293 K. The high dislocation density and nanocrystalline grains in PR HEAs result in an ultrahigh yield strength. The limited uniform elongation is attributed to the early plastic instability, which results from the limited strain-hardening capability of the deformation microstructure with highly prestored dislocations (as shown in Fig.1), where it is difficult to introduce more dislocations [31]. When the deformation temperature decreases to 77 K, the PR HEA shows a very high cryogenic strengthening response with an increase in yield strength by 482 MPa and the yield strength approaches to 1692 MPa. Meanwhile, the uniform elongation increases unprecedently from 1.1% to 10.3% upon decreasing temperature. Fig. 2b demonstrates the tensile true stress-strain curves of the PR HEAs deformed at 77 K and 293 K before necking and the related strain-hardening curves. At 293 K, it is observed that the strainhardening rate decreases steeply and necking appears quickly posterior to yielding for PR HEA, while the strain-hardening rate of the PR HEA has been remarkably enhanced at 77 K. This 4



observation indicates that the strain-hardening capacity of the PR HEA is greatly enhanced with decreasing the temperature. In that case, it is essential to reveal the deformation microstructures to explore mechanisms for the remarkable improvements of yield strength and uniform elongation at cryogenic temperature.

Fig. 3. TEM micrographs showing microstructure evolution with tensile strain in the PR HEA at 293 K and 77 K.

The deformation microstructures of PR CoCrFeMnNi HEAs were characterized by TEM observation after tensile tests at 293 K and 77 K, respectively. It is found that profuse dislocations can be detected in the recrystallized grain interior in the post-tensile microstructures at 293 K, as illustrated in Fig. 3a and b, which is similar to the recrystallized HEAs [30]. The PR HEA exhibits a higher yield strength than the recrystallized UFG HEAs [30], which could be attributed to the high dislocation density in the unrecrystallized areas, as shown in the Fig. 3a. However, it is proposed that the unrecrystallized areas are hardly deformed during tensile deformation due to the prestored high-density dislocations, which results in the limited uniform elongation. Furthermore, deformation twins were not observed in the recrystallized grains at 293 K, which is induced by the increased twinning stress with grain refinement [32]. In contrast to the deformation activities at 293 K, the good strain-hardening capacity of the PR HEA at 77 K is highly correlated with the transition of the deformation mechanisms. At a low strain of 2% at 77 K, Fig. 3c exhibits that the unrecrystallized areas contain high dislocation density, which can provide high yield strength during the tension process at 77 K. The temperature dependence of the yield strength is usually observed in FCC alloys, which can be expressed as the sum of thermal (σth) and athermal (σath) components [33,34]: σYS = σth + σath (1) where σth is thermal contribution, which corresponds to thermal activation of dislocation motion, and σath is athermal contribution, which is independence of the temperature. Owing to the 5



dislocation nature of thermally activated glide for overcoming obstacles, σth should be highly sensitive to the temperatures. At cryogenic temperatures, the energy available to assist dislocations in overcoming obstacles becomes essentially insignificant, leading to a prominent increase of σth. Especially, the high dislocation density and nanocrystalline grains in the unrecrystallized areas result in a high σth, which leads to a remarkable increment of 482 MPa as achieved for the yield strength of PR HEA. For the recrystallized grains, the cross slip of dislocation should be highly inhibited at 77 K, facilitating the efficient storage of dislocations. Fig. 3d exhibits that dislocation slip in planar mode is clearly captured in the recrystallized grain at a low strain of 2% at 77 K. Fig. 3e shows that a certain dislocations can be detected in the deformation twins. Extensive dislocation-twin boundaries interaction is propitious to the enhancement of the strain-hardening and then ductility [35]. Furthermore, some stacking faults (SF) bundles are formed in the recrystallized grain during the deformation process, as shown in the high resolution TEM image of Fig. 3f, which can be confirmed by the inset fast Fourier transformation pattern. The deformation twins itself can greatly accommodate the plasticity, while newly formed twin boundaries essentially have a “dynamic Hall-Petch” effect to reduce the dislocation mean free path and cause strengthening [36,37].

