3.1. Oxidation Behavior of AlTiNiCuCox Series High-Entropy Alloys at 800 °C
The relationship between the oxidation weight per unit area and time at high temperature can be expressed using the following equation:
where ∆
m is the weight gain per unit surface area,
K1 is the rate constant,
t is time, and
n is the time index.
In terms of the oxidation reaction of a metal alloy, there are two extreme cases [
20,
21,
22]. The first occurs when oxygen is in direct contact with the surface. Upon contact, the rate control mechanism of the oxidation reaction is a gas–metal interface reaction. Under this condition, the mass gain per unit surface area increases linearly with oxidation time, and the time index is
n = 1. The second extreme case occurs when a crack-free stable surface oxide film is formed, and the alloy does not directly make contact with gaseous oxygen. Under this condition, oxidation layer diffusion becomes the rate-controlling process of the oxidation reaction. In this case, the oxidation rate decreases with time (
n = 0.5).
Table 1 shows the oxidation weight gain per unit area of AlTiNiCuCo
x (x = 0.5, 1.0, 1.5) series HEAs at different oxidation times. Through comparison, it was found that after oxidation for 100 h, the weight gain per unit area of the AlTiNiCuCo
0.5 alloy was 1.096 mg/cm
2, compared with 0.503 and 0.144 mg/cm
2 for the AlTiNiCuCo
1.0 and AlTiNiCuCo
1.5 alloys, respectively. Therefore, for the same oxidation time, the oxidation weight per unit area decreased with increasing Co content in AlTiNiCuCo
x series HEAs. The oxidation kinetic curve of AlTiNiCuCo
x (x = 0.5, 1.0, 1.5) series HEAs is shown in
Figure 2. It can be seen that at 800 °C, the oxidation weight gain trends of the three alloys were similar. The oxidation weight gain gradually increased with the extension of oxidation time, and the process mainly belonged to the first oxidation case. The average oxidation rates, as obtained by fitting straight lines to the three kinetic curves, were 0.012 mg/(cm
2.h) (AlTiNiCuCo
0.5), 0.006 mg/(cm
2.h) (AlTiNiCuCo
1.0), and 0.002 mg/(cm
2.h) (AlTiNiCuCo
1.5). This indicated that the oxidation resistance of the AlTiNiCuCo
x series HEAs at high temperature was closely related to their Co content.
3.3. Surface Morphology of The Oxide Film
Figure 4 presents SEM micrographs of the oxide film surface of the AlTiNiCuCo
0.5 alloy oxidized at 800 °C for 50 and 100 h.
Table 2 and
Table 3 present EDS analysis results of the corresponding regions in the two SEM images.
In
Figure 4, it is clear that the oxide film after 50 h oxidation had a more uniform and compact structure and smaller crystal grain size. However, after oxidation for 100 h, the crystal grain size became bigger, and the crystal grain size uniformity was worse.
According to the EDS analysis, the contents of Ti, Al, and O were relatively high after oxidation at 800 °C for 50 h. From this, combined with the results of XRD analysis of the AlTiNiCuCo0.5 alloy oxidation film surface after oxidation at 800 °C for 50 h, it can be concluded that TiO2 and Al2O3 were formed. When the oxidation time was 100 h, according to the XRD analysis on the surface of the oxide layer, a new oxide, CoAl2O4, was also generated. The content of Ti also increased for 100 h oxidation samples. Combined with the results of XRD analysis, this indicates that, in addition to TiO2, Ti and O also generated the compound Ti0.928O2. This indicates that with increasing oxidation time, the overall oxide content increases.
Figure 5 presents SEM micrographs of the oxide film of the AlTiNiCuCo
1.0 alloy oxidized at 800 °C for 50 and 100 h, and
Table 4 and
Table 5 present EDS analysis results of the corresponding regions in the two SEM images.
It can be seen that after 50 h oxidation, the microstructure had the same characteristics of fine grains, uniformity, and compactness as that with 0.5 Co content. After oxidation for 100 h, the microstructure obviously consisted of two parts: a black area and a large gray area.
