In recent times, there has been a significant volume of work on Eutectic High Entropy Alloys (EHEAs) owing to their remarkable castability combined with excellent mechanical properties , which aids in clearing obstacles for their technological applications. One of the most common EHEAs, which has been of huge interest, at present, p rimarily owing to its solidification and tensile behavior , is AlCoCrFeNi 2.1. However, in order to aim for high temperature applications, oxidation behaviour of material is one of the major aspects which needs to be extensively investigated . To this end, t he present work aims to study the phases evolved during oxidation at elevated temperatures as high as 950 and 1000⁰C in AlCoCrFeNi 2.1 using XRD and also to determine the rate law followed for isothermal oxidation of this alloy at 950 and 1000⁰C, in order to understand the role of Al 2O3 phase formed during isothermal oxidation at 950 and 1000 ˚C. Keyword s: Oxidation, Eutectic High -Entropy Alloys (EHEAs) , Optical microscopy (OM), X - Ray Diffraction (XRD), Scanning Electron Micros copy (SEM) 1. Introduction High -entropy alloys (HEAs), or multi -principal -element alloys , differentiate with conventional alloys in that they have at least four principal elements, instead of one or at mo st, two in the latter. This has led to a paradigm shift in alloy de sign strate gies, and opened a new path for exploration of new materials . A signifi cant volume of work has been d one on understanding solidification behaviour of Eutectic HEAs (EHEAs), especially, due to the excellent castability of these alloy s [1-4], arising due to meagre chances of segregation and shrinkage cavit ies during sol idificatio n . Accordingly, if eutec tic HEAs with the dual phase FCC + BCC micro structure may be 2 synthesised , they may be expected to possess the combined advantages o f excellent mechanical properties and castability of eutectic alloys coupled w ith a remarkable strength -ductility optimizatio n arising out of a dual phase FCC+BCC microstructure , especially when both FCC and BCC phases are ordered [1-9]. This has primarily been t he ideology behind designing EHE As, as reported by a number of r ecent stud ies. In this conte xt, AlCoCrFeNi 2.1 is a pop ular EHEA, in which quite a signific ant volume of work has been done on correlating the dual phase microstructure comprising of ordered FCC ( L12) and orde red BCC (B2) phases with the o verall solidification behavio ur and mech anical response of the allo y, synthesized using different techniques [1-8, 35]. With regard to oxidation, there have been a number of publications which have reported the oxidation behaviour of HEAs. For instance, Daoud et al. [10] examined the oxidation behaviour of Al 8Co17Cr17Cu8Fe17Ni33, Al 23Co15Cr23Cu8Fe15Ni15, and Al17Co17Cr17Cu17Fe17Ni17 HEAs at 800◦C and 1000◦C in air and reported that at 800◦C, th at the alloy with lower Al content forms NiO along with other oxides , namely, Fe2O3, Cr2O3, and Al2O3, unlike at 1000 ˚C, where the same alloy preferentially forms Cr2O3 over Al2O3, in contrast to the oxida tion beh aviour of HEAs with higher Al content , forming Al2O3 scales at both temperatures (800 ˚C and 1000 ˚C). Holcomb et al. [11] investigated the oxidation behaviour of 8 CoCrFeMnNi -based HEAs, along with different conv entional alloys and reported that the Cr and Mn containing HEAs preferentially form both Cr 2O3 and Mn oxides. Senkov et al. [1 2] examined the isoth ermal oxidation behaviour of an arc -melted NbCrMo 0.5Ta0.5TiZr refractory HEA (RHEA) at 1000◦C ad rep orted a superior oxidation resistanc e of the aforementioned alloy as compared to that of similar Nb-based refractory alloys . Similarly , Gorr et al. [13, 14] investigated the influence o f Si on the high temperature oxidation behaviour of a 20Nb -20Mo - 20Cr -20Ti -20Al HEA and reported that, in general, for HEA s with no Si conten t, a linear oxide growth rate law is followed with the formation of porous and non-protective oxides whereas, the additio n of even 1 at % Si promotes parabolic oxide growth kinetics, to a certain extent, and leads to formation of a thin, continuous and protectiv e Al and Cr rich oxide scale . However, in view of lim ited understanding on th e oxidation behaviour of AlCoCrFeNi 2.1 EHEA upto 1200⁰C , present work is aimed at understanding the nature of Al2O3 phase f ormed during isothermal oxidation of AlCoCrFeNi 