Isothermal Oxidation of Aluminized Coatings on High-Entropy Alloys

entropy

Article Isothermal Oxidation of Aluminized Coatings on High-Entropy Alloys

Che-Wei Tsai 1,*, Kuen-Cheng Sung 1, Kzauki Kasai 2 and Hideyuki Murakami 2

1 Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan; [email protected] 2 Surface and Interface Kinetics Group, National Institute for Materials Science, Sengen 1-2-1, Tsukuba, Ibaraki 305-0047, Japan; [email protected] (K.K.); [email protected] (H.M.) * Correspondence: [email protected]; Tel.: +886-3-571-5131 (ext. 35370)

Academic Editor: Kevin H. Knuth Received: 31 July 2016; Accepted: 14 October 2016; Published: 20 October 2016

Abstract: The isothermal oxidation resistance of Al0.2Co1.5CrFeNi1.5Ti0.3 high-entropy alloy is analyzed and the microstructural evolution of the oxide layer is studied. The limited aluminum, about 3.6 at %, leads to the non-continuous alumina. The present alloy is insufﬁcient for severe circumstances only due to chromium oxide that is 10 µm after 1173 K for 360 h. Thus, the aluminized high-entropy alloys (HEAs) are further prepared by the industrial packing cementation process at 1273 K and 1323 K. The aluminizing coating is 50 µm at 1273 K after 5 h. The coating growth is controlled by the diffusion of aluminum. The interdiffusion zone reveals two regions that are the Ti-, Co-, Ni-rich area and the Fe-, Cr-rich area. The oxidation resistance of aluminizing HEA improves outstandingly, and sustains at 1173 K and 1273 K for 441 h without any spallation. The alumina at the surface and the stable interface contribute to the performance of this Al0.2Co1.5CrFeNi1.5Ti0.3 alloy.

Keywords: high entropy alloys; aluminizing; isothermal oxidation

1. Introduction High-entropy alloys (HEAs) were brought forward two decades ago by Professor Yeh’s group. The brief concept and outcomes are discussed on speciﬁc microstructures, with resistance to annealing softening and outstanding mechanical properties at high temperature, in the ﬁrst paper in 2004 [1]. The excellent performance of HEAs is contributed to by the four core effects, and they are the high-entropy effect, the lattice-distortion effect, the sluggish effect, and the cocktail effect [2]. The HEAs provide countless compositions to study their physical phenomena and structural properties for functional uses. More and more researchers study the phenomena in different HEAs systems. The equiatomic multicomponent AlCoCrFeNiTi high-entropy solid solution alloy has been fabricated by vacuum arc melting and studied on the effect of the aluminum molar ratio. It reveals that only a face-centered cubic crystal structure phase is observed in the CoCrFeNiTi alloy. The phase composition transforms to stabilized body-centered cubic structure phases when Al is added [3]. The alloy is also synthesized by mechanical alloying with a high-energy planetary ball mill. Two solid solution phases, supersaturated BCC (body center cubic) and FCC (face center cubic), appear when the blended powder is ball milled more than 18 h [4]. The AlCoCrFeNiTi high-entropy alloy contains more intermetallic compounds after annealing treatment, and there are many compounds, including CoTi2 and FeTi, in CoCrFeNiTi alloy even without aluminum [5,6]. Above all, it means the aluminum is a strong BCC former. The molar ratio of titanium also needs to be reduced to avoid compounds. The non-equiatomic multicomponent AlxCo1.5CrFeNi1.5Tiy (x = 0 and 0.2, y = 0.5 and 1) had been investigated systematically by our group [7]. The amount of Al and Ti affect the formation of hard eta phase (Ni, Co)3Ti, although it enhances the two-times-higher wear resistance of HEAs than

Entropy 2016, 18, 376; doi:10.3390/e18100376 www.mdpi.com/journal/entropy Entropy 2016, 18, 376 2 of 8

