Title Page
ABSTRACT
Contents
Chapter 1. Introduction 15
1.1. Nuclear fusion reactor 15
1.2. Tungsten as a plasma-facing material 18
1.3. Research objectives and structure of thesis 20
1.4. References 21
Chapter 2. Effect of yttrium-doping on tungsten 23
2.1. Introduction 23
2.2. Experimental procedures 25
2.2.1. Materials and methods 25
2.2.2. Characterization 26
2.3. Results and discussion 28
2.3.1. Characterization of powders and specimens 28
2.3.2. Effect of mechanical alloying and internal oxidation 45
2.3.3. Effect of yttrium-doping on sinterability of tungsten 49
2.3.4. Effect of yttrium-doping on plasma-facing properties of tungsten 57
2.3.5. Effect of yttrium-doping on microscopic mechanical behavior of tungsten 66
2.4. Conclusions 74
2.5. References 75
Chapter 3. Effect of potassium-doping on tungsten 81
3.1. Introduction 81
3.2. Experimental procedures 84
3.2.1. Materials and methods 84
3.2.2. Characterization 86
3.2.3. Simulations 88
3.3. Results and discussion 89
3.3.1. Fabrication of potassium-doped tungsten specimen 89
3.3.2. Characterization of specimens 93
3.3.3. Effect of potassium-doping on mechanical behavior of tungsten 105
3.3.4. Molecular dynamics simulations for nanoindentation 115
3.3.5. Dislocation dynamics simulation for nanoindentation 117
3.3.6. Effect of potassium-doping on plasma-facing properties of tungsten 125
3.4. Conclusions 130
3.5. References 131
Chapter 4. Conclusions and Outlook 136
국문 초록 138
Table 1. Characterization of two-step Pure W, two-step W-Y, and one step W-Y specimens. 44
Table 2. Measured densities and fitting parameters for nonlinear regression of densification data. 56
Table 3. Sample characterization. 102
Table 4. Input values of the parameters (a, b, c, and d) for determining q and Ch00 for the pure screw and edge components in tungsten.[이미지참조] 103
Table 5. Dislocation density and mean distance between the dislocations. 104
Table 6. Diameter of contact surface at the pop-in event and maximum shear stress of as- received W, annealed W, and K-doped W. Only those indentations inside the grains and a... 114
Table 7. Von Mises Stress at indentation depth of 10 nm were calculated by FEM simulations with various diameters and depths of bubble. 123
Table 8. Maximum shear stresses were calculated by DD simulations 124
Figure 1. (a) Global energy consumption and (b) CO₂ emission from fossil fuel. 17
Figure 2. SEM images of W-Y powders milled with different milling durations; (a) initial, (b) 0.5 h milled, (c) 1 h milled, and (d) 2 h milled powder. 34
Figure 3. (a) XRD pattern of W-Y powders milled with different milling durations, and (b) crystallite size as a function of milling time. 35
Figure 4. SEM images of ball-milled (a) Pure W and (b) W-Y powders. 36
Figure 5. Particle size distribution of Pure W and W-Y milled for 2 h. 37
Figure 6. Inverse pole figure (IPF) maps along the normal direction (ND) on the top surface of (a) two-step SPSed Pure W, (b) two-step SPSed W-Y, and (c) one-step SPSed W-Y... 38
Figure 7. Vickers hardness of the W-Y₂O₃ alloy developed previously ([8, 13, 15, 19, 20, 39, 41, 46-48]) and in this study. 39
Figure 8. Fractured surfaces of SPSed (a, c) Pure W and (b, d) W-Y alloys. (c) and (d) show high-magnification images of the yellow squares displayed in (a) and (b), respectively. 40
Figure 9. (a) High-angle annular dark-field scanning TEM image of an SPSed W-Y specimen. (b) SAED pattern of the red-circled region in (a). (c) EDS profile of the red-circled region in (a). 41
Figure 10. Size distribution of Y₂O₃ particles in W-Y specimen; (a) size distribution for all particles, (b) size distribution for particles present at the grain boundary and inside the grain. 42
Figure 11. (a) SANS data (symbols) of SPSed Pure W and W-Y samples, and the corresponding fits (lines). (b) Number density and (c) volume fraction of Y₂O₃. The dashed... 43
Figure 12. (a) SEM image on the top surface and (b) fractured surface of W-0.05 wt.%Y₂O₃ specimen. The inset in (b) is a high-magnification image of the fractured surface (white... 47
Figure 13. Size distribution of Y₂O₃ particles in W-Y₂O₃ specimen. The inset is a TEM image of W-Y₂O₃ specimen. 48
Figure 14. Schematic of test setup featuring a mold mounted on an SPS machine. 52
Figure 15. Temperature-related differences in displacement during SPS. 53
Figure 16. Relative density as a function of temperature during SPS. The solid lines were obtained by fitting the densification curves to the sigmoid function. 54
Figure 17. (a) Densification curves of Pure W and W-Y powders. (b) Average grain sizes of Pure W and W-Y. 55
