Effects of neutron irradiation on the brittle to ductile transition in single crystal tungsten

R.G Abernethy,J.S.K.-L Gibson,A Giannattasio,J.D Murphy,O Wouters,S Bradnam,L.W Packer,M.R Gilbert,M Klimenkov,M Rieth,H.-C Schneider,C.D Hardie,S.G Roberts,D.E.J Armstrong

doi:10.1016/j.jnucmat.2019.151799

Abstract

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 × 10−7 - 2.5 × 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 b3 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.

Highlights

Tungsten is the leading candidate as the plasma-facing material for first-wall and divertor applications in future nuclear fusion power plants, due to its high melting point (3695 K), low sputtering rates and good thermal conductivity [1]
These results show that generally unirradiated BCC metals have a brittle to ductile transition temperature (BDTT) of around 0.1-0.15 Tm that increases to a maximum of 0.2-0.3 Tm following neutron irradiation
The level of transmutation in tungsten after neutron irradiation to 1.7 dpa was measured at 1.2 wt% Re, in good agreement with FISPACT-II models, which matched gamma spectroscopy results to a reasonable degree of accuracy

Summary

Introduction

Tungsten is the leading candidate as the plasma-facing material for first-wall and divertor applications in future nuclear fusion power plants, due to its high melting point (3695 K), low sputtering rates and good thermal conductivity [1]. There are concerns over its mechanical properties, in particular its low formability and high brittle to ductile transition temperature (BDTT) [2]. Work is ongoing for developing manufacturing routes for tungsten components such as cold-working [3] and tungsten composites [4] to improve ductility. These approaches have shown increasing promise in recent years [5][6]. There have been several studies of the fracture properties of tungsten [7][8] including its brittle to ductile transition (BDT), there has been comparatively little work on the impact on neutron irradiation on the BDT. Any decrease in ductility as a result of irradiation may influence both operation of the device and end-of-life handling

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