為了提升鈷基合金在高溫下的抗腐蝕能力，故在此研究中設計出一系列的合金。材料的研究主要分為兩種系列，第一系列是根據商用的UMCO50合金為基底並做進一步的合金成份改質；第二系列為含有γ′ 強化相的鈷基超合金(Co-Al-W-Ni-Si)系統。此兩系列合金皆是主要以改變鋁(Al)和矽(Si)的含量來探討其對材料性質的影響。
實驗內容包含鑄造態的微結構分析、析出物熱性質、硬度、沖蝕等，且為了了解鈷基合金對於添加傾向形成氧化層元素的高溫表面穩定性，故測試了在900°C、1000°C、1100°C、1150°C時的恆溫氧化和900°C時的熱腐蝕，並利用XRD、SEM、EDS等儀器分析材料表面和氧化層的組成與厚度。
實驗結果顯示，藉由矽或是鋁的添加而形成的連續氧化鋁保護層有助於提升材料在高溫下的表面穩定性，降低氧化增重的情形。另外，鎢含量的降低也能提升材料在高溫下形成氧化鋁保護層的能力。且經過預氧化處理的材料能有更卓越的抗氧化特性。在熱腐蝕方面，此研究中的合金與商用合金相較之下也有出色的表現。故根據實驗結果，此類型的材料具有在高溫環境下應用潛力的新型鈷基合金。

Co-based superalloys in the past have worse oxidation resistance since the addition of Cr has bad influence on γ′ stability. Furthermore, the protective Cr2O3 layer is tends to volatilize at high temperature. Two sets of Co-based alloys were investigated in this study in order to enhance the high temperature corrosion resistance. One is the modification of composition based on the commercial UMCO50; the other set is γ′-bearing Co-Al-W-Ni-Si alloy system.
Experimental works include microstructure analysis, thermal property, mechanical properties like hardness and erosion. Moreover, isothermal oxidation at 900°C, 1000°C, 1100°C, and 1150°C and hot corrosion at 900°C were conducted to observe the surface stability by adding the oxide-forming elements like Al and Si.
The results indicated that the continuous Al2O3 protective layer formed by addition of Al or Si can have an improvement on the materials’ high temperature surface stability, i.e. lower down the weight gain condition. Moreover, lower W content can enhance material’s ability of forming Al2O3 protective layer at high temperature as well. Pre-oxidation treatment can also make the alloys have outstanding performance. In the aspect of hot corrosion, alloys studied here were many times better than commercial UMCO50. As the result, these type newly developed Co-based alloys may have the potential for high temperature applications.

摘要 I
Abstract II
Acknowledgement III
Content IV
List of Figures VII
List of Tables XIII
1. Introduction 1
1-1 Overview of superalloys 1
1-2 Objective 4
2. Literature Review 6
2-1 High-performance alloys 6
2-2 Overview of Co-based superalloy 10
2-2-1 γ/ structures 10
2-2-2 Secondary phases 12
2-2-3 Alloying element effect 14
2-3 Corrosion problems 16
2-3-1 Oxidation 16
2-3-2 Hot corrosion 19
2-3-3 Erosion 23
3. Material and Experimental Procedure 24
3-1 Alloy design 24
3-2 Preparation of materials 28
3-3 Microstructure observation 28
3-4 X-ray diffractometer (XRD) 29
3-5 Differential thermal analysis (DTA) 29
3-6 Pre-oxidation 29
3-7 Isothermal oxidation 30
3-8 Hardness 32
3-9 Erosion 32
3-10 Hot corrosion 33
3-11 Experimental flow chart 34
4. Result and Discussion 35
4-1 Microstructure of as-cast materials 35
4-1-1 Investigation of eutectic phase by ageing treatment 41
4-2 Thermal property-DTA analysis 43
4-3 Heat treatment 47
4-3-1 Solution heat treatment 47
4-3-2 Ageing heat treatment of -forming alloys 49
4-3-3 Element partitioning effect in γ/ 52
4-4 High temperature oxidation 55
4-4-1 Isothermal Oxidation - 900°C & 1000°C 55
4-4-2 Isothermal Oxidation - 1100°C 62
4-4-3 Pre-oxidation treatment 70
4-4-4 Isothermal Oxidation - 1150°C 83
4-5 Overall Hardness 91
4-6 Erosion 94
4-7 Hot corrosion 97
5. Conclusion 102
6. Future Work 104
References 105

[1] D.R. Gaskell, Introduction to the Thermodynamics of Materials, CRC Press, 2008.
