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Contents lists available at ScienceDirectJournal of Alloys and Compoundsjournal homepage: www.elsevier.com/locate/jalcomMicrostructure evolution and mechanical properties of atmosphereplasma sprayed AlCoCrFeNi high-entropy alloy coatings under postannealingLiangquan Wanga, Fanyong Zhanga,⁎, Shu Yanb,⁎⁎, Guangxing Yua, Jiawen Chena, Jining Hea,Fuxing Yinaa Tianjin Key Laboratory of Materials Laminating Fabrication and Interface Control Technology, Research Institute for Energy Equipment Materials, School ofMaterial Science and Engineering, Hebei University of Technology, Tianjin 300130, Chinab School of Metallurgy, Northeastern University, Shenyang, Liaoning 110819, Chinaa r t i c l e i n f oArticle history:Received 29 December 2020Received in revised form 13 March 2021Accepted 19 March 2021Available online 24 March 2021Keywords:Plasma sprayingHigh entropy alloyCoatingsMicrostructuresMechanical propertiesa b s t r a c tIn this work, atmosphere plasma spraying (APS) was employed to fabricate thick AlCoCrFeNi high entropyalloy (HEA) coatings from commercial HEA powders. Effects of annealing treatments on the phase transformation, microstructure and mechanical properties of HEA coating were systematically investigated. Resultsshowed that the as-sprayed HEA coating exhibited typical lamellar microstructures with some oxidationstrips. The as-sprayed coating was composed of two BCC phases-Fe-Cr rich (A2) and Al-Ni rich phase (B2). Thetransformation from BCC to FCC structure (A1) occurred during post-annealing, which became more apparentwith increasing temperature in the range of 600–900 °C. The rod-shape A2 phase was embedded in theB2 matrix and merged to grow larger with increasing temperature. EPMA analysis revealed that the diffusionof Al and Cr made A2 phase of (Fe, Cr) separate from B2 phase of (Al, Ni), accompanying with the formation ofA1 phase (Fe, Ni). Microhardness and nano-indentation test showed that the hardness and elastic modulus ofas-annealed coatings reached the peak value at 600 °C (576 HV) and 700 °C (232.4 GPa), respectively, thendecreased with further increasing temperature (800–1000 °C). The mechanical performance showed closerelationship with the volume fraction change of A2 and B2 phase.© 2021 Elsevier B.V. All rights reserved.1. IntroductionHigh entropy alloy (HEA), defined as a multicomponent alloy, hasbeen proposed by Ye [1] and Cantor [2] in 2004 and attracted increasing attentions. HEAs are usually composed of five or moreelements with equimolar or near-equimolar composition (rangefrom 5 to 35 at%). Due to the high mixing entropy, these alloys tendto form a solid solution structure instead of intermetallic compounds [3]. The microstructural and componential characteristicsmake HEAs exhibit four key effects, i.e., high entropy effect, latticedistortion, sluggish diffusion effect and cocktail effect [4,5]. Underthe cocktail effect, the overall properties of high entropy alloys depend on the difference in atomic sizes, crystal structure andchemical bonding of the constituent elements and the mutual interaction among constituent elements [6]. The HEAs exhibit manydesirable properties, such as high hardness [7], good wear resistance[8] and superior corrosion resistance [9]. Thus, HEAs wouldbe promising candidates of high-performance coatings in surfaceengineering.As a typical class of HEA system, AlCoCrFeNi has got extensiveinvestigation and it usually consists of Fe-Cr-rich solid solutionphase (disordered BCC), Al-Ni-rich phase (ordered BCC) and Fe-CrNi-rich structure (disordered FCC). The transformation from BCC toFCC or σ structure would occur under high-temperature annealing[10,11]. As reported, AlCoCrFeNi alloy showed high strength, highhardness, and excellent softening resistance [7,12]. The hardness andyield