Fig. 4. Fracture behavior of the PR HEA deformed at (a-c) 293 K and (d-f) 77 K. The observation directions are parallel to (a,d) the transverse direction or (b,c,e,f) rolling direction of the tensile specimen.

The fracture behavior of the HEA was also characterized after tensile tests at 293 K and 77 K, and typical fracture morphologies are shown in Fig. 4. The fracture mode usually consists 6



of the shear fracture and normal fracture, and the shear fracture plane has an angle of about 45o to the loading direction and the normal fracture plane is perpendicular to the loading direction [38]. At 293 K, the HEA exhibits apparent necking and final fracture via shear deformation, as shown in Fig. 4a. In contrast, there is slight necking but apparent fracture via shear deformation after tensile test at 77 K, as shown in Fig. 4d. This is in accordance with the different post necking strains at 293 K and 77 K, as shown in Fig. 2a. Though the HEA suffers apparent shear fracture at 77 K, the reduction in area of 48.8% is slightly larger than 44.3% at 293 K, which is supposed to be induced by the large uniform deformation via enhancing the strain-hardening capability. Further observation on the fracture surfaces revealed evidences of shear deformation and normal deformation, as shown in Fig. 4b and 4e. Fig. 4c and 4f demonstrate that dimples are uniformly distributed after tensile testing at 293 K and 77 K. Precipitates were rarely observed in the dimples, which is different from the previous reports [16].

Fig. 5. Comparison of yield strength and uniform elongation for FCC HEAs (CoCrFeMnNi with SFE of 30±5 mJ m-2, CoCrNi with SFE of 22±4 mJ m-2 [41]) and the PR CoCrFeMnNi HEA at 293 K and 77 K [13-15,41-46].

With respect to the superior mechanical properties of the PR CoCrFeMnNi HEA at 77 K, a comparison of yield strength versus uniform elongation with the other FCC HEAs is shown in Fig. 5. In contrast to the strength-ductility synergy at 293 K, both yield strength and uniform elongation at 77 K can be upgraded to a higher level. This upgrade phenomenon is similar to that existed in the Cu-Al alloy system, which is induced by the enhanced strain-hardening capability via decreasing stacking fault energy (SFE) [39]. Attributing to the decrease of SFE caused by cryogenic temperature, deformation twins have formed during tensile deformation [40]. The stupendous improvement of strength is thus achieved by introducing deformation twins and SF bundles in recrystallized grains at 77 K via the dynamic Hall-Petch effect. It is noteworthy that the PR CoCrFeMnNi HEA in this work exhibits ultrahigh yield strength at 77 K while possessing adequate uniform elongation due to enhanced strain-hardening capability. The current findings provide us an utilizable strategy by introducing proper recrystallization percentage and controlled deformation mechanisms to design advanced HEAs with superior mechanical properties. 7



4. Conclusions The mechanical properties and deformation mechanisms of a PR CoCrFeMnNi HEA at 293 K and 77 K were investigated. In contrast to the high yield strength of 1210 MPa but limited uniform elongation of 1.1% at 293 K, the PR specimen exhibits unprecedented ultrahigh yield strength of 1692 MPa and a considerable uniform elongation of 10.3% at 77 K. The limited uniform elongation at 293 K is determined by the restrained strain-hardening capability via dislocation slip in the recrystallized grains. In contrast, multiple deformation mechanisms of dislocation slip and deformation twinning are introduced in recrystallized grains at 77 K, leading to the enhanced strain-hardening capability. This work has shed light on the design of adaptive HEAs with PR microstructures for practical applications in cryogenic industries.

Acknowledgements Y.Z.T. acknowledges the IMR Foundation for “Young Merit Scholars”. This work was supported by the National Natural Science Foundation of China (NSFC) under grant Nos. 51501198 and 51331007 and Bureau of Shenyang science and technology (17-107-6-00).

