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Sunflower-like Solidification Microstructure in a Near-eutectic High-entropy AlloySheng Guoa, Chun Ngb & C.T. Liuca Surface and Microstructure Engineering Group, Department of Materials and ManufacturingTechnology, Chalmers University of Technology, SE-412 96, Gothenburg, Swedenb Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hung Hom,Kowloon, Hong Kong, People's Republic of Chinac Center of Advanced Structural Materials, Department of Mechanical and BiomedicalEngineering, City University of Hong Kong, Kowloon, Hong Kong, People's Republic of ChinaPublished online: 27 Sep 2013.

To cite this article: Sheng Guo, Chun Ng & C.T. Liu (2013) Sunflower-like Solidification Microstructure in a Near-eutectic High-entropy Alloy, Materials Research Letters, 1:4, 228-232, DOI: 10.1080/21663831.2013.844737

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Mater. Res. Lett., 2013Vol. 1, No. 4, 228232, http://dx.doi.org/10.1080/21663831.2013.844737

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Sunflower-like Solidification Microstructure in a Near-eutectic High-entropy Alloy

Sheng Guoa, Chun Ngb and C.T. Liuc

aSurface and Microstructure Engineering Group, Department of Materials and Manufacturing Technology,Chalmers University of Technology, SE-412 96, Gothenburg, Sweden; bDepartment of Mechanical Engineering,

The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, Peoples Republic of China;cCenter of Advanced Structural Materials, Department of Mechanical and Biomedical Engineering,

City University of Hong Kong, Kowloon, Hong Kong, Peoples Republic of China

(Received 19 August 2013; nal form 10 September 2013 )

Considering the inherent advantages of eutectic alloys as high-temperature materials, eutectic high-entropy alloys provide a brandnew research direction for developing materials to be used in high-temperature environments. Along this line of thinking, thesolidication microstructure in a near-eutectic Al2CrCuFeNi2 alloy was studied. A unique sunower-like eutectic colony structurewas observed, where theNiAl-rich B2 phase formed as the primary phasewith a spherical or ellipsoidalmorphology, theNiAl-richB2/Cr-rich A2 eutectics grew on the primary phase in a radial manner, and the primary B2 phase further decomposed into nearlycubic particles dispersed in the matrix at lower temperatures.

Keywords: High-Entropy Alloy, Eutectics, Solidication Microstructure, Non-Equilibrium Solidication

Introduction Eutectic alloys are important in manyaspects, including controllable near-equilibriummicrostructures that can resist change up to thereaction temperature, high rupture strength, good high-temperature creep resistance, and sometimes inter-esting and unusual electrical, magnetic, and opticalproperties.[1] Naturally, eutectic alloys are good candi-date materials to be used in high-temperature environ-ments. A representative example is the NiAlCr eutecticalloy, whereNiAl has excellent oxidation resistance, highmelting temperature, high thermal conductivity, and rel-atively low density, while Cr provides enhancements inboth the toughness and the creep strength for the eutecticcomposite material.[2,3] It is also widely known that theaddition of some minor elements, such as Mo, Fe, and W,can aect the microstructure and orientation of the NiAland Cr phases.[2]

High-entropy alloys (HEAs), or multicomponentalloys with equiatomic or close-to-equiatomic com-positions, are newly emerging metallic materials thathave great potential to be used in high-temperatureenvironments.[47] A distinctive feature of HEAs is thestabilization of the solid solution phases by the high con-guration entropy at elevated temperatures,[8] and the

Corresponding author. Email: sheng.guo@chalmers.se

usually simple phase constitutions [9,10] even in such ahighly concentrated multi-component alloy system. Twonotes are raised here for clarication. First, solid solutionsare not necessarily the sole alloying products in HEAs.Depending on the alloy compositions, intermetallic com-pounds can form, and the amorphous phase can alsoform.[912] Second, the solid solutions in the cast HEAsare normally non-equilibrium phases. They are actuallythe rst formed solid phases upon solidication, and arekept to the ambient temperature due to the sluggish diu-sion ofHEAs[13,14]: the time scale of the cooling processis relatively too short for the phase transformation towardits equilibrium state. It is also that this sluggish diu-sion eect that leads to the exceptional high-temperaturestrength and structural stability of HEAs.[7,15] Theeutectic HEAs seem to be an interesting idea for design-ing new high-temperature materials, if the advantages ofboth HEAs and the eutectics can be utilized. Previouswork on eutectic HEAs has been sporadic,[1619] anda comparative study of eutectic HEAs to conventionaleutectic high-temperature alloys is missing. Here, weused a recently developed near-eutectic Al2CrCuFeNi2HEA [20,21] as an example, to compare its solidicationstructure with the NiAlCr eutectic alloys. The formation

2013 The Author(s). Published by Taylor & Francis.This is anOpenAccess article distributed under the termsof theCreativeCommonsAttributionLicense (http://creativecommons.org/licenses/by/3.0),which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The moral rights of thenamed author(s) have been asserted.

