Understanding the microstructural evolution of high entropy alloy coatings manufactured by atmospheric plasma spray processing

Journal Pre-proofs Full Length Article Understanding the Microstructural Evolution of High Entropy Alloy Coatings Manufactured by Atmospheric Plasma Spray Processing Ameey Anupam, Ravi Sankar Kottada, Sanjay Kashyap, Ashok Meghwal, B.S. Murty, C.C. Berndt, A.S.M. Ang PII: DOI: Reference:

S0169-4332(19)32933-2 https://doi.org/10.1016/j.apsusc.2019.144117 APSUSC 144117

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Applied Surface Science

Received Date: Revised Date: Accepted Date:

28 June 2019 30 August 2019 19 September 2019

Please cite this article as: A. Anupam, R.S. Kottada, S. Kashyap, A. Meghwal, B.S. Murty, C.C. Berndt, A.S.M. Ang, Understanding the Microstructural Evolution of High Entropy Alloy Coatings Manufactured by Atmospheric Plasma Spray Processing, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc. 2019.144117

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Understanding the Microstructural Evolution of High Entropy Alloy Coatings Manufactured by Atmospheric Plasma Spray Processing Ameey Anupama,b, Ravi Sankar Kottadaa, Sanjay Kashyapa1, Ashok Meghwalb, B.S. Murtya2, C.C. Berndtb, and A.S.M. Angb a

Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai, 600036, Tamil Nadu, India b

Swinburne University of Technology, Hawthorn, Victoria, 3122, Australia

Abstract Atmospheric plasma spray (APS) of mechanically alloyed equiatomic AlCoCrFeNi high entropy alloy (HEA) results in a complex alloy-oxide coating. All the constituent phases have been identified via extensive microscopy and spectroscopy at various length scales. This microstructural characterization along with the in-flight particle size and temperature measurements and single-pass studies have been used to decode the particle-plasmaatmosphere interaction that resulted in the observed coating microstructure. Particles finer than 5 µm diameter are expected to melt, spheroidize and oxidize completely in-flight when closer to the plasma plume core, whereas those larger than 15 µm only exhibit softening and surface oxidation. Molten particles splat on impact resulting in typical lamellar microstructure, while the unmelted particles either get embedded in the coating or bounce off the substrate. Equiatomic AlCoCrFeNi powder oxidizes differently in plasma in air than the cast alloy during isothermal oxidation, resulting in multiple oxides – alumina, chromia, spinels and residual unoxidized alloy cores. Understanding these phenomena in conjunction with each other enables us to tailor feedstock and spray parameters to get the desired coating properties. Keywords: high entropy alloys, thermal spray, coatings, microstructural characterization Corresponding author at: Faculty of Science, Engineering and Technology, Department of Mechanical Engineering and Product Design Engineering, Swinburne University of Technology, H38, P.O. Box 218, VIC 3122, Hawthorn, Australia Email address: [email protected] (Ameey Anupam) 1

Present Address: BML Munjal University, Gurgaon, 122413, Haryana, India

2

Present Address: Dept. of Materials Science and Metallurgical Engineering, Indian Institute of Technology Hyderabad, Kandi, Sangareddy, Telangana State, 502285, India

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1. Introduction High entropy alloys (HEAs), also known as multi-principal-element alloys or compositionally complex alloys (CCAs), are a recent addition to the materials world. Discovered and developed over the last 15 years, these alloys are not based on a single element matrix; instead, 5 or more elements added in major proportions make up these alloys, which can result in simple phases instead of previously expected complex intermetallics, due to high configurational entropy of these alloys [1], [2], [3]. Research on HEAs has been primarily two pronged – one, trying to decode the fundamental questions about the physical metallurgy [4], [5] and alloying behaviour observed [6], [7]; and second, measuring physical properties of the various compositions, such as strength and ductility [8], fracture toughness [9], plastic behaviour [10], creep resistance [11], corrosion resistance [12], irradiation resistance [13], thermoelectric properties [14], and exploring real-life application avenues for them. The current work is a confluence of both, that is, HEA coatings are being developed towards potential application as bond coats for thermal barrier coatings. This work focuses on the more basic question of understanding how the coating’s microstructure evolves upon the alloy’s interaction with a high temperature plasma in air. The field of HEA coatings is a rapidly expanding one, with over 400 publications available in the public domain. A majority of these are synthesized via the popular laser cladding [15], sputtering [16], and vacuum arc [17] techniques, while a large fraction involves HEA nitride/oxide/carbide thin films. These reports have been summarized in a review article by Li et al. [18]. In comparison, very small fraction of publications deal with thick coatings generated via thermal spray techniques, such as plasma spray [19], high velocity oxygen fuel spray [20], cold spray [21], etc. Thermal spray, being the predominant coating technique in the aviation industry [22], was our method of choice. The equiatomic HEA composition AlCoCrFeNi was selected owing to the extensive literature available on this alloy. Atmospheric plasma spray (APS), which involves generating a high temperature plasma by passing a gas through a high voltage arc causing the molecules of the gas to split into ions and electrons, operates at power levels of 20-30 kW in this study. Alloy HEA powders are introduced into this stream, most of which melt and are susceptible to oxidization during their few microseconds of exposure. These molten particles impact on the substrate and quench, which results in an undulated appearing microstructure composed of individual units termed as splats [23]. The microstructure is often different from that ideally expected because the HEAs are compositionally complex and supersaturated due to the mechanical alloying (MA). Features such as splats of oxides, alloy splats with compositions significantly different from the starting 2

