第四代核反應器之一的超高溫氣冷式反應器(VHTR)，使用氦氣作為冷卻劑，爐心出口溫度大約在1123至1223 K。為了達到更高的產能效率、安全性及長時間的工作環境，高溫抗腐蝕的結構材料，例如：Incoloy 800H合金、Haynes 230合金與Inconel 617合金常被選為使用於核反應器部件當中的候選材料，像是內部結構、管路及熱交換系統。由於高熵合金在高溫展現極佳的抗腐蝕性與機械強度，所以近期吸引了非常多的關注。本研究中，對高熵合金Al4Co3Cr25Cu10Fe25Ni33以及對照材料800H合金在溫度範圍為1073到1223 K的環境下進行氧化行為的研究。高熵合金Al4Co3Cr25Cu10Fe25Ni33是利用真空電弧融煉法製得，隨後的氧化測試採用熱重分析儀系統，在實驗溫度為1073、1123、1173及1223 K含氧氣與氬氣的混合環境下持溫24小時進行氧化。實驗結果顯示，Al4Co3Cr25Cu10Fe25Ni33與800H合金的氧化動力學為拋物線，且隨著溫度上升氧化速率也隨之增加。經計算，Al4Co3Cr25Cu10Fe25Ni33與800H合金的活化能分別為178.45 kJ/mole與231.96 kJ/mole。
此外，在氧化後的Al4Co3Cr25Cu10Fe25Ni33微結構中觀察到沿晶腐蝕與雙層氧化物，外層與內層分別為Cr2O3及Al2O3。而在對照合金800H中觀察到外層氧化物組成為FeCr2O4 / Cr2O3，內層為Al2O3/ SiO2。Al4Co3Cr25Cu10Fe25Ni33合金比起800H合金具有較佳的抗氧化能力，其原因在於800H合金的氧化層中含有FeCr2O4，而鐵離子與鉻離子在FeCr2O4之中的擴散速率遠快於在Cr2O3之中。而在經過溫度1223 K 時間為24小時的氧化後，800H合金表層氧化微結構可以觀察到Fe2O3 與 Fe3O4的生成，此現象是由於鐵在FeCr2O4 與 Cr2O3的擴散速率快於鉻的擴散速率導致。在轉換階段，鐵離子優先取代鉻離子形成磁鐵礦結構，而非直接形成赤鐵礦結構。理由是方晶石結構與磁鐵礦結構較為接近，界面能較低。隨後，持續往外擴散的鐵離子因較高的氧活性更進一步氧化形成完全氧化態的赤鐵礦。更進一步，從橫截面的分析中發現，氧化後的Al4Co3Cr25Cu10Fe25Ni33合金在氧化層與合金的界面形成富銅的相，此現象與選擇性氧化及初期氧化有關。由於初期氧化亞銅形成速率較快，且氧的活性也夠充裕，合金表面會形成氧化物，後來因形成連續的氧化層之後，遵守熱力學原則優先選擇形成自由能較低的氧化鋁與氧化鉻，進而將氧化亞銅的氧搶走導致介面與氧化層內部產生富銅的相。內氧化部分，Al4Co3Cr25Cu10Fe25Ni33合金與800H的晶格氧化展現了不一樣的腐蝕行為，Al4Co3Cr25Cu10Fe25Ni33合金的內氧化物主要為氧化鋁，由表面向內延伸形成不連續的塊狀結構。而800H合金，內氧化物主要為氧化鋁和氧化矽，只在表層氧化物與合金的界面形成一層較連續的氧化層，其原因在於氧化矽的形成能介於氧化鉻與氧化鋁之間，能形成內氧化但是無法像氧化鋁一樣在氧活性更低的環境生成。而氧化矽形成之後會進一步阻擋氧原子往合金內擴散，使得形成氧化鋁更加困難，因此只有表層氧化物和合金介面有內氧化現象產生。
根據鋁的重量百分比與氧化溫度的不同，晶界處的內氧化行為會有所差異，以Al4Co3Cr25Cu10Fe25Ni33合金與800H合金來說，Al4Co3Cr25Cu10Fe25Ni33含有1.91wt% 鋁，因此會在晶界形成連續的內氧化層，能有效地阻擋金屬向外擴散；而800H僅含有0.4wt%鋁，並不會形成連續的內氧化層，因此無法有效地阻擋金屬向外擴散，所以從表面微結構可以發現晶界處的氧化呈現山嶺狀。

The very high temperature reactor (VHTR), which uses helium coolant, is one of the Generation IV nuclear reactors. The outlet temperature of the reactor reaches about 1123 – 1223 K. To reach higher power generation efficiency, safety and long-lived environment, the structure materials with high temperature corrosion resistance, such as Incoloy 800H alloy, Haynes 230 alloy and Inconel 617 alloy, are often regarded as a potential candidate used in the components of nuclear reactors, i.e. the reactor internals, piping and intermediate heat exchangers (IHX).
