Microstructural and mechanical characterization of tungsten based materials for fusion reactors

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(1)Universidad Politécnica de Madrid Escuela Técnica Superior de Ingenieros de Caminos, Canales y Puertos Departamento de Ciencia de Materiales. Microstructural and Mechanical Characterization of Tungsten based Materials for Fusion Reactors Tesis Doctoral Teresa Palacios Garcı́a Ingeniera de Materiales Director de la tesis: José Ygnacio Pastor Caño Doctor en Ciencias Fı́sicas Catedrático de Universidad. 2015.

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(3) Tribunal nombrado por el Magfco. y Excmo. Sr. Rector de la Universidad Politécnica de Madrid, el dı́a ...... de .............. de 2015.. Presidente: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Vocal: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Vocal: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Vocal: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Secretario: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Suplente: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Suplente: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. Realizado el acto de defensa y lectura de la Tesis el dı́a ..... de ............. de 2015 en la E.T.S. de Ingenieros de Caminos, Canales y Puertos de la Universidad Politécnica de Madrid.. Calificación: .................................................. EL PRESIDENTE. LOS VOCALES. EL SECRETARIO.

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(5) A mis padres.

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(7) Abstract. Tungsten (W) and tungsten-based alloys are considered to be the best candidate materials for fabricating the divertor in the next-generation nuclear fusion reactors. This component will experience the highest thermal loads during the operation of a reactor since it directly faces the plasma. In recent years, after thorough analysis that followed a strategy of cost reduction, the ITER Organization decided to built a full-tunsgten divertor before the first nuclear campaigns. Therefore, tungsten will be used not only as a plasma-facing material (PFM) but also in structural applications. Tungsten, due to its the excellent thermo-physical properties fulfils the requirements of a PFM, however, its use in structural applications is compromised due to its inherent brittleness. One of the objectives of this phD thesis is therefore, to find a material with improved brittleness behaviour. The microstructural and mechanical characterisation of different tunsgten-based materials was performed. However, this is a challenging task because of the reduced laboratory-scale size of the specimens provided, their fine microstructure and their brittleness. Consequently, many techniques are required to ensure an accurate measurement of all the mechanical and physical properties. Some of the applied methods have been widely used such as nanoindentation or three-point bending (TPB) tests. However, other methods were specifically developed and implemented during this work such as the measurement of the real fracture toughness of bulk-tunsgten alloys or the in situ fracture toughness measurements of very thin tungsten foils.. I.

(8) Bulk-tunsgten materials with different compositions (W-1% Y2 O3 , W-2% V0.5% Y2 O3 , W-4% V-0.5% Y2 O3 , W-2% Ti-1% La2 O3 and W-4% Ti-1% La2 O3 ) were studied and compared with pure tungsten processed under the same conditions. These alloys, produced by a powder metallurgical route of mechanical alloying (MA) and hot isostatic pressing (HIP), were microstructural and mechanically characterised from 77 to 1473 K in air and under high vacuum conditions. Hardness, elastic modulus, flexural strength and fracture toughness for all of the alloys were measured in addition to other physical and mechanical properties. Finally, the fracture surfaces after the TPB tests were analysed to correlate the micromechanisms of failure with the macroscopic behaviour. The results reveal brittle mechanical behaviour in almost the entire temperature range for the alloys and micromechanisms of failure with no improvement in the ductile-brittle transition temperature (DBTT). To continue the search of a tungsten material with lowered DBTT, a preliminary study of pure tunsgten and 0.005 wt.% potassium (K)-doped tungsten foils was also performed. These foils were industrially produced by sintering and hot and cold rolling. After that, they were annealed from 1073 to 2673 K to analyse the evolution of the microstructural and mechanical properties with increasing annealing temperature. The results revealed the stabilisation of the tungsten grains with increasing annealing temperature in the potassium-doped tungsten foil. However, additional studies need to be performed to gain a better understanding of the microstructure and mechanical properties of these materials such as fracture toughness.. II.

(9) Resumen. El wolframio (W) y sus aleaciones se consideran los mejores candidatos para la construcción del divertor en la nueva generación de reactores de fusión nuclear. Este componente va a recibir las cargas térmicas más elevadas durante el funcionamiento del reactor ya que estará en contacto directo con el plasma. En los últimos años, después de un profundo análisis y siguiendo una estrategia de reducción de costes, la Organización de ITER tomó la decisión de construir el divertor ı́ntegramente de wolframio desde el principio. Por ello, el wolframio no sólo actuará como material en contacto con el plasma (PFM), sino que también tendrá aplicaciones estructurales. El wolframio, debido a sus excelentes propiedades termo-fı́sicas, cumple todos los requerimientos para ser utilizado como PFM, sin embargo, su inherente fragilidad pone en peligro su uso estructural. Por tanto, uno de los principales objetivos de esta tesis es encontrar una aleación de wolframio con menor fragilidad. Durante éste trabajo, se realizó la caracterización microstructural y mecánica de diferentes materiales basados en wolframio. Sin embargo, ésta tarea es un reto debido a la pequeña cantidad de material suministrado, su reducido tamaño de grano y fragilidad. Por ello, para una correcta medida de todas las propiedades fı́sicas y mecánicas se utilizaron diversas técnicas experimentales. Algunas de ellas se emplean habitualmente como la nanoindentación o los ensayos de flexion en tres puntos (TPB). Sin embargo, otras fueron especificamente desarrolladas e implementadas durante el desarrollo de esta tesis como es el caso de la medida real de la tenacidad de fractura en los materiales masivos, o de las medidas in situ de la tenacidad de fractura en las láminas delgadas de wolframio.. III.

(10) Diversas composiciones de aleaciones de wolframio masivas (W-1% Y2 O3 , W-2% V-0.5% Y2 O3 , W-4% V-0.5% Y2 O3 , W-2% Ti-1% La2 O3 y W-4% Ti-1% La2 O3 ) se han estudiado y comparado con un wolframio puro producido en las mismas condiciones. Estas aleaciones, producidas por ruta pulvimetalúrgica de aleado mecánico (MA) y compactación isostática en caliente (HIP), fueron microstructural y mecánicamente caracterizadas desde 77 hasta 1473 K en aire y en alto vacı́o. Entre otras propiedades fı́sicas y mecánicas se midieron la dureza, el módulo elástico, la resistencia a flexión y la tenacidad de fractura para todas las aleaciones. Finalmente se analizaron las superficies de fractura después de los ensayos de TPB para relacionar los micromecanismos de fallo con el comportamiento macroscópico a rotura. Los resultados obtenidos mostraron un comportamiento mecánico frágil en casi todo el intervalo de temperaturas y para casi todas las aleaciones sin mejorı́a de la temperatura de transición dúctil-frágil (DBTT). Con el fin de encontrar un material base wolframio con una DBTT más baja se realizó también un estudio, aún preliminar, de láminas delgadas de wolframio puro y wolframio dopado con 0.005wt.% potasio (K). Éstas láminas fueron fabricadas industrialmente mediante sinterizado y laminación en caliente y en frio y se sometieron posteriormente a un tratamiento térmico de recocido desde 1073 hasta 2673 K. Se ha analizado la evolución de su microestructura y las propiedades mecánicas al aumentar la temperatura de recocido. Los resultados mostraron la estabilización de los granos de wolframio con el incremento de la temperatura de recocido en las láminas delgadas de wolframio dopado con potasio. Sin embargo, es necesario realizar estudios adicionales para entender mejor la microstructura y algunas propiedades mecánicas de estos materiales, como la tenacidad de fractura.. IV.

(11) Motivation of the Work. The rise in worldwide energy consumption due to the growth of some developing countries predicts an increase in energy demands. The use of coal reserves can satisfy such a demand for some years, however, burning coal also involves an increase in CO2 emissions, which contribute to global warming. Therefore, the development of new energy sources that can satisfy such a demand are necessary. Nuclear fusion power plants will be able to cover the worldwide demand with limited environmental effects, however, there are still some problems that need to be solved before fusion power plants can become operational. This work is driven by the necessity of finding tungsten materials with decreased brittleness for their use in the divertor, one of the most challenging components of future nuclear fusion reactors. Since tungsten-based materials have not been considered for structural components until only a few years ago, there are insufficient data about the mechanical properties of tungsten that make it very difficult to select appropriate compositions or manufacturing conditions. This PhD thesis focuses on investigating the microstructural and mechanical properties of several bulk and foil tungsten-based materials to find one with a reduced DBTT and also widen the database for tungsten materials. During operation materials will work under vacuum and therefore, the mechanical properties of the tungsten-based materials at high temperatures have been determined under high vacuum. However, there is a safety concern in case of an accidental scenario occurs such as a loss of coolant or a loss of vacuum. Therefore, to have a knowledge about the reaction between materials and air For that reason tests were also performed in oxidising atmosphere.. V.