strength of nano-crystalline AlCoCrFeNi alloy could reach919 HV and 2700 MPa, respectively [13]. Furthermore, its microstructure and mechanical properties could be tailored by alloyingwith metallic elements like Ti, Mo, Nb and Si [14–17]. In addition, theAlCoCrFeNi was reported to show strong oxidation resistance andstructural stability at 1050 °C [18]. Compared with conventionalhttps://doi.org/10.1016/j.jallcom.2021.1596070925-8388/© 2021 Elsevier B.V. All rights reserved.]]]]]] ]]]]⁎ Correspondence to: Research Institute for Energy Equipment Materials, HebeiUniversity of Technology, No. 8 Guangrong Road, Hongqiao District, Tianjin 300130,China.⁎⁎ Corresponding author.E-mail addresses: [email protected] (F. Zhang), [email protected] (S. Yan).Journal of Alloys and Compounds 872 (2021) 159607NiCoCrAl alloy, AlCoCrFeNi exhibited superior oxidation resistancedue to the lower oxidation rate and better inhibition of interfacerumpling [19].Recently, several technologies have been developed to produceAlCoCrFeNi alloy coatings or films, including magnetron sputtering[20–22], laser cladding [23–25] and thermal spraying [26] (Table 1).The sputtered HEA films usually have nano-crystal structure andpreferred growth orientation, thus yield high hardness [20]. In Liao’swork [21], the hardness of AlxCoCrFeNi films reached 12.28 GPa.Khan [22] found that the deposited AlCoCrCu0.5FeNi film with highAl content showed higher hardness. The deposited HEA film couldreach a smooth and relatively compact layer, but have limitedthickness. Several AlCoCrFeNi based HEA coatings have been fabricated by laser cladding, which usually show dendrite and needle-likemicrostructure. The cladding HEA layer comprised a mixture of Fe-Crrich A2 and Al-Ni rich B2 phases. The variation of alloying elements(Si, Fe, Ti, etc.) was adjusted to control the A2/B2 morphology incladding HEA layers [17]. With the demand for industrial environments, thick and large-scale coatings are required, which could beachieved by thermal spraying.As a well-established technology, thermal spraying shows theadvantage to fast produce various metal and ceramic coatings forindustrial applications. High velocity oxygen fuel spraying (HVOF)and atmosphere plasma spraying (APS) are two most used sprayingmethod. The powder feedstock is melted in the flame to form moltendroplets, then flight and impact the substrate, solidify to generate alayered structure [27]. This distinctive microstructure endows thedesirable mechanical properties such as wear and oxidation behavior under extreme environments [25,26]. Srivastava [28] developeda dense and thick AlCoCrFeNi2 coating (~200 µm) by using HVOFtechnology. The coating contained major FCC phase and minor BCCphase with some oxides, which had the hardness of 600 HV andexhibited good high temperature resistance. As reported [29], thegas-atomized feedstock could achieve a denser coating with lowoxidation. Compared with HVOF, plasma spraying could producehigher flame temperature and cooling rate, as well as wider rangesof spraying materials. Recently, Meghwal [30] prepared AlCoCrFeNicoating (~400 µm) by plasma spraying from mechanically alloyedHEA powders. The coating was composed of FCC and two kinds ofBCC, with excellent wear resistance, thermal stability (500 °C) andcorrosion resistance. The phase fraction in plasma spraying AlCoCrFeNi coating could be controlled by adjusting the particle size of rawpowder. Cheng [31] found that the finer particle size used in feedstock led to forming less FCC phase in sprayed AlCoCrFeNi coating.Even with coarse powders, the BCC phase remained stable at higherspray current. Shi [32] reported the AlCoCrFeNi HEA matrix composite coatings with the addition of Ag and BaF2/CaF2 by using APS.The lubricating additives alleviated the abrasion wear and decreasedthe wear rate of the HEA