3.3 Phase Analysis of High Temperature Oxide Film
XRD analysis is performed on the surface oxides of the samples after being kept at 900℃ and 1000℃ for 125h, and the results are shown in Fig. 5. It can be seen that no matter at which temperature, NiO is the main oxidation produc. After 125h of oxidation at 900°C, the oxidation products formed on the alloy are mainly NiO, Al2O3, NiTa2O6 and TaO. After 125h of oxidation at 1000°C, the oxidation products formed on the alloy are mainly NiO, Al2O3, CrTaO4, Co3O4 and TaO2. At 1000°C, TaO2 replaces TaO in the oxide layer compared with 900°C. And it can also be seen from the figure that with the increase of the oxidation temperature, many new substances are formed, and the spinel compounds increase.
In addition, after oxidation at 900℃ and 1000℃, due to the low Mo content in the alloy, no oxidation occurs or the formed MoO3 oxides are volatilized. the oxides of Re and W are volatile, so they are not observed on the surface of the alloy [16].
3.4 Surface Morphology Analysis of High Temperature Oxides
Figure 6 shows the surface oxidation morphology and micro-domain composition detection results of the samples after being oxidized at 900℃ for 125h. It can be seen that after the samples were oxidized at 900℃, the formed oxide films did not have cracks and bubbling. The oxide film morphologies of the samples are slightly similar but not identical. The oxide film of sample B is a uniformly distributed and irregular block, the size of the larger block oxide is about 2–3 µm, and there are many slightly smaller particles distributed among the larger block oxide. The oxide film of sample C is in the shape of beveled edges growing from the substrate. The morphology of the oxide film of sample A is similar to that of sample B, but it is obvious that the formed oxide film is not dense. This creates a channel for oxygen to enter the matrix, accelerating the oxidation rate of the alloy [17].
According to the analysis of EDS energy spectrum, the constituent elements of the oxide films on the surfaces of thesamples are mainly Ni, O, Al, Cr and so on. The surface oxidation products of sample A are mainly composed of three elements, Ni, Al and O. The surface of sample B has been covered with Ni-rich oxides, and there are many elements such as Ni, O, Al, Ta, Cr, etc., as shown in Fig. 6 (b). Combined with the phase analysis of the oxide film in Chap. 3.3, it is speculated that the oxide film is mainly composed of oxides such as NiO, Al2O3, TaO, and Cr2O3. Among them, NiO and Al2O3 are the main oxidation products. The oxide film of sample C is mainly composed of Ni, O, Ta, and A. The surface of the oxide has also been covered with Ni-rich oxide. The oxide surfaces of the samples all contain a large amount of NiO. Comparing the contents of Cr and Al in the three samples, it is found that the content of these two elements is the highest in sample B. The high content of Cr and Al indicates that it is conducive to the formation of two protective oxide films, Al2O3 and Cr2O3, which have a positive effect on the oxidation resistance of the material [18]. This further explains why the oxidation resistance of sample B is better than that of the other two.
Figure 7 shows the surface oxidation morphologies and micro-domain composition detection results of the samples after being oxidized at 1000℃ for 125h. The oxide film formed by the oxidation of the samples at 1000℃ still has no cracking and bubbling. Compared with the oxidation at 900℃, the size of the oxide particles increased significantly. The oxide film of sample A is slightly connected to each other, and the size is about 3–5 µm. The morphology of the oxide film of sample B is similar to that of sample A, but there are finer particles distributed among the spherical particles. The size of particles is about 400–800 nm. This shows that the connection between the oxides is relatively tight, forming a relatively dense oxide film. The oxide morphology of sample C is a large coral-like shape, which is significantly different from that of the product at 900℃. It can also be seen from the figure that the oxides have different shapes and sizes, and there are obvious gaps between the oxides.
The oxide films of the samples were analyzed by EDS. The surface oxide of sample A is mainly composed of elements such as Ni, O, Al, and Ta. The combined phase analysis shows that the oxides are mainly composed of NiO, Al2O3, TaO and other oxides. The content of Ni is extremely high, indicating that the main component of the oxide film is NiO. The content of Al is 4.02%, which indicates that Al2O3 oxide film can be formed, which further hinders the entry of oxygen into the alloy matrix and improves the oxidation resistance of the alloy. The EDS analysis of sample B showed that the oxides formed on the surface can be roughly divided into two types of marks 1 and 2. It is obvious that the most abundant elements in region 1 are Ni and O, and the most abundant elements in region 2 are Ni, Al, O, Ta, Cr, Co, etc. According to XRD phase analysis, it can be concluded that the oxides in region 1 are mainly composed of NiO, and the main oxides in region 2 are NiO, Al2O3, NiCrO4, CrTaO4, Co3O4 and NiAlO4. Compared with the oxidation at 900℃, the oxidation products are obviously delaminated, and the spinel phase is formed in the oxidation products. The EDS analysis results of sample C show that the oxides are mainly composed of Ni, O, Al, Cr, Ta and other elements. Combined with XRD phase analysis, the main oxides are NiO, Al2O3, NiCrO4, CrTaO4 and Co3O4. The element content and oxidation products are similar to those of sample B. The content of Al and Cr in sample B and sample C is much larger than that in sample A, which is similar to oxidation at 900℃.
3.5 Analysis of Cross-sectional Morphology of High Temperature Oxide
Figure 8 shows the oxidation cross-section morphologies and EDS line scan analysis results of the samples after high temperature oxidation at 900℃ for 125h. It can be seen from the calculation that the thickness of the oxide film of sample A is about 22.11 µm, the thickness of the oxide film of sample B is about 10.77 µm, and the thickness of the oxide film of sample C is about 11.79 µm. By comparison, it can be obtained that sample B has the best oxidation resistance among the three samples, and sample A has the worst oxidation resistance. This conclusion is also consistent with the previous results of oxidation kinetics.