SUJ2 and SKH51 at the same hardness. The SUJ2 bearing steel and SKH51 high-speed steel have good mechanical properties and are proven anti-wear alloys. However, the workability and ductility of AlxCo1.5CrFeNi1.5Tiy needs to be improved for application, so the lower molar ratio of Al and Ti, x + y = 0.5, is studied. As y > 0.4, the eta phase still can be observed at 900 ◦C or 1000 ◦C for 50 h. ◦ In the isothermal oxidation test at 900 C, the continuous Cr2O3 is formed on the surface for protection, but it is still limited for application compared with the superalloy Inconel 718 [8,9]. Inconel 718 is a known chromia former, and has the best oxidation resistance compared to other alloys until now. At x < 0.2, a lower Al molar ratio is insufficient for oxidation resistance. Increasing the Al ratio generally results in forming a BCC solid solution phase in HEAs. The HEAs with a BCC phase generally have low workability and ductility at ambient temperature. Therefore, forming aluminum-rich coatings, built up by a diffusion zone, can protect the surface against oxidation. That is the method for HEAs to confront temperature up to 1400 K and even higher. The same matter is also resolved for superalloys, since they are widely used for blades in turbine engines [10–13]. In aircraft, the gas turbines have been equipped with single-crystal blades. The aluminizing process is an effective coating method frequently used for the protection of metal surfaces against oxidation, via the formation of an α-Al2O3 layer. This technique is provided with a simple apparatus set-up, high cost performance and uniform formation of coatings [12–14]. In this paper, the FCC phase of Al0.2Co1.5CrFeNi1.5Ti0.3 HEA is studied. The isothermal oxidation resistance of HEA and aluminizing HEA are analyzed after for various times at 1173 K and 1273 K. Since the continuous and dense alumina is not formed at the surface, the present alloys are poorly oxidation resistant according to the literature. The evolution of oxide layers is shown in the isothermal testing of HEAs, and the microstructures and distributed region are further discussed. This is the first finding that HEAs are suitable for industrial aluminization. For the aluminizing coating on HEAs, the oxidation resistance is improved and its sustaining temperature is increased up to 1273 K.

2. Materials and Methods

The Al0.2Co1.5CrFeNi1.5Ti0.3 HEA ingots were prepared from the mixtures of the constituent elements with purities higher than 99.5 wt % in an argon atmosphere, and the melting procedure was repeated five times to improve the homogeneity of the ingots by arc-melting. The dimension of slab is 40 × 20 mm with 10 mm in thickness, and further homogenized at 1100 ◦C for 6 h with water quenching in air. After that, it is rolled to 3 mm at ambient temperature with 70% thickness reduction. The plate was machining by electrical discharge machining machine. The samples with 10 mm in diameter are used for isothermal oxidation test, and another with 9 mm in square are for aluminization. For the aluminization, the specimens were embedded in a heat-resistant container with a powder mixture of 24.5 wt % Al (45–106 µm in size), 24.5 wt % Cr (45 µm in size), 49.0 wt % Al2O3 (15–63 µm in size) and 2.0 wt % NH4Cl. This container was placed in a vacuum furnace that was evacuated down to 1.0 × 10−1 Pa at ambient temperature using a rotary mechanical pump. After the furnace was evacuated, Ar was introduced until the pressure reached 1.0 × 105 Pa (1 atm). This process of evacuation and flushing with Ar was repeated three times. Two heating processes, 1273 K and 1323 K, are used respectively. The furnace was heated at a rate of 10 K/min and kept at the temperature for 5 h while Ar was flowed through the furnace at a rate of 200 mL/min. In the isothermal heating test, the specimens of HEAs and aluminized HEAs were kept heated at 1173 K, 1273 K and 1373 K for different endurance time from 1 h to 441 h. All specimen surfaces were grinded before oxidation test and aluminizing process. The oxidation weight gains of specimens were measured by digital weighing scale, and cross-sections were examined. The specimens were prepared by the process of mounting, grinding and polishing. Scanning electron microscopy (SEM, JEOL JSM-6010, JEOL Ltd., Tokyo, Japan) equipped with energy dispersive X-ray spectrometry (EDS, Oxford Instruments, Oxfordshire, UK) is applied to observe and analyze the microstructure and composition of oxide layers. The oxides are identified by an X-ray diffractometer (XRD, RINT2500, Entropy 2016, 18, 376 3 of 8 Entropy 2016, 18, 376 3 of 8

