Influence of Annealing on the Microstructures and Oxidation Behaviors of Al 8 ( CoCrFeNi ) 92 , Al 15 ( CoCrFeNi ) 85 , and Al 30 ( CoCrFeNi ) 70 High-Entropy Alloys

The understanding of the oxidation behaviors of as-cast and annealed high-entropy alloys (HEAs) is currently limited. This work systematically investigates the influence of annealing on the microstructures and oxidation behaviors of AlCoCrFeNi-based HEAs. Annealing was found to alter the distribution of Al-rich phases which caused a change in the oxidation mechanisms. In general, all three of the investigated HEAs displayed some degree of transient oxidation at 1050 ◦C that was later followed by protective, parabolic oxide growth. The respective oxidation behaviors are discussed relative to existing oxide formation models for Ni–Cr–Al alloys.


Introduction
Recently, a novel class of compositionally-complex alloys termed high-entropy alloys (HEAs) have attracted substantial interest in the scientific community [1][2][3][4].In contrast to conventional alloys, HEAs lack a primary constituent and are generally composed of five or more elements in nearly equal proportions [1,2].It is well-known that these alloys typically form phases with simple FCC, BCC, and HCP crystal structures and can contain ordered intermetallic phases [4].HEAs also exhibit the inherent potential to be alloyed with high concentrations of Al and/or Cr to promote enhanced oxidation resistances without necessarily forming various intermetallic phases that can be deleterious to strength [5][6][7].In addition, HEAs have been reported to exhibit sluggish diffusion kinetics [8][9][10] and high thermal stabilities [11][12][13], making them ideal candidates for use in high temperature applications.However, a fundamental understanding of the active oxidation mechanisms are required in order to effectively implement HEAs in these environments.
With regard to oxidation, there have been a number of publications that have reported the oxidation behaviors of HEAs.These studies include both transition metal based [6,7,[14][15][16][17][18][19][20][21][22][23][24][25] and refractory metal based [26][27][28][29] HEA systems.In general, it was reported that elements that commonly oxidize in less-complex, conventional alloy systems (i.e., Ni, Mn, Cr, Al, Ti) also tend to preferentially oxidize in compositionally complex alloys.For example, Daoud et al. [23] Ni 17 HEAs at 800 • C and 1000 • C in air.At 800 • C, the low Al content alloy formed a combination of NiO, Fe-oxide, Cr 2 O 3 , and Al 2 O 3 .However, at 1000 • C, the same alloy preferentially formed Cr 2 O 3 above Al 2 O 3 .As for the other two HEAs, which contained considerably higher concentrations of Al, Al 2 O 3 scales tended to form at both temperatures.Similarly, Holcomb et al. [24] investigated the oxidation behaviors of eight model CoCrFeMnNi-based HEAs, along with various other conventional alloys.It was reported that the Cr and Mn containing HEAs preferentially formed both Cr 2 O 3 and Mn oxides.This idea can also be extended to the realm of refractory HEAs.Senkov et al. [26] examined the 1000 • C oxidation behavior of an arc-melted NbCrMo 0.5 Ta 0.5 TiZr HEA.The oxidation behavior was reported to be superior to that of similar Nb-based refractory alloys, with an oxide consisting of a combination of NbCrO 4 , TiO 2 , and Cr 2 O 3 .Likewise, Gorr et al. [27,28] investigated the high temperature oxidation behaviors of a 20Nb-20Mo-20Cr-20Ti-20Al HEA with and without Si additions.It was reported that in general, the HEA without Si followed a linear oxide growth rate law and formed a porous, non-protective mix of various oxides.Interestingly, a small addition of 1 at % Si promoted parabolic oxide growth kinetics, to some extent, and facilitated the formation of a thinner, nearly continuous scale with Al and Cr rich layers.
Based on the published literature on the oxidation behavior of HEAs, it is evident that they tend to oxidize in a similar manner compared to simple alloys based on the same elements.However, there are few publications that attempt to systematically correlate the oxidation behavior of HEAs with existing oxide formation models [6,7,15].There is also lack of understanding of the influence of annealing on the active oxidation mechanisms, as most published results emphasize as-cast alloys.This work investigates the microstructures and oxidation behaviors of three AlCoCrFeNi based HEAs with various Al contents.This particular alloy system is ideal for a fundamental oxidation study since it contains both Al and Cr, which are known to produce protective Cr 2 O 3 and/or Al 2 O 3 scales [30].Additionally, the AlCoCrFeNi system is one of the most thoroughly investigated to date [1,4,10,14,[31][32][33][34][35][36][37][38][39][40][41][42][43].This paper will systematically report the influence of annealing on the microstructures and active oxidation mechanisms.The results will be discussed relative to existing oxide formation models developed for model Ni-Cr-Al alloys [30].

