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Descripción
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| Tungsten and tungsten (W)-based alloys are considered to be the best candidate materials for the fabrication of the divertor for next-generation nuclear fusion reactors. During the operation of a reactor, this component will experience the highest thermal loads since it directly faces the plasma. In recent years, after a thorough analysis that follows a strategy of cost reduction, ITER Organization decided to built a full-W divertor before the fi?rst nuclear campaigns. Therefore, tungsten will be used not only as a plasma-facing material (PFM) but also in structural applications. Due to the very stringent conditions that the divertor will have to withstand, materials with specifi?c properties are required. Because of the excellent thermo-physical properties of tungsten, it ful?fils the requirements of a PFM, however, its use in structural applications is compromised due to its inherent brittleness. One of the objectives of this work is therefore, to ?find a material with improved brittleness behaviour. In this PhD thesis, the microstructural and mechanical characterisation of di fferent W-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 speci?cally developed and implemented during this work such as the measurement of the real fracture toughness of bulk-W alloys or the in situ fracture toughness measurements of very thin tungsten foils. Bulk-W materials with different compositions (W-1% Y2O3, W-2% V-0.5% Y2O3, W-4% V-0.5% Y2O3, W-2% Ti-1% La2O3 and W-4% Ti-1% La2O3) were studied and compared with a pure tungsten material, 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). For that reason, tungsten foils were also studied. A preliminary study of pure W and potassium (K)-doped tungsten foils industrially produced by sintering and hot and cold rolling was performed. The materials were annealed from 1073 to 2673 K to analyse the evolution of microstructural and mechanical properties with increasing annealing temperature. The results reveal the stabilisation of the tungsten grains with increasing annealing temperature in the 0.005 wt.% K-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. | |
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Internacional
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Si |
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ISBN
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Tipo de Tesis
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Doctoral |
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Calificación
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Sobresaliente cum laude |
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Fecha
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09/07/2015 |