The isothermal oxidation behavior of Ni-xAl (x=9, 10, 11, 12, 13, 14 at.%) alloy in pure oxygen at 900℃ was studied. The scales of Ni-xAl (x=9, 10, 11 at.%) with lower aluminum content are composed of an outer layer of NiO and an inner layer of NiO+NiAl2O4. Internal oxidation of Al occurs beneath the scales, resulting in a faster corrosion rate. The outer layer structure of the scales of Ni-xAl (x=13,14 at.%)with higher aluminum content is similar to that of the Ni-Al alloys with lower aluminum content, but the innermost layer forms a continuous and dense Al2O3 protective layer, providing good protection for the alloy without the internal oxidation of Al. Ni-12Al is very unique, with both internal and external oxidation characteristics, and its oxidation behavior falls between protective and non-protective. Due to slight differences in the composition of the alloy in different regions, its oxidation behavior is similar to Ni-9Al, Ni-10Al, and Ni-11Al in areas with slightly lower aluminum content, while its oxidation behavior is similar to Ni-13Al and Ni-14Al in areas with slightly higher aluminum content. It can be inferred that 12 at.% Al is the critical concentration of the alloy for the transition from internal oxidation to external oxidation.
Research Article
The transition from internal oxidation to external oxidation of Ni-Al alloy and its critical Al content
https://doi.org/10.21203/rs.3.rs-5016153/v1
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Ni-Al alloy
Internal oxidation
External oxidation
Critical concentration
In the field of metal materials, Ni-Al alloys have been widely studied due to their unique physical and chemical properties, and have broad application prospects in aviation, aerospace, energy, and chemical industries [1–3]. However, Ni-Al alloys are prone to oxidation failure in high-temperature environments due to insufficient content of film-forming element Al. There are two types of oxidation: internal oxidation and external oxidation [4]. Internal oxidation refers to the formation of oxides inside the alloy, while external oxidation refers to the formation of oxides on the surface of the alloy [5]. For Ni-Al alloys, when the Al content is sufficient to make the alloy to complete the transition from internal oxidation to external oxidation, Al can provide the required high-temperature oxidation resistance. Therefore, the confirmation of the critical Al concentration of Ni-Al alloys is crucial.
Since the 1990s, extensive research has been conducted on the internal and external oxidation of Ni-Al alloys [6–12], but the critical concentration and mechanism of the transition from internal oxidation to external oxidation of Ni-Al alloys are still unclear. H. M. Hindam et al. [5] investigated the oxidation performance of Ni-2 wt% A1 (Ni-4 at.% A1) and Ni-6 wt% A1 (Ni-12 at.% A1) alloys in 1000 ~ 1300℃ and 1 atm oxygen. The results showed that both alloys underwent internal oxidation, forming a multi-layer oxide film structure. G. C. Wood et al. [8] investigated the isothermal oxidation behavior of Ni-7 wt% Al (Ni-14 at.% A1) and Ni-12.5 wt% Al (Ni-25 at.% A1) in 1 atm oxygen at 800℃, 1000℃, and 1200℃. The results showed that the higher the Al content and temperature of the alloy, the easier it is to form a complete and protective external layer α-A12O3 oxide film. The above studies all indicate that the higher the Al content in Ni-Al alloy, the better the alloy's oxidation resistance, but the critical concentration of Ni-Al alloy for the transition from internal oxidation to external oxidation is not clear. Through simplified analysis, Yan Niu et al. [6], theoretically calculated the critical concentration of Ni-Al alloy for the transition from internal oxidation to external oxidation at 800℃ as 0.114, but believed that the actual critical concentration was much higher than this concentration. Based on this, this article studied the isothermal oxidation behavior of Ni-xAl (x = 9, 10, 11, 12, 13, 14 at.%) alloy in pure oxygen at 900℃ and 0.1MPa, with a focus on exploring the critical concentration of Ni-Al alloy from internal oxidation to external oxidation, and analyzing its transformation mechanism.
Six alloys, Ni-9Al, Ni-10Al, Ni-11Al, Ni-12Al, Ni-13Al, and Ni-14Al, were prepared by repeated melting of appropriate amounts of the two pure (99.99%) components using non- consumable vacuum arc melting process under the protection of argon gas. Subsequently, the alloy ingots were annealed for 24 hours at 900℃ to remove residual stress and promote uniform dispersion of elements. Oxidation specimens with dimensions of 10 mm×10 mm×1 mm were cut from the alloy ingots by wire cut electrical discharge machining. The surface of specimens was subsequently ground using SiC paper down to 2000 grit. Rounded edges were specially prepared to avoid typical edge effects on the oxidation process. Prior to oxidation, all specimens were thoroughly cleaned by ethanol in an ultrasonic bath and finally dried. Samples were oxidized separately under 0.1 MPa pure O2 at 900℃. Continuous mass change of samples is carried out by a Setaram Setsys Evo thermogravimetric analyzer (Setsys Evo TMA, SETARAM Instrumentation Co., Ltd., France) for a duration of 24h.