Materials and Methods
Three bulk alloy buttons with the compositions of Al 8 (CoCrFeNi) 92 , Al 15 (CoCrFeNi) 85 , and Al 30 (CoCrFeNi) 70 (designated: Al 8 , Al 15 , and Al 30 ) were arc-melted from pure elemental constituents on a water-cooled copper hearth in an inert ultra-high purity (UHP) argon atmosphere.Each button was flipped and re-melted five times to promote homogeneity.CALPHAD based thermodynamic modeling using the TCNI8 database in ThermoCalc™ was used in conjunction with microstructural analyses to select the appropriate annealing conditions [44][45][46][47].Annealing was carried out at 1050 • C for 120 h under an inert, dynamic UHP Ar atmosphere.
The microstructures of the as-cast, annealed, and oxidized specimens were characterized using a combination of X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), scanning-transmission electron microscopy-high angle annular darkfield (STEM-HAADF), and energy-dispersive X-ray spectroscopy (EDS) in both SEM and TEM modes.XRD measurements were conducted on a Philips X'pert MPD XRD (Philips, Amsterdam, The Netherlands) using an accelerating voltage of 45 kV, a current of 40 mA, and a step size of 0.1 degrees.SEM images were captured at 20 kV using a spot size of 10 in a JEOL 7000F SEM (JEOL Ltd., Tokyo, Japan).Specimens for SEM were mounted in hot phenolic resin and polished using standard metallographic techniques up to a 0.05 micron finish.STEM-HAADF images and selected area diffraction patterns (SADPs) were captured on a 200-keV FEI Tecnai G 2 F-20 Supertwin scanning-transmission electron microscope.TEM samples were prepared using a variation of the focused-ion-beam (FIB) lift-out method in an FEI Quanta 200 3D Dual Beam FIB microscope (FEI Company, Hillsboro, OR, USA) [48].Discontinuous isothermal oxidation tests of the as-cast and annealed specimens were conducted at 1050 • C in a tube furnace under ambient laboratory air.Prior to testing, specimens were ground to a 1200 grit surface finish followed by ultrasonic cleaning in acetone.Samples were periodically removed from the furnace and weighed using a microbalance (accuracy of 10 −6 g).