alloy, dendritic morphologies within alloy splats, precipitate formation within splats, inter-splat cracks, porosity, cracks, and unmelted particles are commonly observed. Our previous work with AlCoCrFeNi coatings generated via APS reported the composite alloy-oxide microstructure [24], as well as identified its mechanical properties and compared it with traditional MCrAlY coatings. The current study examines the mechanically alloyed particle of AlCoCrFeNi HEA from its introduction into the plasma torch until the formation of the composite coating. A distinctive microstructure evolves during the few microseconds of residence time that the 5-component alloy spends within the plasma flame. While particle-plasma interactions have been studied in the APS literature, these concepts will now be applied to an HEA system. An understanding of the mechanism of this interaction can help us predict and tailor alloy composition and spray parameters; thereby enabling a coating with desirable properties. Detailed characterization of the coatings via electron microscopy and spectroscopy such as EPMA, TEM and EDS has been performed to understand the transformation processes of the HEA powder into the HEA+oxide coating.

2. Materials and Methods Elemental Al, Co, Cr, Fe and Ni powders (>99.5% purity) were alloyed via MA for 10 hours, and the resultant alloy was coated onto mild steel substrates by APS. The details of the MA and APS processes have been reported in the previous work [24]. The as-coated alloy was characterized via X-ray diffraction (XRD), scanning electron microscopy (SEM), electron probe microanalysis (EPMA), transmission electron microscopy (TEM), and nanoindentation. XRD was conducted on a Panalytical X’Pert Pro (Malvern Panalytical Ltd., Royston, United Kingdom) diffractometer with Cu-kα target, using a step size of 0.02° and time per step of 20s. SEM was carried out on FEI Inspect F scanning electron microscope with a working voltage of 20 kV, working distance of 12mm and spot size of 4.5 (beam diameter ~1 µm). EPMA was carried out on a JEOL JXA-8530f wavelength dispersive spectrometer (WDS). TEM characterization was conducted using the FEI TECNAI T20 using 200 kV equipped with an EDAX EDS system. Nanoindentation experiments were carried out on Hysitron TI 950 TriboIndenter with a diamond Berkovich tip. Tests were done in load controlled mode with a peak load of 5000 µN and loading rate of 500 µN/s.

3. Results The as-sprayed APS AlCoCrFeNi coating was first investigated by X-ray diffraction for phase identification. As reported previously [24] and replotted in Fig. 1a, the 10 h 3