coating under high temperature. Actually,thermal sprayed AlCoCrFeNi coatings are usually composed ofmultiphase microstructure with local disparity [33]. Thus, it wouldbe necessary to understand the connection between the microstructure evolution and mechanical performance of HEA coatings.However, thermal sprayed coatings usually contain defects such aslocal segregation, pores and oxidation. The post annealing processcould be an effective route to improve the microstructure and enhance the mechanical properties of sprayed coatings [34–36]. As forAlCoCrFeNi alloy, the microstructure has no obvious change whenannealed below 600 °C, while phase transformation usually occurs at600–1100 °C. In this range, BCC phase transformed into the hard andbrittle σ phase. Furthermore, σ phase would change back to BCCstructure when annealed at 1100 °C [37]. Generally, the increase ofBCC phase could greatly increase the hardness of plasma sprayedAlCoCrFeNi coatings [38]. In Löbel’s study [39], the HVOF-spayed AlCoCrFeNi coating formed a duplex structure comprising large fractionof FCC phase and achieved more uniform grain size after annealing at1050 °C. The ductile FCC phase decreased the hardness but improvedthe wear resistance of HEA coatings. These results inspired us tofurther study the phase structure of sprayed AlCoCrFeNi coatings.In this study, plasma spraying was employed to fabricateAlCoCrFeNi HEA coatings using commercial gas-atomized HEApowders. Then, annealing treatment was conducted on the sprayedHEA coatings at different temperatures to understand the evolutionof microstructures and mechanical properties of AlCoCrFeNi coating.The phase transformation law and its relation to mechanical properties change of HEA coating were investigated.2. Experimental details2.1. Materials and methodsCommercial AlCoCrFeNi powders (Beijing Yanbang New MaterialTechnology Co., Ltd.) prepared by gas atomization were used asfeedstock. The AISI 1045 steel specimens were degreased and gritblasted for spraying. Table 2 shows the plasma spraying parameters.Firstly, the Ni-10 wt% Al self-fluxing alloy powders were pre-deposited onto the substrate as bond layer. Then, AlCoCrFeNi coatingwas sprayed using H2 and Ar as process atmosphere to controlspraying power and current, and Ar was using as carrier gas. The assprayed HEA samples were annealed at 600, 700, 800, 900 and1000 °C under an Ar atmosphere. Samples were placed in a tubefurnace, heated to the required temperature at a rate of 10 °C/min,holding for 4 h and then naturally cooled to room temperature in thefurnace. The as-sprayed and annealed coatings were labeled as H0,H600, H700, H800, H900 and H1000.Table 1Synthesis and properties of AlCoCrFeNi based on HEA coatings.System Phase Properties Method Ref.AlxCoCrFeNi A2 + B2 + FCC Nanohardness(7.13–10.25 GPa), resistivity (191.8–535.9 μΩ cm) MS [20]Al0.3CoCrFeNi BCC + FCC Nanohardness (11.2 GPa), elastic modulus (191 GPa) MS [21]AlCoCrFeNiCu0.5 BCC + FCC Hardness (13 GPa) MS [22]AlCoFeNiCu BCC + FCC Dendritic region (Fe-Co), interdendritic region LC [23]AlCoCrFe1.5/2.5Ni A2 + B2 Hardness (582–614 HV) LC [24]AlCoCrFeNi-TiC A2 + B2 + TiC Hardness reached 1027.5 HV LC [25]AlCoCrFeNi BCC + FCC + Oxides Hardness: alloy phase (~6 GPa), alumina phase(~11 GPa), mixed oxide (~9 GPa) APS [27]AlCoCrFeNi2 FCC + BCC Hardness (~600 HV) HVOF [28]AlCoCrFeNiTi0.5 A2 + B2 Good high-temperature wear resistance up to 900 °C HVOF [29]AlCoCrFeNi BCC + FCC Superior wear resistance at elevated temperature. APS [30]AlCoCrFeNi A2 + B2 Phase-adjustable by particle size of HEA powders and spraying parameters APS [31]AlCoCrFeNi BCC Hardness (~511 HV) APS [32]AlCoCrFeNi A2 + B2 + FCC BCC→σ: 650–975 °C, σ→ BCC: 1100 °C As-cast + Heat treatment [37]AlCrFeCoNi BCC + FCC Improved wear resistance after annealing. HVOF + Heat treatment [39]MS: magnetron