晶界工程為一加工熱處理工藝，藉由多次冷輥壓、熱輥壓與退火製程，改變晶界分佈的構成方式，改善材料性質，並被廣泛應用於超合金與不鏽鋼中。本研究將晶界工程應用於等莫耳合金CoCrFeMnNi，CoCrFeNi，與FeCoNi，其目的為研究高熵合金於晶界工程之晶界反應機制，並探討高熵化是否有助於讓材料更易形成特殊晶界與打斷高角度晶界網路，藉此優化材料特性。研究結果顯示，依不同晶界工程加工工藝，影響特殊晶界的主要因子也隨之改變。於單步再結晶時，FeCoNi擁有最佳化之晶界特徵分佈，而CoCrFeNi擁有最少量之特殊晶界比例。晶界特徵分佈主要受晶界移動速率，與高熵效應中，嚴重晶格扭曲所造成之晶界能量下降之影響。同時，高效效應中的嚴重晶格扭曲與遲緩擴散亦造成材料再結晶溫度的上升。於單步應變退火時，CoCrFeNi擁有最佳化之晶界特徵分佈，而FeCoNi之晶界特徵分佈則為最差。晶界特徵分佈主要受晶界移動速率，非共格Σ3晶界於加工與退火過程中的生成，與高熵效應中，嚴重晶格扭曲和遲緩擴散效應的影響。於多步應變退火時，CoCrFeNi擁有最佳化之晶界特徵分佈，而CoCrFeMnNi則次之。晶界特徵分佈主要受晶界移動速率，非共格Σ3晶界於加工與退火過程中的生成，與高熵效應中，嚴重晶格扭曲和遲緩擴散效應的影響，影響因子和單步應變退火相近。

In present study, grain boundary engineering has been processed on equiatomic CoCrFeMnNi, CoCrFeNi, and FeCoNi alloys. Special boundary fraction, twin density, grain size distribution and misorientation distribution have been evaluated by EBSD. Experimental results indicate that FeCoNi exhibits the highest special boundary fraction and twin density for one-step recrystallization, while CoCrFeNi exhibits the highest special boundary fraction and twin density for one-step and iterative strain annealing. The special boundary increment of one-step recrystallization is mainly affected by grain boundary velocity, while twin density of one-step recrystallization is mainly affected by average grain boundary energy and twin boundary energy. The special boundary increment and twin density for one-step and iterative strain annealing are mainly affected by grain boundary velocity, nontwin Σ3 boundary, severe-lattice-distortion effect, and sluggish-diffusion effect.

Abstract I
摘要 II
Acknowledgements III
Table of Contents VI
List of Figures IX
List of Tables XVI
1. Introduction 1
2. Literature review 4
2.1. Special boundaries 4
2.2. Grain boundary engineering 6
2.3. Mechanism of annealing twin formation 8
2.4. Special boundary interaction 11
2.5. Grain boundary interconnectivity 13
2.6. High entropy alloy 14
2.7. Equiatomic CoCrFeMnNi system 15
3. Experiments 16
3.1. Alloy design 16
3.2. Vacuum arc melting 17
3.3. Solution heat treatment 17
3.4. Grain boundary engineering 18
3.5. Oxidation test 18
3.6. Microstructure analysis 19
3.7. EBSD analysis 19
4. Results and discussion 21
4.1. GBCD of REC 21
4.1.1. REC of CoCrFeMnNi 27
4.1.2. REC of CoCrFeNi 31
4.1.3. REC of FeCoNi 35
4.1.4. REC Comparison 39
4.1.5. Twin density evolution of REC 40
4.2. GBCD of GBE1 43
4.2.1. GBE1 of CoCrFeMnNi 45
4.2.2. GBE1 of CoCrFeNi 50
4.2.3. GBE1 of FeCoNi 55
4.2.4. GBE1 Comparison 58
4.2.5. Twin density evolution 60
4.3. GBCD from GBE2 to GBE5 63
4.3.1. CoCrFeMnNi from GBE2 to GBE5 65
4.3.2. CoCrFeNi from GBE2 to GBE5 69
4.3.3. Comparison from GBE2 to GBE5 73
4.3.4. Twin density evolution 74
4.4. Triple junction analysis 75
5. Conclusion 78
6. Future work 79
7. Appendix 80
7.1. Oxidation test of CoCrFeMnNi 80
7.2. Oxidation test of CoCrFeNi 87
8. Nomenclature 94
Reference 95

