本研究以一款析出強化高熵合金(Al0.2Co1.5CrFeNi1.5Ti0.3，Al02Ti03 HEA)為對象，藉由擇區雷射熔融製程與後熱處理，並添加不同含量碳化鈮以研究其微觀結構變化及機械性質。
添加碳化鈮後，本合金的織構與晶粒大小被有效的降低。此外，本研究利用直接時效熱處理、固溶與時效熱處理改善微結構。FCC+L12基底相、L21與MC碳化物析出共存於其微結構中。值得注意的是:在直接時效熱處理製程下，因為擇區雷射熔融而生成之胞狀結構會出現在微結構中，而固溶熱處理則會完全消除這樣的偏析現象。
本研究聚焦於不同的碳化鈮含量對合金性質的影響。隨著碳化鈮添加量上升，晶粒大小與胞狀結構間距皆會顯著下降，而析出物的數量與大小則會隨之上升。在本研究的最後，將會導入模型計算各強化機制對性質的貢獻。

A precipitate strengthened high entropy alloy (HEA), Al0.2Co1.5CrFeNi1.5Ti0.3 (Al02Ti03 HEA), with various addition of niobium carbide (NbC), processed by selective laser melting (SLM) and post heat treatment is developed in this research. After NbC addition and post heat treatment, the grain size and texture of SLM NbC/Al02Ti03 HEA can be effectively reduced. Furthermore, direct ageing (DA) and solution heat treatment plus ageing (SHTA) are separately conducted to modify microstructure. FCC+L12 matrix, L21, MC carbide precipitates are identified after post heat treatment. It is unveiled that cell structure, which is a consequence of micro-segregation due to SLM processing, exists after DA heat treatment, while SHTA processing removes such structures. Most importantly, various addition of NbC brings about discrepancy in microstructure. Simply put, grain size, cell size is reduced, while the fraction and size of precipitates both increase as NbC content is raised, resulting in the strengthening of this alloy. In the end of this research, strengthening models are applied to fit the experimental results.

Abstract I
摘要 II
致謝 III
Table of contents IV
List of Figure VI
List of Table IX
1. Introduction 1
2. Literature Review 3
2.1. Introduction of High Entropy Alloy (HEA) 3
2.1.1 Concept and Characteristics of High Entropy Alloy 3
2.1.2 Alloy of Interest: Al0.2Co1.5CrFeNi1.5Ti0.3 (Al02Ti03) HEA 5
2.2. Present Work on HEA Manufactured by Selective Laser Melting (SLM) 7
2.3. Carbide Addition 10
2.4. Precipitate Strengthening 13
2.5. Internal Stress 17
2.6. Dispersion Strengthening 20
3. Experiments and Methods 22
3.1. Experimental Procedures 22
3.2. Al02Ti03 HEA Powder 23
3.3. NbC/Al02Ti03 HEA Powder 24
3.4. Selective Laser Melting (SLM) 26
3.5. Post Heat Treatment 28
3.6. Analyzing methods 29
3.6.1 Optical Microscope (OM) 29
3.6.2 Scanning electron microscopy (SEM) 29
3.6.3 X-ray Diffractometer (XRD) 29
3.6.4 Electron Backscatter Diffraction (EBSD) 29
3.6.5 Transmitting Electron Microscope (TEM) 30
3.6.6 Tensile Test 30
4. Results and Discuss 31
4.1. Effect of Carbide and Solution Heat Treatment (SHT) on Texture 31
4.1.1 Al02Ti03 HEA under SHT 31
4.1.2 NbC/Al02Ti03 HEA under 1200℃ SHT 33
4.2. Microstructure Characterization 36
4.2.1 NbC/Al02Ti03 HEA under DA process 36
4.2.2 NbC/Al02Ti03 HEA under SHTA process 42
4.3. Mechanical Property 46
4.4. Strengthening Mechanisms 52
5. Conclusion 56
6. Reference 58

[1] C.Y. Yap, C.K. Chua, Z.L. Dong, Z.H. Liu, D.Q. Zhang, L.E. Loh, S.L. Sing, Review of selective laser melting: Materials and applications, Applied physics reviews 2(4) (2015) 041101.
