Publication: Producción, procesado y caracterización de aleaciones de W reforzadas con dispersión de óxidos para reactores de fusión
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2013
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2013-07-12
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Abstract
El objetivo de esta tesis ha sido investigar la producción y caracterización de materiales de base de wolframio. Las aleaciones de wolframio son los materiales candidatos para formar parte de los componentes de la primera pared en los futuros reactores de fusión. En particular, estas aleaciones se consideran los materiales más prometedores en la construcción del divertor enfriado por He para el futuro reactor de fusión de demostración (DEMO) y para los elementos de blindaje de la primera pared de la vasija del reactor directamente expuestos a las mayores cargas térmicas y erosión por partículas energéticas. Las propiedades que se requirieren y hacen a estos materiales adecuados para su uso como material de la primera pared son: una temperatura de fusión elevada, resistencia al choque térmico, buena conductividad térmica, resistencia a fluencia, mínima retención de tritio y resistencia a alta temperatura, junto con una buena resistencia al daño por irradiación y alta resistencia a la erosión por partículas energéticas. El wolframio cumple satisfactoriamente estos requisitos. Sin embargo, la fabricación de componentes de wolframio es difícil, porque el wolframio policristalino es frágil a temperatura ambiente. La temperatura de transición dúctil-frágil (DBTT) y la temperatura de recristalización (RCT) de las aleaciones de tungsteno deben mejorarse, a fin de aumentar su tenacidad a la fractura a bajas temperaturas y de ampliar su rango de temperatura de trabajo (OTW), respectivamente. Para los componentes de base wolframio, la OTW parece estar actualmente establecida entre 800 y 1200 °C. Por lo que un requisito para el diseño del divertor es el desarrollo de aleaciones de tungsteno que tengan una DBTT en este intervalo de temperatura y una RCT por encima de 1300 °C. Además, la DBTT para el wolframio puro ha sido determinada por medio de ensayos Charpy estándar en el intervalo de 300 a 400 °C, haciendo que su comportamiento sea frágil a temperatura ambiente. La DBTT y la RCT así como, la ductilidad del wolframio dependen de su microestructura, los elementos aleantes y del procesado. El refuerzo mediante partículas duras (ODS) puede mejorar las propiedades mecánicas, la RCT y la capacidad para poder procesar las aleaciones de wolframio. La mejora de las propiedades mecánicas a alta temperatura de las aleaciones de wolframio reforzados con ODS se debe a la dispersión de nano-partículas, pues éstas pueden inhibir el movimiento de las dislocaciones y el crecimiento del grano. Otro gran interés del desarrollo de materiales ODS nanoestructurados para fusión es su mayor resistencia a la irradiación, al actuar la gran densidad de partículas dispersas como trampas de los defectos inducidos durante la irradiación. Sin embargo, aún no se ha realizado una caracterización exhaustiva de esta dispersión en las aleaciones de wolframio. Por otra parte, las aleaciones de W-Ti o W-V podrían presentar características más adecuadas para su uso en el divertor, porque tendrían menos activación inducida, facilitarían la soldadura entre las losas de blindaje basadas en W y los elementos estructurales de soporte y podrían presentar mejores propiedades mecánicas y una DBTT más baja. La adición de Ti o V favorece la densificación del wolframio y las aleaciones de wolframio procesadas mediante HIP, y refina el tamaño de grano de la microestructura. Además, una dispersión de ODS en las aleaciones W-Ti o W-V podría reforzar el material sin reducir su ductilidad e inhibir el crecimiento de grano. Es de esperar, por tanto, que ambos efectos contribuyan a mejorar el comportamiento mecánico de las aleaciones de W-Ti o W-V. Dentro del ámbito de este trabajo, una variedad de composiciones de materiales base wolframio (W, W-La₂O₃, W-Y₂O₃, W-Ti, W-Ti-La₂O₃, W-V, W-V-La₂O₃ y W-V-Y₂O₃) fueron producidos mediante técnicas pulvimetalurgicas (P/M) que consisten en una fase de mezcla, un aleado mecánico, seguido de un proceso de desgasificación y prensado isostático en caliente (HIP). Los métodos de preparación para estas aleaciones se optimizaron estudiando el efecto del proceso de la molienda y del sinterizado por HIP, en la composición de la microestructura obtenida y en las propiedades del material resultante. Además, las aleaciones W-V y W-V-Y₂O₃ fueron sometidas a tratamientos térmicos a distintas temperaturas, seguidos de un temple para estudiar la estabilidad de la microestructura. La microestructura de las aleaciones a lo largo de las distintas etapas de procesado se analizó utilizando las técnicas de difracción de rayos