[1]Watanabe T. Res Mechanica 1984;11:47.
[2]Tan L, Sridharan K, Allen TR, Nanstad RK, McClintock DA. Journal of Nuclear Materials 2008;374:270.
[3]Reed BW, Kumar M, Minich RW, Rudd RE. Acta Materialia 2008;56:3278.
[4]Michiuchi M, Kokawa H, Wang ZJ, Sato YS, Sakai K. Acta Materialia 2006;54:5179.
[5]Tan L, Sridharan K, Allen TR. Journal of Nuclear Materials 2007;371:171.
[6]Yang S, Wang Z, Kokawa H, Sato YS. Materials Science and Engineering: A 2008;474:112.
[7]Randle V, Materials Io. The Role of the Coincidence Site Lattice in Grain Boundary Engineering: Institute of Materials, 1996.
[8]Randle V. Mater. Sci. Technol. 2010;26:253.
[9]Kobayashi S, Hirata M, Tsurekawa S, Watanabe T. Procedia Engineering 2011;10:112.
[10]Hu C, Xia S, Li H, Liu T, Zhou B, Chen W, Wang N. Corrosion Science 2011;53:1880.
[11]Randle V. Acta Materialia 1999;47:4187.
[12]Lin P, Palumbo G, Aust KT. Scripta Materialia 1997;36:1145.
[13]Gleiter H. Acta Metallurgica 1969;17:1421.
[14]Pande CS, Imam MA, Rath BB. MTA 1990;21:2891.
[15]Li Q, Cahoon JR, Richards NL. Scripta Materialia 2006;55:1155.
[16]Cahoon JR, Li Q, Richards NL. Materials Science and Engineering: A 2009;526:56.
[17]Song KH, Chun YB, Hwang SK. Materials Science and Engineering: A 2007;454–455:629.
[18]Yeh JW, Chen SK, Lin SJ, Gan JY, Chin TS, Shun TT, Tsau CH, Chang SY. Advanced Engineering Materials 2004;6:299.
[19]Tsai MH, Yeh JW. Materials Research Letters 2014;2:107.
[20]Bhattacharjee PP, Sathiaraj GD, Zaid M, Gatti JR, Lee C, Tsai CW, Yeh JW. Journal of Alloys and Compounds 2014;587:544.
[21]Zaddach AJ, Niu C, Koch CC, Irving DL. JOM 2013;65:1780.
[22]Cantor B, Chang ITH, Knight P, Vincent AJB. Materials Science and Engineering: A 2004;375–377:213.
[23]Wu Z, Bei H, Otto F, Pharr GM, George EP. Intermetallics 2014;46:131.
[24]Siegel DJ. Applied Physics Letters 2005;87:121901.
[25]Gratias D, Portier R. J. Phys. Colloques 1982;43:C6.
[26]Rohrer GS. Structure and Bonding in Crystalline Materials: Cambridge University Press, 2001.
[27]Priester L. Grain Boundaries: From Theory to Engineering: Springer, 2012.
[28]Winning M, Rollett AD. Acta Materialia 2005;53:2901.
[29]Winning M, Gottstein G, Shvindlerman LS. Acta Materialia 2001;49:211.
[30]Winning M, Gottstein G, Shvindlerman LS. Acta Materialia 2002;50:353.
[31]Winning M. Acta Materialia 2003;51:6465.
[32]Read WT, Shockley W. Physical Review 1950;78:275.
[33]Brandon DG. Acta Metallurgica 1966;14:1479.
[34]Gertsman VY, Bruemmer SM. Acta Materialia 2001;49:1589.
[35]Shimada M, Kokawa H, Wang ZJ, Sato YS, Karibe I. Acta Materialia 2002;50:2331.
[36]Mahajan S, Pande CS, Imam MA, Rath BB. Acta Materialia 1997;45:2633.
[37]Schwartz A. JOM 1998;50:50.
[38]Romero RJ, Murr LE. Acta Metallurgica et Materialia 1995;43:461.
[39]Owen G, Randle V. Scripta Materialia 2006;55:959.
[40]Randle V, Davies P, Hulm B. Philosophical Magazine A 1999;79:305.
[41]Thomson CB, Randle V. Acta Materialia 1997;45:4909.
[42]Fullman RL, Fisher JC. Journal of Applied Physics 1951;22:1350.
[43]Dash S, Brown N. Acta Metallurgica 1963;11:1067.
[44]Meyers MA, Murr LE. Acta Metallurgica 1978;26:951.
[45]Kumar M, King WE, Schwartz AJ. Acta Materialia 2000;48:2081.
[46]Kumar M, Schwartz AJ, King WE. Acta Materialia 2002;50:2599.
[47]Tasan CC, Deng Y, Pradeep KG, Yao MJ, Springer H, Raabe D. JOM 2014;66:1993.
[48]Otto F, Hanold NL, George EP. Intermetallics 2014;54:39.
[49]Lee C. Study on deformation behaviors of equimolar alloys from Ni to CoCrFeMnNi. Department of Materials Science and Engineering, vol. Master: National Tsing Hua University, 2012. p.137.
[50]Otto F, Dlouhý A, Somsen C, Bei H, Eggeler G, George EP. Acta Materialia 2013;61:5743.
[51]Chen Yj, Hjelen J, Roven HJ. Transactions of Nonferrous Metals Society of China 2012;22:1801.
[52]Mingard KP, Roebuck B, Bennett EG, Gee MG, Nordenstrom H, Sweetman G, Chan P. International Journal of Refractory Metals and Hard Materials 2009;27:213.
[53]Jin Y, Lin B, Bernacki M, Rohrer GS, Rollett AD, Bozzolo N. Materials Science and Engineering: A 2014;597:295.
[54]Randle V, Horton D. Scripta Metallurgica et Materialia 1994;31:891.
[55]Lee S, Yoon D, Hwang N, Henry M. Metall and Mat Trans A 2000;31:985.
[56]Wang W, Brisset F, Helbert AL, Solas D, Drouelle I, Mathon MH, Baudin T. Materials Science and Engineering: A 2014;589:112.
[57]Hosford WF. Mechanical Behavior of Materials: Cambridge University Press, 2005.
[58]Rath BB, Imam MA, Pande CS. Materials Physics and Mechanics 2000:61.