mechanically alloyed powder had major BCC and minor FCC phases in addition to minor WC contamination from the milling media. In the APS coating however, the FCC phase dominated while the BCC phase was present in minor amounts. Peaks corresponding to AB2O4 (A=Ni/Co/Fe, B=Cr/Al) type mixed oxides were also observed. This transformation has been attributed to in-flight oxidation of the HEA particles during spraying; thereby depleting Al, which is a known BCC-phase stabilizing element [25], for oxide formation. Micrographs corresponding to the 10h MA HEA powder and the APS coating cross section are illustrated in Fig. 1b and 1c for reference. In order to identify these phases, SEM and EDS were carried out on the coating cross section, which revealed multiple contrasts and compositions (Fig. 2a). Point EDS on phases presenting similar atomic number contrast revealed that there were essentially two categories of phases – alloys and oxides, as illustrated in Fig. 2b. The alloy phases exhibited a ‘white’ contrast, some of which appeared ‘plain’, corresponding to slightly Al depleted Al14(CoCrFeNi)86 composition (i.e., 14 at.% Al and Co, Cr, Fe, Ni distributed nearly equi-atomically in the remaining 86% at 21.5 at.% each), while others appeared ‘mottled’, having a nearly equiatomic quaternary Al2(CoCrFeNi)98 composition. The oxide phases appeared either as ‘grey’ – a mix of various Al-Cr-Fe oxides, or as ‘black’ alumina splats. It is important to note that a number of fine regions in the microstructure could not be resolved accurately via EDS owing to limitations on the electron beam size used in EDS. One such feature is illustrated in Fig. 3a, where z-contrast reveals phase separation at a sub-micron and nano length scale. Fig. 3b additionally indicates the various microstructural features observed in the coating cross section such as multiple types of splats, unmelts and micron-sized spherical features.

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Figure 1 (a) Phases observed on XRD of AlCoCrFeNi mechanically alloyed for 10 hours showing a major BCC and minor FCC, vs. phases observed in the corresponding APS coating, comprised of major FCC, minor BCC and spinel oxide peaks; (b) AlCoCrFeNi HEA powder particles after 10 h of mechanical alloying showing irregular morphology and wide particle size range; (b) cross sectional view of the as-sprayed coating on stainless steel substrate.

Figure 2 (a) The as-sprayed AlCoCrFeNi coating cross section viewed at a higher magnification and (b) the phase compositions identified via EDS.

Figure 3 (a) High magnification micrograph of the as sprayed coating showing phases in the sub-micron and nano-size range whose composition cannot be resolved via SEM-EDS, and (b) various splat morphologies constituting the HEAAPS coating microstructure.

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The coating cross section was analyzed via EPMA to capture complex features and more accurately assess the elemental distribution across the coating. The strength of this method is the wavelength dispersive X-ray spectrometer (WDS) that has a higher spectral resolution than EDS [26]. Figure 4a is an elemental map generated after subjecting the coating cross section to WDS, revealing in clear colours the elemental distribution across the various phases. The colour bar in Fig. 4a indicates that the concentration of an element in a phase can range between a maximum ‘pink’ (high concentration) to a minimum ‘blue-black’ (i.e., negligible concentration) colour code in the map. An analysis of the 6 maps for each element reveals that the white contrast corresponds to regions rich in Ni, Co, Fe, Cr, and Al. WDS resolved fine striations of one phase within another, thereby enabling an understanding of the evolution of such a microstructure. The oxide phases, corresponding to high oxygen concentration (pink/red in O map) have a correspondingly high concentration of either only Al, indicating alumina, or, Al, Cr and some Fe, indicating mixed oxides. The average of 5 spot-WDS per phase is reported in Fig. 4b. Interestingly, WDS resolved the regions corresponding to equiatomic AlCoCrFeNi, which was previously unseen with EDS. This phase falls under the ‘white mottled’ phase, which, as pointed out in our previous work, is composed of multiple phases that EDS could not resolve. Some of those regions may correspond to an Al-depleted CoCrFeNi with fine oxide precipitates, while other regions are unmelted and/or unoxidized alloy powder particles that underwent phase decomposition at a very fine scale [27], resulting in a similar mottled appearance. In addition WDS also captured another mixed oxide with nearly equal amounts of Al, Cr, Fe, Co and Ni, which EDS did not resolve.

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Figure 4 (a) Elemental map of the as sprayed AlCoCrFeNi APS coating generated via EPMA using a colour spectrum with ‘pink’ showing the maximum concentration of an element in a phases while ‘blue’ indicating its absence, along with (b) phase identification using WDS to measure elemental compositions in them, revealing the previously unidentified equiatomic AlCoCrFeNi phase in the coating.