sputtering, LC: laser cladding, HVOF: high velocity oxygen fuel spraying, APS: atmosphere plasma spraying.L. Wang, F. Zhang, S. Yan et al. Journal of Alloys and Compounds 872 (2021) 15960722.2. CharacterizationThe phase structure of composite coatings was examined byX-Ray Diffraction (XRD) in a Rigaku D/max 2500 system with Cu Kαradiation. The compositions and morphologies of coatings wereobserved on JSM7100F scanning electron microscope (SEM) attachedwith energy dispersive X-ray Spectroscopy (EDS, EDAX Genesis2000). The elements distribution was analyzed by using a JXA-8530electron probe micro-analyzer (EPMA). The Vickers hardness ofcoatings was tested on a HMV-G hardness system, which was conducted under 0.98 N for 15 s. The average value for each sample wascalculated by using 10 indentations. The nano-indentation test wascarried out by using an Anton Paar nanoindenter equipped with aBerkovich diamond indenter (triangular pyramid) under the standard continuous stiffness mode. The phase volume fraction of thecoatings were calculated using the ImageJ software and SEM images.3. Results and discussions3.1. Characterization of powder feedstockFig. 1 shows SEM morphologies and phase structure of raw HEApowders. As shown in Fig. 1a and b, the atomized powders haveregular spherical shape and smooth surface, with the particle size of~50 µm, which were suitable for plasma spraying. In Fig. 1c, equiaxedgrains with uniform size can be observed on the surface of atomizedpowders. According to the EDS results in Table 3, the five principalcomponents are evenly distributed in the measured area, which isclose to the equal molar ratio, indicating that the principal components are well mixed and meet the experimental expectation. Thephase composition of atomized powder is displayed in Fig. 1d. Thepowder is a two-phase structure containing ordered and disorderedBCC. The peak position is similar to that reported by Cheng [31], butthe intensity of ordered BCC (B2) is higher and coincides with that ofdisordered BCC (A2).3.2. Phase transformations during annealingFig. 2 shows the X-ray diffraction patterns of HEA coatings beforeand after annealing at 600–1000 °C. The as-sprayed coating (H0)consists of mainly disordered A2 phase (Fe, Cr) and ordered B2 phase(Al, Ni), which has no obvious difference with the feedstock. It reveals that the phase constituents in feedstock have excellent stability under the ultra-high flame temperature during plasma spraying.The formation of complex intermetallic was inhibited due to thehigh entropy effect of the multi-component alloys [2]. After annealing, obvious new peak at 43° is observed, which is correspond to(111) plane of FCC phase (A1). All the annealed coatings exhibit thethree-phase structures of disordered FCC, ordered BCC and disordered BCC, assigning to the space groups of Im-3m (229), Pm-3m(221) and Fm-3m(225), respectively. As seen from Fig. 2, duringTable 2Plasma spraying parameters for AlCoCrFeNi coatings.Process parameter Unit NiAl transitionlayerHEA coatingsProtective gas flow rate (Ar) L/min 80 80Secondary gas flowrate (H2)L/min 25 30Carrier gas flow rate (Ar) L/min 0.32 0.32Current A 500 500Voltage V 60 70Power kW 30 35Gun-substrate distance mm 110 110Fig. 1. SEM morphologies (a)-(c) and XRD spectrum (d) of AlCoCrFeNi atomized powder.Table 3EDS results of the marked area in Fig. 1.Area Composition (at%)Al Cr Fe Co Ni1 17.5 21.7 23.3 19.8 17.72 20.4 22.0 21.3 18.2 18.03 17.8 21.2 23.1 19.6 18.4L. Wang, F. Zhang, S. Yan et al. Journal of Alloys and Compounds 872 (2021) 1596073annealing at 600–900 °C, the peak intensity ratio of I(111)FCC/ I(110)BCCincreases remarkably with the increase of temperature, reaching themaximum at 900 °C. This indicates the high temperature promotesthe phase transition from BCC to FCC [40]. However, the peak intensity ratio of