高熵合金突破了以單一元素為主的傳統合金設計理念，由多種溶質元素且無主要溶劑元素所組成的合金。高熵合金經由適當的熱處理，可獲得極佳的機械與物理性質，例如：高熵合金具有高硬度、高強度、抗腐蝕與抗氧化等。本論文探討由真空熔煉的Ni2CoCrFe和Ni2CoCrFeAl0.35高熵合金，觀察從1050℃固溶處理與低溫恆溫處理後的相變化以及微觀結構改變，發現Ni2CoCrFeAl0.35合金經熱處理後發生spinodal相分離與有序化反應。我們以X光繞射以及同步輻射分析，得知相結構由單相的沃斯田體(γ)所組成。經由高溫冷卻過程發生spinodal相分離與有序化相變化。其中沃斯田體相的相變化形式如下：高溫沃斯田體於冷卻過程中分解為二個低溫沃斯田體(γ′ + γ′′)，而高濃度的γ′′相於更低溫時，經由有序化反應而相轉變為L12相。其總反應式為：γ → γ′ + γ′′ → γ′ + L12，並在低溫恆溫處理時晶界處發現有層狀析出物。

High-entropy alloys (HEAs) are composed of at least five principal elements. These alloys show some unusual features, such as excellent mechanical and high temperature properties, and some of them are yet to be discovered. However, the development of the HEAs requires knowledge of the phase transformations that occur during the alloy making processes.
Phase transformations of Ni2CoCrFe and Ni2CoCrFeAl0.35 alloys have been studied. The alloy underwent hot forging, cold rolling, annealing at 1323 K and quenching. This was followed by isothermal holding at 1073K to 873K. The results of our study show that Ni2CoCrFe is single phase austenite after cooling from high temperature. In CoCrFeNi alloy, Cr-rich phase precipitates at the grain boundary around 973K. The results of our study show that the spinodal decompositions occur in Ni2CoCrFeAl0.35 after heating and cooling from 1323 K, the high temperature austenite (γ) decomposes into low temperature solute-lean austenite (γ′) and solute-enriched austenite (γ′′). The solute-enriched austenite phase also transforms into the L12 phase via the ordering reaction upon cooling to lower temperature. Therefore, fine particles of L12 phase precipitate homogeneously in the austenite grains. The occurrence of spinodal decomposition and ordering reaction in the austenite phase of the HEA can be written as follows. γ → γ′ + γ′′ → γ′ + L12. Coherent fine particles of L12-type precipitate homogeneously in the austenite. In addition, lamellae of austenite and L12 phase nucleate and grow from the grain boundary.

1.M.C. Gao, J.W. Yeh, P.K. Liaw, Y. Zhang, High-Entropy Alloys, 1st edition (2016).
2.D.A. Porter, K.E. Easterling, M. Y. Sherif, Phase Transformations in Metals and Alloys, 3rd edition (2009).
3.W.C. Cheng, Phase Transformations of an Fe-0.85 C-17.9 Mn-7.1 Al Austenitic Steel After Quenching and Annealing, JOM, 66( 9), 1809 (2014).
4.J.W. Cahn, On spinodal decomposition, Acta Metall., 9, 795 (1961).
5.J.W. Cahn, On spinodal decomposition in cubic crystals, Acta Metall., 10, 179 (1962).
6.K.H. Huang, J.W. Yeh, A study on multicomponent alloy systems containing equal-mole elements, M.S. Thesis, Thesis National Tsing Hua University (1996).
7.M.H. Tsai, J.W. Yeh, High-Entropy Alloys: A Critical Review, Mater. Res. Lett. (2014).
8.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, Adv. Eng. Mat., 6(5), 299 (2004).
9.Y.F. Ye, Q. Wang, J. Lu, C.T. Liu, Y. Yang, High-entropy alloy: challenges and prospects, Mater. Today, 19(6), 349 (2016).
10.C. Zhang, F. Zhang, H. Diao, M.C. Gao, Z. Tang, J.D. Poplawsky, P.K. LiawUnderstanding phase stability of Al-Co-Cr-Fe-Ni high entropy alloys, Mater. Des., 109 (2016).
11.B. Gludovatz, A. Hohenwarter, D. Catoor, E.H. Chang, E.P. George, R.O. Ritchie, A fracture-resistant high-entropy alloy for cryogenic applications, Science, 345( 6201), 1153 (2014).
12.H.I. Aaronson, M. Enomoto, J.K. Lee, Mechanisms of Diffusional Phase Transformations in Metals and Alloys, Ch. 6 (2016).
13.Y.S. Lim, D.J. Kim, S.S. Hwang, H.P. Kim, S.W. Kim, M23C6 precipitation behavior and grain boundary serration in Ni-based Alloy 690, Mater. Charact., 96, 28 (2014).
14.Y. Li, Y. Gao, B. Xiao, T. Min, Y. Yang, S. Ma, D. Yi, J. The electronic, mechanical properties and theoretical hardness of chromium carbides by first-principles calculations, Alloys Compd., 509, 5242 (2011).
15.T.M. Butler, Phase stability and oxidation behavior of Al-Ni-Co-Cr-Fe based high-entropy alloys, Ph.D. Thesis, The University of Alabama (2016).
16.K.S. Lee, B. Bae, J.H. Kang, K.R. Lim, Y.S. Na, Multi-phase refining of an AlCoCrFeNi high entropy alloy by hot compression, Mater. Lett. 198, 81 (2017).
17.Z. Tang, O.N. Senkov, C.M. Parish, C. Zhang, F. Zhang, L.J. Santodonato, G. Wang, G. Zhao, F.Q. Yang, P.K. Liawa, Mat. Sci. Eng. A, 647, 229 (2015).