Entropyspectrometry2016, 18, 376(EDS, Oxford Instruments, Oxfordshire, UK) is applied to observe and analyze3 the of 8 spectrometry (EDS, Oxford Instruments, Oxfordshire, UK) is applied to observe and analyze the microstructure and composition of oxide layers. The oxides are identified by an X-ray microstructure and composition of oxide layers. The oxides are identified by an X-ray diffractometer (XRD, RINT2500, Rigaku, Tokyo, Japan) with Cu-target radiation. The specimens are diffractometer (XRD, RINT2500, Rigaku, Tokyo, Japan) with Cu-target radiation.θ The specimens◦ are Rigaku,analyzed Tokyo, at 2θ angles Japan) from with Cu-target20° to 100° radiation. with a scanning The specimens rate of 2 aredeg./min. analyzed at 2 angles from 20 to 100analyzed◦ with at a scanning2θ angles rate from of 20° 2 deg./min. to 100° with a scanning rate of 2 deg./min. 3. Results 3. Results Results 3.1. Isothermal Oxidation of HEA without Aluminizing Coating 3.1. Isothermal Oxidation of HEA without Aluminizing Coating The Al0.2Co1.5CrFeNi1.5Ti0.3 HEA contains aluminum and chromium, and the amount of both is The Al0.2Co1.5CrFeNi1.5Ti0.3 HEA contains aluminum and chromium, and the amount of both is aboutThe 21.8 Al at0.2 Co%. 1.5It CrFeNiis well 1.5knownTi0.3 HEA that containsit forms aluminumalumina and and chromium chromium, oxide and to the protect amount the of alloys both about 21.8 at %. It is well known that it forms alumina and chromium oxide to protect the alloys isgenerally. about 21.8 However, at %. It isthe well weight known gain that of itoxidation forms alumina at 1173 and K and chromium 1273 K oxidewas shown to protect to be the poor alloys in generally. However, the weight gain of oxidation at 1173 K and 1273 K was shown to be poor in generally.Figure 1. At However, 1173 K the the curve weight is similar gain of to oxidation that found at 1173in the K previous and 1273 literature K was shown [9]. That to be oxidation poor in Figure 1. At 1173 K the curve is similar to that found in the previous literature [9]. That oxidation Figurelayer will1. At detach 1173 K from the curve the substrate is similar and to that the found weight in gain the previous decreases literature after 100 [ 9 h.]. ThatUnder oxidation only 1273 layer K layer will detach from the substrate and the weight gain decreases after 100 h. Under only 1273 K willfor 50 detach h, the from layer the detaches substrate from and the the substrate, weight gain and decreasesthe spallation after is 100 more h. Under serious only than 1273 that K at for 1173 50 h,K. for 50 h, the layer detaches from the substrate, and the spallation is more serious than that at 1173 K. theFigure layer 1a detaches shows the from surface the substrate, morphology and theof samples spallation at is1273 more K seriousfor 180 thanh. Thus, that atthe 1173 weight K. Figure gains1 ofa Figure 1a shows the surface morphology of samples at 1273 K for 180 h. Thus, the weight gains of showsthe present the surface alloys morphologydecrease drastically, of samples because at 1273 the K oxide for 180 layers h. Thus, are thespalled weight both gains at the of thetop presentside or the present alloys decrease drastically, because the oxide layers are spalled both at the top side or alloysthe bottom decrease side. drastically, because the oxide layers are spalled both at the top side or the bottom side. the bottom side.