I(111)FCC/ I(110)BCC decreases with further increasingtemperature to 1000 °C. Note that additional peaks occur between40° and 50° (2θ) when annealing above 800 °C, which are related to σphase. This phase was Fe-Cr precipitates with hard and brittlenature, the peaks of which begin to weaken after 975 °C [37]. According to the research of Yeh [41], the A2 (Fe, Cr-rich) transformsinto σ and A1 above 597 °C after the diffusion of multi-element.However, σ phase disappears at above 962 °C. Compared with bulkHEA, the relative peak intensity of FCC phase for H900 and H1000 arestronger, and the precipitation temperature range of σ phase is wider(800–1000 °C).In general, high mixing entropy will inhibit the formation of intermetallic compounds and other phases. During annealing, the increase of enthalpy would reduce the thermal stability of the systemand promote the formation of B2, but it has no obvious effect on theGibbs free energy, that is, it can still maintain the solid solutionstructure at high temperature. As seen from Fig. 2, the sprayed HEAcoating can still keep the solid solution structure after annealing atwide temperature range (600–1000 °C).3.3. Microstructure observationsFig. 3 displays the cross-sectional morphologies of HEA coatingsbefore and after annealing. As seen from Fig. 3a, the as-sprayedcoating has the thickness of ~600 µm, exhibiting typical laminatedstructure with a small amount of pores. The overview of the microstructure looks homogenous and some black strips dispersed inthe coating. According to EDS results in Table 4, the light area(marked A) of the HEA coating contains 19.9Al-19.9Cr-19.6Fe-21.0Co-19.7Ni (at%), approximately equaling to the molar ratio,indicating the constitution of A2 and B2 phase. The black strips(area B) contains 32.6Al-28.0Cr-20.3Fe-11.0Co-8.2Ni (at%), indicatingthe Al-Cr-rich phase. The area C close to the black strips contains4.4Al-88.2Cr-2.6Fe-2.8Co-2.0Ni (at%), indicating the Cr-rich phase.According to the above results, the as-sprayed coating is composedof relatively uniform microstructure with A2 and B2 phase withsome Al-Cr-rich regions. The stacking deposition of metal dropletsgenerate a typical lamellar structure of HEA coating, which wasdisplayed more clearly after annealing treatment (Fig. 3b–f). However, the lamellar structure does not change significantly with theincrease of annealing temperature, showing good high temperaturestability.Fig. 4 displays the backscattered electron (BSE) images of thepolished surfaces of HEA coatings. The chemical compositions ofeach coating is summarized in Table 5. As seen from Fig. 4a, the assprayed coating is rather dense and shows a uniform matrix. It implies that the alloy powder is well melted during the depositionprocess. The spot A in the matrix has a composition with equal molaratomic ratio, indicating the BCC phase (A2 and B2). The gray defectsof spot B and spot C are Al-Cr-rich and Cr-rich phase, respectively. Asmentioned in Fig. 3, the large black defect (spot D) is Al-rich oxides.However, the gray defects disappear gradually while these oxideshardly change in the as-annealed coatings (Fig. 4b–f). Comparedwith other main components, Al has a lower melting point and iseasier to convert into liquid phase, thus segregates at the dropletsurface. Consequently, the in-flight oxidation occurs in the plasmaflame and these oxides deposit on the substrate to form lath aluminastructure, which will be further characterized in the next section.Similar oxides are also reported in Ang’s report on sprayed AlCoCrFeNi coating from mechanical-alloyed HEA powders [33]. However,the present HEA coatings show more uniform microstructure andless oxidation than that in Andrew’s report. The atomized HEAfeedstock with round shape and even particle size (~50 µm) contribute to the homogenous microstructure.As seen from Fig. 4a, the HEA matrix shows smooth and uniformstructure, which might contain the mixed BCC phase (A2 + B2). Afterannealing treatment, interestingly, herringbone structure with distinct white and gray regions is generated in the HEA matrix(Fig. 4b–f). It implies the phase separation related to the elementsdiffusion, which can be verified from EDS results in Table 5. WithFig. 