[2] B. Song, X. Zhao, S. Li, C. Han, Q. Wei, S. Wen, J. Liu, Y. Shi, Differences in microstructure and properties between selective laser melting and traditional manufacturing for fabrication of metal parts: A review, Frontiers of Mechanical Engineering 10(2) (2015) 111-125.
[3] B. Zhang, J. Chen, C. Coddet, Microstructure and transformation behavior of in-situ shape memory alloys by selective laser melting Ti–Ni mixed powder, Journal of Materials Science & Technology 29(9) (2013) 863-867.
[4] E. Sallica-Leva, A. Jardini, J. Fogagnolo, Microstructure and mechanical behavior of porous Ti–6Al–4V parts obtained by selective laser melting, Journal of the mechanical behavior of biomedical materials 26 (2013) 98-108.
[5] S. Saedi, A.S. Turabi, M.T. Andani, C. Haberland, H. Karaca, M. Elahinia, The influence of heat treatment on the thermomechanical response of Ni-rich NiTi alloys manufactured by selective laser melting, Journal of Alloys and Compounds 677 (2016) 204-210.
[6] M. Xia, D. Gu, G. Yu, D. Dai, H. Chen, Q. Shi, Selective laser melting 3D printing of Ni-based superalloy: understanding thermodynamic mechanisms, Science Bulletin 61(13) (2016) 1013-1022.
[7] Y. Shi, K. Yang, S.K. Kairy, F. Palm, X. Wu, P.A. Rometsch, Effect of platform temperature on the porosity, microstructure and mechanical properties of an Al–Mg–Sc–Zr alloy fabricated by selective laser melting, Materials Science and Engineering: A 732 (2018) 41-52.
[8] M.-H. Tsai, J.-W. Yeh, High-entropy alloys: a critical review, Materials Research Letters 2(3) (2014) 107-123.
[9] Y. Ye, Q. Wang, J. Lu, C. Liu, Y. Yang, High-entropy alloy: challenges and prospects, Materials Today 19(6) (2016) 349-362.
[10] D.B. Miracle, O.N. Senkov, A critical review of high entropy alloys and related concepts, Acta Materialia 122 (2017) 448-511.
[11] A. Zaddach, C. Niu, C. Koch, D. Irving, Mechanical properties and stacking fault energies of NiFeCrCoMn high-entropy alloy, Jom 65(12) (2013) 1780-1789.
[12] B. Schuh, F. Mendez-Martin, B. Völker, E.P. George, H. Clemens, R. Pippan, A. Hohenwarter, Mechanical properties, microstructure and thermal stability of a nanocrystalline CoCrFeMnNi high-entropy alloy after severe plastic deformation, Acta Materialia 96 (2015) 258-268.
[13] N. Stepanov, D. Shaysultanov, G. Salishchev, M. Tikhonovsky, Structure and mechanical properties of a light-weight AlNbTiV high entropy alloy, Materials Letters 142 (2015) 153-155.
[14] W. Chen, Z. Fu, S. Fang, H. Xiao, D. Zhu, Alloying behavior, microstructure and mechanical properties in a FeNiCrCo0. 3Al0. 7 high entropy alloy, Materials & Design 51 (2013) 854-860.
[15] Y.-J. Chang, A.-C. Yeh, The evolution of microstructures and high temperature properties of AlxCo1. 5CrFeNi1. 5Tiy high entropy alloys, Journal of Alloys and Compounds 653 (2015) 379-385.
[16] J.P. Collier, S.H. Wong, J.K. Tien, J.C. Phillips, The effect of varying AI, Ti, and Nb content on the phase stability of INCONEL 718, Metallurgical Transactions A 19(7) (1988) 1657-1666.
[17] J. Brooks, P. Bridges, Metallurgical stability of Inconel alloy 718, Superalloys 88 (1988) 33-42.