(a) (b) (a) (b) Figure 1. The isothermal oxidation of Al0.2Co1.5CrFeNi1.5Ti0.3 alloys: (a) surface morphology of Figure 1.1. TheThe isothermalisothermal oxidationoxidation ofof AlAl0.20.2CoCo1.51.5CrFeNi1.5TiTi0.30.3 alloys:alloys: (a (a) )surface surface morphology ofof samples at 1273 K for 180 h; (b) the weight gain curves at 1273 K and 1173 K. samples atat 12731273 KK forfor 180180 h;h; ((bb)) thethe weightweight gaingain curvescurves atat 12731273 KK andand 11731173 K.K. The X-ray curves of the oxidation samples at 1173 K are shown in Figure 2. The Al oxide, Cr The X-ray curves of the oxidation samples at 1173 K are shown in Figure 2. The Al oxide, Cr oxideThe and X-ray Ti oxide curves have of been the oxidationidentified and samples detect ated 1173 after K 1 areh. The shown precipitation in Figure phase2. The of Al NiTi oxide, was oxide and Ti oxide have been identified and detected after 1 h. The precipitation phase of NiTi was Crformed oxide after and Ti360 oxide h. The have intensities been identiﬁed of those and oxides detected increase after with 1 h. Thetime. precipitation Otherwise, phasethe matrix of NiTi phase was formedformed afterafter 360360 h.h. TheThe intensitiesintensities of of those those oxides oxides increase increase with with time. time. Otherwise, Otherwise, the the matrix matrix phase phase of of Al0.2Co1.5CrFeNi1.5Ti0.3 HEA is FCC solid solution, and the peaks are detectable in 100 h. However, of Al0.2Co1.5CrFeNi1.5Ti0.3 HEA is FCC solid solution, and the peaks are detectable in 100 h. However, Althose0.2Co oxide1.5CrFeNi layers1.5 areTi0.3 tooHEA thick is and FCC hide solid the solution, matrix FCC and thephase peaks after are 360 detectable h. in 100 h. However, thosethose oxideoxide layerslayers areare tootoo thickthick andand hidehide thethe matrixmatrix FCCFCC phasephase afterafter 360360 h.h.

Figure 2. The X-ray curves of isothermal oxidation Al0.2Co1.5CrFeNi1.5Ti0.3 alloys at 1173 K for various Figure 2. The X-ray curves of isothermal oxidation Al0.2Co1.5CrFeNi1.5Ti0.3 alloys at 1173 K for various Figuretimes. 2. The X-ray curves of isothermal oxidation Al0.2Co1.5CrFeNi1.5Ti0.3 alloys at 1173 K for varioustimes. times.

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EntropyEntropyThe microstructural 20162016,, 18,, 376376 evolution of the oxide sample is further observed. The microstructures44 of of 8 8 of oxidation layers at 1173 K are shown in Figure3. The thin and different oxide layers formed after 1 h, The microstructural evolution of the oxide sample is further observed. The microstructures of and the oxideThe microstructural layers are Ti oxide, evolution Cr oxide, of the andoxide Al sample oxide fromis further thesurface observed. in sequence.The microstructures As it is heating of oxidationoxidation layers at 1173 K are shown in Figure 3. The thinthin andand differentdifferent oxideoxide layerslayers formedformed after after 1 1 for 10 h, the oxide layer became thicker and grew in the deeper area. The thickness of the Ti oxide h,h, andand