Microstructures of the As-Cast and Annealed HEAs
Table 1 shows the bulk compositions, as measured using EDS.The actual compositions were found to be in good agreement with the intended compositions.Figure 1 shows representative microstructures for the as-cast, (a-c), and annealed, (d-f), Al 8 , Al 15 , and Al 30 HEAs.All of the as-cast microstructures shown here are taken from [7,15].Figure 1a shows a backscattered electron (BSE) image of the as-cast Al 8 HEA, which consisted of a bright, high atomic number contrast (high-Z), matrix with dark, low atomic number Z contrast (low-Z), interdendritic regions.Some AlN inclusions were also present and were attributed to impurities in the starting materials.
Table 1 shows the bulk compositions, as measured using EDS.The actual compositions were found to be in good agreement with the intended compositions.Figure 1 shows representative microstructures for the as-cast, (a-c), and annealed, (d-f), Al8, Al15, and Al30 HEAs.All of the as-cast microstructures shown here are taken from [7,15].Figure 1a shows a backscattered electron (BSE) image of the as-cast Al8 HEA, which consisted of a bright, high atomic number contrast (high-Z), matrix with dark, low atomic number Z contrast (low-Z), interdendritic regions.Some AlN inclusions were also present and were attributed to impurities in the starting materials.annealed (d-f) conditions, respectively.The inset images in (b,c) are higher resolution scanning-transmission electron microscopy-high angle annular darkfield (STEM-HAADF) images of the Al15 and Al30 HEAs.The as-cast BSE images are taken from [7,15].
Annealing of the Al8 HEA resulted in the decomposition of the low-Z interdendritic regions into a dense distribution of low-Z precipitates with platelet-like morphologies, as is shown in the BSE image in Figure 1d.Although not detailed here, TEM analysis was also conducted on the as-cast Al8 HEA [7,15].It was determined that the continuous matrix phase was FCC, while the low-Z interdendritic regions were multiphase consisting of an ordered B2 phase interspersed with high-Z BCC precipitates.The phase compositions are shown in Table 1.These BCC precipitates were not observed in the annealed Al8 HEA. Figure 2a shows a STEM-HAADF image for the annealed Al8 HEA.The matrix was found to have an FCC crystal structure, while the low-Z interdendritic regions were found to have a B2 crystal structure, as evident from the selected area electron diffraction patterns (SADPs) in Figure 2b,c, respectively.The B2 regions were enriched in Ni and Al, while the FCC region was Cr and Fe rich (See Table 1).
Figure 1b shows a representative BSE image and inset STEM-HAADF image for the as-cast Al15 HEA.The as-cast HEA consisted of colonies of high-Z contrast Widmanstätten-like lamellae and highly transformed, multiphase regions, Figure 1b.TEM analysis revealed the high-Z contrast phase to be FCC, whereas the transformed regions consisted of a BCC matrix with cuboidal B2 precipitates [7,15].The phase chemistries are listed in Table 1. Figure 1e shows a representative BSE image for the annealed Al15 HEA.The annealed microstructure consisted of a semi-continuous mixture of coarsened high-Z contrast lamellae and globular regions intermixed with a discontinuous low-Z phase.
Table 1.Chemistries of the as-cast HEAs in at % (SEM-EDS).The respective phase chemistries for each as-cast HEA and after annealing for 120 h/1050 °C are also shown.** As-cast chemistries taken from [7,15].Annealing of the Al 8 HEA resulted in the decomposition of the low-Z interdendritic regions into a dense distribution of low-Z precipitates with platelet-like morphologies, as is shown in the BSE image in Figure 1d.Although not detailed here, TEM analysis was also conducted on the as-cast Al 8 HEA [7,15].It was determined that the continuous matrix phase was FCC, while the low-Z interdendritic regions were multiphase consisting of an ordered B2 phase interspersed with high-Z BCC precipitates.The phase compositions are shown in Table 1.These BCC precipitates were not observed in the annealed Al 8 HEA. Figure 2a shows a STEM-HAADF image for the annealed Al 8 HEA.The matrix was found to have an FCC crystal structure, while the low-Z interdendritic regions were found to have a B2 crystal structure, as evident from the selected area electron diffraction patterns (SADPs) in Figure 2b,c, respectively.The B2 regions were enriched in Ni and Al, while the FCC region was Cr and Fe rich (See Table 1).