[18] B. AlMangour, M.-S. Baek, D. Grzesiak, K.-A. Lee, Strengthening of stainless steel by titanium carbide addition and grain refinement during selective laser melting, Materials Science and Engineering: A 712 (2018) 812-818.
[19] B. AlMangour, D. Grzesiak, T. Borkar, J.-M. Yang, Densification behavior, microstructural evolution, and mechanical properties of TiC/316L stainless steel nanocomposites fabricated by selective laser melting, Materials & Design 138 (2018) 119-128.
[20] N. Kang, W. Ma, L. Heraud, M. El Mansori, F. Li, M. Liu, H. Liao, Selective laser melting of tungsten carbide reinforced maraging steel composite, Additive Manufacturing 22 (2018) 104-110.
[21] D. Gu, H. Wang, D. Dai, F. Chang, W. Meiners, Y.-C. Hagedorn, K. Wissenbach, I. Kelbassa, R. Poprawe, Densification behavior, microstructure evolution, and wear property of TiC nanoparticle reinforced AlSi10Mg bulk-form nanocomposites prepared by selective laser melting, Journal of Laser Applications 27(S1) (2015) S17003.
[22] J.W. Yeh, S.K. Chen, S.J. Lin, J.Y. Gan, T.S. Chin, T.T. Shun, C.H. Tsau, S.Y. Chang, Nanostructured high‐entropy alloys with multiple principal elements: novel alloy design concepts and outcomes, Advanced Engineering Materials 6(5) (2004) 299-303.
[23] B. Cantor, I. Chang, P. Knight, A. Vincent, Microstructural development in equiatomic multicomponent alloys, Materials Science and Engineering: A 375 (2004) 213-218.
[24] M.-H. Chuang, M.-H. Tsai, W.-R. Wang, S.-J. Lin, J.-W. Yeh, Microstructure and wear behavior of AlxCo1. 5CrFeNi1. 5Tiy high-entropy alloys, Acta Materialia 59(16) (2011) 6308-6317.
[25] C.-Y. Hsu, C.-C. Juan, W.-R. Wang, T.-S. Sheu, J.-W. Yeh, S.-K. Chen, On the superior hot hardness and softening resistance of AlCoCrxFeMo0. 5Ni high-entropy alloys, Materials Science and Engineering: A 528(10-11) (2011) 3581-3588.
[26] H. Daoud, A. Manzoni, N. Wanderka, U. Glatzel, High-temperature tensile strength of Al 10 Co 25 Cr 8 Fe 15 Ni 36 Ti 6 compositionally complex alloy (high-entropy alloy), Jom 67(10) (2015) 2271-2277.
[27] Y.-K. Kim, J. Choe, K.-A. Lee, Selective laser melted equiatomic CoCrFeMnNi high-entropy alloy: Microstructure, anisotropic mechanical response, and multiple strengthening mechanism, Journal of Alloys and Compounds 805 (2019) 680-691.
[28] S.-P. Wang, J. Xu, TiZrNbTaMo high-entropy alloy designed for orthopedic implants: As-cast microstructure and mechanical properties, Materials Science and Engineering: C 73 (2017) 80-89.
[29] S. Guan, D. Wan, K. Solberg, F. Berto, T. Welo, T.M. Yue, K. Chan, Additive manufacturing of fine-grained and dislocation-populated CrMnFeCoNi high entropy alloy by laser engineered net shaping, Materials Science and Engineering: A 761 (2019) 138056.
[30] D. Karlsson, A. Marshal, F. Johansson, M. Schuisky, M. Sahlberg, J.M. Schneider, U. Jansson, Elemental segregation in an AlCoCrFeNi high-entropy alloy–a comparison between selective laser melting and induction melting, Journal of Alloys and Compounds 784 (2019) 195-203.
[31] I.-T. Ho, Y.-T. Chen, A.-C. Yeh, C.-P. Chen, K.-K. Jen, Microstructure evolution induced by inoculants during the selective laser melting of IN718, Additive Manufacturing 21 (2018) 465-471.