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(13) Contents. Abstract. I. Resumen. III. Motivation of the Work. V. Contents. VII. List of Figures. XVI. List of Tables. XVII. 1 Introduction. 1. 1.1. Nuclear fusion process and devices . . . . . . . . . . . . . . . . . .. 2. 1.2. ITER . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 4. 1.3. Materials for divertor applications . . . . . . . . . . . . . . . . . .. 9. 1.3.1. Tungsten materials . . . . . . . . . . . . . . . . . . . . . . .. 11. 1.3.2. Tungsten-based alloys . . . . . . . . . . . . . . . . . . . . .. 13. 1.3.2.1. Tungsten-based alloys with rhenium . . . . . . . .. 13. 1.3.2.2. Tungsten-based alloys with tantalum . . . . . . .. 14. 1.3.2.3. Tungsten-based alloys with vanadium . . . . . . .. 15. 1.3.2.4. Tungsten-based alloys with titanium . . . . . . . .. 16. Dispersion-strengthened tungsten alloys . . . . . . . . . . .. 17. 1.3.3.1. 18. 1.3.3. ODS tungsten with yttria . . . . . . . . . . . . . .. VII.

(14) Contents. 2 Experimental Methods 2.1. 2.2 2.3. 2.4. 2.5. Microstructural characterisation . . . . . . . . . . . . . . . . . . . .. 21. 2.1.1. Energy dispersive X-ray spectroscopy . . . . . . . . . . . .. 25. 2.1.2. Electron backscatter diffraction . . . . . . . . . . . . . . . .. 25. Nanomechanical characterisation . . . . . . . . . . . . . . . . . . .. 27. 2.2.1. Nanoindentation tests . . . . . . . . . . . . . . . . . . . . .. 27. Micromechanical characterisation . . . . . . . . . . . . . . . . . . .. 30. 2.3.1. Microindentation tests . . . . . . . . . . . . . . . . . . . . .. 30. 2.3.2. Microtensile tests . . . . . . . . . . . . . . . . . . . . . . . .. 31. Macromechanical characterisation . . . . . . . . . . . . . . . . . . .. 32. 2.4.1. Impulse excitation technique . . . . . . . . . . . . . . . . .. 32. 2.4.2. Three-point bending tests . . . . . . . . . . . . . . . . . . .. 33. 2.4.2.1. Flexural strength tests. . . . . . . . . . . . . . . .. 35. 2.4.2.2. Fracture toughness tests . . . . . . . . . . . . . . .. 36. Other physical characterisations . . . . . . . . . . . . . . . . . . . .. 37. 2.5.1. Density . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 37. 2.5.2. Oxidation tests . . . . . . . . . . . . . . . . . . . . . . . . .. 37. 3 Advanced Experimental Measurements 3.1. Methods:. Fracture. Toughness 39. Bulk-tungsten samples . . . . . . . . . . . . . . . . . . . . . . . . .. 39. 3.1.1. 41. Techniques to introduce the notch . . . . . . . . . . . . . . 3.1.1.1. Single edge notched beam specimens using a diamond disc . . . . . . . . . . . . . . . . . . . . .. 42. Single edge notched beam specimens using a diamond wire . . . . . . . . . . . . . . . . . . . . .. 42. 3.1.1.3. Single edge V-notched beam specimens . . . . . .. 43. 3.1.1.4. Single edge laser-notched beam specimens . . . . .. 47. Influence of the notch root radius . . . . . . . . . . . . . . .. 48. Tungsten foils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 51. 3.2.1. Geometry selection . . . . . . . . . . . . . . . . . . . . . . .. 51. 3.2.1.1. The double-edge notched specimen . . . . . . . . .. 51. 3.2.1.2. The single-edge notched specimen . . . . . . . . .. 52. 3.1.1.2. 3.1.2 3.2. 21. VIII.

(15) Teresa Palacios Garcı́a. 4 Bulk-Tungsten Materials. 55. 4.1. Materials and samples . . . . . . . . . . . . . . . . . . . . . . . . .. 55. 4.2. Microstructure at room temperature . . . . . . . . . . . . . . . . .. 57. 4.2.1. Microstructure of pure tungsten. . . . . . . . . . . . . . . .. 57. 4.2.2. Microstructure of W1Y . . . . . . . . . . . . . . . . . . . .. 59. 4.2.3. Microstructure of W2V0.5Y . . . . . . . . . . . . . . . . . .. 61. 4.2.4. Microstructure of W4V0.5Y . . . . . . . . . . . . . . . . . .. 64. 4.2.5. Microstructure of W2Ti1La . . . . . . . . . . . . . . . . . .. 67. 4.2.6. Microstructure of W4Ti1La . . . . . . . . . . . . . . . . . .. 69. Micromechanical and physical characterisation . . . . . . . . . . .. 72. 4.3.1. Density . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 72. 4.3.2. Elastic modulus . . . . . . . . . . . . . . . . . . . . . . . . .. 74. 4.3.3. Hardness . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 76. 4.3.4. High temperature stability in air . . . . . . . . . . . . . . .. 77. Fracture behaviour . . . . . . . . . . . . . . . . . . . . . . . . . . .. 81. 4.4.1. Fracture behaviour of pure tungsten . . . . . . . . . . . . .. 82. 4.4.2. Fracture behaviour of W1Y . . . . . . . . . . . . . . . . . .. 88. 4.4.3. Fracture behaviour of W2V0.5Y . . . . . . . . . . . . . . .. 92. 4.4.4. Fracture behaviour of W4V0.5Y . . . . . . . . . . . . . . .. 97. 4.4.5. Fracture behaviour of W2Ti1La . . . . . . . . . . . . . . . . 102. 4.4.6. Fracture behaviour of W4Ti1La . . . . . . . . . . . . . . . . 106. 4.3. 4.4. 4.5. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 4.5.1. W-1%Y2 O3 alloy - Effect of the addition of yttria. 4.5.2. W-V-Y2 O3 alloys . . . . . . . . . . . . . . . . . . . . . . . . 115. 4.5.3. 4.5.4. . . . . . 110. 4.5.2.1. W-2%V-0.5%Y2 O3 alloy. . . . . . . . . . . . . . . 115. 4.5.2.2. W-4%V-0.5%Y2 O3 alloy. . . . . . . . . . . . . . . 119. 4.5.2.3. Effect of the addition of vanadium and yttria . . . 123. W-Ti-La2 O3 alloys . . . . . . . . . . . . . . . . . . . . . . . 127 4.5.3.1. W-2%Ti-1%La2 O3 alloy . . . . . . . . . . . . . . . 127. 4.5.3.2. W-4%Ti-1%La2 O3 alloy . . . . . . . . . . . . . . . 131. 4.5.3.3. Effect of the addition of titanium and lanthana . . 135. Effect of the alloying elements on the bulk-tungsten . . . . 139. IX.

(16) Contents. 5 Tungsten Foils. 143. 5.1. Materials and samples . . . . . . . . . . . . . . . . . . . . . . . . . 144. 5.2. Microstructural characterisation . . . . . . . . . . . . . . . . . . . . 144. 5.3. Mechanical characterisation . . . . . . . . . . . . . . . . . . . . . . 148 5.3.1. Indentation tests . . . . . . . . . . . . . . . . . . . . . . . . 148. 5.3.2. Microtensile tests . . . . . . . . . . . . . . . . . . . . . . . . 150. 5.3.3. Three-point bending tests . . . . . . . . . . . . . . . . . . . 153. 5.3.4. Fracture surfaces after the TPB tests . . . . . . . . . . . . . 153. 6 Conclusions and Perspectives 6.1. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157 6.1.1. Experimental methods . . . . . . . . . . . . . . . . . . . . . 158. 6.1.2. Bulk-tungsten alloys . . . . . . . . . . . . . . . . . . . . . . 159. 6.1.3 6.2. 157. 6.1.2.1. Influence of the addition of yttria . . . . . . . . . 159. 6.1.2.2. Influence of the addition of vanadium and yttria . 160. 6.1.2.3. Influence of the addition of titanium and lanthana 161. Tungsten foils . . . . . . . . . . . . . . . . . . . . . . . . . . 162. Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164. References. i. List of Publications. xv. Nomenclature. xvii. Agradecimientos. xix. X.

(17) List of Figures. 1.1. Types of magnetic confinement configurations . . . . . . . . . . . .. 3. 1.2. Artistic view of JET and ITER . . . . . . . . . . . . . . . . . . . .. 4. 1.3. ITER cutaway model . . . . . . . . . . . . . . . . . . . . . . . . . .. 5. 1.4. Cutaway of the ITER vacuum vessel . . . . . . . . . . . . . . . . .. 7. 1.5. Three-dimensional view of a divertor cassette . . . . . . . . . . . .. 8. 1.6. Operation of ITER . . . . . . . . . . . . . . . . . . . . . . . . . . .. 10. 1.7. Some elements used for alloying tungsten . . . . . . . . . . . . . .. 13. 1.8. Binary phase diagram of W-Re . . . . . . . . . . . . . . . . . . . .. 14. 1.9. Binary phase diagram of W-Ta . . . . . . . . . . . . . . . . . . . .. 15. 1.10 Binary phase diagram of W-V . . . . . . . . . . . . . . . . . . . . .. 16. 1.11 Binary phase diagram of W-Ti . . . . . . . . . . . . . . . . . . . .. 17. 1.12 Binary phase diagram of Y-O . . . . . . . . . . . . . . . . . . . . .. 18. 1.13 Binary phase diagram of W-Y . . . . . . . . . . . . . . . . . . . . .. 19. 2.1. Interaction volume of high-energy electrons with solid materials . .. 22. 2.2. Effect of topography on SE2 detector signal . . . . . . . . . . . . .. 23. 2.3. Schematic of the GEMINI lens from Zeiss with the InLens detector. 24. 2.4. Schematic set-up of the EBSD detector used inside the FE-SEM .. 26. 2.5. High-resolution Kikuchi pattern of a single crystal . . . . . . . . .. 26. 2.6. Berkovich geometry. . . . . . . . . . . . . . . . . . . . . . . . . . .. 28. 2.7. Typical load-displacement curve obtained by nanoindentation . . .. 29. 2.8. Vickers geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 30. 2.9. Miniaturised tensile testing machine for in situ tensile tests . . . .. 31. XI.