Figure 1b shows a representative BSE image and inset STEM-HAADF image for the as-cast Al 15 HEA.The as-cast HEA consisted of colonies of high-Z contrast Widmanstätten-like lamellae and highly transformed, multiphase regions, Figure 1b.TEM analysis revealed the high-Z contrast phase to be FCC, whereas the transformed regions consisted of a BCC matrix with cuboidal B2 precipitates [7,15].The phase chemistries are listed in Table 1. Figure 1e shows a representative BSE image for the annealed Al 15 HEA.The annealed microstructure consisted of a semi-continuous mixture of coarsened high-Z contrast lamellae and globular regions intermixed with a discontinuous low-Z phase.Figure 2b shows a STEM-HAADF image collected from the annealed Al 15 HEA.The high-Z, semi-continuous matrix was found to have an FCC crystal structure, while the low-Z regions had B2 crystal structures, as can be seen evident in the SADPs in Figure 2e,f.No precipitates were visually evident within the B2 phase.However, extra reflections were observed in the SADP, indicating the possibility of a nanoscale phase, Figure 2f.Like in the annealed Al 8 HEA, the FCC phase was found to be Cr and Fe rich, while the B2 phase was found to be Ni and Al rich (see Table 1).
Figure 1c shows a representative BSE image and inset STEM-HAADF image for the as-cast Al 30 HEA.Remnants of dendritic segregation are evident in the BSE image.However, STEM-HAADF imaging revealed the entire microstructure to be composed of a fine-scale distribution of high-Z contrast precipitates with BCC crystal structures dispersed in a low-Z contrast B2 matrix [7,15].The as-cast phase chemistries are shown in Table 1. Figure 2b shows a STEM-HAADF image collected from the annealed Al15 HEA.The high-Z, semi-continuous matrix was found to have an FCC crystal structure, while the low-Z regions had B2 crystal structures, as can be seen evident in the SADPs in Figure 2e,f.No precipitates were visually evident within the B2 phase.However, extra reflections were observed in the SADP, indicating the possibility of a nanoscale phase, Figure 2f.Like in the annealed Al8 HEA, the FCC phase was found to be Cr and Fe rich, while the B2 phase was found to be Ni and Al rich (see Table 1).
Figure 1c shows a representative BSE image and inset STEM-HAADF image for the as-cast Al30 HEA.Remnants of dendritic segregation are evident in the BSE image.However, STEM-HAADF imaging revealed the entire microstructure to be composed of a fine-scale distribution of high-Z contrast precipitates with BCC crystal structures dispersed in a low-Z contrast B2 matrix [7,15].The as-cast phase chemistries are shown in Table 1.Similar BCC+B2 microstructures have been reported in as-cast HEAs with similar compositions by Wang et al. [32,35,39].Similar to the Al15 HEA, annealing caused significant coarsening of the microstructure.Figure 1f shows a BSE image of the annealed Al30 HEA, which consisted of an Similar BCC+B2 microstructures have been reported in as-cast HEAs with similar compositions by Wang et al. [32,35,39].Similar to the Al 15 HEA, annealing caused significant coarsening of the microstructure.Figure 1f shows a BSE image of the annealed Al 30 HEA, which consisted of an apparent mixture high-Z contrast and low-Z contrast regions.TEM analysis was conducted on the annealed Al 30 HEA (Figure 2g).The low-Z contrast regions were found to be precipitate-free, with a B2 crystal structure that was Ni and Al rich, Figure 2i and Table 1.The high-Z contrast regions were found to be composed of two phases; a high-Z contrast matrix phase with a BCC structure that was Cr and Fe rich and a lower-Z contrast precipitate phase with a B2 crystal structure, Figure 2g,h.Differentiation between the two crystal structures (BCC matrix and B2 precipitate) was determined by observing the <001> B2 || <001> BCC and <011> B2 || <011> BCC SADPs near the centers and outer edges of the high-Z phase.Near the center, which contained a large number of precipitates, the diffraction patterns contained clear superlattice reflections that were consistent with (100) B2 reflections.However, diffraction patterns taken near the outer edges of the high-Z phase, which was precipitate free, had no superlattice reflections inferring that the high-Z phase is BCC and that the precipitates are B2.
To better elucidate the phase evolution in each HEA post annealing, experimental phase fractions were determined using two-dimensional image contrast thresholding.The phases were sorted based on their particular contrast and segmented to determine a respective area phase fraction.This was then converted into a three-dimensional value assuming an equivalent spherical radius [49].The results are shown in Table 2 along with the thermodynamically predicted phase fractions.In the Al 8 and Al 15 HEAs, the annealing process appears to have destabilized the BCC phase, as indicated by a decrease in the respective phase fractions.This observation is consistent with the thermodynamic predictions which showed no BCC phase to be stable at the 1050 • C annealing temperature.In general, the estimated phase fractions for the annealed HEAs were found to be reasonably accurate.It is also important to note that the phase chemistries for the as-cast HEAs, reported in [7,15], were not significantly different from the corresponding phases in the annealed condition.This infers that annealing does not significantly alter the phase chemistries, but does tend to change the phase size and morphology.The phase fraction of B2 increased for all of the HEAs after annealing.

