*3.2. Oxidation Behavior*

The oxidation behavior of the as-solidified Al76Co24 and Al71Co29 alloys was studied in flowing synthetic air at 773, 973 and 1173 K. The microstructure and chemical composition of the oxide scale were investigated by SEM/EDS. The cross-section of the Al71Co29 alloy after oxidation at 1173 K is presented in Figure 4. The oxide scale was homogeneous. EDS element maps are included in Figure 4b–d. The scale was found to be composed of aluminum oxide.

The cross-section image of the Al76Co24 alloy after oxidation at 1173 K is presented in Figure 5. The thickness of the oxide scale was approximately 1 μm after 30 h of oxidation. The chemical composition of the oxide scale was studied by EDS analysis. Results presented in Figure 5b show that the scale was predominantly composed of Al2O3. A small amount of Co (~ 2 at.%) was also detected in the scale, however, this result is attributable to a possible interference of the bulk alloy signal.

**Figure 4.** Cross section of the Al71Co29 alloy after oxidation at 1173 K for 30 h (**a**) and energy-dispersive x-ray spectrometer X-max (EDS) element maps for O (**b**), Al (**c**) and Co (**d**).

**Figure 5.** Cross section of the Al76Co24 alloy after oxidation at 1173 K for 30 h (**a**) and EDS element maps for O (**b**), Al (**c**) and Co(**d**).

The phase constitution of the oxide scale was studied by room temperature X-ray diffraction. The diffraction patterns of the Al71Co29 alloy are presented in Figure 6. In the alloy, peaks corresponding to θ-Al2O3 have been identified. θ-Al2O3 is a metastable alumina phase [64,65]. θ-Al2O3 structures are based on a cubic close packing of oxygen anions. The cubic close packing of oxygen anions of θ-Al2O3 is deformed monoclinic. The thermodynamically stable α-Al2O3 adopts a corundum structure. θ-Al2O3 is a transition phase. It transforms into stable forms of alumina during long term annealing [65]. Metastable θ-Al2O3 has been found in oxidized Al-Cu-Fe alloys studied previously [66]. It was formed initially with an orientational relationship to the substrate. At 1173 K, θ-Al2O3 was found to slowly transform into α-Al2O3 with an increasing oxidation time (70 h, [66]).

At 1173 K, an orientation of θ-Al2O3 in (002) crystallographic plane has been found (Figure 6). The same behavior was also observed for the Al76Co24 alloy (Figure 6). This observation indicates a preferential crystal growth. The morphology of alumina scale formed on the Al71Co29 and Al76Co24 alloys after oxidation at 1173 K for 30 h is given in Figure 7. The scale had a blade-like structure. The platelet-like scale morphology is indicative of rapid outwards growth. Alumina scales grow by counter-diffusion of aluminum and oxygen [67]. The ions, however, diffuse faster in polycrystalline alumina at near-atmospheric oxygen partial pressures. The scale morphology is indicative of rapid diffusion through the scale. A grain boundary diffusion was probably the preferred transport path for the Al3<sup>+</sup> and O2<sup>−</sup> ions in the scale.

**Figure 6.** Room temperature XRD patterns of the scales formed on the Al71Co29 and Al76Co24 alloys. For the discussion of the peak marked with an asterisk (\*), please refer to the article text.

**Figure 7.** Blade-like morphology of alumina scale formed on the oxidized Al71Co29 alloy (**a**) and Al76Co24 alloy (**b**) during oxidation at 1173 K.

The un-indexed peak next to θ-Al2O3 (002), marked with an asterisk in Figure 6, is an alumina peak. The closest match was found for hexagonal form of Al2O3 (reference code 98-017-3713, [68]). Nevertheless, it should also be mentioned that θ-Al2O3 has a disordered structure [69–72]. As such, the peak could also be a result of stacking faults (twinning) or other structural defects in θ-Al2O3 [73–75]. The precise peak assignment was not possible, owing to the difficulty to unambiguously distinguish the various Al2O3 polymorphs by XRD technique alone.

The alumina scale was well adherent to the substrate (Figures 4a and 5a). Nevertheless, locally, a detachment of the scale on the Al71Co29 alloy was observed (Figure 8). A scale delamination was found preferentially around β-AlCo dendrites. The situation is shown in Figure 8c. An explanation of the layer spallation could reside in a mechanical stress developed during oxide growth. The stress is formed due to different molar volumes of the oxide and the underlying original metal substrate [76]. The stress generated in the oxide during excessive growth may lead to crack formation in the scale. The pores, cracks and other defects facilitate the access of molecular oxygen to the metal substrate.

**Figure 8.** Scale microstructure around β-AlCo dendrites of the Al71Co29 alloy: (**a**) an overview; (**b**) a detailed view; (**c**) cross section image of the scale.