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mechanism of the unique sunower-like microstructurein Al2CrCuFeNi2 was given particular attention.

Experimental The Al2CrCuFeNi2 alloy was preparedby arc-melting a mixture of constituent elements withpurity higher than 99.9wt% in a Ti-gettered high-purityargon atmosphere. Subscripts in the alloy compositionsindicate the atomic portion of each individual element.The melting was repeated ve times to improve thechemical homogeneity, and nally the molten alloywas drop-cast into a 10mm diametered copper mold.The phase constitutions of the alloy were examined bythe X-ray diractometer (XRD) using the Co radiation(Bruker AXS D8 Discover). To facilitate the microstruc-ture observation, the sample surfaces were sequentiallypolished down to the 0.3m grit alumina suspen-sion furnish, and then electrochemically etched usingthe Cica-Reagent Electrolyte A (Kanto Chemical). Themicrostructures of the alloys were examined using theeld emission scanning electron microscope (FE-SEM,Zeiss 1550VP, equippedwith anX-ray energy-dispersivespectrometer (EDS)). Transmission electron microscope(TEM) specimens were prepared by mechanical thin-ning followed by the twin-jet electrochemical polishingwith the 10 vol% HClO4-90 vol% ethanol solution. Thechemical composition was analyzed using the TEM-EDS(Philips CM20) method.

Results and discussion Themicrostructure of the alloyis given in Figure 1(a), where copious eutectic grains orcolonies can be seen. According to Li and Kuribayashi[22] when discussing the free solidication behavior ofundercooled eutectics, a single eutectic colony shall bethe basic unit rather than the bulk. This methodologywas adopted here to study the eutectic structure. Essen-tially, each eutectic colony has a similar microstructurein that the lamellae grow on the spherical or ellipsoidalphase in a radial manner, and copious nearly cubic par-ticles appear within the spherical or ellipsoidal phase. Atypical microstructure of the eutectic colony is shown inFigure 1(b). Interestingly, it resembles much the struc-ture of a sunower. The lamellae constitute the petalsand inter-petals, the spherical phase as the disk oretand the particles form the seeds. This sunower-likemicrostructure can basically represent the morphology ofall the eutectic colonies, with the main dierences amongcolonies being the size and shape of the disk-oret phase,and whether the collision of neighboring colonies wouldprevent the full development of an individual colony. Forthe eutectic colony shown in Figure 1(b), the disk-oretphase has a diameter of4m, the petals are1.4m inlength and 300 nm in width, with the spacing betweenpetals ranging from75 to350 nm, and the seeds havean average size of 230 nm. Depending on the growthcondition (for both the oret disk and the petals), these

Figure 1. (color online) (a) Secondary electron image of theetched Al2CrCuFeNi2 alloy and (b) the enlarged view of atypical sunower-like microstructure. The inset in (a) gives theXRD pattern for the alloy.

sizes vary among dierent eutectic colonies but thesemeasurements give an idea of the magnitude.

Figure 2 shows an SEM-EDS map for a typicaleutectic colony, and it gives a glimpse of the elemen-tal distribution for the eutectic structure. The petals andseeds are both enriched in Cr and Fe, while the inter-petals and the disk oret are both enriched in Ni and Al.Interestingly, the seeds form in a concentric region to thedisk oret, which is highlighted in the secondary electronimage. The preliminary chemical information indicatesthat the petals and seeds could be the same phase, theinter-petals and the disk oret also belong to the samephase. To conrm this conjecture, the chemical composi-tions were analyzed by TEM-EDSwith amuch improvedspatial resolution (with the electron beam size of 40 nm),and the results are given in Table 1. Indeed, the inter-petals and the disk oret (matrix, excluding the seeds)have almost the exact chemical compositions. A morecareful observation of their compositions suggests thatthey can bewritten in the form of (Ni, Fe, Cr)50(Al, Cu)50,orwith the composition of aNiAl-type solid solution. Thepetals and seeds do not have the same compositions, butthey are both enriched in Cr and Fe, and the enrichment ismore signicant in the petals. Considering that the XRD