the oxide layers are Ti oxide, Cr oxide, andd AlAl oxideoxide fromfrom thethe surfacesurface inin sequence.sequence. AsAs itit isis layer was almost unchanged at the surface, but it formed an NiTi-rich layer beneath the Cr oxide layer. heatingheating for 10 h, the oxide layer became thicker and grewgrew inin thethe deeperdeeper area.area. TheThe thicknessthickness of of the the Ti Ti However, the alumina layer is discontinuous, even lasting for 100 h, so only the Cr oxide layer is oxideoxide layer was almost unchanged at the surface, butbut itit formedformed anan NiTi-richNiTi-rich layerlayer beneathbeneath thethe CrCr insufﬁcientoxideoxide layer. for theHowever, alloys tothe suffer alumina at 1173 layer K is or disconti even anuous,nuous, higher even temperature.even lastinglasting forfor Therefore, 100100 h,h, soso the onlyonly thickness thethe CrCr of eachoxideoxide oxide layer layer is keepsinsufficient growing, for the and alloys the interdiffusionto suffer atat 11731173 area KK oror is eveneven over aa 100 higherhigherµm temperature.temperature. in Figure3d Therefore, afterTherefore, heating for 360thethe h.thicknessthickness By the EDSof each mapping oxide layer results, keeps three growing,growing, regions, andand the thethe Al-rich, interdiffusioninterdiffusion the Ti-rich, areaarea and isis theoverover Cr-rich 100100 μμmm regions, inin areFigure comparedFigure 3d after in Figureheating4 .for The 360 totalh. By thicknessthe EDS mapping of the reaction results,results, threethree layer regions,regions, is also thethe shown. Al-rich,Al-rich, The the the Ti-rich, oxidationTi-rich, andandand interdiffusion thethe Cr-rich areregions, both are considered. compared The in Figure Cr-rich 4.4. regionTheThe totaltotal is mainlythicknessthickness the ofof oxide thethe reactionreaction layer, andlayerlayer it isis is alsoalso clearly directed.shown.shown. The TheThe thickness oxidation is and about interdiffusion 10 µm after are 360 both h. considered.considered. The Al-rich TheThe regions Cr-richCr-rich are regionregion divided is is mainly mainly into the the the oxidationoxide oxide layer, and it is clearly directed. The thickness is about 10 μm after 360 h. The Al-rich regions are arealayer, and interdiffusionand it is clearly area. directed. The aluminaThe thickness formed is about a porous 10 μ andm after discontinuous 360 h. The Al-rich layer, andregions dispersed are divided into the oxidation area and interdiffusion area. The alumina formed a porous and closedivided to the surface.into the Becauseoxidation ofarea the depletionand interdiffusion of Al, the area. NiTi The compound alumina phaseformed is a formed porous and and that discontinuous layer, and dispersed close to the surface. Because of the depletion of Al, the NiTi enlargesdiscontinuous the Ti-rich layer, region and in dispersed Figure3d close [8]. to the surface. Because of the depletion of Al, the NiTi compoundcompound phase is formed and that enlarges thethe Ti-richTi-rich regionregion inin FigureFigure 3d3d [8].[8].