[2] R.B. Jack Bedder, Into the melting pot : the superalloy market and its impact on minor metals Minor Metals Conference, London, (2013).
[3] N. Eliaz, G. Shemesh, R.M. Latanision, Hot corrosion in gas turbine components, Engineering Failure Analysis, 9 (2002) 31-43.
[4] A. Passerone, R. Sangiorgi, E. Ricci, Penetration kinetics of liquid sulphides in nickel and cobalt, Metallurgical Science and Tecnology, 4 (2013).
[5] R.C. Reed, The superalloys: fundamentals and applications, Cambridge university press, 2006.
[6] M.S. Titus, A. Suzuki, T.M. Pollock, High Temperature Creep of New L12 Containing Cobalt‐Base Superalloys, Superalloys 2012, (2012) 823-832.
[7] T. Nishizawa, K. Ishida, The Co (cobalt) system, Journal of Phase Equilibria, 4 (1983) 387-390.
[8] R.A. Rapp, Hot corrosion of materials: a fluxing mechanism?, Corrosion Science, 44 (2002) 209-221.
[9] C.T. Sims, N.S. Stoloff, W.C. Hagel, superalloys II, (1987).
[10] R. Cahn, P. Siemers, J. Geiger, P. Bardhan, The order-disorder transformation in Ni3Al and Ni3Al-Fe alloys—I. Determination of the transition temperatures and their relation to ductility, Acta metallurgica, 35 (1987) 2737-2751.
[11] J. Sato, T. Omori, K. Oikawa, I. Ohnuma, R. Kainuma, K. Ishida, Cobalt-base high-temperature alloys, Science, 312 (2006) 90-91.
[12] K. Shinagawa, T. Omori, J. Sato, K. Oikawa, I. Ohnuma, R. Kainuma, K. Ishida, Phase Equilibria and Microstructure on γ' Phase in Co-Ni-Al-W System, Materials transactions, 49 (2008) 1474-1479.
[13] A. Mottura, A. Janotti, T.M. Pollock, Alloying Effects in the γ′ Phase of Co‐Based Superalloys, Superalloys 2012, (2012) 683-693.
[14] S. Kobayashi, Y. Tsukamoto, T. Takasugi, Phase equilibria in the Co-rich Co-Al-W-Ti quaternary system, Intermetallics, 19 (2011) 1908-1912.
[15] H.-Y. Yan, V. Vorontsov, D. Dye, Alloying effects in polycrystalline γ' strengthened Co–Al–W base alloys, Intermetallics, 48 (2014) 44-53.
[16] J.V. Giacchi, C.N. Morando, O. Fornaro, H.A. Palacio, Microstructural characterization of as-cast biocompatible Co–Cr–Mo alloys, Materials Characterization, 62 (2011) 53-61.
[17] L. Klein, M.S. Killian, S. Virtanen, The effect of nickel and silicon addition on some oxidation properties of novel Co-based high temperature alloys, Corrosion Science, 69 (2013) 43-49.
[18] L. Klein, A. Bauer, S. Neumeier, M. Göken, S. Virtanen, High temperature oxidation of γ/γ'-strengthened Co-base superalloys, in: Corrosion Science, 2011, pp. 2027-2034.
[19] A. Bauer, S. Neumeier, F. Pyczak, M. Göken, Microstructure and creep strength of different γ/γ'-strengthened Co-base superalloy variants, Scripta Materialia, 63 (2010) 1197-1200.
[20] A. Bauer, S. Neumeier, F. Pyczak, R.F. Singer, M. Göken, Creep properties of different γ'-strengthened Co-base superalloys, Materials Science and Engineering: A, 550 (2012) 333-341.
[21] C. Zenk, S. Neumeier, H. Stone, M. Göken, Mechanical properties and lattice misfit of γ/γ' strengthened Co-base superalloys in the Co–W–Al–Ti quaternary system, Intermetallics, 55 (2014) 28-39.
[22] F. Xue, H. Zhou, X. Ding, M. Wang, Q. Feng, Improved high temperature γ' stability of Co–Al–W-base alloys containing Ti and Ta, Materials Letters, 112 (2013) 215-218.