TEM-EDS was carried out on cross-sectioned samples to probe the phases in the APS HEA coating and confirm the findings from EDS and WDS analysis as well as discover additional phases. Multiple regions were probed to determine the phase chemistry and crystal structure and are summarized in Fig. 5a and 5b. Regions corresponding to Al-depleted alloy phases – Al4(CoCrFeNi)96 and an Al and Cr depleted Ni-Co-Fe phase were observed. Among oxides, there was evidence to confirm the presence of alumina and the Al-Cr-Fe mixed oxide as observed from EDS and WDS. ‘Mixed oxide 1’ of composition ~62 at.% O, and 10 at.% each of Al, Cr, Fe, 5 at.% Co and 3 at,% Ni was observed via TEM-EDS. ‘Mixed oxide 2’ was observed with ~21 at.% of O, Ni, Co, and Fe, slightly lower Cr (~13 at.%) and negligible (2 at.%) Al. Dark and bright field images and corresponding selected area diffraction patterns taken on grains belonging to the alloy phases are illustrated in Figs. 6a through d, revealing an expected FCC crystal structure [28]. Figs. 6e through g illustrate a region of amorphous alumina that was observed to undergo in-situ crystallization during a 15-min long exposure to the electron beam [29]. This is evidence of the rapid melting and solidification processes that occur during APS.

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Figure 5 (a) TEM micrographs of as-sprayed APS AlCoCrFeNi coating and (b) TEM-EDS data representing elemental compositions of phases observed, confirming phases identified using SEM-EDS and WDS while also revealing Ni-Co-Fe alloy phase and multiple mixed oxide phases.

Figure 6 Phase specific TEM-EDS, micrographs and diffraction patterns for (a) Al4(CoCrFeNi)96 with FCC phase and (b) Al2O3. Amorphous alumina was observed to undergo in-situ crystallization during a 15-min exposure to the electron beam.

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A nanoindentation investigation of the cross section characterized the hardness and elastic modulus of the multiphase coating. Indents were carried out on only the ‘black’, ‘grey’ and ‘white’ regions since no distinction between mottled and plain white phases, or the various mixed oxide phases could be made because the indent region was selected by optical microscopy at 200× magnification. The result of an average of 15 indents/phase is shown in Fig. 7a, showing a clear difference in the phase hardnesses. The black alumina phase exhibits the highest hardness of 11+3 GPa, the grey mixed oxide had an intermediate hardness of 9+2 GPa, and the white alloy phase the least hardness of 6+2 GPa. For comparison, bulk alumina exhibits a hardness of 27 GPa [30]. The reduced elastic modulus of the three phases revealed general trends but were numerically overlapping within the error range of each other. Scanning probe (SPM) images of the phases, taken by raster scanning the selected region with the Berkovich tip, provides an insight into the phase morphologies (Fig. 7b). The alloy phase, being softer, is polished preferentially, thus appearing as valleys, while the harder oxide phases present as ridges and plateaus. The SPM technique captured the grain boundaries of the oxide phases, thereby illustrating coarse columnar as well as fine equiaxed, possibly nanocrystalline, grains; an indication of the varied cooling rates attained during solidification upon impact.

Figure 7 Nanoindentation results for the multi-phase alloy showing (a) phase-wise distribution of hardness and reduced elastic modulus, and (b) a scanning probe microscope (SPM) image of the cross section of the coating revealing coarse columnar and fine equiaxed grains in the oxide phase likely indicative of varying cooling rates during coating. The impression of one indent on the oxide phase has been encircled.

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4. Discussion Extensive spectroscopy of the HEA coating at various length scales revealed that there is a multitude of alloy and oxide phases distributed randomly across the coating. These results are summarized in Table 1. Further analysis of the coating microstructure across length scales provides morphological information concerning the splat behaviour of the HEA powder during APS. As mentioned earlier, Fig. 3b exhibits the following splat morphologies: (i) thick wave-like splats (~20×5 µm), usually of alloy composition; (ii) fine string-like thin splats (~20×2 µm), both of alloy and oxide compositions; (iii) discontinuous thick splats, which are usually oxides; (iv) large unmelted particles, which are usually of the alloy composition; and (v) microscopic circular features (<2 µm diameter) distributed randomly throughout the coating.

Table 1: Phases in the AlCoCrFeNi APS coating as identified via the characterization technique.