From the results of EDS energy spectrum analysis, it can be seen that the Ti, Al, and O contents were still high (
Table 4 and
Table 5). From the XRD analysis of the oxide film above, it can be concluded that two oxides, TiO
2 and Al
2O
3, were formed in the oxide film. However, compared with the AlTiNiCuCo
0.5 alloy oxidized for 50 h, the EDS analysis showed significantly increased Cu content in AlTiNiCuCo
1.0, which indicates that the oxide film was thinner due to Cu in the substrate during EDS analysis. Comparing Parts A and B in
Table 5 and combining the results with those from XRD analysis, we can infer that there were two oxides, TiO
2 and Al
2O
3, at A, and Cu was enriched at A, where it was intergranular. This indicates that at this time, the oxide film was also thinner. In Part B, the Ti and O contents were very high, and it was concluded there was a large amount of TiO
2. From the analysis above, we conclude that the oxide films of AlTiNiCuCo
1.0 alloy oxidized at 800 °C for 50 and 100 h were thinner.
Figure 6 presents SEM micrographs of the oxide film of the AlTiNiCuCo
1.5 alloy oxidized at 800 °C for 50 and 100 h, and
Table 6 and
Table 7 present the EDS analysis results of the corresponding regions in the two SEM images.
Compared with the microstructural morphology of the oxide films of AlTiNiCuCo0.5 and AlTiNiCuCo1.0 after 50 h oxidation, the microstructure distribution was very uneven and consisted of three obvious parts, namely, Part A with uniform and fine black spots, Part B consisting of large raised areas, and Part C consisting of small raised areas.
The EDS analysis indicated that Part A contained more Al, Co, and O. In XRD analysis, CoAl2O4 and Al2O3 were found in this part. The contents of O and Cu in Part B were more than 50%, which indicates that the Cu phase was enriched there. The contents of Ti, Co, and O in Part C were relatively high. We speculated that TiO2 and CoAl2O4 might be present in this small raised part. After 100 h oxidation, the microstructural morphology obviously consisted of two parts. In Part A, the particle size, which was uniform and fine after 50 h oxidation, became different, and there was also a large raised area in Part B. Compared with that after oxidation for 50 h, the Ti content in the Part A region was significantly increased. From this, combined with XRD analysis results, we presumed that TiO2 and CoAl2O4 were the two oxides in this region. The large raised area in Part B was basically the same as that after oxidation for 50 h, with high contents of Cu and O and low contents of other elements. There was still a solid solution rich in the Cu phase and possibly a small amount of CuO in Part B.
In summary, by comparing the microstructural morphology and EDS analysis results of AlTiNiCuCox (x = 0.5, 1.0, 1.5) series HEAs, we conclude the following: With the extension of the oxidation time, the grain size of each component of the alloys in the microstructure became larger, and the distribution of the microstructure became increasingly uneven, which indicates that the high-temperature oxidation resistance became progressively worse. The oxide films of the AlTiNiCuCo1.0 alloy oxidized for 50 and 100 h were thinner. At the same time, with the extension of oxidation time, the Ti content in the HEAs increased. From the XRD analysis results on the surface of the oxide film, it was found that other oxides composed of Ti and O were generated in addition to TiO2.
3.4. Cross-Sectional Morphology of The Oxide Film
Figure 7 presents line scanning energy spectrum diagrams of the cross section of the AlTiNiCuCo
0.5 alloy oxide layer after oxidation for 50 and 100 h at 800 °C. It can be seen that after oxidation for 50 h, the oxide film was relatively continuous and uniform in thickness. When the oxidation time was 100 h, the thickness of the oxide film increased significantly, and the film morphology and thickness were uneven. In
Figure 7, representing 50 h, it can be seen that there were high contents of Al, Ti, and O in the oxide layer. Combining this with the XRD analysis results of the oxide layer, it was concluded that the main components of the oxide layer were Al
2O
3 and TiO
2. After oxidation for 100 h, the contents of Al and Ti in the oxide layer increased, and their distribution was more uniform. XRD analysis indicated that in addition to Al
2O
3 and TiO
2, Ti
0.928O
2 and CoAl
2O
4 oxides also appeared.
Figure 8 presents line scanning energy spectrum diagrams of the cross section of the AlTiNiCuCo
1.0 alloy oxide layer after oxidation for 50 and 100 h at 800 °C. After 100 h oxidation, the oxide film was thicker than that after 50 h, but the change in thickness was not apparent. This indicates that the oxide film of an HEA of this composition under high-temperature oxidation is relatively dense and plays a certain protective role for the alloy; thus, the alloy has better high-temperature oxidation resistance. It can be seen from
Figure 8a, representing 50 h oxidation, that the oxide layer still contained large amounts of Al, Ti, and O elements. It was presumed that there were mainly two oxides, Al
2O
3 and TiO
2, in the oxide layer. After oxidation for 100 h, the contents of Al and Ti in the oxide layer were greater, and the content of Co was significantly increased. Combining this with XRD analysis results, it was found that there were three oxides present: CoAl
2O
4, Al
2O
3, and TiO
2.