X-ray diffraction (XRD-6100, Shimadzu Instruments Co., Ltd., Japan) and scanning electron microscopy (Leo 1530 FEG SEM, Carl Zeiss AG Co., Ltd., Germany) in combination with energy-dispersive X-ray spectroscopy (INCA EDS, Oxford Instruments Co., Ltd., Japan) were used to characterize the alloy microstructure and the constitution of oxides formed.
3.1 The composition and phase of Ni-Al alloys
Table 1 shows the actual composition of Ni-Al alloys measured by EDS, and the results show that the composition of Ni-Al alloys is basically consistent with the design. Figure 1 shows the Ni-Al binary phase diagram drawn by FactSage program, with the red marked line indicating the composition region of samples of this study. Figure 2 shows the XRD patterns of Ni-Al alloy at room temperature. The results show that the six Ni-Al alloys are single-phase nickel aluminum solid solutions at 900℃, but fall into the Ni3Al-Ni-Al two-phase region at room temperature, as indicated by Fig. 2.
|
Alloy |
Ni-9Al |
Ni-10Al |
Ni-11Al |
Ni-12Al |
Ni-13Al |
Ni-14Al |
|---|---|---|---|---|---|---|
|
Ni |
91.07 |
89.80 |
89.25 |
87.99 |
86.71 |
85.24 |
|
Al |
8.93 |
10.20 |
10.75 |
12.01 |
13.29 |
14.76 |
3.2 Oxidation kinetics
The kinetic curves for the oxidation of individual samples of the Ni-Al alloys under 0.1 MPa pure O2 at 900℃ are shown in Fig. 3(a). The corresponding parabolic plots are given in Fig. 3(b). Approximate parabolic rate constants of Ni-Al alloys are listed in Table 2. The oxidation behavior of six Ni-Al alloys can be described in two groups: Ni-9Al, Ni-10Al, and Ni-11Al alloys with lower aluminum content corrode more seriously and the active component Al does not provide sufficient protection for the alloys. The three alloys approximately follow a parabolic law throughout the entire oxidation time, with the corresponding parabolic rate constants of 1.70×10− 8, 1.58×10− 8, and 1.58×10− 8 g2·cm− 4·min− 1, respectively. The three values of the three alloys differ very little, and their kinetic curves almost overlap. In fact, the corrosion rate slightly decreases with the increase of aluminum content, and the weight gain is 4.81, 4.66, and 4.60 mg/cm2, respectively after 24 h oxidation. Ni-12Al, Ni-13Al, and Ni-14Al alloys with high aluminum content present good oxidation resistance, and the active component Al provides good protection for the alloys by forming a protective oxide film.
|
Ni-9Al |
Ni-10Al |
Ni-11Al |
Ni-12Al |
Ni-13Al |
Ni-14Al |
|---|---|---|---|---|---|
|
1.70×10− 8 (0-1440 min) |
1.58×10− 8 (0-1440 min) |
1.54×10− 8 (0-1440 min) |
1.15×10− 8 (0-900 min) |
7.03×10− 9 (0-700 min) |
3.38×10− 9 (0-360 min) |
|
5.58×10− 9 (900–1440 min) |
9.23×10− 10 (700–1440 min) |
1.98×10− 10 (360–1440 min) |
The oxidation process of the three alloys can be divided into two stages: transient and steady-state. In the transient oxidation stage, the protective oxide film has not yet fully formed, and its corrosion rate is relatively high. The oxidation kinetics of Ni-12Al, Ni-13Al, and Ni-14Al alloys at this stage also approximate a parabolic law, with the corresponding parabolic rate constants of 1.15×10− 8, 7.03×10− 9, and 3.38×10− 9 g2·cm− 4·min− 1, respectively. However, the corrosion rate significantly decreases with the increase of aluminum content. In the steady-state oxidation stage, a protective Al2O3 layer has formed, and its corrosion rate rapidly decreases. The oxidation kinetics of the three alloys at this stage also approximate the parabolic law, with the corresponding parabolic rate constants of 5.58×10− 9, 9.23×10− 10, and 1.98×10− 10 g2·cm− 4·min− 1, respectively. The corrosion rate also decreases with the increase of aluminum content.