Reference [1] B. Cantor, I.T.H. Chang, P. Knight, A.J.B. Vincent, Microstructural development in equiatomic multicomponent alloys, Mater. Sci. Eng. A 375–377 (2004) 213-218. [2] 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 utcomes, Adv. Eng. Mater. 6 (2004) 299-303. [3] D.B. Miracle, O.N. Senkov, A critical review of high entropy alloys and related concepts, Acta Mater. 122 (2017) 448-511. [4] W. Zhang, P.K. Liaw, Y. Zhang, Science and technology in high-entropy alloys, Sci. China Mater. 61 (2018) 2-22. [5] C.L. Wu, S. Zhang, C.H. Zhang, H. Zhang, S.Y. Dong, Phase evolution and cavitation erosioncorrosion behavior of FeCoCrAlNiTix high entropy alloy coatings on 304 stainless steel by laser surface alloying, J. Alloys Compd. 698 (2017) 761-770. [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] Z. Li, K.G. Pradeep, Y. Deng, D. Raabe, C.C. Tasan, Metastable high-entropy dual-phase alloys overcome the strength–ductility trade-off, Nature 534 (2016) 227-230. [8] Z. Tang, T. Yuan, C.-W. Tsai, J.-W. Yeh, C.D. Lundin, P.K. Liaw, Fatigue behavior of a wrought Al0.5CoCrCuFeNi two-phase high-entropy alloy, Acta Mater. 99 (2015) 247-258. [9] G. Qin, S. Wang, R. Chen, X. Gong, L. Wang, Y. Su, J. Guo, H. Fu, Microstructures and mechanical properties of Nb-alloyed CoCrCuFeNi high-entropy alloys, J. Mater. Sci. Technol. 34 (2018) 365369. [10] Y. Lu, X. Gao, Y. Dong, T. Wang, H.-L. Chen, H. Maob, Y. Zhao, H. Jiang, Z. Cao, T. Li, S. Guo, Preparing bulk ultrafine-microstructure high-entropy alloys via direct solidification, Nanoscale 10 (2018) 1912-1919. [11] Z. Fu, B.E. MacDonald, Z. Li, Z. Jiang, W. Chen, Y. Zhou, E.J. Lavernia, Engineering heterostructured grains to enhance strength in a single-phase high-entropy alloy with maintained ductility, Mater. Res. Lett. 6 (2018) 634-640.

8



[12] B. Gludovatz, A. Hohenwarter, D. Catoor, E.H. Chang, E.P. George, R.O. Ritchie, A fractureresistant high-entropy alloy for cryogenic applications, Science 345 (2014) 1153-1158. [13] D. Li, C. Li, T. Feng, Y. Zhang, G. Sha, J.J. Lewandowski, P.K. Liaw, Y. Zhang, High-entropy Al0.3CoCrFeNi alloy fibers with high tensile strength and ductility at ambient and cryogenic temperatures, Acta Mater. 123 (2017) 285-294. [14] Y.H. Jo, S. Jung, W.M. Choi, S.S. Sohn, H.S. Kim, B.J. Lee, N.J. Kim, S. Lee, Cryogenic strength improvement by utilizing room-temperature deformation twinning in a partially recrystallized VCrMnFeCoNi high-entropy alloy, Nat. Commun. 8 (2017) 15719. [15] F. Otto, A. Dlouhý, C. Somsen, H. Bei, G. Eggeler, E.P. George, The influences of temperature and microstructure on the tensile properties of a CoCrFeMnNi high-entropy alloy, Acta Mater. 61 (2013) 5743-5755. [16] B. Schuh, F. Mendez-Martin, B. Völker, E.P. George, H. Clemens, R. Pippan, A. Hohenwarter, Mechanical properties, microstructure and thermal stability of a nanocrystalline CoCrFeMnNi highentropy alloy after severe plastic deformation, Acta Mater. 96 (2015) 258-268. [17] J.Y. He, H. Wang, H.L. Huang, X.D. Xu, M.W. Chen, Y. Wu, X.J. Liu, T.G. Nieh, K. An, Z.P. Lu, A precipitation-hardened high-entropy alloy with outstanding tensile properties, Acta Mater. 102 (2016) 187-196. [18] H. Shahmir, J. He, Z. Lu, M. Kawasaki, T.G. Langdon, Effect of annealing on mechanical properties of a nanocrystalline CoCrFeNiMn high-entropy alloy processed by high-pressure torsion, Mater. Sci. Eng. A 676 (2016) 294-303. [19] N. Stepanov, M. Tikhonovsky, N. Yurchenko, D. Zyabkin, M. Klimova, S. Zherebtsov, A. Efimov, G. Salishchev, Effect of cryo-deformation on structure and properties of CoCrFeNiMn high-entropy alloy, Intermetallics 59 (2015) 8-17. [20] Z. Fu, W. Chen, H. Wen, D. Zhang, Z. Chen, B. Zheng, Y. Zhou, E.J. Lavernia, Microstructure and strengthening