High entropy alloys (HEA) have recently attracted considerable interest and attention due to their novel properties of good corrosion resistance and superior mechanical strength at high temperature. Thus, in this study, we investigated the oxidation behaviors of Al4Co3Cr25Cu10Fe25Ni33 HEA at the temperatures ranging from 1073 to 1223 K under the Ar and O2 atmosphere. Meanwhile, the high temperature corrosion behavior of 800H alloy was also studied for comparison. Firstly, the HEA with composition of Al4Co3Cr25Cu10Fe25Ni33 was fabricated by Vacuum Arc Melting (VAM) method, and then the oxidation tests were performed by using the thermo-gravimetric analyzer (TGA) at 1073, 1123, 1173K, and 1223 K under the atmosphere of Ar and O2 for 24 hours. The results showed that the oxidation kinetics of Al4Co3Cr25Cu10Fe25Ni33 and 800H alloys followed a parabolic regime, and their rate constants steadily increased with increasing the temperature. The activation energy of Al4Co3Cr25Cu10Fe25Ni33 alloy and 800H alloy was found to be 178.45 kJ/mole and 231.96 kJ/mole, respectively. These results suggest that Al4Co3Cr25Cu10Fe25Ni33 alloy exhibited better oxidation resistance than 800H alloy at high temperature.
In addition, preferential intergranular oxidation and the bilayer scales, Cr2O3/Al2O3, were found for Al4Co3Cr25Cu10Fe25Ni33 after oxidation process. However, for 800H alloy, FeCr2O4 / Cr2O3 was formed at the external scale and Al2O3/ SiO2 was formed at the internal scale. Al4Co3Cr25Cu10Fe25Ni33 alloy possessed better antioxidant capacity than 800H alloy because chromium and iron ions diffused faster in FeCr2O4, which was formed at the external oxide of 800H alloy, than in Cr2O3. Fe2O3 and Fe3O4 were also observed in some regions of the external oxide layer at 1223 K. The formation mechanism of Fe2O3 and Fe3O4 was because the diffusivity of iron was larger than that of chromium in both Cr2O3 and FeCr2O4. At this transition stage, iron ions replaced chromium ions to form magnetite, but did not become hematite directly, because of the lower energy barrier related to the interfacial energy. The continuously outwardly diffused iron ions were fully oxidized as hematite, which favorably formed on the outer layer with high oxygen activity. Moreover, from cross-section analysis, the formation of the copper-rich phase at the scale-alloy interface in Al4Co3Cr25Cu10Fe25Ni33 alloy after oxidation was related to initial oxidation and then to selective oxidation. Since the initial cuprous oxide formation rate was fast and the oxygen activity was sufficient, cuprous oxide was formed on the surface of the alloy. Later, a continuous oxide layer was formed with aluminum oxide and chromium oxide, which had lower Gibbs free energy. Following Thermodynamics, these two oxides robbed the oxygen of cuprous oxide and result in a copper-rich phase at the scale-alloy interface. As for internal oxide, Al4Co3Cr25Cu10Fe25Ni33 alloy and 800H alloy exhibited different corrosion behaviors. For Al4Co3Cr25Cu10Fe25Ni33 alloy, the internal oxide was aluminum oxide, which extended inward from the surface to form a discontinuous massive