Another interesting feature was the porosity of β-AlCo dendrites located beneath the spalled scale of the Al71Co29 alloy (Figure 8b). The voids in β-AlCo were not observed before oxidation (Figure 2). The voids were thus formed during reaction, probably because of rapid aluminum outward diffusion from the metal surface. During oxidation, metallic species diffuse out from the alloy bulk to the alloy/oxide interface. The Al atoms, however, leave behind their vacant sites. The vacancies may have coalesced into larger defects, giving rise to the observed macroscopic porosity. The oxide spallation has not been observed on the Al76Co24 alloy. The Al76Co24 alloy is composed of structurally complex intermetallic phases (Z-Al3Co, m-Al13Co4 and Al9Co2, Figure 3). The surfaces of these phases are typically Al-rich [77]. Aluminum necessary for the scale growth was readily available at the surface of complex intermetallics. As such, the diffusion of Al atoms from these phases was less rapid compared to β-AlCo.

The scale of the Al76Co24 alloy did not show any spallation. It was well adherent to the substrate and had a wave-like morphology (Figure 5). The wave-like morphology of the scale could be indicative of epitaxial growth. The XRD pattern showed a preferential orientation of θ-Al2O3 grains in (002) crystallographic plane (Figure 6). The θ-Al2O3 (400) peak was not observed. The Al76Co24 alloy was primarily composed of m-Al13Co4, with small amounts of Z-Al3Co and Al9Co2 (Table 1). A preferred orientation is typical for m-Al13Co4 phase [26,78]. It is therefore likely that the θ-Al2O3 phase was formed with an orientation relationship to the m-Al13Co4 phase. This may be reflected in the preferential orientation of the θ-Al2O3 grains, as a result of which the intensities of the peaks of this phase change and some may disappear [79,80].

The oxide layer on the Al71Co29 alloy, on the other hand, was more uniform (Figure 4). The preferential layer growth is a self-limited process driven by atomic diffusion, and surface energy minimization [81,82]. It requires a certain concentration and certain mobility of Al atoms on the surface. The Al71Co29 alloy was primarily composed of Al5Co2. The Al76Co24 alloy, on the other hand, was mainly composed of m-Al13Co4 (Table 1). The surface of Al5Co2 is terminated at specific bulk layers (Al-rich puckered layers) where various fractions of specific sets of Al atoms are missing [83]. The surface of m-Al13Co4 has a higher density of Al atoms when compared to Al5Co2. Previous

investigations on Al13Co4 (100) showed that it was terminated by a dense aluminum topmost layer [84]. Therefore, the conditions for the atomic diffusion and subsequent oxide growth on the Al76Co24 and Al71Co29 alloys were different. The preferential oxide growth was less favored on the Al71Co29 alloy.

The mass gain of the samples was recorded by simultaneous thermogravimetry (TGA). Thermogravimetric curves of the Al71Co29 and Al76Co24 alloys are presented in Figures 9 and 10. The mass gain of the samples was increasing with increasing time. The kinetic curves obeyed a parabolic behavior.

**Figure 9.** Mass gain of the Al71Co29 alloy during isothermal oxidation in flowing synthetic air.

**Figure 10.** Mass gain of the Al76Co24 alloy during isothermal oxidation in flowing synthetic air.

The parabolic oxidation could be described by the following equation

$$\left(\frac{\Delta \mathbf{m}}{\mathbf{S}}\right)^2 = \mathbf{k\_{\tilde{P}}}\mathbf{t} + \mathbf{C} \tag{1}$$

In this equation, <sup>Δ</sup><sup>m</sup> <sup>S</sup> represents the specific mass gain (mass increase per unit area), kp is the parabolic constant, t is the annealing time and C is the integration constant. The plot of Δm S 2 versus t was linear and the slope of the line represented the parabolic rate constant. The kp values are collected in Table 2. The rate constants of the alloys are found between 1.6 <sup>×</sup> 10−<sup>14</sup> and 2.5 <sup>×</sup> 10−<sup>11</sup> g2 cm−<sup>4</sup> s<sup>−</sup>1. The obedience of the parabolic behavior shows that the oxidation process of the Al71Co29 and Al74Co26 alloys was controlled by ionic diffusion in the scale.


**Table 2.** Parabolic rate constants of the Al71Co29 and Al76Co24 alloys in air.

Oxidation is a thermally activated process. The parabolic rate constants increase with increasing temperature. The activation energy of oxidation thus could be obtained from the following equation

$$
\log \text{k}\_{\text{P}} = \log \text{A} - 0.434 \frac{\text{E}\_{\text{A}}}{\text{RT}} \tag{2}
$$

In this equation, A is a constant, EA is the activation energy, R is the molar gas constant (8.3144 J K−<sup>1</sup> mol−1) and T is the absolute temperature (in K). The plot of rate constants at different temperatures is presented in Figure 11. The rate constants follow the Arrhenius-type behavior. Activation energy has been found from the slope of lines given in Figure 11. The activation energy of the Al71Co29 alloy is 90 kJ mol−<sup>1</sup> and the activation energy of the Al76Co24 alloy is 129 kJ mol−1. These activation energies are comparable to activation energies for Al oxidation reported by previous studies [85,86].