It can be seen from Fig. 8 (a) that the oxide film of sample A has obvious delamination phenomenon. The outer film is coarse, loose and porous. Although the inner oxide film is relatively dense, it is discontinuous. Such an oxide film composition cannot effectively prevent the entry of oxygen, and the protection is weak. According to EDS, it can be seen that the coarse structure of the outer layer is mainly composed of NiO, Cr2O3 and AlNi3, but the content of Cr2O3 is very small. The inner layer is mainly a dense and protective oxide film of Al2O3.
Figure 8 (b) shows that the thickness of the oxide film of sample B is not very uniform, but the thickness of the oxide film at the thickest part is not more than 20 µm, which also shows the excellent oxidation resistance of sample B. It can also be seen from the figure that the oxide film is divided into two layers, similar to sample A. The content of O, Al, Cr, Co, Ni and Ta in the outer film is very high, indicating that a large amount of oxides of NiO, Cr2O3, TaO and Co3O4 are formed. The main elements of the inner layer are Al, Cr and O. This will form two dense protective films (Cr2O3 and Al2O3) in the inner layer, which will hinder the oxidation reaction and play the role of protecting the matrix [19].
Figure 8 (c) shows that the oxide film of sample C is also divided into two distinct layers. But the oxide film is not dense enough to provide a channel for oxygen. It can be seen from EDS that the content of O, Cr, Co and Ni in the outer layer of the oxide film is relatively high. Combined phase analysis ,it shows that the main oxides on the outside are NiO, Cr2O3 and Co3O4. In the part close to the matrix, the content of Al and O is very high, indicating that a dense Al2O3 oxide film is formed on the inner side of the oxide. However, it can be seen from the figure that the Al2O3 oxide film is thin and discontinuous in some places, which cannot effectively prevent the further entry of oxygen.
Figure 9 shows the oxidation cross-section morphologies and EDS line scan analysis results of the samples after high temperature oxidation at 1000℃ for 125h. It can be seen from the calculation that the thickness of the oxide film of sample A is about 46.32 µm, the thickness of the oxide film of sample B is about 26.32 µm, and the thickness of the oxide film of sample C is about 31.28 µm. Compared with the oxidation at 900℃, the thickness of the oxide film is significantly increased. It shows that the increase of temperature increases the oxidation rate of the alloy. By comparing the thickness of the oxide film at the same temperature and the same oxidation time, it can be obtained that sample B has the best oxidation resistance among the three alloys, and sample A has the worst oxidation resistance. Consistent with the previous conclusions on oxidation kinetics.
First of all, it can be clearly seen from the figure that the oxide film after oxidation at 1000°C has changed from two layers to three layers. The outermost particle size of the oxide layer of sample A is very coarse, loose and porous. The middle layer is an oxide with slightly finer particles, but it is not dense and has deep cracks with the outermost layer, indicating that the oxide film is not tightly connected. At the position of the innermost layer close to the matrix, there is a thin layer of oxide, but its thickness is uneven and it is discontinuous. Combined with EDS analysis, the content of Ni and O in the outermost layer is high, indicating that the outermost layer is loose, porous and very coarse granular NiO. There are many O, Al, Cr, Ni, Ta, W and Re in the intermediate layer, indicating that the intermediate layer forms W-rich and Re-rich phases and some spinel phases. The presence of Re will reduce the rate of Al out-diffusion, and it is easy to form an α-Al2O3 oxide film in the inner layer. This is consistent with the EDS analysis results of the innermost layer. The elements with more content in the innermost layer are O and Al, which also shows that the innermost layer of the oxide film is Al2O3 oxide film. But the Al2O3 oxide film is very thin and discontinuity, which can affect the oxidation resistance of the alloy.
The oxide layer of sample B is two layers. The outermost layer is a slightly coarse granular oxide, and the connection between particles is not tight, loose, porous, weak in oxidation resistance, uneven in thickness and uneven in the interface of the oxide layer. In the middle of the oxide layer, the darkest part is resin, which is intertwined with the outermost particles in the process of inlaying. There is no obvious boundary between resin and the outermost layer, which also shows that the thickness of the outermost oxide film is not uniform, and the particles are coarse. The innermost layer is about a quarter of the thickness of the entire oxide layer and is thick and dense. Combined with EDS analysis, the content of Ni, Cr and O in the outermost layer is high, indicating that the outermost layer is loose and porous, very coarse granular NiO and Cr-rich oxides. The elements with more content in the innermost layer are only O and Al, indicating that the inner layer of the oxide film is Al2O3 oxide film, and the Al2O3 oxide film is thick and continuous, which greatly improves the oxidation resistance of the alloy.
The morphology of the oxide film of sample C is similar to that of sample A, and it is divided into three layers. The outermost particles are very coarse in size, loose and porous. The oxide particles in the middle layer are small but not dense, and there is a deep gap between the oxide film and the outermost layer, indicating that the oxide film is not tightly connected. At the position of the innermost layer close to the matrix, there is a thin layer of oxide, but it is discontinuous, and there is no straight boundary between it and the matrix. Combined with EDS analysis, only the Ni and O are high in the outermost layer, indicating that the outermost layer is coarse, loose and porous granular NiO. Except for Cr and Co, there are no elements with prominent contents in the interlayer, indicating that Cr-rich and Co-rich oxide phases and some spinel phases are formed in the interlayer. The elements with more content in the innermost layer are O and Al, and the combined phase analysis shows that the innermost layer of the oxide film is Al2O3 oxide film. Although the Al2O3 oxide film is very thin but continuous, it also makes the alloy have better antioxidant properties.