(a) (b) (a) (b)

(c) (d) (c) (d) Figure 3. SEM-EDS mapping of isothermal oxidation samples at 1173 K for various times: (a) 1 h; (b) FigureFigure 3. SEM-EDS3. SEM-EDS mapping mapping ofof isothermalisothermal oxidatio oxidationn samples samples at 1173 at 1173 K for K various for various times: times: (a) 1 h; ( a(b)) 1 h; 10 h; (c) 100 h; and (d) 360 h. (b) 1010 h;h; (c) 100 h; h; and and (d (d) )360 360 h. h.

Figure 4. The thicknesses of the oxidation and interdiffusion layer are shown after the isothermal FigureFiguretest. 4. TheThe 4. numbersThe thicknesses thicknesses on the of figure theof the oxidation are oxidation the summation and and interdiffusion interdiffusion of these three layer layer regions. are are shown shown after after the the isothermal isothermal test. Thetest. numbers The numbers on the ﬁgureon the arefigure the are summation the summation of these of these three three regions. regions.

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Entropy 2016, 18, 376 5 of 8 3.2.Entropy Microstucture 2016, 18, 376 of Aluminizing Layers 5 of 8 3.2. Microstucture of Aluminizing Layers 3.2.There Microstucture is generally of Aluminizing Al coating Layers and a IDZ (interdiffusion zone) with aluminization, and the present alloy alsoThere shows is generally a similar structureAl coating in and Figure a 5IDZ. In (interdiffusion this study, the aluminizingzone) with aluminization, layer and interdiffusion and the zonepresent areThere 50 alloyµ mis andgenerallyalso 18.3showsµ Alm a respectively coatingsimilar andstructure undera IDZ in 1273 (interdiffusionFigu Kre for 5. 5In h. this Thezone) study, other with the one aluminization, aluminizing is 71.3 µm andlayer and 24.6 andtheµ m present alloy also shows a similar structure in Figure 5. In this study, the aluminizing layer and respectivelyinterdiffusion under zone 1323 are K 50 for μ 5m h. and The 18.3 composition μm respectively of the aluminizingunder 1273 K layers for 5 ish. almostThe other the one same, is 71.3 and it interdiffusion zone are 50 μm and 18.3 μm respectively under 1273 K for 5 h. The other one is 71.3 revealsμm and that 24.6 the μ temperaturem respectively of 1273under K 1323 is good K for to fulﬁll5 h. The the composition aluminization. of the The aluminizing surface morphology layers is μalmostm and the 24.6 same, μm respectivelyand it reveals under that the1323 temperature K for 5 h. Theof 1273 composition K is good of to the fulfill aluminizing the aluminization. layers is and EDS mapping are shown in Figure6. The aluminizing layer is uniform at the surface. From the almostThe surface the same, morphology and it reveals and EDS that mapping the temperature are shown of in 1273 Figure K is 6. good The aluminizingto fulfill the layer aluminization. is uniform EDS mapping results, Ti-rich particles that are less than 3 µm are distributed in the aluminizing layer. Theat the surface surface. morphology From the and EDS EDS mapping mapping results,are shown Ti-rich in Figure particles 6. The that aluminizing are less layerthan is3 uniformμm are There are two areas in the IDZ region. One is rich in Co, Ni, and Ti, and the other is rich in Fe and Cr. atdistributed the surface. in the From aluminizing the EDS layer.mapping There results, are two Ti-rich areas inparticles the IDZ that region. are Oneless isthan rich 3in μ Co,m areNi, Figure7 shows the X-ray curve of the aluminization layer at 1273 K. Al(CoCrFeNiTi) is the main phase. distributedand Ti, and in the the other aluminizing is rich in layer.Fe and There Cr. Figure are two 7 shows areas thein the X-ray IDZ curve region. of theOne aluminization is rich in Co, layer Ni, Al Ti, Al Ni and NiTi are the minor phases. 3andat 1273 Ti,3 andK. Al(CoCrFeNiTi) the other is rich is in the Fe mainand Cr. phase. Figure Al3 Ti,7 shows Al3Ni the and X-ray NiTi curveare the of minor the aluminization phases. layer at 1273 K. Al(CoCrFeNiTi) is the main phase. Al3Ti, Al3Ni and NiTi are the minor phases.

(a) (b) (a) (b) Figure 5. The microstructure of the aluminizing layer is shown, and the compositional profiles of Figure 5. The microstructure of the aluminizing layer is shown, and the compositional proﬁles of each Figureeach element 5. The aremicrostructure also inserted ofafter the the aluminizing aluminizing layer process is shown, composition and the at (acompositional) 1273 K and ( bprofiles) 1323 K. of element are also inserted after the aluminizing process composition at (a) 1273 K and (b) 1323 K. each element are also inserted after the aluminizing process composition at (a) 1273 K and (b) 1323 K.

(a) (b) (a) (b)

(c) (c) Figure 6. (a,b) The surface morphology after aluminizing coating at 1273 K; (c) the EDS mapping of Figure 6. (a,b) The surface morphology after aluminizing coating at 1273 K; (c) the EDS mapping of Figureeach 6.element(a,b) The by cross-section. surface morphology after aluminizing coating at 1273 K; (c) the EDS mapping of each element by cross-section. each element by cross-section.

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FigureFigure 7.7. TheThe X-ray X-ray curve curve of of thethe aluminizationaluminization layerlayer atat 12731273 K.K.