Table 2.
Respective phases from each HEA along with the estimated phase fractions for the as-cast and annealed states.For comparison, the thermodynamically predicted phase fractions (at 1050 • C) as predicted from ThermoCalc are also shown.

Oxidized Microstructures
Figure 3 shows representative BSE images of the as-cast and annealed HEAs after 50 h of oxidation at 1050 • C. In contrast to the as-cast HEAs [7,15], all of the annealed HEAs exhibited higher degrees of scale spallation.Oxidation of the as-cast Al 8 HEA resulted in the formation of an outer Cr 2 O 3 scale, along with a sublayer of oxygen-enriched alloy (labeled "metal") atop an internal, semi-continuous Al 2 O 3 scale, Figure 3a,b [7,15].Oxidation of the annealed Al 8 HEA resulted in the formation of Al 2 O 3 , (Al+Cr) 2 O 3 , and an Fe+Cr+Ni-rich spinel, as shown by the plan-view and cross-sectional BSE images in Figure 3c,d.Spinel phases of this type are commonly observed on M-Cr-Al alloys during the initial stages of oxidation [30].
Figure 3e,f show plan-view and cross-sectional BSE images for the as-cast Al 15 HEA after oxidation [7,15] Figure 3i,j show plan-view and cross-sectional BSE images for the as-cast Al 30 HEA after 50 h of oxidation [7,15].An external Cr 2 O 3 scale formed above an underlying, continuous scale of Al 2 O 3 .In this particular case, no spinel was observed and very few AlN precipitates were present, Figure 3j.Oxidation of the annealed Al 30 HEA resulted in the sole formation of Al 2 O 3 , Figure 3k,l.In contrast to the Al 8 and Al 15 HEAs, no internal oxidation was observed.
To verify the post oxidation phases in the annealed/oxidized HEAs, XRD analysis was done in plan-view on bulk specimens.Figure 4a  AlN precipitates were also observed beneath the internal Al2O3 scale.These nitrides are consistent with the observations of Zhang et al. [22] who reported the formation of AlN precipitates in Al0.5FeCoCrNi and Al0.5CoCrFeNiSi0.2HEAs oxidized in air at 900 °C.Oxidation of the annealed Al15 HEA resulted in the formation of Al2O3 with an underlying Fe+Cr+Ni-rich spinel, Figure 3g,h.Similar observations have been reported in austenitic Fe-Ni-Cr-Al alloys [50].Figure 3i,j show plan-view and cross-sectional BSE images for the as-cast Al30 HEA after 50 h of oxidation [7,15].An external Cr2O3 scale formed above an underlying, continuous scale of Al2O3.In this particular case, no spinel was observed and very few AlN precipitates were present, Figure 3j.Oxidation of the annealed Al30 HEA resulted in the sole formation of Al2O3, Figure 3k,l.In contrast to the Al8 and Al15 HEAs, no internal oxidation was observed.
To verify the post oxidation phases in the annealed/oxidized HEAs, XRD analysis was done in plan-view on bulk specimens.Figure 4a shows representative XRD patterns for the annealed HEAs after 50 h of oxidation.Each alloy contained peaks associated with a combination of Al2O3, spinel (NiCr2O4), and FCC, BCC, and/or B2 type phases.Additionally, some peaks associated with Cr2O3 were present and are likely associated with the Cr-rich regions observed in some regions of the Al2O3 scales.