It is obvious that when the aluminum content of Ni-Al alloy reaches or exceeds 12 at.%, the oxidation behavior of the alloy changes from non-protective to protective, indicating 12 at.% Al is the critical concentration at which the oxidation behavior of alloys changes. Moreover, the time of Ni-12Al, Ni-13Al and Ni-14Al from non-protective transient oxidation stage to protective steady-state oxidation stage is rapidly shortened as 900 min, 700 min and 360 min, respectively, with the increase of aluminum content.
3.3 Scale microstructure and composition
Surface morphology of Ni-Al alloys oxidized under 0.1 MPa pure O2 at 900℃ for 24h is shown in Fig. 4. and the XRD patterns of Ni-Al alloys oxidized are shown in Fig. 5. The results showed that a dense NiO oxide film has formed on the surface of all the six alloys. SEM images of cross sections of Ni-Al alloys oxidized are shown in Fig. 6. The scales of Ni-9Al, Ni-10Al, and Ni-11Al alloys with lower aluminum content are composed of an outer layer of NiO and an inner layer of NiO + NiAl2O4. An internal oxidation zone (I.O.Z) forms under the scales, with a continuous thin layer of Al2O3 (which is not dense and cannot provide good protection for the alloy) present at the front of the I.O.Z. The scales of Ni-13Al and Ni-14Al alloys with higher aluminum content are significantly different from those of the three alloys with lower aluminum content. A continuous and dense Al2O3 protective layer has formed as the innermost layer in addition to the outer layer of NiO and the inner layer of NiO + NiAl2O4. Moreover, due to the formation of the protective Al2O3 layer, the alloys do not undergo internal oxidation of Al, and the oxidation behavior changes greatly. Ni-12Al alloy is quite special, exhibiting different scales structures in different positions, and internal oxidation only occurs in local areas. Due to the slight variations of the composition in different areas, Ni-12Al has formed a similar scales structure in areas with slightly lower aluminum content to those of the Ni-9Al, Ni-10Al and Ni-11Al alloys, while a similar scales structure in areas with slightly higher aluminum content to those of the Ni-13Al and Ni-14Al alloys. It is easy to infer that 12 at.% Al is the critical concentration for the change of oxidation behavior. When the aluminum content of Ni-Al alloys exceeds 12 at.%, the oxidation behavior of the alloy changes from non-protective to protective, which is completely consistent with the conclusion of kinetics.
The interface between the outer layer of NiO and the inner layer of NiO + NiAl2O4 is flat, indicating that this interface is the original interface of the alloy. Since the presence of Al was not found in the NiO layer, it can be inferred that the NiO layer is formed by the outward diffusion of Ni and a further reaction with oxygen. Meanwhile, the voids appearing inside the alloys further confirm the outward diffusion process of the Ni element. The line scan image of Ni-10Al after oxidation (Fig. 7) shows that Al depletion occurs beneath the Al2O3 thin layer formed at the front of I.O.Z. This is due to the formation of the Al2O3 layer causing obvious consumption of the Al element below it.
Before discussing the oxidation behavior of Ni-Al alloys, it is necessary to review the high-temperature oxidation behavior of pure nickel and pure aluminum. Atkinson et al. [13] conducted isotopic oxidation experiments on pure nickel, and the results showed that after the oxidation, the outer layer of the oxide film was formed by the outward diffusion of Ni, while the inner layer was formed by the inward diffusion of oxygen. The situation of the Ni-Al alloys studied in this article is similar, that is, the outer layer of NiO is formed by the outward diffusion of Ni, and the NiO in the inner layer of NiO + NiAl2O4 is formed by the inward diffusion of oxygen. Due to the slow growth rate of the oxide film on pure aluminum and the thin thickness of the oxide film, it is difficult to conduct labeling experiments. However, a large number of experimental results [14–16] tend to support the dominance of oxygen inward transport. The situation of the Ni-Al alloys studied in this article is similar, that is, Al2O3 layer in the innermost layer of scales and the internal oxidation zone is formed by oxygen inward diffusion.
The Δ G values of the oxidation of nickel and aluminum at different temperatures calculated by the HSC software are shown in Fig. 8. At 900℃, the Δ G values of the oxidation of nickel and aluminum are − 63.9 kJ and − 207.7 kJ, respectively.