2.1 EHEA at 950 and 1000⁰C. 2. Experiment Initially, a single sample (in the shape of a button) of AlCoCrFeNi 2.1 is prepared from Suction Casting of pure components (purity > 99.9%) (on a water -cooled Cu hearth, under Ar inert atmosphere and using Ti as getter) . The bar with a rectangular cross -section, so obtained was flipped and remelted multiple times to promote homogeneity and then cut into 8 samples of 3 average dimensions ~10 by 6 mm2 (approximate thickness ~ 1.2 mm) using EDM (Electrode Discharge Machining) . After standard metallographic polishing , the 1st and 2nd samples were subjected to isothermal oxidation at 1000⁰C, 500h in a muffle furnace (CWF -1300). The 3rd sample was subjected to isothermal oxidation at 1000⁰C, 100h in Al 2O3 crucible, using Kanthal wire as heating element and a static oxidation environment. The 4th and 8th sample were similarly put in the same furnace at 1000 and 950 ⁰C, respectively and each of them was taken out after 4, 18, 25, 50 and 100h . Similarly, samples 5 and 6 were oxidised at 950⁰C, 500h and sample 7 at 950⁰C, 100h. Phase a nalysis of the base and oxidized samples was carri ed out usi ng XRD (X-Ray Diffraction, Bruker) using Cu kα radiation at 45 mV and 30 mA and, at a scanning rate of 0.02⁰/min between the scanning range of 20 to 80⁰ and peak fitting was done using Xpert Hi-score analysis software , in order to determine the phases prese nt in the oxidised layer, at different temperatures and time of oxidation. For microstruc tural characterization, the base sample was etched with aqua regia (90% C2H5OH+10% HNO 3). The cross section al area and difference in weight (before and after oxidati on) of every sample were measured to determine the oxidation rate law at different temperatures of oxidation. 3. Results and discussions 3.1 Phase analysis during oxidation using X-Ray Diffraction (XRD) Fig 1. Stacked XRD plots of as-cast (at the bottom, marked as black) to oxidised samples at different temperatures and time of isothermal oxidation (marked as red, blue, pink, green, deep blue, violet indigo and maroon) . # # # # * * 4 Fig. 1 shows the X -Ray diffractograms of as -cast along with oxidised sample s. The variation In terms of angular Wavenumber (k), with the exception of 1000⁰C, 100h sample, is observed from 1 to 5. Whereas, the variation in the same quantity is observed between 2 and 5 in 1000⁰C, 500h sample . Moreover, It is evident (from Fig. 1) that in as-cast sample, there is a dual-phase microstructure comprising of B2 (ordered BCC) and L1 2 (ordered FCC) phases (lattice parameters: 3.6011Ǻ and 5.9375 Ǻ , respectively ), with the most intense (111) superlattice peak at 2θ = 43.5⁰, corresponding to L1 2 phase. Besides, it is also observed that in as -cast sample, there are more peaks of L1 2 phase as compar ed to that of B2. After isothermal oxidation at varying tempe ratures and times of oxidation till 1000⁰C, 500h, there are three phases detected by XRD analysis: L1 2, Al 2O3 and Fe 2O3. After 25h of isothermal oxidation at 950⁰C, Cr2O3 phase is also observed . However, af ter 100h of oxidation , it is clearly observed that there are 2 oxides pres ent (similar to that observed after isothermal o xidation at 950⁰C for 25h) namely , Al2O3 and Fe2O3 phases , with the most i ntense (113) peak of Fe2O3 at 2θ = 43.5⁰, indicating that Fe2O3 is the predominant oxide phase at 950⁰C, 100h sample. Unlike Al 2O3, Fe2O3 phase forms a non -protective oxide layer but the exact information about oxides may be gained only by looking at cross -sections using SEM [15-17] which will be taken up in subsequent publications . After 50 0h of isothermal oxidation at 950˚C, the most intense (113) peak corresponds to that of Al2O3 at exactly 2θ = 43.5⁰ indicating that Al2O3 phase form s the matrix with 2nd phase as Fe2O3, similar to the microstructure after 50 h of oxidation at 950⁰C. Besides, after 10 00⁰C, 100h of isothermal oxidation, the volume fraction determination of individual phases based on the relative intensities of XRD peaks clearly indicate s that Fe2O3 phase forms the matrix and Al2O3 forms the 2nd phase in a dual p hase Al2O3 + Fe2O3 microstructure , similar to that in samples oxidized at 950˚C for 25 and 100h . However, after 500h of isothermal oxidation at 1000⁰C , the most int ense (113) peak of Al 2O3 with its position (2θ) shifted slightly to the right i.e. at 2θ = 43.7⁰, indicates a dual phase microstructure comprising of Al2O3 (matrix) and Fe 2O3 phases, similar to that in samples after oxidation at 950⁰C for 50 and 50 0h and tha t after oxida tion at 1000⁰C for 25 and 50h although the detailed phase analysis, is beyond the scope of discussion of the present work, as because XRD , being a bulk characterization technique, cannot detect phases with vol. fraction of less than 0.05. 