(18) List of Figures. 2.10 Experimental set-up for TPB tests at 77 K . . . . . . . . . . . . .. 34. 2.11 Experimental set-up for TPB tests at high temperature . . . . . .. 34. 2.12 Schematic of the smooth TPB test specimen . . . . . . . . . . . . .. 35. 2.13 Schematic of the SELNB TPB test specimen . . . . . . . . . . . .. 36. 3.1. Microstructure of the W4Ti1La nanostructured alloy . . . . . . . .. 40. 3.2. Vickers microindentation . . . . . . . . . . . . . . . . . . . . . . . .. 41. 3.3. Micrographs of a SENB-D specimen . . . . . . . . . . . . . . . . .. 42. 3.4. Micrographs of a SENB-W specimen . . . . . . . . . . . . . . . . .. 43. 3.5. Razor blade machine to create the SEVNB specimens . . . . . . .. 45. 3.6. Micrographs of a SEVNB specimen . . . . . . . . . . . . . . . . . .. 46. 3.7. Micrograph of a SELNB notch . . . . . . . . . . . . . . . . . . . .. 47. 3.8. SELNB notch root . . . . . . . . . . . . . . . . . . . . . . . . . . .. 48. 3.9. Influence of the notch root radius on the fracture toughness . . . .. 50. 3.10 Geometry of the double-edge notched specimen . . . . . . . . . . .. 51. 3.11 Geometry of the single-edge notched specimen . . . . . . . . . . . .. 53. 4.1. Microstructure of pure tungsten . . . . . . . . . . . . . . . . . . . .. 58. 4.2. Histogram of the equivalent grain size distribution for pure tungsten 58. 4.3. EBSD measured tungsten grains for pure tungsten . . . . . . . . .. 59. 4.4. Microstructure of W1Y . . . . . . . . . . . . . . . . . . . . . . . .. 60. 4.5. High magnification BSE-SEM for W1Y with the EDX spectral data. 60. 4.6. Micrograph of W1Y with EDX spectral data . . . . . . . . . . . .. 61. 4.7. Microstructure of W2V0.5Y under low magnification . . . . . . . .. 62. 4.8. Microstructure of W2V0.5Y . . . . . . . . . . . . . . . . . . . . . .. 62. 4.9. Histogram of the equivalent grain size distribution for W2V0.5Y .. 63. 4.10 EBSD measured tungsten grains for W2V0.5Y . . . . . . . . . . .. 63. 4.11 Micrograph of W2V0.5Y with EDX elemental mapping . . . . . . .. 64. 4.12 Microstructure of W4V0.5Y under low magnification . . . . . . . .. 65. 4.13 High resolution micrograph of W4V0.5Y . . . . . . . . . . . . . . .. 65. 4.14 Histogram of the equivalent grain size distribution for W4V0.5Y .. 66. 4.15 EBSD measured tungsten grains for W4V0.5Y . . . . . . . . . . .. 66. 4.16 EDX linescan of a vanadium pool of W4V0.5Y . . . . . . . . . . .. 67. XII.

(19) Teresa Palacios Garcı́a. 4.17 Microstructure of W2Ti1La under low magnification . . . . . . . .. 68. 4.18 Etched micrograph of W2Ti1La under low magnification . . . . . .. 68. 4.19 High resolution micrograph of W-1%La . . . . . . . . . . . . . . .. 69. 4.20 Micrograph of W2Ti1La with EDX elemental mapping . . . . . . .. 70. 4.21 Microstructure of W4Ti1La under low magnification . . . . . . . .. 71. 4.22 Microstructure of W4Ti1La . . . . . . . . . . . . . . . . . . . . . .. 71. 4.23 Density of the bulk materials . . . . . . . . . . . . . . . . . . . . .. 72. 4.24 Microstructure of the bulk materials at low magnification . . . . .. 73. 4.25 Elastic modulus for the bulk materials . . . . . . . . . . . . . . . .. 75. 4.26 Hardness for the bulk materials . . . . . . . . . . . . . . . . . . . .. 76. 4.27 Mass gain versus temperature for samples oxidised in air . . . . . .. 78. 4.28 Oxide scale developed for the bulk materials in air at 1273 K . . .. 79. 4.29 Evolution of the oxide scale in W2V0.5Y with increasing temperature 80 4.30 Flexural σ- curves for pure tungsten . . . . . . . . . . . . . . . . .. 82. 4.31 Average flexural strength as a function of temperature for pure tungsten . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 83. 4.32 Flexural force-displacement curves for SELNB specimens for pure tungsten . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 84. 4.33 Average fracture toughness as a function of temperature for pure tungsten . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 85. 4.34 Fracture surfaces of pure tungsten at 77 and 300 K . . . . . . . . .. 85. 4.35 Fracture surfaces of pure tungsten at 873 K . . . . . . . . . . . . .. 86. 4.36 Fracture surfaces of pure tungsten at 1273 K . . . . . . . . . . . .. 86. 4.37 Transversal sections of pure tungsten after TPB tests at 1473 K in smooth and notched specimens . . . . . . . . . . . . . . . . . . . .. 87. 4.38 Flexural σ- curves for W1Y . . . . . . . . . . . . . . . . . . . . . .. 88. 4.39 Average flexural strength as a function of temperature for W1Y . .. 89. 4.40 Flexural force-displacement curves for SELNB specimens for W1Y. 89. 4.41 Average fracture toughness as a function of temperature for W1Y. 90. 4.42 Fracture surfaces of W1Y at 300 K . . . . . . . . . . . . . . . . . .. 91. 4.43 Fracture surfaces of W1Y at 873 K . . . . . . . . . . . . . . . . . .. 91. 4.44 Fracture surfaces of W1Y at 1273 K . . . . . . . . . . . . . . . . .. 92. XIII.

(20) List of Figures. 4.45 Fracture surface for W1Y at 873 K under vacuum with the spectral data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 92. 4.46 Flexural σ- curves for W2V0.5Y . . . . . . . . . . . . . . . . . . .. 93. 4.47 Average flexural strength as a function of temperature for W2V0.5Y 94 4.48 Flexural force-displacement curves for SELNB specimens for W2V0.5Y . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 94. 4.49 Average fracture toughness as a function of temperature for W2V0.5Y 95 4.50 Fracture surfaces of W2V0.5Y at 77 K and 300 K . . . . . . . . . .. 96. 4.51 Fracture surfaces of W2V0.5Y at 873 K . . . . . . . . . . . . . . .. 96. 4.52 Fracture surfaces of W2V0.5Y at 1273 K . . . . . . . . . . . . . . .. 97. 4.53 Fracture surface of W2V0.5Y at 1473 K . . . . . . . . . . . . . . .. 97. 4.54 Flexural σ- curves for W4V0.5Y . . . . . . . . . . . . . . . . . . .. 98. 4.55 Average flexural strength as a function of temperature for W4V0.5Y 99 4.56 Average fracture toughness as a function of temperature for W4V0.5Y100 4.57 Fracture surface of W4V0.5Y at 300 K . . . . . . . . . . . . . . . . 100 4.58 Fracture surfaces of W4V0.5Y at 873 K . . . . . . . . . . . . . . . 101 4.59 Fracture surfaces of W4V0.5Y at 1273 K . . . . . . . . . . . . . . . 101 4.60 Flexural σ- curves for W2Ti1La . . . . . . . . . . . . . . . . . . . 102 4.61 Average flexural strength as a function of temperature for W2Ti1La 103 4.62 Flexural force-displacement curves for the SELNB specimens for W2Ti1La . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 4.63 Average fracture toughness as a function of temperature for W2Ti1La104 4.64 Fracture surface of W2Ti1La at 300 K . . . . . . . . . . . . . . . . 105 4.65 Fracture surfaces for W2Ti1La at 873 K . . . . . . . . . . . . . . . 105 4.66 Fracture surfaces of W2Ti1La at 1273 K . . . . . . . . . . . . . . . 106 4.67 Flexural σ- curves for W4Ti1La . . . . . . . . . . . . . . . . . . . 106 4.68 Average flexural strength as a function of temperature for W4Ti1La 107 4.69 Average fracture toughness as a function of temperature for W4Ti1La108 4.70 Fracture surface of W4Ti1La at 300 K . . . . . . . . . . . . . . . . 108 4.71 Fracture surfaces of W4Ti1La at 873 K . . . . . . . . . . . . . . . 109 4.72 Fracture surfaces of W4Ti1La at 1273 K . . . . . . . . . . . . . . . 109 4.73 Microstructure of the W1Y alloy . . . . . . . . . . . . . . . . . . . 110 4.74 TPB test results for the smooth specimens for W1Y . . . . . . . . 111 XIV.