2. XRD spectrums of plasma sprayed AlCoCrFeNi coatings under different annealing temperatures.L. Wang, F. Zhang, S. Yan et al. Journal of Alloys and Compounds 872 (2021) 1596074increasing temperature (> 800 °C), the separation becomes moreclear and larger short rods grow in the white region. According to theEDS results, the white rods (spots L, N and Q) shows Fe-Cr-rich, related to A2 phase (disordered BCC), while the adjacent gray area(spot R) is Al-Ni-rich with Al/Ni atomic ratio of 1:1, identified as B2phase (ordered BCC). Generally, Fe and Cr are easy to form infinitesolid solution due to their small difference of atomic radius andelectronegativity between them. The AlNi phase is also easy to formdue to their higher mixing entropy. Therefore, the above phase examination is in good agreement with XRD results and previous reports [42].Note that some large white regions (spots F, J and O) occur, withthe composition of equal molar atomic ratio Fe-Co-Ni-Cr and Aldeficient, referred to FCC phase [43]. This may be due to thesegregation and dilution of Cr during annealing, which diffuse fromthe Al-rich region to Fe-Ni-rich region, thus forming separated Fe-Crphase (disordered BCC, A2) and Fe-Ni-Co-Cr phase (disordered FCC,A1). This is consistent with previous reports [44,45] that Ni, Co areidentified as FCC phase common ingredients and γ-phase stabilizerswhile Cr is BCC phase common ingredients and α-phase stabilizers.Moreover, the further increasing temperature (900–1000 °C) leads tomore intense diffusion of Al, Cr and Ni, thus promote the precipitation of FCC structure [46,47], thus obvious peaks of FCC phaseoccur in H900 and H1000 coating (Fig. 2).Thus, the above analysis has confirmed the as-annealed coatingsare mainly composed of herringbone matrix with separated A2 (FeCr-rich, white rods) and B2 phase (Al-Ni, gray region), some A1phase (Fe-Ni-Co-Cr, bright region) and black oxide strips (Al rich).Fig. 5 points out these phases and reveals the morphology evolutionduring annealing treatments. Obviously, the initial highly-mixed A2/B2 phase (Fig. 5a) tend to separate after annealing at 600 °C, formingshort rod-shaped A2 embedded in B2 matrix (Fig. 5b). With increasing temperature, the A2 short rods grow larger (Fig. 5c–e,700–900 °C) and merge to form the polygonal long rods (Fig. 5e–f,900–1000 °C). Moreover, the rods become more uniform in distribution, regular in shape and uniform in size. However, the sizeand shape of oxide strip defects seems no obvious change duringFig. 3. BSE images of cross-sectional morphologies of plasma sprayed AlCoCrFeNi coating after annealed at different temperatures: (a) H0, (b) H600, (c) H700, (d) H800, (e) H900 and(f) H1000.Table 4EDS results of the marked area in Fig. 3.Coatings Area Composition (at%) Possible phaseAl Cr Fe Co NiH0 A 21.3 19.3 20.2 19.6 19.7 A2 + B2B 28.5 14.6 19.9 18.9 18.2 Al-rich phaseC 1.2 16.3 26.8 28.6 27.2 Al-poor phaseL. Wang, F. Zhang, S. Yan et al. Journal of Alloys and Compounds 872 (2021) 1596075annealing. In order to reveal the mechanism of microstructureevolution during annealing, EPMA analysis was conducted in thenext section.3.4. Discussions of microstructure evolution(1) Evolution mechanism of major phaseThe atom diffusion and rearrangement activated by annealinglead to the phase transformation in the HEA coatings, whichcould be revealed by EPMA in Fig. 6. Obviously, all the components in H0 shows rather even distribution (Fig. 6a), thusforming a