structure. For 800H alloy, the internal oxide contained mainly aluminum oxide and silicon oxide, which formed a rather continuous oxide layer at the interface between the external oxide and the alloy. The reason was that the formation energy of silicon oxide was between chromium oxide and aluminum oxide, so silicon oxide could form as internal oxide, but could not form at low oxygen activity environment like aluminum oxide. Since silicon oxide blocked the inward diffusion of oxygen, aluminum oxide was hard to form further inward. Therefore, internal oxidation appeared only at the interface between the external oxide and the alloy. Furthermore, the internal oxidation behaviors at the grain boundaries were also different between Al4Co3Cr25Cu10Fe25Ni33 and 800H alloys. Al4Co3Cr25Cu10Fe25Ni33 had 1.91wt% aluminum and could form a continuous internal oxide layer at the grain boundary that could effectively block the metal from diffusing outwardly to form an oxide. While 800H alloy only had 0.4wt% aluminum, so the internal oxide layer was not continuous and could not effectively block the outward diffusion of metal ions. Therefore, an oxide ridge was formed at the grain boundary. In this study, the microstructure and oxidation behavior of high entropy alloy, Al4Co3Cr25Cu10Fe25Ni33, were investigated with two purposes. One was to verify the oxidation mechanism of Al4Co3Cr25Cu10Fe25Ni33 alloy among 1073, 1123, 1173,1223 K in oxygen-containing atmosphere for 24 hours. The other was to investigate the effect of alloying elements on the microstructure and oxidation resistance, which would provide a reference for the optimization of the alloy compositions.

[1] X. L. Yan, "3 - Very high-temperature reactor," in Handbook of Generation IV Nuclear Reactors, I. L. Pioro, Ed., ed: Woodhead Publishing, 2016, pp. 55-90.
[2] S. Fujikawa, H. Hayashi, T. Nakazawa, K. Kawasaki, T. Iyoku, S. Nakagawa, et al., "Achievement of Reactor-Outlet Coolant Temperature of 950°C in HTTR," Journal of Nuclear Science and Technology, vol. 41, pp. 1245-1254, 2004/12/01 2004.
[3] G. Locatelli, M. Mancini, and N. Todeschini, "Generation IV nuclear reactors: Current status and future prospects," Energy Policy, vol. 61, pp. 1503-1520, 2013/10/01/ 2013.
[4] J.-W. Yeh, "Alloy Design Strategies and Future Trends in High-Entropy Alloys," JOM, vol. 65, pp. 1759-1771, 2013/12/01 2013.
[5] W. Kai, W.-L. Jang, R.-T. Huang, C. C. Lee, H. Hsieh, and C. F. Du, "Air Oxidation of FeCoNi-Base Equi-Molar Alloys at 800–1000°C," Oxidation of Metals, vol. 63, pp. 169-192, 01/04 2005.
[6] W. Kai, C. C. Li, F. P. Cheng, K. P. Chu, R. T. Huang, L. W. Tsay, et al., "The oxidation behavior of an equimolar FeCoNiCrMn high-entropy alloy at 950°C in various oxygen-containing atmospheres," Corrosion Science, vol. 108, pp. 209-214, 2016/07/01/ 2016.
[7] W. Chen, X. Ding, Y. Feng, X. Liu, K. Liu, Z. P. Lu, et al., "Vacancy formation enthalpies of high-entropy FeCoCrNi alloy via first-principles calculations and possible implications to its superior radiation tolerance," Journal of Materials Science & Technology, vol. 34, pp. 355-364, 2018/02/01/ 2018.