Figure 18. Microstructural changes in tungsten specimens with annealing temperature: (a) Pure W, (b) W-Y, (c) commercially obtained pure tungsten. 60
Figure 19. EBSD analysis conducted along the RD surface of as-received commercially obtained pure tungsten at room temperature. (a) KAM and (b) grain-boundary maps. The... 61
Figure 20. Hardness of (a) Pure W, (b) W-Y, and (c) commercially obtained pure tungsten after annealing at various temperatures. 62
Figure 21. Surface morphologies of (a, b) Pure W, (c, d) W-Y, and (e, f) commercially obtained tungsten specimens (a, c, e) before and (b, d, f) after deuterium irradiation. 63
Figure 22. TDS results of Pure W, W-Y, and commercially obtained tungsten specimens. 64
Figure 23. SEM images after He + irradiation on SPSed Pure W, SPSed W-Y and commercially obtained pure tungsten; (a, b) incident surface and cross-section of SPSed Pure W, (c, d)... 65
Figure 24. Load-displacement curves of (a) Pure W, (b) W-Y, and (c) commercially obtained pure tungsten acquired during nanoindentation tests. The purple dotted lines were obtained... 69
Figure 25. Maximum shear stress for yielding in (a) Pure W, (b) W-Y, and (c) commercially obtained pure tungsten. The purple lines (P1, P2, P3, and P4) were obtained by fitting the... 70
Figure 26. Cumulative probability distribution of maximum shear stress for yielding in Pure W, W-Y and commercially obtained pure tungsten; The raw data are consistent with Fig. 9 in... 71
Figure 27. Cumulative probability distributions of nanohardness for Pure W, W-Y, and commercially obtained pure tungsten. 72
Figure 28. Nanohardness in (a) Pure W, (b) W-Y, and (c) commercially obtained pure tungsten; The raw data are consistent with Fig. 10 in the manuscript. 73
Figure 29. SEM image of initial K-doped W powder. 85
Figure 30. the inverse pole figures along the normal direction of the center of top surface of the K-doped W sintered with difference conditions; (a) sintered at 1700℃ without Ta foil, (b)... 91
Figure 31. the SEM images about a region near the cylindrical surface of top surface of the K-doped W specimens sintered with applying Ta foil at (a) 1700℃, and (b) 1800℃. 92
Figure 32. the inverse pole figures along the normal direction of the observed surfaces of the specimens: (a) As-received W, (b) Annealed W, and (c) K-doped W. 98
Figure 33. (a) High angle annular dark field scanning TEM image of K-doped W sample; (b) Size distribution of pores; (c-d) EDS results about the yellow points in (a). 99
Figure 34. Modified Williamson-Hall fitting for K-doped W specimen. 100
Figure 35. XRD profile and CMWP fitted line for K-doped W. 101
Figure 36. Engineering stress-strain curves of uniaxial tensile tests; (a) As-received W, (b) Annealed W, and (c) K-doped W. ((a) and (b) were reproduced utilizing the data obtained... 108
Figure 37. Fractured surfaces of samples after uniaxial tensile tests; (a, d) As-received W, (b, e) Annealed W, and (c, f) K-doped W. 109
Figure 38. The toughness of the specimens obtained from uniaxial tensile tests. (As-received W and Annealed W data were utilized from [42].) 110
Figure 39. Load-displacement curves of (a) As-received pure W, (b) Annealed pure W, and (c) K-doped W during nanoindentation tests. The purple dotted lines are obtained by fitting the... 111
Figure 40. The maximum shear stress for yielding in As-received W, Annealed W, and K- doped W. 112
Figure 41. The nanohardness in As-received W, Annealed W, and K-doped W. 113
Figure 42. Snapshots at the time of pop-in event during molecular dynamics simulations; (a) perfect single crystal of tungsten, (b) single crystal of tungsten with a nano-sized bubble. (c)... 116
Figure 43. Schematic diagram of FEM simulations 119
Figure 44. Distribution of von Mises stress in a perfect single crystal and single crystals with a nano-sized bubble simulated by FEM; diameters and depths of the bubble in the FEM... 120
Figure 45. the load-displacement curves of the dislocation dynamics simulations for perfect single crystal, single crystal with a bubble, and single crystal with four bubbles. 121
Figure 46. the load-displacement curves of the dislocation dynamics simulations for perfect single crystal, and single crystal with pre-existing dislocations (ρ=10¹⁴m⁻²). 122
Figure 47. the inverse pole figures along the normal direction of the observed surface of heat- treated K-doped W specimens; (a) initial, (b) 3 h heat-treated, and (c) 10 h heat-treated sample. 127
Figure 48. Surface morphologies of K-doped W (a, b) before and (c, d) after deuterium irradiation. 128
Figure 49. TDS results of SPSed Pure W, SPSed K-doped W, and As-received W specimens. (the SPSed Pure W specimen is the same as the one described in Chapter 2) 129