SEM-

Phase / Technique

XRD

Al4(CoCrFeNi)96





Al14(CoCrFeNi)86





EDS

WDS

  

AlCoCrFeNi (Al,Cr)2O3

B=Al,Cr)

EDS 

NiCoFe

AB2O4 (A=Ni,Co,Fe;

TEM-



















(AlCoCrFeNi)-O

The concepts governing the plasma spray process and HEA phase structure give rise to the homogeneously distributed features that confer the composite nature of the HEA-APS microstructure. . Fundamental work on the thermal and chemical history of a feedstock particle through flight, impact and coating formation [31], [32], [33], provide in-depth understanding. Further knowledge from this study has introduced case-specific parameters; that are (i) an oxygen sensitive 5 component equiatomic HEA composition, (ii) the MA synthesis technique, and (iii) a particle size range lower than typical for APS. The influence of particle size and thermal history through the plasma plume can be investigated by implementing an experimental design based on a particle size vs. particle temperature study that resolves the particle in-flight and impact behaviour. Knowledge of the

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mechanisms responsible for converting an AlCoCrFeNi particle into a composite coating can serve to optimize conditions for spraying mechanically alloyed HEAs. 4.1

Behaviour of particles in flight

4.1.1 Effect of temperature Different particles experience heating to different temperatures, as presented schematically in Fig. 8a, depending on the size of the particles, the energy of the plasma and the distance of the particle from the center of the plume (core). In-flight measurement of particle velocity and temperature is an invaluable tool to predict particle behaviour and finetune the coating properties [34]. The DPV 2000 (Tecnar Automation Ltd., Quebec, Canada) has been employed [24] to determine the particle temperature, velocity and size distribution along the axis of the plasma plume. AlCoCrFeNi particles at the plume core experience the highest temperatures, which was measured to be 2353 + 160 °C. Particle temperatures (Tp) in the medium energy zone, termed as the intermediate zone, reach 1200–1300 °C, whereas those in the outermost, peripheral region might experience up to 500 °C. The thermal history of individual particles depends on the particle size, surface area, thermal conductivity, and any chemical reactions that may occur in flight. As a result, depending on the melting point of the alloy, particles in the hottest zone certainly melt, with the finer particles being spheriodized further down the stream in a fashion similar to plasma atomization [23]. In the present case, the melting point of AlCoCrFeNi ranges from 1372 to 1388 °C [35]. In addition, the 5–10 nm nanocrystalline character of the powders would further decrease the melting point. Therefore, particles closer to the plasma core would melt, those in the intermediate region would not melt but might be softened, whereas those in the periphery would only be heated.

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Figure 8 (a) Schematic of plasma spray set up illustrating the 3 energy zones – periphery (particle temperature (Tp) <500 °C), intermediate (500
4.1.2 Effect of oxygen in the processing environment Oxygen concentration across the APS plume is relatively uniform. Thus, distance from the plasma plume center affects oxidation in terms of particle temperature, and the effective elemental diffusion and chemical reaction rates. The in-flight oxidation phenomenon [36], [37] is summarized as follows. (i) The extent of particle oxidation depends on particle diameter, velocity and shrouding gas. (ii) Oxygen adsorption after particle melting is usually the dominating oxidation mechanism rather than convective oxidation. (iii) Smaller sized particles are oxidized completely whereas larger particles only experience thin surface oxide formation. (iv) Oxidation kinetics competes with thermodynamics. Thus, thermodynamically metastable oxides often observed, but keeping in mind that this behaviour depends on the alloy composition and APS conditions. It is important to note that although the residence time for any particle in the plasma plume is tenths of milliseconds, this reaction time and temperature are still sufficient for the observation of oxide layers of several micrometers on deposited splats. In addition, for molten particles, oxygen pickup would be aided by faster diffusion. As well, in the present case for nanocrystalline mechanically alloyed powder, additional diffusion pathways are available in the form of nanocrystalline grain boundaries that would facilitate oxidation in solid or partially molten particles.

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In-flight oxidation can be largely avoided by employing vacuum plasma spray (VPS) or controlled atmospheric plasma spray (CAPS) methods rather than APS. Although APS is not the preferred process to manufacture low oxide coatings, it is possible to reduce the extent of oxidation by adjusting the APS parameters. 4.2