Oxidation Behavior
The relative mass changes for the as-cast/oxidized and annealed/oxidized HEAs are shown in Figure 4b.Each of the as-cast and annealed HEAs exhibited some degree of transient oxidation that transitioned to parabolic oxide growth after approximately 5 to 15 h depending on the alloy composition.The behaviors were comparable to those of various Ni-Cr-Al alloys and some wrought Ni-based superalloys [7,30,51].The as-cast Al15 and Al30 HEAs appeared to plateau after 30 h and 10 h, respectively.As expected, the highest Al content HEA, Al30, exhibited the smallest mass change.The parabolic oxide growth constants (kP) for each HEA are shown in Table 3.All of the values were between those reported for model Group II and Group III Ni-Cr-Al alloys [30].There were only small variations in values between the as-cast and annealed conditions, which is surprising since the post oxidation microstructures were quite different.However, the oxidation behaviors indicate that all of the HEAs exhibit some degree of oxidation resistance.

Oxidation Behavior
The relative mass changes for the as-cast/oxidized and annealed/oxidized HEAs are shown in Figure 4b.Each of the as-cast and annealed HEAs exhibited some degree of transient oxidation that transitioned to parabolic oxide growth after approximately 5 to 15 h depending on the alloy composition.The behaviors were comparable to those of various Ni-Cr-Al alloys and some wrought Ni-based superalloys [7,30,51].The as-cast Al 15 and Al 30 HEAs appeared to plateau after 30 h and 10 h, respectively.As expected, the highest Al content HEA, Al 30 , exhibited the smallest mass change.The parabolic oxide growth constants (k P ) for each HEA are shown in Table 3.All of the values were between those reported for model Group II and Group III Ni-Cr-Al alloys [30].There were only small variations in values between the as-cast and annealed conditions, which is surprising since the post oxidation microstructures were quite different.However, the oxidation behaviors indicate that all of the HEAs exhibit some degree of oxidation resistance.

Discussion
As reported in [7,15], the as-cast HEAs were found to exhibit post-oxidation microstructures indicative of the sequential oxidation reported for model Group II and Group III Ni-Cr-Al alloys [30,52].Neglecting transient oxide formation, Cr2O3 tends to form first, and evolves in tandem with Al2O3 formation.Based on the microstructural observations, one can infer that increasing the Al content of the as-cast HEAs enhances the oxidation resistance by promoting the formation of a morecontinuous Al2O3 scale with/without Cr2O3 in contrast to forming a discontinuous, internal Al2O3 scale at low Al concentrations.This is not the case for the annealed HEAs, which formed a combination of predominantly Al2O3 and spinel phases.Interestingly, in most cases, the spinel phase was observed to form beneath the outer Al2O3 scale.Since the phase chemistries between the as-cast and annealed HEAs were not significantly different, the change in oxidation mechanism is most likely related to the relative phase fractions and morphologies.
Annealing was found to alter both of these characteristics promoting increased phase fractions of the Ni+Al-rich B2 phase in all three HEAs.It is proposed that this increased phase fraction and the associated more uniform distribution of the B2 phase within the microstructure provided a larger Al reservoir near the alloy surfaces, creating a more favorable condition for the formation of Al2O3.Delaunay et al. [50] demonstrated that the oxidation resistance of austenitic Fe-Ni-Cr-Al alloys could be improved by annealing the alloys to induce precipitation of the B2 β-NiAl phase.During prolonged oxidation exposure, the Al2O3 scale depletes the underlying alloy of Al resulting in near surface regions with relatively high concentrations of Fe, Co, and Cr.It is suggested that once the Al content is depleted to a critical level so that the favorability of Al2O3 formation is diminished, the formation of the underlying spinel phase takes precedence.The slightly higher mass gain exhibited by the annealed alloys, most prominently at longer oxidation times, can be explained by the formation of the spinel phase, which does not provide oxidation resistance.The spinel phase was not