(21) Teresa Palacios Garcı́a. 4.75 TPB test results for the SELNB specimens for W1Y . . . . . . . . 112 4.76 Average flexural strength as a function of temperature for W0.5Y and W1Y . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113 4.77 Evolution of the fracture surfaces of W1Y after TPB tests . . . . . 114 4.78 Microstructure of the W2V0.5Y alloy . . . . . . . . . . . . . . . . . 115 4.79 TPB test results for the smooth specimens for W2V0.5Y . . . . . . 116 4.80 TPB test results for the SELNB specimens for W2V0.5Y . . . . . 117 4.81 Evolution of the fracture surfaces of W2V0.5Y after TPB tests . . 118 4.82 Microstructure of the W4V0.5Y alloy . . . . . . . . . . . . . . . . . 119 4.83 TPB test results for the smooth specimens for W4V0.5Y . . . . . . 120 4.84 TPB test results for the SELNB specimens for W4V0.5Y . . . . . 121 4.85 Evolution of the fracture surfaces of W4V0.5Y after TPB tests . . 122 4.86 TPB test results for the smooth specimens for W2V0.5Y and W4V0.5Y . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124 4.87 TPB test results for the SELNB specimens for W2V0.5Y and W4V0.5Y . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126 4.88 Microstructure of the W2Ti1La alloy . . . . . . . . . . . . . . . . . 127 4.89 TPB test results for the smooth specimens for W2Ti1La . . . . . . 128 4.90 TPB test results for the SELNB specimens for W2Ti1La . . . . . . 129 4.91 Evolution of the fracture surfaces of W2Ti1La after TPB tests . . 130 4.92 Microstructure of the W4Ti1La alloy . . . . . . . . . . . . . . . . . 131 4.93 TPB test results for the smooth specimens for W4Ti1La . . . . . . 132 4.94 TPB test results for the SELNB specimens for W4Ti1La . . . . . . 133 4.95 Evolution of the fracture surfaces of W4Ti1La after TPB tests . . 134 4.96 TPB tests for the smooth specimens for W2Ti1La and W4Ti1La . 137 4.97 TPB tests for the SELNB specimens for W2Ti1La and W4Ti1La . 138 4.98 Flexural strength of bulk alloys in air. . . . . . . . . . . . . . . . . 139. 4.99 Flexural strength of bulk alloys under vacuum . . . . . . . . . . . . 140 4.100Fracture toughness of bulk alloys in air . . . . . . . . . . . . . . . . 141 4.101Fracture toughness of bulk alloys under vaccum . . . . . . . . . . . 142 5.1. Specimen dimensions of tungsten foils . . . . . . . . . . . . . . . . 144. 5.2. Cross-sections of the foils in the as-received condition. XV. . . . . . . . 145.

(22) List of Figures. 5.3. EBSD cross-section of the foils annealed 1 h at 1173 K . . . . . . . 146. 5.4. EBSD cross-section of the foils annealed 1 h at 1673 K . . . . . . . 146. 5.5. EBSD cross-section of the foils annealed 1 h at 2473 K . . . . . . . 147. 5.6. Evolution of the microstructure with increasing annealing temperature for the foils . . . . . . . . . . . . . . . . . . . . . . . . 148. 5.7. Hardness of the foils . . . . . . . . . . . . . . . . . . . . . . . . . . 149. 5.8. FE-SEM image of the double-edge notched specimen before tensile test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150. 5.9. Force-displacement curves for in situ tensile tests of double-edge notched specimens . . . . . . . . . . . . . . . . . . . . . . . . . . . 151. 5.10 Fracture surface after in situ tensile tests of double-edge notched specimen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151 5.11 Force-displacement curves for in situ tensile tests of single-edge notched specimens . . . . . . . . . . . . . . . . . . . . . . . . . . . 152 5.12 Foils status after TPB tests for pure tungsten and WVM foils . . . 153 5.13 Fracture surfaces at 1273 K . . . . . . . . . . . . . . . . . . . . . . 154 5.14 Fracture surfaces at 1373 K . . . . . . . . . . . . . . . . . . . . . . 154 5.15 Fracture surfaces at 1473 K . . . . . . . . . . . . . . . . . . . . . . 155 5.16 Fracture surfaces at 1573 K . . . . . . . . . . . . . . . . . . . . . . 155 5.17 Fracture surfaces at 1673 K . . . . . . . . . . . . . . . . . . . . . . 156 5.18 Fracture surfaces at 1873 K . . . . . . . . . . . . . . . . . . . . . . 156. XVI.

(23) List of Tables. 3.1. Mean values of the measure fracture toughness with the standard error for the four methods . . . . . . . . . . . . . . . . . . . . . . .. 49. 4.1. Weight % composition of the bulk alloys . . . . . . . . . . . . . . .. 56. 4.2. Features of the starting powders . . . . . . . . . . . . . . . . . . .. 56. 4.3. Density of the constituent powders . . . . . . . . . . . . . . . . . .. 72. 4.4. Estimated porosity for the bulk materials . . . . . . . . . . . . . .. 74. 4.5. Volume fraction for the bulk materials . . . . . . . . . . . . . . . .. 74. 4.6. Thickness of the oxide scale as a function of temperature . . . . . .. 77. 4.7. Comparison of the physical and micromechanical properties of the previously reported W2V, W4V and W0.5Y with the pure W, W2V0.5Y and W4V0.5Y . . . . . . . . . . . . . . . . . . . . . . . . 125. 4.8. Comparison of the physical and micromechanical properties of the previously reported W4Ti and W1La with pure W, W2Ti1La and W4Ti1La . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136. XVII.

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(25) CHAPTER. Introduction. A rise in the worldwide population during last few decades forecasts an increased energy demand of 37% by 2040. The growth of some economies outside the Organization for Economic Cooperation and Development such as China or India plays a decisive role in this increase, which also involves an increase in global greenhouse-gas emissions. In terms of reducing these CO2 emissions, which are mainly caused by burning fossil fuels that contribute to a long-term global warming of 3.6 o C, and satisfying the demand, new sources of energy need to be developed [IEA, 2014]. Even though renewable energy technologies, helped by global subsidies, are rapidly gaining ground, other alternative clean sources have been developed in the last few decades. One of the most promising sources is nuclear fusion power. It will have the capacity to cover worldwide energy demand with limited environmental effects and almost unlimited natural sources. But, although several experiments have been successfully performed, additional research should be conducted before nuclear fusion is implemented. Up until now, the materials are the most challenging and unsolved key point because of the extreme conditions that they will have to support under operation [IAEA, 2001].. 1. 1.

(26) Chapter 1. Introduction. 1.1. Nuclear fusion process and devices. Nuclear fusion is a process in which two light atomic nuclei collide to form one heavier atom and release large amounts of energy. It is an analogous process than that produced in the core of the Sun when hydrogen (H) nuclei combine to form helium (He). But to achieve sufficient fusion reaction rates and to make fusion useful as an energy source requires that the fuel (Deuterium (D) and Tritium (T)) be heated to temperatures over 100 million ◦ C to overcome the Coulomb barrier so they can initiate the fusion reaction [CCFE, 2015]. There are several nuclear reactions, but one of them (eq. 1.1) requires a lower ignition temperature and is more likely to occur [ITER-India, 2015]. 2 1D. +31 T !42 He (3.5 MeV) +10 n (14.1 MeV). (1.1). Although neither deuterium nor tritium are directly available on Earth, they can be obtained to have a sustainable nuclear fuel cycle. Deuterium can be found in water, while tritium, a very rare and expensive radioactive hydrogen isotope, can be produced within the fusion device via neutron irradiation of lithium targets. This plasma, however, is extremely thin and fragile and must be contained to minimise contamination and prevent cooling from coming into contact with the vessel surfaces. Such a confinement system can be gravitational, which is only possible in stars, inertial or magnetic. In inertial confinement, the fuel is quickly heated with a laser that produces a plasma implosion that reaches very high pressures and temperatures. If the fuel is dense and hot enough, the fusion reaction rate will burn a significant fraction of the fuel before it is dissipated. However, to achieve these extreme conditions, the initially cold fuel must be explosively compressed. In magnetic confinement on the other hand, the plasma is confined by powerful magnetic fields. This is the most developed and promising confinement system for designing future fusion reactors and energy production [CCFE, 2015]. The confinement is achieved by the generation of a strong toroidal doughnut-shaped field and a poloidal field that finally produce a helicoidal field. The type of fusion reactor – stellarator or tokamak (fig. 1.1) – is determined by the design of the poloidal field. In a stellarator reactor (fig. 1.1, left), the poloidal field is produced by meticulously designed external field coils. This design yields a more stable plasma with continuous operation, but the cost of achieving this intricate coil system is high and the proper design of the coils is challenging. An example of stellarator 2.