BCC structure, in which A2 and B2 are hardly distinguished. Under low annealing temperature, there is no obvious atoms migration (Fig. 6b). The increasing temperatureleads to remarkable atoms movement from 700 °C, especially forthe Al, Cr and Ni atoms (Fig. 6c–f). On the contrast, the distribution of Fe and Co atoms have no obvious change. Consequently, this results in separated regions rich and poor in Al, Crand Ni, meanwhile changing the relative content of Fe and Co.Due to the negative mixing enthalpy (-22 kJ/mol) [48], Al and Niatoms have high affinity, thus easy to combine with each otherto form ordered BCC phase (Al-Ni, B2). As is known, Fe, Co, Niand Cr have large solid solution degree with each other. However, the atomic size variety also leads to formation of differentsolution type, which can be inferred by formula (1):= 100% ixi(1 ri/r)2 , r = ixixi (1)Fig. 4. BSE images of surface morphologies of plasma sprayed AlCoCrFeNi coating after annealed at different temperatures: (a) H0, (b) H600, (c) H700, (d) H800, (e) H900 and (f) H1000.Table 5EDS results of the marked area in Fig. 4.Coatings Spot Composition (at%) Possible phaseAl Cr Fe Co Ni OH0 A 19.9 19.9 19.6 21.0 19.7 —— A2 + B2B 32.6 28.0 20.3 11.0 8.2 —— Al-Cr rich phaseC 4.4 88.2 2.6 2.8 2.0 —— Cr rich phaseD 42.3 2.0 1.4 1.0 0.7 52.6 Al oxidesH600 E 21.0 19.9 20.0 20.3 18.8 —— A2 + B2F 0.5 14.0 27.4 30.2 28.0 —— FCCG 40.4 4.8 1.3 0.8 0.6 52.1 Al oxidesH700 H 19.2 17.6 22.5 20.6 20.1 —— A2 + B2I 19.4 18.4 20.3 20.2 21.8 —— A2 + B2J 5.0 19.3 25.6 24.0 26.1 —— FCCK 40.5 6.6 7.2 7.5 7.6 30.7 Al oxidesH800 L 4.9 45.0 22.8 20.1 7.3 —— A2M 53.1 14.4 10.3 11.6 10.6 —— Al rich phaseH900 N 3.6 46.3 23.7 19.3 7.1 —— A2O 8.5 20.2 24.2 23.7 23.3 —— FCCP 45.4 1.3 0.6 0.4 0.3 52.0 Al oxidesH1000 Q 8.8 25.0 31.4 22.0 12.9 —— A2R 29.2 10.1 13.2 19.4 28.1 —— B2L. Wang, F. Zhang, S. Yan et al. Journal of Alloys and Compounds 872 (2021) 1596076Where δ is the atomic size difference in the alloy, R is the atomicradius. Compared with close packing FCC structure, the relativeloose BCC structure could provide large space for atom replacement. Moreover, the latter results in less lattice distortion in thesolution, thus BCC structure might be easier than FCC to form inFe-Co-Ni-Cr system. In addition, Al and Ni have largest atomicsize difference among the main components so that this effectkeep more obvious for Al [49]. Therefore, Al should be regardedas FCC destabilizing agent. In the annealing process, Al and Niwould first form ordered B2 structure and the residual components prefer to form the disordered BCC phase (Fe-Cr-rich),embedding in the ordered B2 matrix [50]. As seen from maps inFig. 6, Al, Cr and Ni show significant composition change withincreasing annealing temperature, indicating their high diffusionability in the HEA coatings. Especially, the diffusion of Cr plays animportant role in phase transformation which is in accordancewith Chen’s study [51]. In addition, Co is also reported to haveslow diffusion ability in AlCoCrFeNi based HEAs [52], whichexplains why the Co element can still maintain a relativelyuniform distribution (Fig. 6) under present high temperatures.As seen from Fig. 6e and f, the Fe, Cr and Ni maps are clearlyoverlapped with increasing temperature (>900 °C), forming theFCC phase [52].