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Figure 2. (color online) SEM-EDS elemental map of a typicalsunower-like microstructure. The dash-dotted lines imposedon the image are drawn to guide the eyes.

analysis only shows the existence of two phases in thisalloy, bcc (A2) and ordered bcc (B2) phase, the abovechemical analysis leads us to presume that the petals andseeds have the A2 structure, while the inter-petals and thedisk-oret matrix have the B2 structure. This presump-tion is further supported by the selected area diractionpatterns (SADP). Figure 3(a) shows the morphology ofa eutectic colony under TEM, where the interface of theoret disk and the radially grown petals on the oret diskcan be clearly observed. The SADPs for both the seedsand the petals, shown in Figures 3(b) and 3(c), conrmthat they have the A2 structure.

Combining the XRD analysis, the microstructuralfeatures, the chemical composition, together with thestructural characterization by TEM, we came up withthe formation mechanism of the unique sunower-likemicrostructure, shown in Figure 4. During the solid-ication, the B2 phase with the composition of (Ni,Fe, Cr)50(Al, Cu)50 forms rst from the liquid phaseas the primary phase. The primary phase grows almostisotropically, not forming a dendritic structure. When

Figure 3. (color online) (a) TEM image showing the sun-ower-like microstructure, with some sketch lines drawn tohighlight the shape of the petals and seed particles. (b) and(c) are representative SADPs for the seed parties and petals.

reaching the eutectic temperature, the alternating B2 andA2 phases form the lamellae growing on the primaryphase, and the growth direction of the lamellae is nor-mal to the boundary of the primary phase, thus forminga radial pattern. A eutectic colony is then formulated andthe growth of each eutectic colony stops when the solid-ication ends or it collides with neighboring colonies.The nearly cubic A2 phases form at a lower temperature,decomposing from the primary B2 phase, very possiblydue to the occurrence of the spinodal decomposition. Theseparation of the NiAl-rich B2 phase (matrix) and theCrFe-rich A2 phase (seeds) seems to lend support to thishypothesis. Spinodal decompositions forming the com-positionally dierent A2 and B2 phases are frequentlyseen in HEAs,[23,24] and it is interesting to note that inthe AlCoCrCuFeNi HEA, the products of the spinodaldecomposition are also NiAl-rich B2 phase and CrFe-rich A2 phase.[24] The cubic form of the A2 phase,rather than the modulated plates as in References,[23,24]shall originate from the need tominimize the elastic strain

Table 1. TEM-EDS analysis for the sunower-like structure.

Element (at%)

Region Al Cr Cu Fe Ni Comment

Nominal 28.57 14.28 14.28 14.28 28.57Petal 3.63 53.65 1.27 38.15 3.30 (Cr, Fe) rich, A2 phaseInter-petal 35.08 1.37 15.43 7.01 41.12 (Ni, Fe, Cr)50(Al, Cu)50, B2 phaseDisk-oret: matrix 35.55 1.29 16.54 6.27 40.35 (Ni, Fe, Cr)50(Al, Cu)50, B2 phaseDisk-oret: seed 15.26 32.64 6.30 29.69 16.11 (Cr, Fe) rich, A2 phase

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Figure 4. (color online) Sketch showing the proposed forma-tion mechanism of the sunower-like structure. Symbols usedhere: L for the liquid phase, for the primary B2 phase and theB2 phase in the lamellae, for the A2 phase in the lamellae, and for the B2 and A2 phase out of the spinodal decom-position, respectively, dierentiating them with the B2 and A2phase mentioned above.

energy.[25] The nature of the connement of the regionwhere the spinodal decomposition occurs, for exampleto within the region circled by the dash-dotted lines inFigure 2, is unclear but it could be related to the boundaryeect, as the spinodal decomposition free region locatesnicely along the phase boundary between the primaryphase and the eutectics.