[23] L. Klein, Y. Shen, M.S. Killian, S. Virtanen, Effect of B and Cr on the high temperature oxidation behaviour of novel γ/γ'-strengthened Co-base superalloys, Corrosion Science, 53 (2011) 2713-2720.
[24] F. Pyczak, A. Bauer, M. Göken, U. Lorenz, S. Neumeier, M. Oehring, J. Paul, N. Schell, A. Schreyer, A. Stark, F. Symanzik, The effect of tungsten content on the properties of L12-hardened Co–Al–W alloys, Journal of Alloys and Compounds, 632 (2015) 110-115.
[25] P.S. Liu, K.M. Liang, High-Temperature Oxidation Behavior of the Co-Base Superalloy DZ40M in Air, Oxidation of Metals, 53 (2000) 351-360.
[26] R. Bedworth, N. Pilling, The oxidation of metals at high temperatures, J. Inst. Met., 29 (1923) 529-582.
[27] M.G. BirksN, Introduction to High Temperature Oxidation of Metals, in, 1982.
[28] R. Prescott, M. Graham, The formation of aluminum oxide scales on high-temperature alloys, Oxidation of metals, 38 (1992) 233-254.
[29] G. Wallwork, A. Hed, Mapping of the oxidation products in the ternary Co-Cr-Al system, Oxidation of Metals, 3 (1971) 213-227.
[30] P. Berthod, Kinetics of High Temperature Oxidation and Chromia Volatilization for a Binary Ni–Cr Alloy, Oxidation of Metals, 64 (2005) 235-252.
[31] L. Klein, A. Zendegani, M. Palumbo, S.G. Fries, S. Virtanen, First approach for thermodynamic modelling of the high temperature oxidation behaviour of ternary γ'-strengthened Co–Al–W superalloys, Corrosion Science, 89 (2014) 1-5.
[32] A. Sato, Y.L. Chiu, R.C. Reed, Oxidation of nickel-based single-crystal superalloys for industrial gas turbine applications, Acta Materialia, 59 (2011) 225-240.
[33] A. Sato, H. Harada, Y. Koizumi, T. Kobayashi, K. Kawagishi, H. Imai, Oxidation resistances of silicon-containing 5th generation Ni-base single crystal superalloys, Journal of The Japan Institute of Metals, 70 (2006) 180-183.
[34] J. Nowotny, Diffusion in solids and high temperature oxidation of metals, Diffusion and defect data. Solid state data. Part B, Solid state phenomena, (1992).
[35] R.A. Rapp, K. Goto, The hot corrosion of metals by molten salts, in: Proceedings of the Second International Symposium on Molten Salts, Physical Electrochemistry Division, Electrochemical Society, (1981), pp. 159.
[36] S.B. Mishra, S. Prakash, K. Chandra, Studies on erosion behaviour of plasma sprayed coatings on a Ni-based superalloy, Wear, 260 (2006) 422-432.
[37] V. Chawla, A. Chawla, D. Puri, S. Prakash, P.G. Gurbuxani, B.S. Sidhu, Hot corrosion & erosion problems in coal based power plants in India and possible solutions–a review, Journal of minerals and materials characterization and Engineering, 10 (2011) 367.
[38] H.-t. WANG, G.-l. ZHANG, H.-s. YU, S.-q. WANG, G.-h. MIN, Effects of Chromium, Aluminium and Silicon on Oxidation Resistance of Fe-base Superalloy [J], Journal of Materials Engineering, 12 (2008) 73-77.
[39] C.-F. Cheng, Designing Si bearing Co-base superalloys containing γ' phases, in, Tsing-Hua University, 2014.
[40] M. Heilmaier, M. Krüger, H. Saage, J. Rösler, D. Mukherji, U. Glatzel, R. Völkl, R. Hüttner, G. Eggeler, C. Somsen, Metallic materials for structural applications beyond nickel-based superalloys, Jom, 61 (2009) 61-67.
[41] B. Pieraggi, Calculations of parabolic reaction rate constants, Oxidation of metals, 27 (1987) 177-185.
[42] D. Monceau, B. Pieraggi, Determination of parabolic rate constants from a local analysis of mass-gain curves, Oxidation of metals, 50 (1998) 477-493.
[43] V. Nagarajan, J. Stringer, D. Whittle, The hot corrosion of cobalt-base alloys in a modified Dean's rig-II. Co-Cr-Al alloys, Corrosion Science, 22 (1982) 429-439.