**Figure 11.** Temperature dependence of parabolic rate constants of the Al-Co alloys.

Results presented above show that a protective scale has been formed on the surface of complex metallic alloys. The rate constants were relatively low, and a thin alumina scale was found on the surface (Figures 4 and 5). Alumina is formed by the following reaction

$$\frac{4}{3}\text{Al} + \text{O}\_2 \rightarrow \frac{2}{3}\text{Al}\_2\text{O}\_3\tag{3}$$

The Gibbs energy (ΔG) of reaction (3) is given in Figure 12. ΔG(Al2O3) is very low which indicates a strong affinity of aluminum towards oxygen. In principle, cobalt oxidation in the Al-Co alloys is also possible. This reaction can be given by the following equation

$$\text{2Co} + \text{O}\_2 \rightarrow \text{2CoO} \tag{4}$$

**Figure 12.** Gibbs free energies of metal oxidation reactions at elevated temperatures, redrawn from [54].

Nevertheless, ΔG reaction (4) is considerably larger compared to Gibbs free energy for aluminum oxidation (Figure 12). A further oxidation of CoO is even more energetically demanding [87,88]. Therefore, CoO tends to decompose in reaction with Al. The reaction can be expressed by the following equation

$$
\frac{4}{3}\text{Al} + 2\text{CoO} \rightarrow \frac{2}{3}\text{Al}\_2\text{O}\_3 + \text{Co}\tag{5}
$$

In this disproportionation reaction, CoO is reduced, and Al oxidized. The Gibbs energy of reaction (5) is negative. Therefore, the selective oxidation of aluminum in the Al-Co alloys is thermodynamically possible.

The oxidation of Co-rich Al-Co alloys was previously studied by Irving et al. [61]. The alloys were studied in the as-cast state. The authors studied several Co-xAl alloys with x = 0–32 at.%. Alloys with small Al concentration (< 10 at.%) formed a single CoO layer. Cobalt oxide layer grew with a considerably higher corrosion rate compared to Al2O3. At intermediate Al concentrations (10–20 at.%), the authors found that an inner layer of Al2O3 started to form below the outer CoO scale. With increasing aluminum concentration, a continuous Al2O3 scale has been developed. The comparison of the present results with those from literature is given in Figure 13. Our data complement the previous results of Irving et al. The continuous Al2O3 scale forms a barrier to cobalt diffusion. It hinders the nucleation and growth of cobalt oxides. Irving et al. found that a protective alumina scale can be formed when Al concentration 24 at.% at 1173 K is reached. Comparable minimum Al concentrations required to form the external alumina scale were also found for the Ni-Al and Fe-Al alloys [89].

The comparison of the present results with previously studied complex metallic alloys is provided in Figure 14. Kinetics of oxidation of Al-Cu-Fe and Al-Pd-Mn quasicrystal surfaces was studied in synthetic air [66,90]. High temperature oxidation kinetics of Al-Cr-Fe complex metallic alloys was studied in pure oxygen [91]. Our data are comparable to Al-Cu-Fe and Al-Pd-Mn alloys. The parabolic rate constants of the Al-Cr-Fe complex metallic alloys are lower compared to the remainder of the alloys. The oxidation resistance of the Al-Fe-TM (TM = Cr, Cu) alloys is related to the chemical composition of the oxide scale. The scale found in Al-Cu-Fe alloys was alumina. The scale formed in Al-Cr-Fe complex metallic alloys, however, was composed of Al2O3 and (Al0.9Cr0.1)2O3. The second scale component provided an additional barrier against corrosion. Chromium as a third alloying element may improve the overall oxidation resistance of the alloy. The corrosion resistance of alumina forming alloys alloyed with chromium is higher compared to alloys without Cr. When a sufficient Cr concentration is available, a complete chromia scale can be formed on top of the alumina scale [92,93]. The duplex Al2O3/Cr2O3 scale has an outstanding corrosion resistance.

**Figure 13.** Variation of parabolic rate constants of Al-Co alloys with increasing aluminum atomic fraction.

**Figure 14.** Parabolic rate constants for metal oxidation of Al-TM complex metallic alloys.

Previous authors also studied the microstructure evolution of the oxide scale. In early oxidation stages, γ-Al2O3 on the Al63Cu25Fe12 alloy was formed with an orientational relationship to the underlying Al-Cu-Fe quasicrystal [66]. γ-Al2O3 continued to grow as θ-Al2O3 until the oxide layer of several hundred nanometers has been formed. θ-Al2O3 was later transformed into the thermodynamically stable α-Al2O3. α-Al2O3 continued to grow with a nodular morphology. The oxide-nodule formation changed the growth mechanism. A protective layer formation was no longer observed. A massive spallation occurred after few days of oxidation [66]. A spallation of Al2O3

scale from the oxidized Al-Cr-Fe surfaces at high temperatures was also observed [91]. The massive oxide spallation has not been observed in the present study. However, the oxide spallation was observed locally on β-AlCo dendrites (Figure 8). It is possible that further stresses in the scale could develop during long term annealing. Therefore, further experiments on the Al-Co complex metallic alloys are required to study the effects of long-term annealing and/or thermal cycling on the oxidation behavior. These studies could shed further light into the long-term oxidation resistance for practical applications of the Al-Co alloys at high temperatures.