(27) Teresa Palacios Garcı́a. is the currently operational TJ-II in Madrid (Spain) that produced its first plasma in 1997. This reactor focuses on investigating the physics of the device and contributing to the study of magnetic confinement for fusion [CIEMAT, 2015]. Another example is the advanced Stellarator Wendelstein 7-X in Greifswald (Germany) that investigates the suitability of using a stellarator for a power plant and wants to demonstrate its essential property: continuous operation [IPP-W7-X, 2015].. Fig. 1.1 Types of magnetic confinement configurations: segment of the steel vessel of W7-X stellarator (left) and cutaway graphic of the tokamak configuration of the JET vessel (right) [EUROfusion, 2015].. Another type of fusion device design, a tokamak reactor (fig. 1.1, right), is the most advanced and best investigated configuration. In it, the plasma flow is induced by an increasing current in the poloidal coils. Once the current reaches its maximum value, the induction and consequently the pulse will cease. Therefore, unlike a stellarator reactor, a tokamak design has to operate in a pulsed mode. It became the dominant concept in the late 1950s after pioneering work in the Soviet Union [New-Scientist, 2009]. But it was not until the early 1990s when the first fusion device, the Tokamak Fusion Test Reactor in Princeton, New Jersey (United States), started to operate with the actual fusion fuel (D+T). It achieved plasma temperatures of several hundred million ◦ C and major progresses in confinement time and energy density, but it was shut down in 1997 [PPPL, 2015]. In Japan, the JT-60, in operation since 1985, holds the record for the highest values of the three key parameters in which fusion depends: density, temperature and confinement time [JAEA, 2015]. In the meantime, the Tore Supra, a EURATOM-CEA installation in operation since 1988 in France, has achieved its goal of creating an exceptionally long-duration plasma [EURATOM-CEA, 2015]. At the moment, however, the Joint European Torus (JET) in Oxfordshire (United Kingdom) is the largest and most powerful tokamak in the world and the only one capable of producing fusion energy. JET was designed to study plasma behaviour in conditions and dimensions approaching those required in a fusion 3.

(28) Chapter 1. Introduction. reactor. It produced its first plasma in 1983 and the first controlled release of fusion power in the world in 1991. Its current primary task is to provide the knowledge for the construction and operation of ITER, the latest worldwide collaboration project before the demonstration power plant (DEMO) in the 2030s [EUROfusion, 2015; New-Scientist, 2009].. Fig. 1.2 Artistic view of JET and ITER by Russel Perry [EUROfusion, 2015].. 1.2. ITER. ITER is a large-scale collaboration project for the construction of a tokamak fusion reactor. It aims to demonstrate the technological and scientific feasibility of nuclear fusion energy for commercial purposes. The name ITER, originally an acronym for the International Thermonuclear Experimental Reactor, is the Latin word meaning ”the way”. This name was chosen by participants in the conceptual design activities (CDA) between 1988 and 1992 to express their common hope of creating an international cooperation project for the development of a new form of energy [IAEA, 1988; ITER, 2015]. The project established its roots towards the end of the Cold War when Mikhail Gorbachev of the former Soviet Union and President Ronald Reagan of the United States agreed to develop an international project to create fusion energy for peaceful purposes. Soon after, Europe and Japan joined the project. In 2003 entered China and South Korea, and finally in 2005 India became a member. Together, these countries represent over half of the world population [ITER, 2015].. 4.

(29) Teresa Palacios Garcı́a. Under the support of the International Atomic Energy Agency (IAEA), the project began its CDA in 1988. Its success lead to the detailed engineering design activities that produced a complete documentation of the ITER final design report in 1992 that was approved by its members in 2001 [IAEA, 2001]. This report includes not only the approval for the construction but also a full analysis of costs and a safety assessment for the facility now under construction at Cadarache in southern France [Ikeda, 2010]. The machine itself (fig. 1.3) is a complex device based on the tokamak concept of plasma magnetic confinement. Inside the vacuum vessel temperatures of approximately 150 million ◦ C will persist, and a strong helicoidal magnetic field created by the superconducting magnets and the plasma itself is used to keep the plasma away from the walls.. v acu u mv es el. cr y os t at. s u per con du ct i n gmagn et s t h er mal s h i el d. bl an k et. di v er t or. el ect r omagn et Fig. 1.3 ITER cutaway model [New-Scientist, 2009].. 5.

(30) Chapter 1. Introduction. The scientific goals of ITER are to release 10 times as much energy as it will need to initiate the fusion reaction, i.e. a ratio of fusion to input power of 10 (Q 10), with an inductive burn duration between 300 and 500 s and a steady state operation of Q 5. In terms of engineering and testing performance, ITER will be used to test tritium breeding module concepts with 14 MeV-neutron power loads on the first wall. The reactor will furthermore demonstrate the availability and integration of fusion technologies and components for a future commercial power plant [IAEA, 2001]. The vacuum vessel (figs. 1.3 and 1.4) is a hermetically-sealed steel container inside a cryostat that houses the plasma and acts as a first safety barrier. Its size determines the volume of the fusion plasma; in ITER has dimensions of 6 metres diameter by 11 metres height. The inner components of the machine (fig. 1.4), commonly known as plasma-facing components (PFCs), include the blanket system and the divertor. They directly face the fusion plasma over an area of approximately 850 m2 and are one of the most critical and technically challenging components. PFCs consist of a PFM mounted onto a heat sink, which acts as armour. This PFM is supported by a structural material that acts as shielding. According to Merola et al. (2010), the main functions of the PFC: Absorb radiation, conduct heat from the plasma and contribute to the absorption of neutronic heating. Minimise the impurities from the plasma. Provide limiting surfaces that define the plasma boundary during start-up and shutdown. Contribute to the plasma passive stabilisation. The blanket system (fig. 1.4) covers the inner surface of the vacuum vessel. Its basic function is to provide heat and neutron shielding to the vessel but also to external components. The blanket system includes blanket modules, which provide the bulk of the shielding, and blanket manifolds, which route the water coolant to the blanket modules [Merola et al., 2014]. In a future fusion power plant, neutrons will be slowed down in the blanket and their kinetic energy will be transformed into heat energy and collected by the coolants for its use as electrical power production. At a later stage of the project, some of these blanket modules will be also used to test materials for tritium breeding concepts [ITER, 2015]. Such test blanket modules will provide the first experimental data of the performance of the breeding blankets in the integrated fusion nuclear environment since data about tritium 6.

(31) Teresa Palacios Garcı́a. self-sustainment are essential to design and predict the performance of DEMO and other future fusion reactors [Giancarli et al., 2012].. bl an kets y s t em. di v er t or. Fig. 1.4 Cutaway of the ITER vacuum vessel [ITER, 2015].. The divertor (fig. 1.4) is located at the very bottom of the vacuum vessel. Its main function is to exhaust the scrape-off layer power from the plasma volume while maintaining an acceptable level of core plasma contamination due to α-particles (the so-called helium-ash) and transmutation impurities. However, at the same time, the divertor must tolerate high heat loads (approximately 15% of the total fusion power) and provide neutron shielding to the vacuum vessel and magnet coils in its vicinity [Norajitra et al., 2008; Merola et al., 2010]. Its design, which should be capable of removing a high heat load of at least 10 MW/m2 , is based on a modular construction of a helium-cooled finger unit that helps to reduce thermal stresses [Norajitra et al., 2007]. The success of the divertor strongly depends on the effectiveness of the cooling technology and on the availability of appropriate structural materials. The divertor consists of 54 remotely-removable cassettes (fig. 1.5), each of which includes a cassette body and three PFCs: the inner vertical target (IVT), the outer vertical target (OVT) and the dome [Merola et al., 2014]. 7.

(32) Chapter 1. Introduction. Fig. 1.5 Three-dimensional view of a divertor cassette [Merola et al., 2014].. The operation of ITER can be explained as follows (fig. 1.6) [New-Scientist, 2009]: A) Electricity flows through the electromagnet and produce a voltage across the gas while a blast of deuterium and tritium is injected into the vacuum vessel. Due to the voltage, the deuterium and tritium atoms are transformed into charged atoms (ions) within a few microseconds, thereby forming the plasma. B) The plasma, locked inside the vacuum vessel due to the magnetic confinement, is quickly heated by the superconducting magnetic coils to 10 million ◦ C. However, even at this temperature the plasma is not hot enough for fusion to occur; a temperature of 100–200 million ◦ C is required. Therefore, radio and microwaves are fired into the plasma along with high-energy beams of deuterium atoms. C) High-energy neutrons and helium particles deposit their energy into the plasma and keep it hot. As these materials turn into ash, they are removed though the divertor. However, these neutrons and other particles also bombard the tiles on the PFCs and heat them. D) Deuterium and tritium must be continuously refuelled to continue the process. Unburnt fuel is recovered from the gaseous exhaust and the reaction can be fine-tuned by firing frozen pellets of fusion fuel deep into the plasma. 8.

