W-foil: DuctilityIdentification of the mechanism of plastic deformation

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Tungsten (W) is he metal with the highest melting point of all metals ((TS = 3422°C) and is therefore often considered as primary candidate for high temperature applications in energy conversion systems. However W has two major drawbacks: (i) a catastrophic oxidation behavior beyond 600°C as well as (ii) a low fracture toughness, KIC, or a high brittle-to-ductile transition temperature (BDTT) measured by Charpy. Especially for the last problem an interesting solution was found. This solution consists of the ductilisation of W through the synthesis of a W laminate (W multilayer) made of W foils. W foils have a high fracture toughness and are even ductile at room temperature. By assembling and joining we succeeded in expanding the toughness of the foil to the bulk.Many publications point out that W can be ductilised by cold working. However this is only described by a phenomenological approach and there is no understanding of the underlining mechanism. Especially the mechanism of the plastic deformation of W foil or in general of bcc UFG materials is not understood in its whole.Within the framework of this proposal the influence of (i) the high amount of mobile edge dislocations, (ii) the grain refinement, as well as (iii) the dislocation annihilation on the free surface on the extraordinary ductility of W foil, thickness 0.1 mm, will be assessed. Doing this the mechanism of the plastic deformation will be determined in a direct (tensile tests, TEM) as well as in an indirect manner (SRS, activation volume) and the results obtained will be correlated.