[8] F. Granberg, "Mechanism of radiation damage reduction in equiatomic multicomponent single phase alloys," Phys. Rev. Lett., vol. 116, 01/01 2016.
[9] I. L. Pioro, R. B. Duffey, P. L. Kirillov, and R. Panchal, "1 - Introduction: A survey of the status of electricity generation in the world∗," in Handbook of Generation IV Nuclear Reactors, I. L. Pioro, Ed., ed: Woodhead Publishing, 2016, pp. 1-34.
[10] W. Hoffelner, Materials for nuclear plants: From safe design to residual life assessments, 2014.
[11] I. L. Pioro, "2 - Introduction: Generation IV International Forum," in Handbook of Generation IV Nuclear Reactors, I. L. Pioro, Ed., ed: Woodhead Publishing, 2016, pp. 37-54.
[12] N. OECD, "GIF R&D Outlook for Generation IV Nuclear Energy Systems," 2009.
[13] T. O’Connor, "Gas Reactors–A Review of the Past, an Overview of the Present and a View of the Future," in GIF Symposium, Paris (France), 2009, pp. 9-10.
[14] W. Van Rooijen, "Gas-cooled fast reactor: A historical overview and future outlook," Science and Technology of Nuclear Installations, vol. 2009, 2009.
[15] I. A. E. AGENCY., High Temperature Gas Cooled Reactor Fuels and Materials: International Atomic Energy Agency, 2010.
[16] F. Otto, Y. Yang, H. Bei, and E. P. George, "Relative effects of enthalpy and entropy on the phase stability of equiatomic high-entropy alloys," Acta Materialia, vol. 61, pp. 2628–2638, 04/01 2013.
[17] O. N. Senkov, J. M. Scott, S. V. Senkova, D. B. Miracle, and C. F. Woodward, "Microstructure and room temperature properties of a high-entropy TaNbHfZrTi alloy," Journal of Alloys and Compounds, vol. 509, pp. 6043-6048, 2011/05/19/ 2011.
[18] K. Y. Tsai, M. H. Tsai, and J. W. Yeh, "Sluggish diffusion in Co–Cr–Fe–Mn–Ni high-entropy alloys," Acta Materialia, vol. 61, pp. 4887-4897, 2013/08/01/ 2013.
[19] S. Wang, "Paracrystalline property of high-entropy alloys," AIP Advances, vol. 3, p. 102105, 10/01 2013.
[20] S. Ranganathan, "Alloyed pleasures: Multimetallic cocktails," Current Science, vol. 85, 11/25 2003.
[21] W. Kai, C. C. Li, F. P. Cheng, K. P. Chu, R. T. Huang, L. W. Tsay, et al., "Air-oxidation of FeCoNiCr-based quinary high-entropy alloys at 700–900 °C," Corrosion Science, vol. 121, pp. 116-125, 6/1 2017.
[22] C. Wagner, "Beitrag zur theorie des anlaufvorgangs," Zeitschrift für physikalische Chemie, vol. 21, pp. 25-41, 1933.
[23] P. Kofstad, "High temperature corrosion," Elsevier Applied Science Publishers, Crown House, Linton Road, Barking, Essex IG 11 8 JU, UK, 1988., 1988.
[24] D. R. Gaskell and D. E. Laughlin, Introduction to the Thermodynamics of Materials: CRC press, 2017.
[25] H. J. T. Ellingham, "Reducibility of oxides and sulfides in metallurgical processes," J Soc Chem Ind, vol. 63, pp. 125-133, 1944.
[26] H. S. Ray and S. Ray, "Non-ideal Conditions and Complex Reactions," in Kinetics of Metallurgical Processes, H. S. Ray and S. Ray, Eds., ed Singapore: Springer Singapore, 2018, pp. 193-237.