Therefore, based on the Vant Hoff isotherm equation, the minimum oxygen pressure required for the oxidation of nickel and aluminum can be calculated by the change of Gibbs free energy of the reaction. Since the activity of solid substances is 1, the equation can be written as:
When ΔG = 0 kJ, the reaction is in equilibrium, and the equation can change to:
From this, it can be calculated that the minimum oxygen pressure required for the oxidation of nickel and aluminum is 1.4×10− 3 Pa and 5.6×10− 10 Pa, respectively. Due to the extremely low oxygen pressure required for the oxidation of Al, Al2O3 can form as an internal oxidation zone below the scales/alloy interface (with an oxygen pressure of 1.4×10− 3 Pa).
Schematic diagram of scales grown on Ni-Al alloys under 0.1 MPa pure O2 at 900℃ is shown in Fig. 9. Internal oxidation of Ni-9Al, Ni-10Al, and Ni-11Al are illustrated in Fig. 9a. It can be seen that when Ni-Al alloys oxidize, Ni diffuses outward to form a NiO layer on the surface of the alloy, and new oxides form on the outer side of the NiO layer. Meanwhile, due to the inward diffusion of oxygen, NiO also forms inside the alloys until the oxygen pressure drops below 1.4×10− 3 Pa. Similarly, because the oxidation of Al is dominated by the oxygen inward transfer, Al2O3 is generated below the alloy interface until the oxygen pressure drops below 5.6×10− 10 Pa. It is worth noting that due to the presence of concentration gradient and the lower system energy after oxidation, the Al element inside the alloy diffuses outward and oxidizes at the oxygen pressure of 5.6×10− 10 Pa, resulting in the enrichment of Al2O3 at this oxygen pressure, followed by the formation of a relatively continuous Al2O3 layer.
Single external oxidation of Ni-13Al and Ni-14Al are illustrated in Fig. 9b. Compared with internal oxidation, the main feature of single external oxidation of Ni-Al alloy is the absence of internal oxidation zone of Al2O3. This is due to the increased Al content in the alloy, resulting in the formation of a protective Al2O3 layer at an oxygen pressure greater than 1.4×10− 3 Pa. The formation of a dense Al2O3 layer causes a sudden drop of oxygen pressure lower than 5.6×10− 10 Pa. An oxygen pressure zone higher than 5.6×10− 10 Pa and lower than 1.4×10− 3 Pa like Fig. 9a does not exist, so internal oxidation of Al cannot happen. The alloys of Ni-13Al and Ni-14Al achieve a transition from internal oxidation to external oxidation due to the increase of Al content. In the near future, further research will be conducted on the specific process and mechanism of Ni-Al alloy's transition from transient oxidation to steady-state oxidation.
(1) Three alloys with low aluminum content, Ni-9Al, Ni-10Al, and Ni-11Al, undergo internal oxidation of Al. Although a continuous thin layer of Al2O3 forms at the front of the internal oxidation zone, it is not dense and can't provide good protection for the alloys.
(2) The Ni-13Al and Ni-14Al alloys with high aluminum content form a continuous and dense Al2O3 protective layer, providing good protection for the alloy, so the alloys do not undergo internal oxidation of Al.
(3) The scales of Ni-12Al alloy exhibits different structures in different regions. Due to the slight variations of the composition in different areas, Ni-12Al has formed a similar structure in areas with slightly lower aluminum content to those of the Ni-9Al, Ni-10Al and Ni-11Al alloys, while a similar structure in areas with slightly higher aluminum content to those of theNi-13Al and Ni-14Al alloys. It is easy to infer that 12 at.% Al is the critical concentration for the transition from protective to non- protective.
Author contributions
Heng Zhang, Junhuai Xiang, Lingyun Bai: Investigation, Methodology, Writing-original draft, Software, Formal analysis. Botao Xiao: Visualization, Data curation, Funding acquisition. Lingyun Bai: Formal analysis, Visualization. Heng Zhang: Writing - original draft. Lingyun Bai: Software, Resources. Botao Xiao: Funding acquisition. Heng Zhang, Junhuai Xiang, Lingyun Bai, Botao Xiao: Conceptualization, Validation, Supervision, Writing - review and editing, Funding acquisition. All authors listed have made a substantial, direct, and intellectual contribution to the work and approved it for publication.
Acknowledgments
The authors gratefully acknowledge the financial supports from the National Natural Science Foundation of China (Nos. 51865014), the Science and Technology Project of Jiangxi Education Department, china (GJJ211130), and the Open research fund of State Key Laboratory of Material Forming and Mould Technology, china (P2019-012).
Data availability statement
The raw/processed data required to reproduce these findings can not be shared at this time as the data also forms part of an ongoing study.
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No competing interests reported.