This study aims to compare the effects of neutron and self-ion irradiation on the mechanical properties and microstructural evolution in W. Neutron irradiation at the HFR reactor to 1.67 dpa at 800 °C resulted in the formation of large Re and Os rich clusters and voids. The post-irradiation composition was measured using APT and verfified against FISPACT modelling. The measured Re and Os concentration was used to create alloys with equivalent concentrations of Re and Os. These alloys were exposed to self-ion irradiation to a peak dose of 1.7 dpa at 800 °C. APT showed that self-ion irradiation leads to the formation of small Os clusters, wheras under neutron irradiation large Re/Os clusters form. Voids are formed by both ion and neutron irradiation, but the voids formed by neutron irradiation are larger. By comparing the behaviour of W-1.4Re and W-1.4Re-0.1Os, suppression of Re cluster formation was observed. Irradiation hardening was measured using nanoindentation and was found to be 2.7 GPa, after neutron irradiation and 1.6 GPa and 0.6 GPa for the self-ion irradiated W-1.4Re and W-1.4Re-0.1Os. The higher hardening is attributed to the barrier strength of large voids and Re/Os clusters that are observed after neutron irradiation.

Tungsten samples have been irradiated with 3 MeV protons with dose rates of 1×10-04 to 5×10-05 dpa/s to doses of 0.01–0.67 dpa at 360 K in a pilot experiment. Micro- and macro-indentation were used to measure irradiation hardening in the samples. An initial irradiation hardening of 1.23 ± 0.09 GPa and 1.88 ± 0.83 GPa was measured by micro and macro indentation. The irradiation hardening was observed to saturate at 0.03 dpa damage. Dislocation loops were identified using TEM suggesting an increasing loop size with dose.

Only limited data exist on the effect of neutron irradiation on the brittle to ductile transition (BDT) in tungsten. This work investigates the increase in brittle to ductile transition temperature (BDTT) following neutron irradiation to 1.67 displacements per atom, using four-point bend tests over a range of temperatures (623-1173 K) and strain rates (3.5 x 10(-7) - 2.5 x 10(-5) s(-1)). The BDTT was found to increase by 500 K after irradiation. The activation energy for the BDT was determined using Arrhenius analysis of the four-point bend tests. Nanoindentation strain-rate jump tests were used to characterise the activation volume for dislocation motion. These were quantified as 1.05 eV and 4.6 b(3) respectively, very close to values found for unirradiated tungsten. This suggests that kink-pair formation is the controlling mechanism for the BDT before and after irradiation. This work also carries out a unique verification of inventory-code-modelling (via FISPACT-II) of transmutation of tungsten to rhenium and osmium under neutron irradiation using two independent techniques (X-ray and gamma-ray spectroscopy). These results show that modelling can correctly predict this transmutation, provided that an accurate neutron spectrum is used. This is a critical result given the widespread use of inventory codes such as FISPACT-II, and the associated nuclear data libraries, for modelling transmutation of tungsten. (C) 2019 The Authors. Published by Elsevier B.V.

A viable fusion power station is reliant on the development of plasma facing materials that can withstand the combined effects of high temperature operation and high neutron doses. In this study we focus on W, the most promising candidate material. Re is the primary transmutation product and has been shown to induce embrittlement through cluster formation and precipitation below its predicted solubility limit in W. We investigate the mechanism behind this using a kinetic Monte Carlo model, implemented into Stochastic Parallel PARticle Kinetic Simulator (SPPARKS) code and parameterised with a pairwise energy model for both interstitial and vacancy type defects. By introducing point defect sinks into our simulation cell, we observe the formation of Re rich clusters which have a concentration similar to that observed in ion irradiation experiments. We also compliment our computational work with atom probe tomography (APT) of ion implanted, model W-Re alloys. The segregation of Re to grain boundaries is observed in both our APT and KMC simulations.

High temperature, neutron irradiated single crystal tungsten,with a post irradiation composition of W-1.20±0.11at.%Re-0.11±0.05at.%Os0.03±0.01at.%Ta was characterised using a combination of Atom ProbeTomography (APT) and Scanning Tunnelling Electron Microscopy (STEM). APTshowed that within nanoscale clusters of Re/Os, the atomic density wasabove the theoretical limit. Complimentary High Angle Annular Dark Field(HAADF) imaging shows that some clusters contain voids at their centrewhich are leading to APT aberrations and enhancing the atomic density.High resolution Energy Dispersive X-ray (EDX) spectroscopy shows thatvoids are decorated with a shell of rhenium with a small osmium clusterto one side.

Tungsten is the leading candidate material for plasma facing components in future fusion devices. In this environment tungsten will be exposed to high neutron fluxes (up to 14 MeV energies) that will cause both chemical changes through transmutation and displacement damage. This will lead to hardening and potentially embrittlement, limiting the lifetime of tungsten in a reactor. Neutron irradiation of single crystal tungsten to 1.7 dpa has been carried out in the Petten test reactor with a thermal neutron spectrum. Transmutation of tungsten to rhenium was also measured using energy dispersive x-ray spectroscopy. This was found to match the levels predicted by modelling, using inventory code FISPACT, at 1.4 wt% Re. This is similar to the levels of transmutation expected in a fusion neutron spectrum after ~18 months [1].

Tungsten has been proposed for use in the divertor of future fusion devices. In this environment, it will be exposed to high heat fluxes, neutron damage and hydrogen and helium implantation. This review covers previous experimental and modelling work to establish our ability to predict the performance of tungsten in a fusion environment. Surrogates for high-energy neutrons have been used to predict the change in mechanical properties of tungsten, including fission neutron and self-ion exposure. These studies are critically analysed, with a focus on the difference in results from neutron and ion studies. Further studies to improve our ability to predict the performance of tungsten are proposed as a critical part of the path towards a working fusion reactor.

Tungsten has been proposed for use in the divertor of future fusion devices. In this environment, it will be exposed to high heat fluxes, neutron damage and hydrogen and helium implantation. This review covers previous experimental and modelling work to establish our ability to predict the performance of tungsten in a fusion environment. Surrogates for high-energy neutrons have been used to predict the change in mechanical properties of tungsten, including fission neutron and self-ion exposure. These studies are critically analysed, with a focus on the difference in results from neutron and ion studies. Further studies to improve our ability to predict the performance of tungsten are proposed as a critical part of the path towards a working fusion reactor. This review was submitted as part of the 2016 Materials Literature Review Prize of the Institute of Materials, Minerals and Mining run by the Editorial Board of MST. Sponsorship of the prize by TWI Ltd is gratefully acknowledged