[32] R. Wang, G. Zhu, C. Yang, W. Wang, D. Wang, A. Dong, D. Shu, L. Zhang, B. Sun, Nano-size carbide-reinforced Ni matrix composite prepared by selective laser melting, Nano Materials Science (2019).
[33] A. Amar, J. Li, S. Xiang, X. Liu, Y. Zhou, G. Le, X. Wang, F. Qu, S. Ma, W. Dong, Additive manufacturing of high-strength CrMnFeCoNi-based High Entropy Alloys with TiC addition, Intermetallics 109 (2019) 162-166.
[34] J. Li, S. Xiang, H. Luan, A. Amar, X. Liu, S. Lu, Y. Zeng, G. Le, X. Wang, F. Qu, Additive manufacturing of high-strength CrMnFeCoNi high-entropy alloys-based composites with WC addition, Journal of Materials Science & Technology 35(11) (2019) 2430-2434.
[35] B. Li, L. Zhang, Y. Xu, Z. Liu, B. Qian, F. Xuan, Selective laser melting of CoCrFeNiMn high entropy alloy powder modified with nano-TiN particles for additive manufacturing and strength enhancement: Process, particle behavior and effects, Powder Technology 360 (2020) 509-521.
[36] E. Nembach, G. Neite, Precipitation hardening of superalloys by ordered γ′-particles, Progress in Materials Science 29(3) (1985) 177-319.
[37] B. Reppich, W. Kühlein, G. Meyer, D. Puppel, M. Schulz, G. Schumann, Duplex γ′ particle hardening of the superalloy Nimonic PE 16, Materials Science and Engineering 83(1) (1986) 45-63.
[38] T.H. Courtney, Mechanical behavior of materials, Waveland Press2005.
[39] E. Nembach, S. Schänzer, K. Trinckauf, The antiphase boundary energy of γ precipitates in nickel-based superalloys, Philosophical Magazine A 66(5) (1992) 729-738.
[40] R.C. Reed, The superalloys: fundamentals and applications, Cambridge university press2008.
[41] E.S. Huron, R.C. Reed, M.C. Hardy, M.J. Mills, R.E. Montero, P.D. Portella, J. Telesman, Superalloys 2012, John Wiley & Sons2012.
[42] B. Reppich, Some new aspects concerning particle hardening mechanisms in γ'precipitating Ni-base alloys—I. Theoretical concept, Acta Metallurgica 30(1) (1982) 87-94.
[43] D. Collins, H. Stone, A modelling approach to yield strength optimisation in a nickel-base superalloy, International Journal of Plasticity 54 (2014) 96-112.
[44] L. Parry, I. Ashcroft, R.D. Wildman, Understanding the effect of laser scan strategy on residual stress in selective laser melting through thermo-mechanical simulation, Additive Manufacturing 12 (2016) 1-15.
[45] A.E. Patterson, S.L. Messimer, P.A. Farrington, Overhanging features and the SLM/DMLS residual stresses problem: Review and future research need, Technologies 5(2) (2017) 15.
[46] Y. Lu, S. Wu, Y. Gan, T. Huang, C. Yang, L. Junjie, J. Lin, Study on the microstructure, mechanical property and residual stress of SLM Inconel-718 alloy manufactured by differing island scanning strategy, Optics & Laser Technology 75 (2015) 197-206.
[47] D. Wang, S. Wu, Y. Yang, W. Dou, S. Deng, Z. Wang, S. Li, The Effect of a scanning strategy on the residual stress of 316L steel parts fabricated by selective laser melting (SLM), Materials 11(10) (2018) 1821.
[48] G.I. Taylor, The mechanism of plastic deformation of crystals. Part I.—Theoretical, Proceedings of the Royal Society of London. Series A, Containing Papers of a Mathematical and Physical Character 145(855) (1934) 362-387.