[44] D. McKee, D. Shores, K. Luthra, The effect of SO2 and NaCl on high temperature hot corrosion, Journal of the Electrochemical Society, 125 (1978) 411-419.
[45] C.-C. Hsieh, W. Wu, Overview of Intermetallic Sigma (σ) Phase Precipitation in Stainless Steels, ISRN Metallurgy, 2012 (2012).
[46] R.L. Plaut, C. Herrera, D.M. Escriba, P.R. Rios, A.F. Padilha, A Short review on wrought austenitic stainless steels at high temperatures: processing, microstructure, properties and performance, Materials Research, 10 (2007) 453-460.
[47] X. Tang, Sigma phase characterization in AISI 316 stainless steel, Microscopy and Microanalysis, 11 (2005) 78-79.
[48] B.F.O. Costa, J.M. Loureiro, G. Le Caër, Phase transformations of σ-FeCr induced by ball milling, in: P.E. Lippens, J.C. Jumas, J.M.R. Génin (Eds.) ICAME 2005, Springer Berlin Heidelberg, (2007), pp. 107-112.
[49] S. Miura, K. Ohkubo, T. Mohri, Mechanical properties of Co-based L12 intermetallic compound Co3(Al, W), Materials transactions, 48 (2007) 2403-2408.
[50] H.Y. Yan, V.A. Vorontsov, D. Dye, Alloying effects in polycrystalline γ' strengthened Co–Al–W base alloys, Intermetallics, 48 (2014) 44-53.
[51] T. Pollock, J. Dibbern, M. Tsunekane, J. Zhu, A. Suzuki, New Co-based γ-γ' high-temperature alloys, JOM, 62 (2010) 58-63.
[52] S. Meher, H.-Y. Yan, S. Nag, D. Dye, R. Banerjee, Solute partitioning and site preference in γ/γ' cobalt-base alloys, Scripta Materialia, 67 (2012) 850-853.
[53] S. Mrowec, A. Stokłosa, Calculations of parabolic rate constants for metal oxidation, Oxidation of Metals, 8 (1974) 379-391.
[54] A. Yeh, K. Kawagishi, H. Harada, T. Yokokawa, Y. Koizumi, T. Kobayashi, D. Ping, J. Fujioka, T. Suzuki, Development of Si-bearing 4th generation Ni-base single crystal superalloys, Superalloys 2008, (2008) 619-628.
[55] F. Golightly, F. Stott, G. Wood, The Relationship Between Oxide Grain Morphology and Growth Mechanisms for Fe‐Cr‐Al and Fe‐Cr‐Al‐Y Alloys, Journal of the Electrochemical Society, 126 (1979) 1035-1042.
[56] J.L. Smialek, Discussion of “The Relationship Between Oxide Grain Morphology and Growth Mechanisms for Fe‐Cr‐Al and Fe‐Cr‐Al‐Y Alloys”[FA Golightly, FH Stott, and GC Wood (pp. 1035–1042, Vol. 126, No. 6)], Journal of The Electrochemical Society, 126 (1979) 2275-2276.
[57] B. Seiser, R. Drautz, D.G. Pettifor, TCP phase predictions in Ni-based superalloys: Structure maps revisited, Acta Materialia, 59 (2011) 749-763.
[58] A. Suzuki, A.J. Elliott, M.F.X. Gigliotti Jr, K.B. Morey, J.C. Schaeffer, P. Subramanian, Alumina-forming cobalt-nickel base alloy and method of making an article therefrom, in, Google Patents, 2015.
[59] S. Chang, Y. Hung, T. Chuang, Joining alumina to Inconel 600 and UMCo-50 superalloys using an Sn10Ag4Ti active filler metal, Journal of materials engineering and performance, 12 (2003) 123-127.
[60] P. Jose, D. Gupta, R.A. Rapp, Solubility of α‐Al2 O 3 in Fused Na2 SO 4 at 1200 K, Journal of The Electrochemical Society, 132 (1985) 735-737.
[61] D.Z. Shi, R.A. Rapp, Solubility of SiO2 in fused Na2SO4 at 900 C, J. Electrochem. Soc.;(United States), 133 (1986).
[62] S. Bose, High temperature coatings, Butterworth-Heinemann, (2011).