[27] H. Daoud, "Oxidation Behavior of Al8Co17Cr17Cu8Fe17Ni33, Al23Co15Cr23Cu8Fe15Ni15, and Al17Co17Cr17Cu17Fe17Ni17 Compositionally Complex Alloys (High-Entropy Alloys) at Elevated Temperatures in Air," Advanced Engineering Materials, vol. 17, 06/15 2015.
[28] C. Jang, D. Kim, D. Kim, I. Sah, W.-S. Ryu, and Y.-s. Yoo, "Oxidation behaviors of wrought nickel-based superalloys in various high temperature environments," Transactions of Nonferrous Metals Society of China, vol. 21, pp. 1524-1531, 2011/07/01/ 2011.
[29] H.-M. Tung and J. F. Stubbins, "Incipient oxidation kinetics of alloy 617 and residual stress of the oxide scale formed in air at temperatures between 850 and 1000°C," Journal of Nuclear Materials, vol. 424, pp. 23-28, 2012/05/01/ 2012.
[30] D. J. Young, High temperature oxidation and corrosion of metals vol. 1: Elsevier, 2008.
[31] R. A. Rapp, "Kinetics, Microstructures and Mechanism of Internal Oxidation - Its Effect and Prevention in High Temperature Alloy Oxidation," CORROSION, vol. 21, pp. 382-401, 1965/12/01 1965.
[32] H. C. Yi, S. W. Guan, W. W. Smeltzer, and A. Petric, "Internal oxidation of NiAl and NiAlSi alloys at the dissociation pressure of NiO," Acta Metallurgica et Materialia, vol. 42, pp. 981-990, 1994/03/01/ 1994.
[33] N. Birks, G. H. Meier, and F. S. Pettit, Introduction to the High Temperature Oxidation of Metals, 2 ed. Cambridge: Cambridge University Press, 2006.
[34] N. N. Greenwood and A. Earnshaw, Chemistry of the Elements: Elsevier, 2012.
[35] "25 - Environmental Damage at High Temperature," in High Temperature Deformation and Fracture of Materials, J.-S. Zhang, Ed., ed: Woodhead Publishing, 2010, pp. 330-348.
[36] E. N’dah, M. Hierro, K. Borrero, and F. Perez, "Study of the cyclic oxidation resistance of superalloy IN-625: lifetime predicted by COSP-modelling program," Oxidation of metals, vol. 68, pp. 9-21, 2007.
[37] C. Tedmon, "The effect of oxide volatilization on the oxidation kinetics of Cr and Fe‐Cr alloys," Journal of the Electrochemical Society, vol. 113, pp. 766-768, 1966.
[38] J. R. Davis, Nickel, cobalt, and their alloys: ASM international, 2000.
[39] H. Buscail, S. Perrier, and C. Josse, "Oxidation mechanism of the Inconel 601 alloy at high temperatures," Materials and Corrosion, vol. 62, pp. 416-422, 2011.
[40] P. Nanni and F. Gesmundo, "Scaling Behavior of an Fe-4.54 Wt% Cu Alloy in the Range 700 to 1000 C Under 1 Atm Oxygen," Corrosion, vol. 36, pp. 119-125, 03/01 1980.
[41] J. Dąbrowa, G. Cieślak, M. Stygar, K. Mroczka, K. Berent, T. Kulik, et al., "Influence of Cu content on high temperature oxidation behavior of AlCoCrCuxFeNi high entropy alloys (x = 0; 0.5; 1)," Intermetallics, vol. 84, pp. 52-61, 2017/05/01/ 2017.
[42] K. Shibata, S.-J. Seo, M. Kaga, H. Uchino, A. Sasanuma, K. Asakura, et al., "Suppression of Surface Hot Shortness due to Cu in Recycled Steels," MATERIALS TRANSACTIONS, vol. 43, pp. 292-300, 2002.
[43] Y. Kondo, "Suppression of Surface Hot Shortness Caused by Copper during Hot Rolling," Materials Science Forum, vol. 696, pp. 183-188, 2011.