(33) Teresa Palacios Garcı́a. 1.3. Materials for divertor applications. As a part of the PFCs, the divertor directly faces the plasma. It is also the highest thermally loaded component that will have to withstand very demanding conditions during the reactor operation. The most important requirements of the divertor are to be resistant, at least up to a certain point, against radiation damage such as the evolution of transmutation elements and He/H production that leads to swelling and embrittlement of the materials. Furthermore, it is essential for the PFMs to maintain their integrity within the operation temperature window (OTW) of the reactor, which is currently estimated to be 1073–1473 K. Such an OTW is defined in all body-centred cubic (bcc) and most face-centred cubic metals by the DBTT at the lower limit and the recrystallisation temperature (RCT) at the upper limit [Norajitra et al., 2004; Bolt et al., 2004]. Since recrystallisation in structural materials should be avoided in any case, the DBTT is the main focus of the design. It is strongly dependent on the chemical composition and the microstructural state and hence the manufacturing history (particularly for refractory alloys). However, the determination of the DBTT strongly depends on the testing method [Nakamura et al., 1994; Rieth and Dafferner, 2005]. Additionally, PFCs conditions beyond ITER (e.g. DEMO) will be more stringent due to the increase in operational lifetime and also because the surface power to be removed from the plasma, the neutron fluencies and the operational temperature will all increase. Furthermore, PFCs should allow the operation under off-normal events such as plasma disruptions and high transient heat loads during strong edge localised modes of the plasma [Bolt et al., 2002]. All of these requirements therefore, demand the development of materials and alloys that broaden the range of applications and usefulness. During the 1990s, plasma performance was the most important aspect of fusion research, therefore the utilization of low-Z PFMs such as beryllium, graphite or boron was preferred [Noda et al., 1997]. The experience during plasma operation showed that the use of low-Z PFMs greatly reduced radiation losses from the plasma compared with medium and high-Z PFMs [Bolt et al., 2002]. Additionally, low-Z PFMs significantly improved the reactor performance by contributing to a steady increase in the fusion triple product of density, temperature and energy confinement [Philipps et al., 2000]. The use of beryllium is not suitable as a PFM because of its low power handling capacity and high physical sputtering yield at low particle energies [Neu, 2003]. Carbon-based materials meanwhile, have very advantageous thermo-mechanical 9.

(34) Chapter 1. Introduction. A. pl as ma. B. f us i onr eact i on. C. D. Fig. 1.6 Operation of ITER [New-Scientist, 2009].. 10.

(35) Teresa Palacios Garcı́a. properties, good availability, good machinability, low activation and low radiation losses from carbon at plasma temperatures [Garcı́a-Rosales, 1994]. A significant advantage of these carbon-based materials is that they do not melt under off-normal heat loads. However, the biggest concern is erosion under normal operating conditions, which most serious consequence would be the subsequent redeposition of carbon in remote locations. The solution to these problems is to develop an efficient cleaning method for layer removal or to omit carbon as a PFM [Neu, 2003]. Therefore, these carbon-based materials should be used in combination with special wall conditioning procedures such as boronisation, siliconisation, lithium injection or beryllium evaporation, which reduce oxygen impurities and improve density control [Philipps et al., 2000]. Originally, the ITER divertor baseline was slated to use carbon fibre-reinforced carbon (CFC) for the lower part of the IVTs and OVTs, which would be replaced with a full-tungsten divertor prior to the start of nuclear operation (hence 3–4 years of operation). However, due to the potential risk of rapid accumulation of tritium by codeposition with eroded CFCs or deep migration into the bulk-carbon, CFC were excluded from the present ITER licensing [Merola et al., 2014]. Consequently, following a strategy of cost reduction by the utilisation of a single divertor from the start of operation until the nuclear phase, in September 2011 the ITER Council proposed to investigate the possibility of beginning operation with a full-tungsten armoured divertor compatible with nuclear operations. This proposal was analysed and completed successfully and November 2013 it was decided that ITER would operate with a full-tungsten divertor [Carpentier-Chouchana et al., 2014]. Recently, some authors such as Brooks et al. (2015) have focused their attention on broadening the options for high-Z PFM beyond tungsten to avoid relying on one single material. Five potential alternative materials (zirconium, niobium, molybdenum, hafnium and tantalum) were examined as full-thickness structural materials or thinner coating materials. Even so, research is still focused on tungsten as a PFM for divertor applications.. 1.3.1. Tungsten materials. The use of tungsten, a bbc refractory metal, as a PFM is mainly motivated by its superior thermo-mechanical properties and low erosion under steady state operation conditions. These favourable properties include, among others, high melting point (3410 ◦ C), good thermal conductivity (155 W/mK at room temperature), low vapour pressure (1.3 10−7 Pa at its melting point), relatively low 11.

(36) Chapter 1. Introduction. thermal expansion (4.6 10−6 K−1 at room temperature; ASM Handbook Vol.02 (1991)), high creep resistance and high temperature strength, low sputtering yield, low tritium retention and low hydrogen solubility [Norajitra et al., 2004]. Nevertheless, many other properties that are relevant in a fusion reactor environment are required besides these specific advantages. Each property has to be addressed through specific experimental and theoretical approaches to specify its behaviour in an integrated approach. All of the properties noted above make tungsten a suitable PFM. However, its structural applications are still restricted by its inherent brittleness. The safety and efficiency of a fusion reactor will depend on its materials since their properties should be maintained inside the OTW. Therefore it is convenient to use materials with a DBTT as low as possible and a suitable toughness and ductility at low temperatures for preventing an accidental brittle fracture when the PFCs are below the lower bound of the OTW. Pure tungsten, as a material with bbc structure, has a DBTT that ranges 473–673 K depending on the characterisation method. Therefore, to enable its use as structural material, its DBTT needs to be decreased. Many efforts have been made to lower the DBTT of tungsten. The main strategies that have been employed thus far include conventional techniques such as alloying with other elements or strengthening with dispersed particles and thermo-mechanical treatments. Although in general no substantial improvements have been reported, it was found that oxide-dispersion-strengthened (ODS) or Al-K-Si doping enhances the mechanical strength and increases the RCT of tungsten but unfortunately without decreasing its DBTT [Rieth and Dafferner, 2005; Faleschini et al., 2007]. In the last few years, several other methods such as the sinterising of tungsten composites from tungsten or doped tungsten foils have been developed with promising improvements [Reiser et al., 2013a]. More recently, active extrinsic toughening mechanisms of energy dissipation that has been widely used in ceramic fibre-reinforced ceramics [Jones and Henager, 2005], has being implemented in tungsten fibre-reinforced tungsten composites with successful results [Riesch et al., 2014]. In this phD thesis, the main strategies to enhance fracture toughness have been alloying with elements such as titanium or vanadium and ODS particles for the case of bulk-tungsten materials (chapter 4) and annealing heat treatment for the tungsten foils (chapter 5). In the following sections a more specific description of some of the reported attempts to ductilize tungsten are provided.. 12.

(37) Teresa Palacios Garcı́a. 1.3.2. Tungsten-based alloys. The most common alloying elements used for alloying tungsten are shown in fig. 1.7. Some of these alloys have been studied because of their declared good properties (e.g. W-Ta). However, others such as W-Re that were rejected for economic or practical reasons are still under study because of the transmutation of tungsten during irradiation conditions that gives rise to W-Re or W-Os alloys. Sc Sc andi um. Y Yt t r i um. La Lant hanum. Ti Ti t ani um. Zr Zi r c oni um. Hf Haf ni um. V Vanadi um. Nb Ni obi um. Ta Tant al um. Cr. Mn. Chr omi um. Manganes e. Mo Mol ybdenum. W Tungs t en. Tc Tec hnet i um. Re Rheni um. Fe I r on. Ru Rut heni um. Os Os mi um. Fig. 1.7 Some elements used for alloying tungsten.. 1.3.2.1. Tungsten-based alloys with rhenium. The addition of rhenium enhances the ductility of tungsten by shifting its DBTT to lower values. An improvement in the room temperature fabricability together with an increased in the high temperature strength of the alloy was first reported by Geach and Hughes (1956). In addition, an increase in the RCT was reported by Mutoh et al. (1995). Such characteristics would made W-Re alloys very promising materials for fusion applications were if not for the fact that transmutation under irradiation conditions produce drastic embrittlement and hardening by precipitation and therefore compromises the structural integrity of the PFCs [Herschitz and Seidman, 1984]. Nonetheless, despite its high price of approximately $5000 (USD) per kilogram [MetalPrices, 2015], W-Re alloys have drawn significant attention over the years. Rhenium, with a hexagonal-close-packed (hcp) crystal structure, has the second highest melting point after tungsten (3180 ◦ C; ASM Handbook Vol.02 (1991)). Its complex binary diagram with tungsten (fig. 1.8) exhibits σ and χ phases that when precipitated are responsible for the embrittlement under neutron irradiation. The highest solubility of Re in tungsten is pproximately 37 at.% although the highest W-Re alloy reported is W-26% Re [Nemoto et al., 2000; Wurster et al., 2010]. The hardening effects of neutron damage during operation have been reported by several authors (e.g. Armstrong et al. (2013) and Xu et al. (2015) in the case of W-5 wt.%). 13.