Discussion
As reported in [7,15], the as-cast HEAs were found to exhibit post-oxidation microstructures indicative of the sequential oxidation reported for model Group II and Group III Ni-Cr-Al alloys [30,52].Neglecting transient oxide formation, Cr 2 O 3 tends to form first, and evolves in tandem with Al 2 O 3 formation.Based on the microstructural observations, one can infer that increasing the Al content of the as-cast HEAs enhances the oxidation resistance by promoting the formation of a more-continuous Al 2 O 3 scale with/without Cr 2 O 3 in contrast to forming a discontinuous, internal Al 2 O 3 scale at low Al concentrations.This is not the case for the annealed HEAs, which formed a combination of predominantly Al 2 O 3 and spinel phases.Interestingly, in most cases, the spinel phase was observed to form beneath the outer Al 2 O 3 scale.Since the phase chemistries between the as-cast and annealed HEAs were not significantly different, the change in oxidation mechanism is most likely related to the relative phase fractions and morphologies.
Annealing was found to alter both of these characteristics promoting increased phase fractions of the Ni+Al-rich B2 phase in all three HEAs.It is proposed that this increased phase fraction and the associated more uniform distribution of the B2 phase within the microstructure provided a larger Al reservoir near the alloy surfaces, creating a more favorable condition for the formation of Al 2 O 3 .Delaunay et al. [50] demonstrated that the oxidation resistance of austenitic Fe-Ni-Cr-Al alloys could be improved by annealing the alloys to induce precipitation of the B2 β-NiAl phase.During prolonged oxidation exposure, the Al 2 O 3 scale depletes the underlying alloy of Al resulting in near surface regions with relatively high concentrations of Fe, Co, and Cr.It is suggested that once the Al content is depleted to a critical level so that the favorability of Al 2 O 3 formation is diminished, the formation of the underlying spinel phase takes precedence.The slightly higher mass gain exhibited by the annealed alloys, most prominently at longer oxidation times, can be explained by the formation of the spinel phase, which does not provide oxidation resistance.The spinel phase was not observed in the annealed Al 30 HEA because the Al content, in comparison to the Al 8 and Al 15 HEAs, was likely high enough to sustain Al 2 O 3 formation, thus suppressing the formation of spinel.

Conclusions
The influences of pre-oxidation annealing on the microstructures and oxidation behaviors of three AlCoCrFeNi HEAs were investigated.The following conclusions can be drawn: (1) The as-cast Al 8 and Al 15 HEAs consisted of a combination of FCC, BCC, and B2 solid solution phases.Annealing at 1050 • C for 120 h resulted in significant coarsening of the FCC and B2 phases and dissolution of the primary BCC phase.The as-cast Al 30 HEA consisted of a uniform distribution of sub-micron sized BCC precipitates with spherical morphologies dispersed in a B2 matrix.Annealing of the Al 30 HEA also caused substantial microstructural coarsening and the formation of additional nano-scale B2 precipitates inside the BCC phase.In all cases, annealing increased the phase fraction of the B2 phase at the expense of the FCC and/or BCC phases.Although annealing had a large impact on the post oxidation microstructures, it seemed to have lesser influence on the parabolic growth rate constants.

Figure 1 .
Figure 1.Representative backscattered electron (BSE) images of the Al 8 , Al 15 , and Al 30 high-entropy alloys (HEAs) in as-cast (a-c) and annealed (d-f) conditions, respectively.The inset images in (b,c) are higher resolution scanning-transmission electron microscopy-high angle annular darkfield (STEM-HAADF) images of the Al 15 and Al 30 HEAs.The as-cast BSE images are taken from [7,15].