X, microscopía electrónica de barrido (SEM) y microscopía electrónica de transmisión (TEM). La distribución del tamaño de partícula de los polvos se midió por dispersión de luz láser. Los contenidos de C y O fueron determinados mediante LECO TC-500 y CS-200. La estabilidad de la microestructura tras los tratamientos térmicos se investigó por medio de EBSD (difracción de electrones por retrodispersión) y SEM. La dispersión de óxidos en la matriz de W se investigó mediante dispersión de neutrones de pequeño ángulo (SANS) y TEM. Las propiedades mecánicas se determinaron mediante medidas de microdureza y ensayos a flexión en cuatro puntos a diferentes temperaturas. La resistencia al desgaste y coeficiente de fricción de las aleaciones también fueron investigados mediante ensayos Pin-on-disk. Este último estudio tribológico se realizó para explorar la posible aplicación de estos materiales de base W en otras áreas tecnológicas en las que la superficie del material se encuentra sometida a fricción, como rodamientos, componentes mecánicos o herramientas de corte. La tesis doctoral se divide en seis capítulos. - El capítulo 1 incluye una introducción sobre las características fundamentales que han de cumplir los materiales para su aplicación en los reactores de fusión según las actuales directivas del proyecto internacional ITER (International Thermonuclear Experimental Reactor). En él se describen las ventajas y desventajas de los materiales base wolframio. - El capítulo 2 presenta las composiciones del wolframio y las aleaciones de wolframio investigadas, así como, las diferentes técnicas pulvimetalurgicas usadas para producir las aleaciones y las técnicas de caracterización utilizadas junto con las condiciones en que se realizaron. - El Capítulo 3 contiene la caracterización de los diferentes materiales en términos de microestructura, distribución del tamaño de partícula de los polvos, contenidos de C y O, densidad y microdureza, durante el proceso de fabricación y tras la consolidación por HIP. Se analizan los resultados con el fin de encontrar las condiciones óptimas de fabricación del wolframio y que dan lugar a una mejora de la microestructura y propiedades mecánicas. Estos resultados se compararon con los datos encontrados en la lliteratura. - El capítulo 4 incluye, primero, los resultados obtenidos por SANS de las aleaciones W-La₂O₃ y W-Y₂O₃. Posteriormente se presentan los resultados relativos a la estabilidad de la microestructura tras los tratamientos térmicos realizados en las aleaciones W-V y W-V-Y₂O₃ y que fueron investigados mediante EBSD. Finalmente, se presenta el análisis de los ensayos a flexión en cuatro puntos a diferentes temperaturas y velocidades de ensayo, también realizado en las aleaciones W-V y W-V-Y₂O₃. - El capítulo 5 presenta los resultados de los ensayos Pin-on-disc. En ellos se estudia el coeficiente de desgaste y el coeficiente de fricción a distintas cargas de ensayo. - El capítulo 6 presenta las principales conclusiones de este trabajo y las perspectivas para futuras actividades. ---------------------------------------------------------------------------------------------------------------------------------------------------------------
The purpose of this PhD thesis has been to investigate the production and the characterization of tungsten-base materials. Tungsten alloys are candidate materials for the production of plasma facing components (PFMs, Plasma Facing Materials) in the future fusion reactors. In particular, these materials are considered the most promising materials in the construction of the He-cooled divertor of the future demonstration fusion reactor (DEMO) and as armor material for the first wall of the reactor’s vessel. The properties required to the PFMs are high melting temperature, thermal shock resistance, good thermal conductivity, creep strength, minimal tritium retention, high temperature strength along with low sputtering and erosion rates. Furthermore, a suitable PFM should contain elements of reduced activation. These considerations reduce the availability of material candidates to that base on C, V, Ti, W and Be. Pure tungsten fulfills most of the previous requirements. However, the fabrication of tungsten components is difficult because polycrystalline tungsten is brittle at room temperature. The ductile–brittle transition temperature (DBTT) and recrystallization temperature (RCT) of the tungsten alloys have to be enhanced in order to widen the operating temperature window (OTW) of the tungsten base components, which appears to be presently established between 800 and 1200 °C. The DBTT of pure tungsten has been determined by standard Charpy tests in the range of 300 – 400 °C, and is very sensitive to the impurity content and to the route used to produce the material. An objective of the present EFDA programme or any programme directed to achieve