[44] M. Torkar, "Effect of trace and residual elements on the hot brittleness, hot shortness and properties of 0.15-0.3 % c al-killed steels with a solidification microstructure," Materiali in Tehnologije, vol. 44, pp. 327-333, 11/01 2010.
[45] Y. Wouters, G. Bamba, A. Galerie, M. Mermoux, and J. P. Petit, "Oxygen and Water Vapour Oxidation of 15Cr Ferritic Stainless Steels with Different Silicon Contents," Materials Science Forum, vol. 461-464, pp. 839-848, 2004.
[46] W. Zhang, J. Pu, B. Chi, and L. Jian, "NiMn2O4 spinel as an alternative coating material for metallic interconnects of intermediate temperature solid oxide fuel cells," Journal of Power Sources, vol. 196, pp. 5591-5594, 2011/07/01/ 2011.
[47] G. Jalilvand and M.-A. Faghihi-Sani, "“Fe doped Ni–Co spinel protective coating on ferritic stainless steel for SOFC interconnect application”," International Journal of Hydrogen Energy, vol. 38, pp. 12007-12014, 2013/09/10/ 2013.
[48] P. Kofstad, Nonstoichiometry, diffusion, and electrical conductivity in binary metal oxides: New York (N.Y.) : Wiley-Interscience, 1972.
[49] A. Takeuchi and A. Inoue, "Classification of Bulk Metallic Glasses by Atomic Size Difference, Heat of Mixing and Period of Constituent Elements and Its Application to Characterization of the Main Alloying Element," MATERIALS TRANSACTIONS, vol. 46, pp. 2817-2829, 2005.
[50] D. P. Whittle, Y. Shida, G. C. Wood, F. H. Stott, and B. D. Bastow, "Enhanced diffusion of oxygen during internal oxidation of nickel-base alloys," Philosophical Magazine A, vol. 46, pp. 931-949, 1982/12/01 1982.
[51] I. Barin, "Thermochemical data of pure substances," VCH, 1989.
[52] A. Galerie, "High Temperature Corrosion of Chromia-forming Iron, Nickel and Cobalt-base Alloys," ed, 2010, pp. 583-605.
[53] M. G. C. Cox, B. McEnaney, and V. D. Scott, "A chemical diffusion model for partitioning of transition elements in oxide scales on alloys," The Philosophical Magazine: A Journal of Theoretical Experimental and Applied Physics, vol. 26, pp. 839-851, 1972/10/01 1972.
[54] R. E. Lobnig, H. P. Schmidt, K. Hennesen, and H. J. Grabke, "Diffusion of cations in chromia layers grown on iron-base alloys," Oxidation of Metals, vol. 37, pp. 81-93, 1992/02/01 1992.
[55] J. Gilewicz-Wolter, Z. Żurek, J. Dudala, J. Lis, M. Homa, and M. Wolter, "Diffusion Rates of51Cr,54Mn and 59Fe in MnCr2O4 and FeCr2O4 Spinels," Advances in Science and Technology, vol. 46, pp. 27-31, 2006.
[56] F. H. Stott and F. I. Wei, "Comparison of the effects of small additions of silicon or aluminum on the oxidation of iron-chromium alloys," Oxidation of Metals, vol. 31, pp. 369-391, 1989/06/01 1989.
[57] F. H. Stott, G. J. Gabriel, F. I. Wei, and G. C. Wood, "The development of silicon-containing oxides during the oxidation of iron-chromium-base alloys," Materials and Corrosion, vol. 38, pp. 521-531, 1987/09/01 1987.
[58] Y. Shida, F. H. Stott, B. D. Bastow, D. P. Whittle, and G. C. Wood, "Development of preferential intergranular oxides in nickel-aluminum alloys at high temperatures," Oxidation of Metals, vol. 18, pp. 93-113, 1982/10/01 1982.