Figure 2 .
Figure 2. STEM-HAADF images and corresponding selected area diffraction patterns (SADPs) for the annealed Al8 (a-c); Al15 (d-f); and Al30 (g-i) HEAs.The open circles indicate the locations from which the SADPs were collected.

Figure 2 .
Figure 2. STEM-HAADF images and corresponding selected area diffraction patterns (SADPs) for the annealed Al 8 (a-c); Al 15 (d-f); and Al 30 (g-i) HEAs.The open circles indicate the locations from which the SADPs were collected.
. An external NiCr 2 O 4 spinel formed on top of an underlying layer of Cr 2 O 3 .Additionally, an internal scale of semi-continuous Al 2 O 3 was observed directly below the Cr 2 O 3 scale.AlN precipitates were also observed beneath the internal Al 2 O 3 scale.These nitrides are consistent with the observations of Zhang et al. [22] who reported the formation of AlN precipitates in Al 0.5 FeCoCrNi and Al 0.5 CoCrFeNiSi 0.2 HEAs oxidized in air at 900 • C. Oxidation of the annealed Al 15 HEA resulted in the formation of Al 2 O 3 with an underlying Fe+Cr+Ni-rich spinel, Figure 3g,h.Similar observations have been reported in austenitic Fe-Ni-Cr-Al alloys [50].
shows representative XRD patterns for the annealed HEAs after 50 h of oxidation.Each alloy contained peaks associated with a combination of Al 2 O 3 , spinel (NiCr 2 O 4 ), and FCC, BCC, and/or B2 type phases.Additionally, some peaks associated with Cr 2 O 3 were present and are likely associated with the Cr-rich regions observed in some regions of the Al 2 O 3 scales.Metals 2016, 6, 222 6 of 10

Figure 3 .
Figure 3. Plan-view and cross-sectional BSE micrographs for the as-cast and annealed Al 8 (a-d); Al 15 (e-h); and Al 30 (i-l) HEAs after 50 h of oxidation at 1050 • C.

Figure 4 .
Figure 4. XRD patterns for the annealed HEAs after 50 h of oxidation at 1050 °C in air (a); and normalized mass change curves for the as-cast/oxidized and annealed/oxidized HEAs (b).

Figure 4 .
Figure 4. XRD patterns for the annealed HEAs after 50 h of oxidation at 1050 • C in air (a); and normalized mass change curves for the as-cast/oxidized and annealed/oxidized HEAs (b).

( 2 )
Discontinuous, isothermal oxidation tests at 1050 • C showed the as-cast HEAs to oxidize in the same way as model Group II and Group III Ni-Cr-Al alloys, with the initial formation of transient oxides and external Cr 2 O 3 , followed by the development of an internal Al 2 O 3 scale.As expected, increased Al concentrations led to enhanced oxidation resistances.In contrast, the annealed HEAs tended to form external Al 2 O 3 scales during oxidation with or without underlying spinel phases depending upon Al content.The change in oxidation mechanism was attributed to the increased phase fraction of Ni+Al-rich B2 phase.It is suggested that the enhanced distribution of B2 phase near the alloy free surfaces promoted the formation of Al 2 O 3 and thus modifying the subsequent stages of oxidation.(3)The parabolic oxidation behaviors of all of the HEAs in both the as-cast and annealed states indicate various levels of protection.The calculated parabolic oxide growth rate constants (k P ) for all of the HEAs correspond to those expected for Group II and Group III Ni-Cr-Al alloys.

Table 3 .
Parabolic oxide rate constants (kP) calculated for the as-cast and annealed HEAs.

Table 3 .
Parabolic oxide rate constants (k P ) calculated for the as-cast and annealed HEAs.