Line scanning energy spectrum diagrams of the cross section of the AlTiNiCuCo
1.5 alloy oxide layer after oxidation at 800 °C for 50 and 100 h are presented in
Figure 9. In
Figure 9, it can be seen that from the surface layer to the core, the sample was composed of an oxide layer, transition layer, and matrix. Combining this with the XRD phase analysis results, we concluded that for 50 h oxidation, three oxides—CoAl
2O
4, Al
2O
3, and TiO
2—were produced in the oxide layer. The Co content in the transition layer increased significantly and CoAl
2O
4 was formed. With increasing oxidation time, the transition layer and oxide layer became thicker. Among them, the brighter part of the outer layer of the HEA appeared after 100 h oxidation, that is, the outer oxide layer, which contained more Cu and Ti. It was presumed that CuO and TiO
2 were present in this part, and the surface was uneven. It was presumed that the peeling phenomenon of the outer oxide layer occurred due to the particle size of TiO
2 in the outermost layer continuously growing.
By comparing the cross-sectional morphologies of oxide films across
Figure 7,
Figure 8 and
Figure 9, we conclude the following: With the extension of the oxidation time, the oxides in the alloys increased. With increasing Co content, a transition layer gradually appeared. There was mainly Al
2O
3 and TiO
2 in the oxide layer, and a small amount of CoAl
2O
4 was present. CoAl
2O
4 appeared in the transition layer.
3.5. Oxidation Mechanism
In the cross-sectional scanning images of AlTiNiCuCo
x (x = 0.5, 1.0, 1.5) series HEAs after oxidation for 50 and 100 h at 800 °C, an oxide layer was present in all samples. According to the analysis of
Figure 7 and
Figure 8, mainly the oxides Al
2O
3 and TiO
2 were present in the oxide layer. This is because the oxidation activity sequence for the five constituent elements of the alloy in decreasing order of activity was Al, Ti, Co, Ni, and Cu. Among these, Al and Ti were highly active and were easily and preferentially oxidized. At the same time, according to the Equation (2)
where Δ
G is Gibbs free energy, Δ
H is the enthalpy changes,
T is the temperature, Δ
S is the Entropy change.
The free energy of the oxidation reaction of Al and Ti was lower than 0 and relatively low, so the reaction could proceed spontaneously, and stable Al2O3 and TiO2 were formed in the oxide layer. The oxidation layer was mainly composed of Al and Ti as alloy components that react with oxygen via direct contact. At this time, the interfacial reaction dominated.
In the oxidation layer cross section of the AlTiNiCuCo
1.5 alloy after oxidation for 50 h and 100 h at 800 °C, we can see both a significant oxidation layer and transition layer. According to our analysis of
Figure 9, the oxide CoAl
2O
4 was mainly present in the transition layer. This is due to the gradual outward diffusion of Co ions and Al ions meeting the gradual inward diffusion of O ions, whereupon CoAl
2O
4 oxide was formed in the transition layer. It can be seen that as the alloy was continuously oxidized, the oxide film continued to grow and gradually played an increasingly significant role, so the process transformed from the original interface reaction to the diffusion process, and diffusion became the dominant oxidation factor. This resulted in the appearance of one or more alloy oxides, mainly CoAl
2O
4.
Our analysis indicates that in the process of high-temperature oxidation, metal elements and oxygen in the alloy formed ions through direct contact reaction, and ions diffused and were transported in the oxide film, which is consistent with the high-temperature oxidation mechanism of NbCrMo
0.5Ta
0.5TiZralloy studied by Senkov et al. [
16], US Air Force Laboratory. Al and Ti ions diffused out of the oxide film through the interface, and the distribution of Al and Ti ions gradually decreased from the oxide layer to the transition layer and to the matrix. Oxygen ions diffused through the oxide film to the substrate, and the concentration of oxygen ions gradually decreased from the oxide layer to the transition layer to the substrate. The concentration of Co ions increased gradually from the matrix to the transition layer. The distribution of Ni and Cu ions increased gradually from the transition layer to the matrix.
By analyzing the variation in the composition and concentration of various compounds in the oxide layer, transition layer, and matrix over time, we deduced the above oxidation mechanism, which is in excellent agreement with the actual observed results.