fusion power reactors, is the development of novel tungsten alloys having a lower DBTT and a RCT above 1300 °C. The DBTT and RCT as well as the ductility of tungsten depend on the microstructure, alloying elements and production history. Reinforcement by fine dispersion of oxide particles (ODS) can enhance the mechanical properties, RCT and workability of the tungsten alloys. These improvements in the performance of W base materials might be enhanced if the reinforced particles have a nanometric size. A material with a fine dispersion of nanoparticles would exhibit a higher resistance to the irradiation due to the high density of traps for the defects induced by irradiation. However, a comprehensive characterization of this dispersion in the tungsten alloys has not been accomplished so far. W-Ti or W-V alloys might exhibit more suitable characteristics for divertor applications because they would have less induced activation, an enhanced resistance to irradiation and open the possibility of joining by brazing the W base materials to the structural components of the reactor (this is a hot issue for the design of the plasma confinement vessel). It is found that the addition of Ti or V favors the densification of the HIP-processed tungsten-base materials and refines the grain structure. Also, a dispersion of ODS particles in W-Ti or W-V alloys strengthens the material without lowering its ductility and inhibits the grain growing. It should be expected that both effects contribute to improve the mechanical behaviour of W-Ti or W-V heavy alloys. Within the scope of this work, a wide variety of compositions of tungsten-base materials (W, W-La₂O₃, W-Y₂O₃, W-Ti, W-Ti-La₂O₃, W-V, W-V-La₂O₃ y W-V-Y₂O₃)were produced by powder metallurgy (P/M) techniques consisting of blending, mechanical alloying, followed by a degassing process and hot isostatic pressing (HIP) treatment. The preparation methods for the alloys were optimized by investigating the effect of the milling and HIP treatment conditions on the microstructure, composition and properties of the materials produced. In addition, W-V and W-V-Y₂O₃ alloys were subjected to annealing and quenching thermal treatments at different temperatures, to study the stability of their microstructure and mechanical properties. The microstructure of the alloys in the course of the successive processing stages was analyzed using the techniques of X-ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The particle size distribution of the powders was measured by laser light scattering, and the O and C contents in the alloys were determined in LECO TC-500 and CS- 200 elemental analyzers respectively. The stability of the microstructure after thermal treatment was investigated by means of electron backscatter diffraction (EBSD) and SEM. The dispersion of the ODS in the W matrix was investigated by small angle neutron scattering (SANS) and TEM. The mechanical properties were determined by microhardness measurements and four points bending tests at different temperatures. The wear resistance and friction coefficient of the alloys were also investigated by Pin-on-disk tests. The wear resistance of the W based material has been performed to explored their possible interest to another industrial applications like ball bearings, wear resistant components, as well as drilling and cutting tools. The manuscript is divided into six chapters. - Chapter 1 includes an introduction to the key issues of materials for fusion power reactors and describes the advantages and drawbacks of tungsten-base materials. - Chapter 2 presents the compositions of the tungsten materials investigated as well as, the various powder metallurgy techniques used to produce the alloys and the various characterization techniques and conditions that were used. - Chapter 3 contains the results obtained for the produced materials in terms of microstructure, the particle size distribution of the powders, the C and O contents in the alloys, density and microhardness, in order to study the materials in the manufacturing process and consolidation process. We have discussed the main results to establish the optimal manufacturing conditions of tungsten-base materials, i.e. the microstructure and mechanical properties of the produced materials, and we have compared these with the original expectations and literature data. - Chapter 4 includes the results obtained by SANS for W-La₂O₃ y W-Y₂O₃and W-Y₂O₃alloys. The stability of the microstructure was investigated by EBSD on W-V and W-V-Y₂O₃ alloys. Finally four point bending tests were performed on W-V and W-V-Y₂O₃ alloys. - Chapter 5 presents results of the Pin-on-disk tests. The wear coefficient and friction coefficient at different normal load has been studied. - Chapter 6 contains the main conclusions of this work and a number of perspectives for future research activities.