18.A. Munitz, S. Salhov, S. Hayun, N. Frage, J. Tensile ductility of an AlCoCrFeNi multi-phase high-entropy alloy through hot isostatic pressing (HIP) and homogenization, Alloys Compd., 683, 221 (2016).
19.T.M. Butler, M.L. Weaver, J. Oxidation behavior of arc melted AlCoCrFeNi multi-component high-entropy alloys, Alloys Compd., 674, 229 (2016).
20.B. Gwalani, D. Choudhuri, V. Soni, Y. Ren, M. Styles, J.Y. Hwang, S. J. Nam, H. Ryu, S.H. Hong, R. Banerjee, Cu assisted stabilization and nucleation of L12 precipitates in Al0.3CuFeCrNi2 fcc-based high entropy alloy, Acta Mater., 129, 170 (2017).
21.T.M. Butler, M.L. Weaver, J. Investigation of the phase stabilities in AlNiCoCrFe high entropy alloys, Alloys Compd., 691, 119 (2017).
22.D.B. Miracle, O.N. Senkov, A critical review of high entropy alloys and related concepts, Acta Mater., 122, 448 (2017).
23.Y. Lv, R. Hu, Z.H. Yao, J. Chen, D.P. Xu, Y. Liu, X.H. Fan, Cooling rate effect on microstructure and mechanical properties of AlxCoCrFeNi high entropy alloys, Mater. Des., 132, 392 (2017).
24.J. Chen, P.Y. Niu, Y.Z. Liu, Y.K. Lu, X.H. Wang, Y.L. Peng, J.N. Liu, Effect of Zr content on microstructure and mechanical properties of AlCoCrFeNi high entropy alloy, Mater. Des., 94, 39 (2016).
25.W.C. Cheng, C.Y. Cheng, C.W. Hsu, D.E. Laughlin, Phase transformation of the L12 phase to kappa-carbide after spinodal decomposition and ordering in an Fe–C–Mn–Al austenitic steel, Mat. Sci. Eng. A, 642, 128 (2015).
26.D.E. Laughlin, Spinodal decomposition in age hardening copper-titanium alloys, Acta Metall., 23, 329 (1975).
27.W.A. Soffa, D.E. Laughlin, Prog. High-strength age hardening copper–titanium alloys, Mat. Sci., 49, 347 (2004).
28.W.A. Soffa, D.E. Laughlin, Decomposition and ordering processes involving thermodynamically first-order order→disorder transformations, Acta Metall., 37, 3019 (1989).
29.R. Oshima, C.M. Wayman, Fine structure in quenched Fe-Al-C steels, Metall. Trans., 3, 2163 (1972).
30.K.H. Han, J.C. Yoon, W.K. Choo, TEM evidence of modulated structure in Fe-Mn-Al-C austenitic alloys, Scripta Metall., 20, 33 (1986).
31.K. Sato, K. Tagawa, Y. Inoue, Age hardening of an Fe-30Mn-9Al-0.9C alloy by spinodal decomposition, Scripta Metall., 22, 899 (1988).
32.K. Sato, K. Tagawa, Y. Inoue, Spinodal decomposition and mechanical properties of an austenitic Fe-30wt.%Mn-9wt.%Al-0.9wt.%C alloy, Mat. Sci. Eng. A, 111, 45(1989).
33.K. Sato, K. Tagawa, Y. Inoue, Metall. Modulated structure and magnetic properties of age-hardenable Fe-Mn-Al-C alloys, Trans. A, 21, 5 (1990).
34.K.H. Han, On the coarsening of the modulated structure during aging of austenitic Fe-Mn-Al-C alloys prepared by the rapid solidification process, Mat. Sci. Eng. A, 197, 223 (1995).
35.W.K. Choo, J.H. Kim, J.C. Yoon, Microstructural change in austenitic Fe-30.0wt%Mn-7.8wt%Al-1.3wt%C initiated by spinodal decomposition and its influence on mechanical properties, Acta Mater., 45, 4877 (1997).
36.M.C. Li, H. Chand, P.W. Kao, D. Gan, The effect of Mn and Al contents on the solvus of κ phase in austenitic Fe-Mn-Al-C alloys, Mater. Chem. Phys., 59, 96 (1999).
37.C.S. Wang, C.N. Hwang, C.G. Chao, T.F. Liu, Phase transitions in an Fe–9Al–30Mn–2.0C alloy, Scripta Mater, 57, 809 (2007).
38.H. Warlimont, Order-Disorder Transformation in Alloys, 58 (1974).
39.T.F. Liu, C.M. Wan, DO3 structure in an Fe-Al-Mn-Cr alloy, Scripta Metall., 19, 805 (1985).
40.C.J. Sung, W.C. Cheng, A study of spinodal decomposition and ordering reaction in Al0.5CoCrFeNi2 high entropy alloys, M.S. Thesis, National Taiwan University of Science and Technology (2018).
41.Bhattacharjee, T., Wani, I.S., Sheikh, S., Clark, I.T., Okawa, T., Guo, S., Bhattacharjee, P.P. Simultaneous Strength-Ductility Enhancement of a Nano-Lamellar AlCoCrFeNi2.1 Eutectic High Entropy Alloy by Cryo-Rolling and Annealing, Sci. Rep. 8, 3276 (2018).
42.Wani, I. S. et al. Tailoring nanostructures and mechanical properties of AlCoCrFeNi2.1 eutectic high entropy alloy using thermo-mechanical processing, Sci. Eng. A 675, 99 (2016).
43.C.M. Kuo, C.W. Tsai, Effect of cellular structure on the mechanical property of Al0.2Co1.5CrFeNi1.5Ti0.3 high-entropy alloy, Mater. Chem. Phys. 210, 103 (2017).