Splat behaviour of feedstock particles

4.2.1 Molten drop impacts Molten particles will splat and deposit as a thin layer before solidifying. Bonding is facilitated via mechanical interlocking with the rough substrate. Upon splatting there can be two major outcomes based on factors that include the (i) substrate temperature (Ts), (ii) thermal resistance between splat and substrate, (iii) substrate roughness, (iv) impact velocity, and (v) density of splat material. The splat spreads uniformly in the shape of a disc for a hot and smooth substrate [38]. The critical temperature of a specific substrate will be related to the feedstock. The molten particle on impacting the hot substrate does not attain sufficient undercooling and thus crystallization is delayed while spreading occurs. Gradually, as sufficient cooling occurs, along with loss of latent heat of solidification, the splat solidifies into a thin disc shaped splat. If the substrate is rough, the physical barriers to spreading can result in thicker splats. If the substrate is cold (Ts < critical temperature), crystallization occurs sooner than for a hot substrate. As a result, uniform spreading cannot take place and, instead, finger formation is observed around a central core region of the splat, a phenomenon termed as ‘fingering’ (please refer figure 2 in [38]). In addition to splat formation, liquid metal and oxide droplets will often result in microscopic droplets that splash away from the particle on impact. These <2 µm particles decorate the peripheral regions of the splat and are termed as ‘satellites’. Satellite splashed droplets solidify faster owing to their smaller size and larger surface area to volume ratio. They are engulfed by the next layer of liquid phase spreading and appear as microscopic circular features distributed randomly within or around splats. 4.2.2 Unmelted particle impacts Complete melting does not occur for particles in the higher size or lower temperature zones and solid particles with some degree of softening impact the substrate. Only particles with sufficient kinetic energy will be embedded in the coating. These particles appear as ‘unmelts’ during metallographic examination of cross sections. They may still undergo oxidation during flight, albeit to a lesser extent than their molten counterparts due to slower 13

diffusion at lower temperatures. This thin oxide layer can be shattered on impact against the substrate at high speeds, and may not be reflected in the coating as a continuous oxide layer surrounding the particle, but as randomly scattered fragments of oxide.

4.3

Splat formation process of AlCoCrFeNi under the current spray parameters Studies on the oxidation of bulk AlCoCrFeNi [39], particularly during isothermal

oxidation in excess of 1000 °C, report the formation of a dual chromia-alumina layer. Transitional oxides, such as NiO, FeO, Cr2O3, Al2O3, CoO form in the first few minutes of oxidation. These give way to the thermodynamically favoured alumina and chromia as oxidation proceeds to a steady state. It is important to note here that the growth of alumina and chromia are governed by diffusivities, either of oxygen inward or Al/Cr outward, which is a function of temperature, diffusion distance and time. The in-flight time scale for APS is of the order of milliseconds and, in addition, the HEA particles are nanocrystalline in character. While Al and Cr will preferentially oxidize, they will also combine with less stable NiO, CoO and FeO to form AB2O4 type spinels (NiCr2O4, FeAl2O4, CoAl2O4), whose enthalpies of formation are more negative than that of alumina and chromia [40]. The preferential oxidation of Al from the alloy will deplete the remaining alloy to Alx(CoCrFeNi), which is known to be FCC structured. The extensive inflight oxidation deteriorates the oxidation resistance expected from the AlCoCrFeNi coating. Thus, it is imperative to adjust the APS parameters to reduce the extent of in-flight oxidation. The above complex and competing phenomena have led to the construction of a particle size (Dp) vs. particle temperature (Tp) matrix to gain a better understanding. The intent is to illustrate the interaction between the HEA particles in the plasma stream with oxygen from atmosphere, during flight, and on impact; all of which together result in the coating microstructure. To reiterate from Fig. 8a, particle temperature ranges have been defined as ‘low’ (Tp<500 °C), ‘intermediate’ (500
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have been denoted by black colour while the alloy phases have been designated blue. The following discussion based on Figs. 9 and 10 elucidates each specific scenario.

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Figure 9 Expected particle behaviour (effect of temperature and oxidizing atmosphere) for various size ranges in-flight; blue denotes metal/alloy and black represents oxide.

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Figure 10 Expected behaviour of particles on impacting the substrate across various particle size and particle temperature zones.

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4.3.1 Small particles (Dp < 5µm) in the core region (1,300
Figure 11a Microstructural evolution of a small (Dp<5µm) AlCoCrFeNi particle in the core (1,300
4.3.2 Small particles (Dp < 5µm) in the intermediate region (500
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relatively longer flight time, allowing some selective oxidation to occur. Particles will undergo almost complete oxidization and form predominantly alumina and spinels, but there may be a small unoxidized Ni-Co core, which will remain within the particle. On impact, the molten, spherical and mostly oxidized droplet will most likely splat too. These distinct phases within the droplet will cool at different rates depending on their thermal conductivities. Hence, the phenomenon of splat fingering with a low degree of flattening is expected for these particles. These mixed oxide splats with some alloy inclusions have been marked ‘2’ and outlined in Fig. 11b.