3.3.3.3. Isothermal Isothermal Oxidation Oxidation withwith AluminizingAluminizing onon HEAHEA

FigureFigure 8 8 8shows showsshows the thethe isothermal isothermalisothermal oxidation oxidationoxidation resistance resistanceresistance of ofof the thethe Al AlAl0.20.20.2CoCoCo1.51.51.5CrFeNiCrFeNiCrFeNi1.51.51.5TiTi0.30.3 HEAHEA withwith aluminizingaluminizing coating coating by by aluminization aluminization at at 1273 1273 K K for for 55 h.h. The The curve curve of of thethe weightweight gaingain isis almostalmost thethe samesame atat 11731173 K,K, andandand thethethe oxidationoxidationoxidation resistanceresistanceresistance ofofof HEAsHEAsHEAs hashashas improvedimprovedimproved obviouslyobviouslyobviously afterafterafter aluminizationaluminizationaluminization comparedcompared to toto that thatthat without withoutwithout aluminization aluminizationaluminization in Figureinin FigureFigure1. As 1.1. the AsAs temperature thethe temperaturetemperature increases increasesincreases to 1273 toto K, 1273 the1273 curve K,K, thethe is ﬂatcurvecurve before isis flatflat 200 beforebefore h. However, 200200 h.h. However,However, the weight thethe gain weightweight reduced gaga ainin little reducedreduced between aa littlelittle 200 betweenbetween h and 441 200200 h; ithh is andand still 441441 sustained h;h; itit isis atstillstill 1273 sustainedsustained K. The samplesatat 12731273 (referK.K. TheThe to samplessamples Supplementary (refer(refer toto Information) SupplementarySupplementary demonstrate Information)Information) that the demonstratedemonstrate aluminizing thatthat coating thethe onaluminizingaluminizing HEAs is still coatingcoating good onon to protectHEAsHEAs isis the stillstill substrate goodgood toto after protprot 441ectect the hthe at substratesubstrate 1173 K and afterafter 1273 441441 K. hh When atat 11731173 testing KK andandat 12731273 1373 K.K. K, theWhenWhen curve testingtesting is shown atat 13731373 to K,K, decrease thethe curvecurve with isis ashownshown wavy to motion,to decreasedecrease because withwith theaa wavywavy coating motion,motion, layer becausebecause is affected thethe bycoatingcoating both oxidationlayerlayer isis affectedaffected and spallation. byby bothboth oxidationoxidation andand spallation.spallation.

FigureFigure 8.8. TheThe isothermalisothermal oxidationoxidation resistanceresistance ofof thethe AlAl0.20.2CoCo1.51.5CrFeNiCrFeNi1.51.5TiTi0.30.3 alloyalloy afterafter aluminizingaluminizing atat Figure 8. The isothermal oxidation resistance of the Al0.2Co1.5CrFeNi1.5Ti0.3 alloy after aluminizing at 11731173 K,K, 12731273 KK andand 13731373 K.K. 1173 K, 1273 K and 1373 K.

4.4. DiscussionDiscussion 4. Discussion ForFor oxidationoxidation resultsresults inin thethe presentpresent HEA,HEA, thethe TiTi oxideoxide formedformed thethe fastestfastest atat thethe surface;surface; For oxidation results in the present HEA, the Ti oxide formed the fastest at the surface; however, however,however, itsits growthgrowth raterate isis inhibitedinhibited byby thethe CrCr oxideoxide layer.layer. TheThe limitedlimited AlAl molarmolar ratioratio leadsleads toto thethe its growth rate is inhibited by the Cr oxide layer. The limited Al molar ratio leads to the non-continuous non-continuousnon-continuous aluminaalumina beneathbeneath thethe CrCr oxideoxide layer,layer, soso thethe CrCr oxideoxide layerlayer isis growinggrowing withwith timetime inin alumina beneath the Cr oxide layer, so the Cr oxide layer is growing with time in Figure3b,d. Therefore, FigureFigure 3b,d.3b,d. Therefore,Therefore, thethe FCCFCC AlAl0.20.2CoCo1.51.5CrFeNiCrFeNi1.51.5TiTi0.30.3 HEAHEA startsstarts toto precipitateprecipitate thethe NiTi-richNiTi-rich phasephase the FCC Al0.2Co1.5CrFeNi1.5Ti0.3 HEA starts to precipitate the NiTi-rich phase because it lacks Al. becausebecause itit lackslacks Al.Al. ForFor thethe CoCo1.51.5CrFeNiCrFeNi1.51.5TiTi0.50.5 HEA,HEA, itit possesspossess mainlymainly aa FCCFCC structure;structure; thethe ηη phasephase For the Co1.5CrFeNi1.5Ti0.5 HEA, it possess mainly a FCC structure; the η phase (Ni3Ti) is at the (Ni(Ni33Ti)Ti) isis atat thethe interdendriticinterdendritic regionregion inin thethe as-cas-castast conditioncondition whichwhich isis stablestable betweenbetween 10731073 KK andand interdendritic region in the as-cast condition which is stable between 1073 K and 1273 K. By the 12731273 K.K. ByBy thethe directionaldirectional solidificationsolidification process,process, itit isis foundfound thatthat Co,Co, Cr,Cr, andand FeFe partitionpartition towardtoward thethe directional solidiﬁcation process, it is found that Co, Cr, and Fe partition toward the dendritic region, dendriticdendritic region,region, whilewhile NiNi andand TiTi partitionpartition towardtoward thethe interdendriticinterdendritic areasareas [8].[8]. ThatThat revealsreveals aa similarsimilar while Ni and Ti partition toward the interdendritic areas [8]. That reveals a similar tendency in this tendencytendency inin thisthis study.study. AsAs thethe aluminumaluminum diffuseddiffused toto thethe surface,surface, thethe compositioncomposition ofof thethe matrixmatrix willwill bebe closeclose toto thatthat ofof CoCo1.51.5CrFeNiCrFeNi1.51.5TiTi0.50.5..