The purpose of this PhD thesis has been to investigate the production and the characterization of tungsten-base materials. Tungsten alloys are candidate materials for the production of plasma facing components (PFMs, Plasma Facing Materials) in the future fusion reactors. In particular, these materials are considered the most promising materials in the construction of the He-cooled divertor of the future demonstration fusion reactor (DEMO) and as armor material for the first wall of the reactor’s vessel. The properties required to the PFMs are high melting temperature, thermal shock resistance, good thermal conductivity, creep strength, minimal tritium retention, high temperature strength along with low sputtering and erosion rates. Furthermore, a suitable PFM should contain elements of reduced activation. These considerations reduce the availability of material candidates to that base on C, V, Ti, W and Be. Pure tungsten fulfills most of the previous requirements. However, the fabrication of tungsten components is difficult because polycrystalline tungsten is brittle at room temperature. The ductile–brittle transition temperature (DBTT) and recrystallization temperature (RCT) of the tungsten alloys have to be enhanced in order to widen the operating temperature window (OTW) of the tungsten base components, which appears to be presently established between 800 and 1200 °C. The DBTT of pure tungsten has been determined by standard Charpy tests in the range of 300 – 400 °C, and is very sensitive to the impurity content and to the route used to produce the material. An objective of the present EFDA programme or any programme directed to achieve fusion power reactors, is the development of novel tungsten alloys having a lower DBTT and a RCT above 1300 °C. The DBTT and RCT as well as the ductility of tungsten depend on the microstructure, alloying elements and production history. Reinforcement by fine dispersion of oxide particles (ODS) can enhance the mechanical properties, RCT and workability of the tungsten alloys. These improvements in the performance of W base materials might be enhanced if the reinforced particles have a nanometric size. A material with a fine dispersion of nanoparticles would exhibit a higher resistance to the irradiation due to the high density of traps for the defects induced by irradiation. However, a comprehensive characterization of this dispersion in the tungsten alloys has not been accomplished so far. W-Ti or W-V alloys might exhibit more suitable characteristics for divertor applications because they would have less induced activation, an enhanced resistance to irradiation and open the possibility of joining by brazing the W base materials to the structural components of the reactor (this is a hot issue for the design of the plasma confinement vessel). It is found that the addition of Ti or V favors the densification of the HIP-processed tungsten-base materials and refines the grain structure. Also, a dispersion of ODS particles in W-Ti or W-V alloys strengthens the material without lowering its ductility and inhibits the grain growing. It should be expected that both effects contribute to improve the mechanical behaviour of W-Ti or W-V heavy alloys. Within the scope of this work, a wide variety of compositions of tungsten-base materials (W, W-La₂O₃, W-Y₂O₃, W-Ti, W-Ti-La₂O₃, W-V, W-V-La₂O₃ y W-V-Y₂O₃)were produced by powder metallurgy (P/M) techniques consisting of blending, mechanical alloying, followed by a degassing process and hot isostatic pressing (HIP) treatment. The preparation methods for the alloys were optimized by investigating the effect of the milling and HIP treatment conditions on the microstructure, composition and properties of the materials produced. In addition, W-V and W-V-Y₂O₃ alloys were subjected to annealing and quenching thermal treatments at different temperatures, to study the stability of their microstructure and mechanical properties. The microstructure of the alloys in the course of the successive processing stages was analyzed using the techniques of X-ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The particle size distribution of the powders was measured by laser light scattering, and the O and C contents in the alloys were determined in LECO TC-500 and CS- 200 elemental analyzers respectively. The stability of the microstructure after thermal treatment was investigated by means of electron backscatter diffraction (EBSD) and SEM. The dispersion of the ODS in the W matrix was investigated by small angle neutron scattering (SANS) and TEM. The mechanical properties were determined by microhardness measurements and four points bending tests at different temperatures. The wear resistance and friction coefficient of the alloys were also investigated by Pin-on-disk tests. The wear resistance of the W based material has been performed to explored their possible interest to another industrial applications like ball bearings, wear resistant components, as well as drilling and cutting tools. The manuscript is divided into six chapters. - Chapter 1 includes an introduction to the key issues of materials for fusion power reactors and describes the advantages and drawbacks of tungsten-base materials. - Chapter 2 presents the compositions of the tungsten materials investigated as well as, the various powder metallurgy techniques used to produce the alloys and the various characterization techniques and conditions that were used. - Chapter 3 contains the results obtained for the produced materials in terms of microstructure, the particle size distribution of the powders, the C and O contents in the alloys, density and microhardness, in order to study the materials in the manufacturing process and consolidation process. We have discussed the main results to establish the optimal manufacturing conditions of tungsten-base materials, i.e. the microstructure and mechanical properties of the produced materials, and we have compared these with the original expectations and literature data. - Chapter 4 includes the results obtained by SANS for W-La₂O₃ y W-Y₂O₃and W-Y₂O₃alloys. The stability of the microstructure was investigated by EBSD on W-V and W-V-Y₂O₃ alloys. Finally four point bending tests were performed on W-V and W-V-Y₂O₃ alloys. - Chapter 5 presents results of the Pin-on-disk tests. The wear coefficient and friction coefficient at different normal load has been studied. - Chapter 6 contains the main conclusions of this work and a number of perspectives for future research activities.
Description
Keywords
Reactores de fusión, Aleaciones de wolframio