Radiation resistance of nano-structured tungsten-rhenium sheet
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Radiation resistance of nano-structured tungsten-rhenium sheet David E.J. Armstrong1 and Steve G. Roberts1 1 Department of Materials, University of Oxford, Parks Road, Oxford, OX1 4JF, United Kingdom, ABSTRACT Tungsten is one the most important material for both plasma facing and structural applications in current designs for advanced divertors. Recent work has shown that composites can be manufactured from nanostructured tungsten foils which show significantly higher toughness than monolithic tungsten, but there is no data on the radiation resistance of such materials. In this study W-5 wt% Re foil in both an as rolled and annealed condition was implanted with 2MeV W+ ions to two damage levels, 0.07 and 0.4 dpa. The change in hardness was measured using nanoindentation. An increase in hardness was seen in both materials at both damage levels, with more hardening seen for the 0.4 dpa implanted samples. However the increase in hardness due to ion implantation was 2.6 times higher in the annealed material as compared to the as rolled material. This is due to the smaller grain size and higher dislocation density providing more sinks for the irradiation produced defects in the as rolled material as compared to the annealed material. Thus showing that unannealed tungsten foils are superior for use in applications in which they will see significant levels of radiation damage. INTRODUCTION Tungsten and its alloys are the most promising class of materials for use in the plasma facing components of the main chamber and the divertor of future nuclear fusion tokamaks such as ITER and DEMO[1]. This is due to its excellent ability to operate at high temperature, good sputtering resistance and low activation. However all materials subjected to 14 MeV neutron fluxes in a fusion device will undergo two major physical processes, transmutation and ballistic damage[2] [3]. These will both result in changes to the mechanical properties of the base alloy. In the case of a pure tungsten sample it has been predicted by Gilbert and Sublet [2] that after 5 years at DEMO full power operation pure W would be transmuted to an alloy with the composition W-3.8at%Re-1.4at%Os. Such changes in composition have been shown to alter the mechanical properties of the tungsten alloy[4,5]. Approximately 20 appm of helium is also produced by transmutation, and may cause further hardening and embrittlement[6]. The ballistic neutron damage causes the formation of dislocation loops by the coalescence of point defects and has the potential to form non-equilibrium phases due to radiation induced clustering[4]. The simulation of neutron damage by ion implantation is a widely utilized technique due to the lower costs than neutron irradiation, faster speed of damage accumulation and nonactivation of samples. In both annealed pure tungsten and tungsten-rhenium alloys W+ ion implantation has been shown to cause the formation of small (2-10 nm) prismatic dislocation loops [7] which cause a significant increase in hardness [4] due to the interaction between loops and
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