[49] H. Conrad, Effect of interstitial solutes on the strength and ductility of titanium, Progress in Materials Science 26(2-4) (1981) 123-403.
[50] G.S. Dyakonov, S. Mironov, I.P. Semenova, R.Z. Valiev, Strengthening mechanisms and super-strength of severely deformed titanium, Nanocrystalline Titanium, Elsevier2019, pp. 123-143.
[51] G. Dyakonov, S. Mironov, I. Semenova, R. Valiev, S. Semiatin, Microstructure evolution and strengthening mechanisms in commercial-purity titanium subjected to equal-channel angular pressing, Materials Science and Engineering: A 701 (2017) 289-301.
[52] P. Rodriguez-Calvillo, J. Cabrera, Microstructure and mechanical properties of a commercially pure Ti processed by warm equal channel angular pressing, Materials Science and Engineering: A 625 (2015) 311-320.
[53] M. Staker, D. Holt, The dislocation cell size and dislocation density in copper deformed at temperatures between 25 and 700 C, Acta Metallurgica 20(4) (1972) 569-579.
[54] L. Murr, D. Kuhlmann-Wilsdorf, Experimental and theoretical observations on the relationship between dislocation cell size, dislocation density, residual hardness, peak pressure and pulse duration in shock-loaded nickel, Acta Metallurgica 26(5) (1978) 847-857.
[55] X. Wang, L.N. Carter, B. Pang, M.M. Attallah, M.H. Loretto, Microstructure and yield strength of SLM-fabricated CM247LC Ni-Superalloy, Acta Materialia 128 (2017) 87-95.
[56] H. Evans, D. Hilton, R. Holm, S. Webstert, The influence of a titanium nitride dispersion on the oxidation behavior of 20% Cr-25% Ni stainless steel, Oxidation of Metals 12(6) (1978) 473-485.
[57] L. Huang, Z. Liu, S. Chen, Y. Guo, Microstructure and tensile properties of yttrium nitride dispersion-strengthened 14Cr–3W ferritic steels, Fusion Engineering and Design 101 (2015) 17-21.
[58] T. Boegelein, S.N. Dryepondt, A. Pandey, K. Dawson, G.J. Tatlock, Mechanical response and deformation mechanisms of ferritic oxide dispersion strengthened steel structures produced by selective laser melting, Acta Materialia 87 (2015) 201-215.
[59] S. Yang, P. Zheng, P. Wang, J. Chen, Mechanical properties and thermal stability of carbide dispersion strengthened CLF-1 steel, Fusion Engineering and Design 136 (2018) 442-446.
[60] T. Kong, B. Kang, H.J. Ryu, S.H. Hong, Microstructures and enhanced mechanical properties of an oxide dispersion-strengthened Ni-rich high entropy superalloy fabricated by a powder metallurgical process, Journal of Alloys and Compounds (2020) 155724.
[61] P. Hazzledine, P. Hirsch, A coplanar Orowan loops model for dispersion hardening, Philosophical Magazine 30(6) (1974) 1331-1351.
[62] J.W. Martin, S. Martin, Micromechanisms in particle-hardened alloys, CUP Archive1980.
[63] P.A. Bland, L.E. Howard, D.J. Prior, J. Wheeler, R.M. Hough, K.A. Dyl, Earliest rock fabric formed in the Solar System preserved in a chondrule rim, Nature Geoscience 4(4) (2011) 244-247.
[64] Y.-J. Chang, A.-C. Yeh, The formation of cellular precipitate and its effect on the tensile properties of a precipitation strengthened high entropy alloy, Materials Chemistry and Physics 210 (2018) 111-119.
[65] I. Manna, S. Pabi, W. Gust, Discontinuous reactions in solids, International Materials Reviews 46(2) (2001) 53-91.
[66] W. Köster, H. Franz, Poisson's ratio for metals and alloys, Metallurgical reviews 6(1) (1961) 1-56.
[67] D. Hull, D.J. Bacon, Introduction to dislocations, Butterworth-Heinemann2001.