Figure 11b Microstructural evolution of a small (Dp<5µm) AlCoCrFeNi particle in the intermediate (500
4.3.3 Small particles (Dp < 5µm) in the peripheral region (Tp< 500 °C) These particles are not expected to experience temperatures above 500 °C. They might undergo low temperature surface oxidation, however, these fine particles are unlikely to reach the substrate, and will probably ‘bounce off the flame’ [41]. They do not get deposited in the coating.

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4.3.4 Medium sized particles (5
Figure 11c Microstructural evolution of a medium (5
4.3.5 Medium sized particles (5
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thin surface oxide layer might shatter on impact and may not be reflected entirely in the coating microstructure. There is also a possibility for convective flow within the molten alloy core, resulting in an alloy splat with trapped oxide debris. Representative splats for this combination have been outlined in Fig. 11d (marked as ‘5’).

Figure 11d Microstructural evolution of a medium (5
4.3.6 Medium sized particles (5
4.3.7 Large particles (15
again splat, with the surface oxide layers fragmenting as the alloy core spreads, resulting in characteristic lamellar splat patterns. Splashing is also possible, which will result in discontinuous alloy/oxide fingers surrounding the central splat region. These splats have been illustrated schematically and representative splats have been identified in the coating cross section in Fig. 11e (marked as ‘7’).

Figure 11e Microstructural evolution of a large (15
4.3.8 Large particles (15
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Figure 11f Microstructural evolution of a large (15
4.3.9 Large particles (15
Vardelle et al. [42] have predicted that in an ideal feedstock injection scenario, around 90% of feedstock particles go through the core region of the plume while only around 10% travel through the periphery. It can thus be summarized that the HEA-APS microstructure is made up predominantly of particles travelling in the high and medium energy zones. In addition, each particle size category contains a range in itself, and the extent of melting and/or oxidation will be affected by it.

5. Summary and Conclusion An AlCoCrFeNi HEA coating was generated using plasma spray and studied extensively via various electron microscopy and spectroscopy techniques. A multitude of phases distributed randomly and homogenously were discovered and an attempt was made to understand the evolution of this composite coating microstructure.

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By applying the mechanisms of particle interaction with plasma and the oxidizing atmosphere, we have predicted how the particle size range and particle temperature of HEA alloys such as AlCoCrFeNi, can have severe consequences on the coating’s final microstructure. It can be correlated from the above behaviour that surface oxidation in cases 5, 7 and 8 (Figs. 9 and 10) may contribute to the relatively higher Al-containing alloy phase (Al14(CoCrFeNi)86) whereas extensive oxidation in cases 2 and 4 may result in nearly Aldepleted Al4(CoCrFeNi)96. It is proposed that a narrow particle size range of feedstock can help improve the coating microstructure, in that a particular type of splat and phase can be promoted. In addition, it is possible to reduce the in-flight oxidation by choosing a larger mean size of feedstock, which largely depends on the powder synthesis technique and parameters. Overall, this experiment gives insights about the experience of AlCoCrFeNi HEA particles through a plasma stream in air through to coating formation. It indicates the various end results possible, helping streamline the powder-plasma interaction for a better coating microstructure.

Acknowledgement The authors would like to acknowledge Dr. Aloke Paul, Dept. of Materials Engineering, Indian Institute of Science, Bengaluru, India for assistance with EPMA. This study was supported by the Australian Research Council (ARC) under the Industrial Transformation Training Centre project IC180100005 that is titled “Surface Engineering for Advanced Materials”, SEAM. The authors would also like to acknowledge the Joint Doctoral Degree Program between Swinburne University of Technology and the Indian Institute of Technology Madras for facilitating this collaboration.

Declaration of interests: none.

References

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Understanding the Microstructural Evolution of High Entropy Alloy Coatings Manufactured by Atmospheric Plasma Spray Processing

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Highlights Complete characterization of AlCoCrFeNi atmospheric plasma sprayed coatings In-flight oxidation results in Al2O3, Cr2O3, spinels, residual unoxidized alloy phases Effect of particle size and position relative to plume core on splatting behavior Particles <5 µm near plume core melt and oxidize completely forming oxide splats Particles >15 µm near plume core melt completely with surface oxides and alloy splats

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