5 3.2 Microstructura l characterization of as-cast sa mple Fig. 2 As-cast sample: (a) Optical image (100x) , (b) Optical image (500x ) of the region highlighted with a red out lined enclosure in part (a); and (c) SEM image (3000x) showing eutectic colo nies with alternate lamellae (of various thickness) of L1 2 and B2 phases (enclo sed within orange outlined rectangular box in part (c)) Both optic al and SEM images reveal eutectic colo nies with alternate lamellae (of various thickness) of L1 2 and B2 phases . Based on image analysis u sing ImageJ software, the average thickness o f L12 phase is found to be nearly 1 μm wh ereas that of B2 lamella is fo und to be nearly 1.2 μm . Besides , the phase faction of L 12 and B2 phases (based on variation in contrast) in the as-cast microstructure is also found to be 0.89 and 0.11 respectiv ely from Figs . 2(a-c), simila r to that inferred from the XRD of as -cast sample in Fig . 1. 6 3.3 I sothermal oxidation kinetics at 950 ˚C and 1000 ˚C Fig. 3 (a) Comparative Isothermal oxidation plots of AlCoCrFeNi 2.1 EHEA at 950 and 1000⁰C between 4h and 500 h of oxidation , (b) Isothermal oxidation plot of AlCoCrFeNi 2.1 EHEA at 950⁰C between 4 h and 50h of oxidation (drawn a s subplot of part (a) ) and (c) Isothermal oxidation plot of AlCoCrFeNi 2.1 EHEA at 1000⁰C (plotted separately for a higher leve l of clarity) between 4 h and 500h of oxidation . From the previous studies of oxidation behaviour of AlCoCrFeNi 2.1 EHEA, [1-8], it has been reported that at elevated temperatures between 900 and 1300⁰C, the weight loss curve for oxidation initially shows a parabolic nature, which saturates with higher periods of oxidation. Fig. 3 (a)), its subplot (Fig. 3(b)) and Fig. 3(c) indicate that irrespective of considerable fluctuations in these weight loss curves due to reas onable amounts of spallation , a linear rate la w is observed at both 950 and 1000˚C, with increase in time of oxidation at both these tem peratures . Com bining the observation on the phase fraction of Al 2O3 and Fe 2O3 phases ( Fig. 1) with the weight los s curves (Figs. 3(a -c)), it may be inferred that a slightl y higher volume fraction of Al2O3 phase (in a dual phase Al2O3 + Fe 2O3 microstructu re) after isothermal oxidation at 1000 ˚C for 500h (that that after oxidation at 950˚C for 500h) leads to a significantly higher amount of weight loss than that after isothermal oxidation at 950˚C for 500h . This further provides an indication that Al 2O3 scale formed during oxidation at elevated temperatures in 7 AlCoCrFeNi 2.1 EHEA aids in fu rther oxida tion unlike the reported nature of Al2O3 scale [1-4]. However, the detailed microstructural investigation suppo rting the present claim on the nature of Al2O3 scale formed during prolonged periods of high temperature oxidation of AlCoCrFeNi 2.1 EHEA , will be discussed in subsequent publications . 4. Conclusions 1. During isothermal oxidation at 950 and 1 000˚C, fo r oxidation perio ds ran ging from 25 to 500h, AlCoCrFeNi 2.1 EHEA (with a dual phase L1 2 + B2 microstruc ture) shows a dual phase Al2O3 + Fe 2O3 microstructu re. 2. At 950 ˚C, there is phase reversion from Al 2O3 to Fe 2O3 when oxidation is p erformed till 50h. However , Fe2O3 reverts back to Al2O3 when oxidation is performed at 950 ˚C for 100 and 500h. Similarly , At 1000˚C, there is phase reversion from Al 2O3 to Fe 2O3 when oxidation is performed till 100h followed by a re -reversion from Fe2O3 to Al2O3 phase for oxidation till 500h at 1000˚C. 3. Irrespective of considerable fluctuations in weight loss curves due to reas onable amounts of spallation , a linear rate la w is observed at both 950 and 1000˚C, with increase in time of oxidation at both these tem peratures from 4h till 500h. 4. Al2O3 scale formed during oxidation at elevated temperatures in AlCoCrFeNi 2.1 EHEA aids in further oxida tion unlike the reported nature of Al2O3 scale . Acknowledgeme nt MS is thankful to the Department of Metallurgical and Materials Engineer ing, NIT Durg apur, for carryi ng ou t the work.