mechanisms

in

an

FCC

structured

single-phase

nanocrystalline

Co25Ni25Fe25Al7.5Cu17.5 high-entropy alloy, Acta Mater. 107 (2016) 59-71. [21] H. Shahmir, J. He, Z. Lu, M. Kawasaki, T.G. Langdon, Evidence for superplasticity in a CoCrFeNiMn high-entropy alloy processed by high-pressure torsion, Mater. Sci. Eng. A 685 (2017) 342-348. [22] I.A. Ovid'ko, R.Z. Valiev, Y.T. Zhu, Review on superior strength and enhanced ductility of metallic nanomaterials, Prog. Mater. Sci. 94 (2018) 462-540. [23] T.H. Fang, W.L. Li, N.R. Tao, K. Lu, Revealing extraordinary intrinsic tensile plasticity in gradient nano-grained copper, Science 331 (2011) 1587-1590. [24] E. Ma, T. Zhu, Towards strength–ductility synergy through the design of heterogeneous nanostructures in metals, Mater. Today 20 (2017) 323-331. [25] X. Wu, M. Yang, F. Yuan, G. Wu, Y. Wei, X. Huang, Y. Zhu, Heterogeneous lamella structure unites ultrafine-grain strength with coarse-grain ductility, Proc. Nat. Acad. Sci. U. S. A. 112 (2015) 1450114505. [26] J. Li, Y. Cao, B. Gao, Y. Li, Y. Zhu, Superior strength and ductility of 316L stainless steel with heterogeneous lamella structure, J. Mater. Sci. 53 (2018) 10442-10456. [27] G. Dini, A. Najafizadeh, R. Ueji, S.M. Monir-Vaghefi, Improved tensile properties of partially recrystallized submicron grained TWIP steel, Mater. Lett. 64 (2010) 15-18. [28] Y.Z. Tian, S. Gao, L.J. Zhao, S. Lu, R. Pippan, Z.F. Zhang, N. Tsuji, Remarkable transitions of yield behavior and Lüders deformation in pure Cu by changing grain sizes, Scr. Mater. 142 (2018) 88-91. [29] Y.M. Wang, E. Ma, R.Z. Valiev, Y.T. Zhu, Tough nanostructured metals at cryogenic temperatures, Adv. Mater. 16 (2004) 328-331.