It is interesting to recognize that the sunower-like microstructure formed in this near-eutecticAl2CrCuFeNi2 HEA bears much similarity to thoseformed in some hypo-eutectic NiAlCr alloys. For exam-ple, a similar structure was seen in an induction melt anddrop-cast 33Ni32Al28Cr3Mo4Ti (at%) alloy,[26]and the phase constitutions of the structural features arealmost close to those in the HEA: NiAl phase forming theprimary phase, alternating NiAlCr(Mo) eutectic lamel-lae grow on the primary NiAl phase in a radial manner,with the Cr(Mo) phase being the petals, and Cr(Mo) par-ticles forming within the primary NiAl phase. However,the sizes of the structural features in the 33Ni32Al28Cr3Mo4Ti alloy are much coarser than those in theHEA. In the former, the petals are 50m in lengthand the seeds are about 1m. It is also worthy point-ing out that in some hyper-eutectic NiAlCr alloy wherethe Cr(Mo) phase was the primary phase, NiAl haloswere present surrounding the primary Cr(Mo) phase,and the eutectics then grew over the halos.[2] Accord-ing to the non-reciprocal nucleation principle,[27] noCr(Mo) halos would form on the primary NiAl phase,and this was experimentally conrmed.[28] The point ofmentioning this is to exclude the possibility of the haloformation in the current HEA, and to state that the spin-odal decomposition-free region, as seen in Figure 2, isnot the halo. NiAlCr alloys are the generally recognizedas a pseudo-binary eutectic system,[29] the solidicationbehavior in the near-eutectic NiAlCr alloys is not sur-prising. However, the solidication of the Al2CrCuFeNi2HEAalso follows a pseudo-binary eutecticmannerwhichis somewhat unexpected, considering such a highly con-centrated alloy system and the high doping of elementsother than Ni, Al and Cr (Table 1). This phenomenoncan possibly be rationalized by two points: the enhanced

solid solutioning by the high conguration entropy at ele-vated temperatures and the non-equilibrium nature of theachieved pseudo-binary eutectics in the HEA. The signif-icance of knowing this structural similarity between theAl2CrCuFeNi2 HEA and NiAlCr alloys is that previousknowledge to optimize the mechanical properties of thelatter can now be transplanted to the former, which wouldcertainly simplify the alloy design for desirable eutecticHEAs.

Theobservationof sunower-like eutecticmicrostruc-ture is rather rare in the literature. To the best knowledgeof the authors, the most similar structures are only seen insome NiAlCr(Mo) hypo-eutectic alloys,[26,30] whichare also essentially equiatomic ternary alloys doped withsmall amounts of alloying elements such as Mo, Ti andrare-earth elements. To achieve the universal sunower-like solidication microstructure as seen in Figure 1(a),all the following four conditions need to be satisedsimultaneously: (1) the existence of copious nucleationsites for the primary phases;[22] (2) the almost isotropicgrowth of the primary phase; (3) the eutectic lamellaegrowing normal to the phase boundary of the primaryphase, hence forming a radial pattern; and (4) the primaryphase decomposing to form copious discontinuous parti-cles at lower temperatures. Certainly, it is highly demand-ing to obtain the perfect sunower-like microstructure,in terms of the alloy compositions [2,21] and solidica-tion conditions.[2,3133] Whether the appearance of asunower-like structure bears any relation to the specicalloy compositions, or in another word whether such aunique structure can be achieved in other HEAs or multi-component alloy systems, for example, when the alloycompositions are not highly concentrated, or when theeutectics are no longer the NiAl-like B2 phase and Cr-rich A2 phase are interesting topics to be explored in thefuture.

Conclusions In summary, the solidication micro-structure in a near-eutectic Al2CrCuFeNi2 HEA wasstudied, with a particular attention to the formation ofthe unique sunower-like eutectic colony structure. Dur-ing the solidication process, the NiAl-rich B2 phaseformed as the primaryphasewith a spherical or ellipsoidalmorphology, followed by the NiAl-rich B2/Cr-rich A2eutectics growingon the primary phase in a radialmanner,and nally the primary B2 phase decomposed at lowertemperatures leading to the formation of copious nearlycubic particles, very possibly via the spinodal decomposi-tion. The structural similarity to NiAlCr hypo-eutecticspaves the way to further optimize the near-eutectic HEAsfor high-temperature applications.

Acknowledgements SG thanks the initiation funding fromthe Area of Advance Materials Science at Chalmers Univer-sity of Technology. The research of CTL was supported by

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Hong Kong RGC Grant CityU/521411 at City University ofHong Kong, P. R. China.

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IntroductionExperimentalResults and discussionConclusionsAcknowledgementsReferences