(38) Chapter 1. Introduction. Fig. 1.8 Binary phase diagram of W-Re [Liu and Chang, 2000].. 1.3.2.2. Tungsten-based alloys with tantalum. Tantalum, with a bcc crystal structure, features a combination of properties that are rarely found in many refractory metals: excellent machinability, a low DBTT and a high melting point (3020 ◦ C) [ASM Handbook Vol.02, 1991]. In combination with tungsten, W-Ta alloys present complete solubility in solid and liquid phases (fig. 1.9). This situation, together with the fact that tantalum does not produce new phases after transmutation, makes it a widely used alloying element to reduce the DBTT in low activation ferritic-martensitic steels [Gilbert and Sublet, 2011]. Early studies by Stephens (1970) showed an effective decrease in DBTT for low alloyed alloys. This decreased in DBTT was very pronounced for W-1 at.% Ta but was already lost for W-2 at.% Ta. These findings indicate an effective solid solution softening at temperatures lower than 10% the melting temperature. Gourdin et al. (1994) meanwhile, have reported a good combination of mechanical properties such as elasticity and corrosion resistance (from the tantalum) and strength (from the tungsten) for tantalum-based alloys with 2.5, 5 and 10% tungsten content. In accordance with these results, Wurster et al. (2011) recently reported a rather good performance of the industrial alloy W-1% Ta with a high decrease in the workability and fracture toughness with the addition of 5 and 10 wt.% Ta. These. 14.

(39) Teresa Palacios Garcı́a. results are also in agreement with data reported by Tejado et al. (2015) from laboratory scale W-5% Ta and W-15% Ta processed using MA and HIP.. Fig. 1.9 Binary phase diagram of W-Ta [ASM Handbook Vol.03, 1992].. All these results together with the data of Rieth et al. (2010), suggest the optimisation of the chemical composition of W-Ta since the alloys that have been already studied did not exhibit improved DBTT values compared with pure tungsten.. 1.3.2.3. Tungsten-based alloys with vanadium. W-V alloys are characterised by an isomorphous phase diagram (fig. 1.10) with a continuous range of solid solution. However, the formation of intermetallic phases even at high temperatures was predicted via ab initio calculations [Muzyk et al., 2013]. Such compounds would involve significant changes in their elastic properties compared with the isotropic elastic properties of pure tungsten. Vanadium-based alloys have been typically used in the aerospace industry, because of their low density, high strength and resistance to high operating temperatures and stress [Shyrokov et al., 2009]; however, little research have been conducted on W-V alloys. In recent years, W-V composites have been investigated using numerical simulation that suggested an enhancement of the 15.

(40) Chapter 1. Introduction. OTW: an improved creep resistance at high temperatures and a reasonable fracture toughness at low temperatures [Hohe and Gumbsch, 2010]. Nonetheless, one of the main problems with vanadium is its low erosion resistance.. Fig. 1.10 Binary phase diagram of W-V [Okamoto, 2010].. 1.3.2.4. Tungsten-based alloys with titanium. Titanium has been used as a sintering activator because of its extreme corrosion resistance and high strength. The W-Ti binary phase diagram (fig. 1.11) shows limited solid solution above 740 ◦ C and complete solid solution at 1255 ◦ C. Although the prerequisite for the formation of a complete solid solution is that the elements possess the same crystal structure and titanium is hcp. However, the formation of a complete solid solution is also affected by other factors, such as the difference in atom size between the alloying elements [Wang et al., 2009]. The mechanical properties of W-Ti alloys have been scarcely investigated. Savoini et al. (2013) reported the formation of some metastable titanium phase such as α0 , α00 or ω that could influence the mechanical behaviour of W-Ti alloys. Meanwhile, Garcı́a-Rosales et al. (2014) studied self-passivating tungsten-based alloys (WCr12Ti2.5) that minimise the formation of volatile, radioactive oxides and the creation of protective barriers in case of a loss of coolant. 16.

(41) Teresa Palacios Garcı́a. Fig. 1.11 Binary phase diagram of W-Ti [ASM Handbook Vol.03, 1992].. 1.3.3. Dispersion-strengthened tungsten alloys. Another method to increase the high temperature performance of tungsten by raising the RCT significantly is doping or the addition of dispersion-strengthened particles. Such an improvement is almost independent of the type of dispersoid [Rieth and Dafferner, 2005]. However, ODS particles inhibit grain growth either during the recrystallisation process or during sintering and contribute to an increase in RCT without decrease in DBTT [Ryu and Hong, 2003]. The dispersion of strengthening TiC particles is the only one that decreases the DBTT [Kitsunai et al., 1999]. tungsten-TiC alloys exhibit superplasticity at high temperatures based on grain boundary sliding above 1673 K, which is strongly dependent on the TiC content as well as the mechanical alloying atmosphere [Veleva, 2011]. The dispersion of thorium dioxide (ThO2 ) particles has been studied from the beginning because these particles possess the highest melting temperature (3473 K) among all metallic oxides [Luo et al., 1991]. Unfortunately, their use has been dismissed due to their radioactive potential, which is an obvious obstacle to nuclear applications [King and Sell, 1965]. Nowadays, the addition of yttria (Y2 O3 ) or lanthana (La2 O3 ) as a second-phase particle is a promising attempt 17.

(42) Chapter 1. Introduction. under study. Although lanthana has been only rarely investigated, it improves the processability, suppresses recrystallisation and exhibits a slight strengthening in creep [Rieth and Dafferner, 2005]. However, the inherently brittle behaviour of pure tungsten increases [Rieth et al., 2010].. 1.3.3.1. ODS tungsten with yttria. The addition of yttria particles provides excellent chemical and thermal stability [Swamy et al., 1998] and creep resistance at high temperatures [Kim et al., 2006]. Additionally, since yttrium possess a high chemical affinity to oxygen, W-Y2 O3 alloys have good corrosion resistance [Itoh and Ishiwata, 1996; Veleva et al., 2009]. The content of the second-phase should be carefully optimised since the addition of yttria to a tungsten matrix generates some contradictory effects in terms of alloy behaviour. Although increased quantities of yttria contribute to increasing the corrosion resistance, yttria also lowers the strength and ductility of the resulting alloy [Itoh and Ishiwata, 1996]. In addition, higher contents of yttria compromise the use of the alloy for structural applications at high temperatures because the melting temperature of yttria (2711 K; Swamy et al. (1998)) is much lower that of tungsten (figs. 1.12 and 1.13).. Fig. 1.12 Binary phase diagram of Y-O [Okamoto, 2011].. 18.

(43) Teresa Palacios Garcı́a. Fig. 1.13 Binary phase diagram of W-Y [Okamoto, 2000].. These alloys are therefore expected to combine a high strength at high temperatures of the ceramic particles with a high ductility as compared to ceramics due to the metallic matrix.. 19.

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(45) CHAPTER. Experimental Methods. Microstructural, mechanical and fractographical characterisation were performed in the tungsten-based materials and alloys using experimental procedures on the nano, micro and macro scales. In this chapter, the experimental techniques employed in both the bulk and the foils together with the procedures used for the sample preparation, are exposed in detail. Nevertheless, particular specifications of each type of material will be clarified in the corresponding chapter (see chapters 4 and 5 for the bulk and foil tungsten materials, respectively).. 2.1. Microstructural characterisation. Microstructural and compositional characterisation of the specimens was performed by analysing transversal sections. Although the standard preparation involves embedding the samples in resin, this method was discarded for the bulk specimens since they were simply placed on an aluminium holder. This preparation allowed a higher conductivity of the samples, which resulted in better quality imagining with the field emission scanning electron microscope (FE-SEM). On the other hand, the transversal sections of foils were embedded in hard resin to produce a tough and stiff compound that prevented the cross-sections from delaminating during the grinding and mechanical polishing [Reiser et al., 2012].. 21. 2.