9



[30] S.J. Sun, Y.Z. Tian, H.R. Lin, X.G. Dong, Y.H. Wang, Z.J. Zhang, Z.F. Zhang, Enhanced strength and ductility of bulk CoCrFeMnNi high entropy alloy having fully recrystallized ultrafine-grained structure, Mater. Des. 133 (2017) 122-127. [31] Y.Z. Tian, Y. Bai, M.C. Chen, A. Shibata, D. Terada, N. Tsuji, Enhanced strength and ductility in an ultrafine-grained Fe-22Mn-0.6C austenitic steel having fully recrystallized structure, Metall. Mater. Trans. A 45 (2014) 5300-5304. [32] S.J. Sun, Y.Z. Tian, H.R. Lin, H.J. Yang, X.G. Dong, Y.H. Wang, Z.F. Zhang, Transition of twinning behavior in CoCrFeMnNi high entropy alloy with grain refinement, Mater. Sci. Eng. A 712 (2018) 603-607. [33] D.T. Pierce, J.A. Jiménez, J. Bentley, D. Raabe, J.E. Wittig, The influence of stacking fault energy on the microstructural and strain-hardening evolution of Fe–Mn–Al–Si steels during tensile deformation, Acta Mater. 100 (2015) 178-190. [34] S. Allain, O. Bouaziz, J.P. Chateau, Thermally activated dislocation dynamics in austenitic FeMnC steels at low homologous temperature, Scr. Mater. 62 (2010) 500-503. [35] L. Lu, X. Chen, X. Huang, K. Lu, Revealing the maximum strength in nanotwinned copper, Science 323 (2009) 607-610. [36] H. Beladi, I.B. Timokhina, Y. Estrin, J. Kim, B.C. De Cooman, S.K. Kim, Orientation dependence of twinning and strain hardening behaviour of a high manganese twinning induced plasticity steel with polycrystalline structure, Acta Mater. 59 (2011) 7787-7799. [37] Z. Zhang, M.M. Mao, J. Wang, B. Gludovatz, Z. Zhang, S.X. Mao, E.P. George, Q. Yu, R.O. Ritchie, Nanoscale origins of the damage tolerance of the high-entropy alloy CrMnFeCoNi, Nat. Commun. 6 (2015) 10143. [38] Z.F. Zhang, G. He, J. Eckert, L. Schultz, Fracture mechanisms in bulk metallic glassy materials, Phys. Rev. Lett. 91 (2003) 045505. [39] Y.Z. Tian, L.J. Zhao, N. Park, R. Liu, P. Zhang, Z.J. Zhang, A. Shibata, Z.F. Zhang, N. Tsuji, Revealing the deformation mechanisms of Cu–Al alloys with high strength and good ductility, Acta Mater. 110 (2016) 61-72. [40] J. Yoo, K. Choi, A. Zargaran, N.J. Kim, Effect of stacking faults on the ductility of Fe-18Mn-1.5Al0.6C twinning-induced plasticity steel at low temperatures, Scr. Mater. 137 (2017) 18-21. [41] G. Laplanche, A. Kostka, C. Reinhart, J. Hunfeld, G. Eggeler, E.P. George, Reasons for the superior mechanical properties of medium-entropy CrCoNi compared to high-entropy CrMnFeCoNi, Acta Mater. 128 (2017) 292-303. [42] B. Gludovatz, A. Hohenwarter, K.V.S. Thurston, H. Bei, Z. Wu, E.P. George, R.O. Ritchie, Exceptional damage-tolerance of a medium-entropy alloy CrCoNi at cryogenic temperatures, Nat. Commun. 7 (2016) 10602. [43] J. Miao, C.E. Slone, T.M. Smith, C. Niu, H. Bei, M. Ghazisaeidi, G.M. Pharr, M.J. Mills, The evolution of the deformation substructure in a Ni-Co-Cr equiatomic solid solution alloy, Acta Mater. 132 (2017) 35-48. [44] D. Li, Y. Zhang, The ultrahigh charpy impact toughness of forged AlxCoCrFeNi high entropy alloys at room and cryogenic temperatures, Intermetallics 70 (2016) 24-28. [45] T. Bhattacharjee, R. Zheng, Y. Chong, S. Sheikh, S. Guo, I.T. Clark, T. Okawa, I.S. Wani, P.P. Bhattacharjee, A. Shibata, N. Tsuji, Effect of low temperature on tensile properties of AlCoCrFeNi2.1 eutectic high entropy alloy, Mater. Chem. Phys. 210 (2018) 207-212. [46] 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.

10