(2) The evolution of oxide defectsAs mentioned in Figs. 4a and 5a, the oxide strips and crescentblocks are distributed homogenously in the HEA matrix. Thechemical composition of these oxide regions mainly containsAl2O3, which is further confirmed by the Al, O maps in Figs. 6a and7a. Similar Al rich oxide phase is also reported in HVOF-sprayedAlCoCrFeNi2 coatings [28]. Thus, the formation mechanism ofoxides could be deduced as follows. Due to the ultra-high temperature of plasma flame, Al in the surface of HEA powders iseasier to volatilize, thus segregate in the shell of HEA droplets toreact with oxygen to form large crescent-shaped blocks. Duringdrop-by-drop deposition, oxidation also occur at the interface toform slender black strips. Partial oxidation of the outer surface ofthe droplet during the spraying process and slight oxidation afterstacking are also reported in Refs. [27–29]. However, the morphology and composition of these oxide strips and blocks keepnearly un-changed after annealing (Fig. 7b and c), indicating hightemperature stability. In the follow-up research, how to reduceand eliminate oxidation would be a key problem.Fig. 5. Surface morphology evolution of plasma sprayed AlCoCrFeNi coating after annealed at different temperatures: (a) H0, (b) H600, (c) H700, (d) H800, (e) H900 and(f) H1000.L. Wang, F. Zhang, S. Yan et al. Journal of Alloys and Compounds 872 (2021) 1596077(3) Overall evolution path mapFig. 8 summarize the overall evolution of HEA coatings underannealing treatment. The as-sprayed HEA coating (H0) is composedof highly mixed A2/B2 phase. During the annealing process, A2phase (bright zone) grows and merges to form rod-shaped structure.Under low temperature (< 700 °C), Cr in B2 phase (dark zone) diffuses to the bright region to form Cr rich and Cr poor regions. Underhigh temperature (> 800 °C), the bright rods have remarkablegrowth. With further increasing temperature, one part of bright rodsis elongated and connected with each other to form large polygonstructure. Meanwhile, short rods continue to form at the end ofthose long rods. According to above descriptions, the microstructureevolution and phase formation mechanism during spraying andannealing is summarized in Fig. 9.3.5. Mechanical performance of coatingsFig. 10 displays the Vicker’s hardness and phase volumefraction (Fig. 10b) of HEA coatings annealed at different temperatures. As seen from Fig. 10a, the hardness of HEA coatings (H0-H1000) is 468, 575, 518, 434, 435 and 433 HV, respectively. Notethat annealing at lower temperatures (600–700 °C) increases themicro-hardness of HEA coating, while high-temperature annealing(>800 °C) leads to slight hardness decrease. However, the hardnessof these coatings (H800, H900 and H1000) shows little change, indicating the good softening resistance to high temperature forplasma-sprayed AlCoCrFeNi coating. Theoretically, the formationof σ phase would lead to the increase of hardness and brittleness[37]. However, the hardness above 800 °C shows a slight downward trend in this study. The possible reason is that the lowFig. 6. EPMA analysis of surface microstructure evolution of plasma sprayed AlCoCrFeNi coating after annealed at different temperatures: (a) H0, (b) H600, (c) H700, (d) H800, (e)H900 and (f) H1000.L. Wang, F. Zhang, S. Yan et al. Journal of Alloys and Compounds 872 (2021) 1596078amount of σ phase cannot prevent the softening caused by theA1phase.The hardness change shows good relationship with the change ofphase volume fraction in Fig. 10b. Actually, the microhardness ofHEA coatings is mainly depended on the content and composition ofBCC phase (A2 and B2). As reported [53,54], the ordered B2 phasecould prevent dislocation movement and improve the strength ofHEA. While the formation of the ductile FCC phase leads to a decrease in hardness [39]. Thus, the relative high hardness of H600might be attributed to the large fraction of B2 phase. However, hightemperature annealing results in the growth and merging of thedisordered A2 phase. The ratio of B2/A2 decreases gradually in therange of 600–900 °C, thus leading to the hardness decrease of HEAcoating. When the annealing temperature exceeds 900 °C, the ratioof B2/A2 have no obvious change, thus the hardness tends to bestable. Since the black oxide strips keep the low volume fraction(

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