(46) Chapter 2. Experimental Methods. Afterwards, the sample surfaces were prepared for observation using a Struers Abramin (Germany) universal grinding and polishing machine. They were progressively more finely ground using 400, 600 and 1000 grit SiC paper and were then polished using a diamond suspension of 3 and 1 µm. Finally, the samples were etched using Murakami reagent (10 g KOH, 10 g K3 Fe(CN)6 in 100 ml distilled water) for 1 min to reveal the microstructure of the materials more clearly, as necessary. High resolution imagining analysis of the microstructure was performed using a FE-SEM from Zeiss (AURIGA with GEMINI electron beam column, Germany) equipped with secondary electrons (SE2, InLens), backscattered electrons (BSE, EBSD) and X-ray (EDX) detectors. For observations of the foils, optical microscopy at very low magnification (up to 200 ) was used due to the large microstructure observed on the samples after the annealing treatment. FE-SEM utilises a very strong magnetic field to draw electrons from a very fine metallic tip to the sample. When the electron beam strikes the specimen, it interacts with some of the material (the interaction volume; fig. 2.1) to produce different signals. The penetration length of the electron beam into the sample is affected by many factors; the most significant are the accelerating voltage (kV) that is applied and the atomic number (Z) of the material. The higher the kV, the greater the penetration; whereas the higher the Z, the lower the penetration. In general, this electron penetration ranged from 1 to 5 µm with the incident beam perpendicular to the sample. e l e c t r on be a m S E 2 S E 1 BS E S E. Xr a y. BS E. c ha r a c t e r i sc X r a y s. s ampl e. Fig. 2.1 Interaction volume of high energy electrons with solid materials [AMMRF, 2014].. 22.

(47) Teresa Palacios Garcı́a. The majority of the incident electrons interact with the specimen atoms and are scattered instead of penetrating the sample. These scattering events can be elastic or inelastic. In elastic scattering, the electron trajectory changes but its kinetic energy and velocity remain essentially constant because of the large difference in the mass of the electron and the masses of the atomic nuclei. Alternatively, in inelastic scattering, the trajectory of the incident electron may be only slightly perturbed but energy is lost via interactions with the orbital electrons of the atoms in the specimen. The most common interaction products used for generating images in SEM are SE1, SE2, BSE and X-rays. They are collected by different detectors to obtain a wide range of information about the materials being studied. The SE2 detector collects low-energy secondary electrons (SE2) ejected from a thin layer near the surface of the sample by the BSEs (fig. 2.1) that have returned to the surface after several inelastic scattering events. Since the electrons come from a surface area that is larger than the spot associated with the incoming electrons, this is a low-resolution signal. The detector is used to show the morphology and surface topography of the sample. The contrast of the image depends on the number of SE2 that reach the detector from each spot of the sample. Such contrast is dominated by the so-called edge effect (fig. 2.2); a higher brightness is produced at the edges because more SE2 are able to leave the sample. In the same way, faces oriented towards the detector will be brighter than those on the opposite side.. Fig. 2.2 Effect of topography on SE2 detector signal [ETH, 2014].. 23.

(48) Chapter 2. Experimental Methods. The BSE detector collects high-energy backscattered electrons (fig. 2.1) coming out of the material via elastic scattering interactions with the specimen’s atoms. This detector is used to detect contrast between areas with different chemical compositions based on the fact that elements with high Z backscatter electrons more strongly (and appear brighter in the image) than low Z elements. However, BSEs are also used to determine the crystallographic orientation of grains using the EBSD detector, as will be explained in section 2.1.2. The InLens detector is an efficiency tool for high-resolution secondary electrons (SE1) imagining. It is located above the objective lens (fig. 2.3) and detects directly in the beam path. SE1 are generated by the incoming electron beam as they enter the surface, and the resolution signal of the InLens detector is only limited by the electron beam diameter. Its efficiency is mainly determined by the electric field of the electrostatic lens, which decreases exponentially with distance. Particularly at low kV and small working distances, it is possible to obtain images with high contrast besides morphology and surface topography information. This capability is possible because the InLens detector features a high lateral resolution unlike images recorded by a conventional SE2 detector in which topographic information is dominant.. E l e c t r oma g ni c a pe r t ur ec ha r g e r F i e l dl e ns. I nL e nsS Ede t e c t or. Ma g ne cl e ns. E l e c t r os t acl e ns. S pe c i me n. Fig. 2.3 Schematic of the GEMINI lens design from Zeiss with the InLens detector [Zeiss, 2012].. 24.

(49) Teresa Palacios Garcı́a. 2.1.1. Energy dispersive X-ray spectroscopy. Energy dispersive X-ray spectroscopy (EDX) is a non-destructive microanalytical technique that provides qualitative and quantitative X-ray analysis information about chemical composition of a material. Electrons from the beam penetrate the sample and interact with the atoms. In this work, the diffracted X-rays (background and characteristic) were captured by the detector (QUANTAX EDX from BRUKER), and the received signal was displayed using the SPRIT software as a spectrum of intensity. The energies of the characteristic X-rays allowed to identify the elements in the sample, while the intensities of the characteristic X-ray peaks provided their quantification. Special sample preparation, other than that required to image inside the FE-SEM, was not required. The different outputs that can be obtained from the EDX analysis (spectrum, mapping, quantitative analysis) were used for microstructural characterisation of the materials.. 2.1.2. Electron backscatter diffraction. Electron backscatter diffraction (EBSD) is a powerful crystallographic SEM tool for microstructural characterisation in which a crystalline sample tilted 70o is struck by an incident electron beam (fig. 2.4). The beam interacts with the crystal lattice within a small volume and coherently diffract BSEs out of the sample (fig. 2.1). The diffracted electrons generate a Kikuchi pattern (fig. 2.5) on the phosphor screen that is characteristic of each crystal’s structure and orientation. This pattern yields information about the absolute crystal orientation with sub-micron resolution [Wilkinson and Britton, 2012]. The Kikuchi patterns were processed and analysed using the ESPRIT software of the QUANTAX CrystAlign EBSD analysis system from BRUKER. These patterns can be used to determine grain size, misorientation, texture, grain boundaries, deformed grains or phases. The detector is equipped with forescattered electron (FSE) detectors and BSE detectors (fig. 2.4) that provide valuable additional information that can be used for efficient microstructure characterisation. The BSE detectors are located to improve the image quality, whereas the FSE make visible the slightest signal variation due to orientation change when a polycrystalline sample is scanned [Bruker, 2014].. 25.

(50) Chapter 2. Experimental Methods. BS Edet ec t or s. e l e c t r on be a m. 7 0 ºl t e d s pe c i me n. F S Edet ec t or s. Fig. 2.4 Schematic set-up of the EBSD the detector used inside the FE-SEM chamber [Bruker, 2014].. Fig. 2.5 High-resolution Kikuchi pattern of a single crystal.. 26.

(51) Teresa Palacios Garcı́a. The preparation of samples for EBSD analysis include an additional final polishing since this technique requires free of relief surfaces to obtain good quality and reliable results. Although this step can be achieved via electropolishing, ion milling or colloidal silica, this last method was preferred because it is the most suitable for high Z materials [Chen et al., 2012]. The step was performed with a VibroMetTM 2 Vibratory Polisher from Buehler (United States) designed to prepare highly reflective polished surfaces with exceptional flatnesses. Its low amplitude vibratory action produces less deformation, flatter surfaces and reduced edge rounding. It also yields a stress-free surface without the use of dangerous electrolytes (often a mixture of acids) associated with electro-polishing. Special care should be taken with the polishing time; it should not be prolonged but rather just sufficient to achieve the desired surface finish without causing excessive relief [Oxford-Instruments, 2014; Buehler, 2014].. 2.2 2.2.1. Nanomechanical characterisation Nanoindentation tests. Nanoindentation tests were performed at room temperature using a NanoIndenter XP from former MTS Systems Corporation (United States) equipped with a continuous stiffness measurement (CSM) module. This module makes it possible to measure the contact stiffness at any point along the loading curve and not just at the point of unloading, which is what is possible using conventional measurements according to Li and Bhushan (2002). The CSM module therefore results in a significant improvement in terms of the information that can be obtained. The nanoindentations were performed using a standard Berkovich tip calibrated with fused silica. Although several indenter geometries can be used (cube corner, spherical, conical, etc), the selected Berkovich tip (fig. 2.6) is the most frequently used for instrumented indentation testing. This three-sided pyramid indenter is preferred over the four-sided Vickers indenter because of its smaller angle between faces that minimises the influence of friction and makes it easier to ground to a sharp point [Hay and Pharr, 2000]. With this equipment, instrumented indentation tests can be performed under load or displacement control. The displacement control tests were conducted at the beginning to determine the optimum test load to avoid the indentation size effect that would affect the mechanical properties results. The force, determined 27.

(52) Chapter 2. Experimental Methods. to be 0.6 N, was applied to the indenter through a calibrated electromagnetic coil with a resolution of 50 nN. Meanwhile, the displacement was measured using an electrostatic transducer with a resolution of 0.01 nm. During the tests, the control unit continuously registered the force and displacement experimented by the indenter when the surface of the material was penetrated.. Fig. 2.6 Geometry of the Berkovich indenter showing its 65.3◦ angle between the centre line and each face (left) and the indentation produced in the material (right).. Based on the load-displacement data obtained (curves as in fig. 2.7), the average values of nanohardness and elastic modulus were calculated according to the method of Oliver and Pharr (1992). Nanoindentation hardness (Hn ) of the materials was defined as the ratio between the peak of applied load (Pmax ) and the projected contact area of the indentation (A) as follows:. Hn =. Pmax A. (2.1). The elastic modulus (En ) was inferred based on the initial unloading contact stiffness (S, fig. 2.7) and the following the formula: p 1 π S p Er = β 2 A. 28. (2.2).