Entropy 2016, 18, 376 7 of 8 study. As the aluminum diffused to the surface, the composition of the matrix will be close to that of Co1.5CrFeNi1.5Ti0.5. By aluminization, the Al coating is stable and contains Al at about 50 at %. In the interdiffusion zone (IDZ), one region is rich in Co, Ti, and Ni, since they have high mixing enthalpy with Al. As Al just diffuses to the interface during aluminization, the diffusivity of Co, Ti and Ni is promoted. Therefore, the other region, rich in Cr and Fe, is formed followed by the previous region in Figure6c. However, the Al is continuously diffused and reached a stable concentration with the other elements for a longer time. At the same time, Ti diffused from the matrix into the coating layers and formed the second phase. Its structure and composition need further study. In Figures5 and7, the coating layer contains the Al(CoCrFeNiTi) phase and Ti-rich phase. As the alumina forms at the surface at high temperature, the high entropy effect will stabilize the aluminizing layer. By the formation of the coating layer in Figure6, the reverse reaction can be inferenced. If the dense and continuous alumina were formed at the surface, the IDZ close to the matrix may transform to the matrix phase again at a temperature over 1273 K. At the moment, the concentration of the aluminum is decreased, and the Ti-rich phase will be dissolved again. It is close to the matrix phase of Al0.2Co1.5CrFeNi1.5Ti0.3. Comparing to Figures1 and8, the oxidation resistance of aluminizing Al 0.2Co1.5CrFeNi1.5Ti0.3 is improved and can be sustained at 1273 K, lasting for 441 h without spallation. It is believed that the coating is covered with alumina at the surface, and is comparable to the substrate at the interface. However, the curve of the isothermal oxidation at 1373 is slightly unstable, but much better than that of HEAs without aluminization. The microstructures of these aluminizing samples after isothermal testing need further observation. The cyclic oxidation test is also worth studying in the future.

5. Conclusions Aluminization by packing cementation, which is a popular industrial process, is suitable for FCC solid solution phase Al0.2Co1.5CrFeNi1.5Ti0.3 HEA. The aluminizing coating layer of 50 µm can be formed at 1273 K for 5 h, and it enhances the oxidation resistance up to 1273 K at least. The phases of the coating layer are Al(CoCrFeNiTi) and a Ti-rich phase. It contributes to the favorable oxidation resistance of the present alloys. Therefore, the alumina is good for surface stability, and the high entropy effect in HEAs further stabilizes the interface between the coating and substrate at high temperature.

Supplementary Materials: The following are available online at www.mdpi.com/1099-4300/18/10/376/s1, Figure S1: The isothermal oxidation resistance of the Al0.2Co1.5CrFeNi1.5Ti0.3 alloy after aluminizing: the surface morphology samples at (a) 1173 K for 200 h and 441 h; and (b) 1273 K for 195 h and 441 h. Acknowledgments: The authors would like to thank the financial support from Ministry of Science and Technology, Taiwan, project grant number: 104-2218-E-007-017. Che-Wei Tsai would like to thank Murakami for discussion on the aluminization of HEAs and for supporting his stay at NIMS in Japan. Author Contributions: Che-Wei Tsai and Kzauki Kasai conceived and designed the experiments; Hideyuki Murakami performed the experiments; Che-Wei Tsai and Kuen-Cheng Sung analyzed the data; Che-Wei Tsai wrote the paper. All authors commented on the manuscript at all stages. All authors have read and approved the final manuscript. Conflicts of Interest: The authors declare no conflict of interest.

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