Hardening and Strain Localisation in Helium-Ion-Implanted Tungsten

45 Pages Posted: 9 Jan 2019

See all articles by Suchandrima Das

Suchandrima Das

University of Oxford - Department of Engineering Science

Hongbing Yu

University of Oxford - Department of Engineering Science

Edmund Tarleton

University of Oxford - Department of Materials

Felix Hofmann

University of Oxford - Department of Engineering Science

Date Written: January 7, 2019

Abstract

Tungsten is the main candidate material for plasma-facing armour components in future fusion reactors. In service, bombardment with energetic neutrons will create collision cascades that leave behind lattice defects. Helium, injected from the plasma and produced by transmutation, strongly interacts with these defects, modifying their behaviour and retention. Helium-ion-implantation provides an effective tool for examining helium-defect interactions and their effect on the properties of tungsten armour. We use nano-indentation to probe the mechanical properties of the shallow helium-ion-implanted layer. Comparison of spherical indents in unimplanted and helium-implanted regions of the same single crystal shows a large increase in hardness and substantial pile-up in the implanted material. The complex lattice distortions beneath indents are probed non-destructively using 3D-resolved synchrotron X-ray micro-diffraction. Reduced lattice rotations and indentation-induced residual strains in the ion-implanted material indicate a more confined plastic zone. This is confirmed by HR-EBSD and TEM observations. Together our results suggest that dislocation motion is initially obstructed by helium-induced defects. The obstacle strength of these defects is reduced by the passage of dislocations, leading to a strain-softening and slip channel formation. A constitutive law for 3D crystal plasticity finite element simulations is developed based on this hypothesis. Simulations of the indentation process successfully capture the helium-implantation induced changes in deformation behaviour observed in experiments. Importantly, the effects we observe are markedly different from previous observations for self-ion-implanted tungsten, highlighting that the exact nature of the implantation damage plays a critical role in determining mechanical property change.

Keywords: tungsten, nano-indentation, micro-beam Laue diffraction, crystal-plasticity, helium-implantation

Suggested Citation

Das, Suchandrima and Yu, Hongbing and Tarleton, Edmund and Hofmann, Felix, Hardening and Strain Localisation in Helium-Ion-Implanted Tungsten (January 7, 2019). Available at SSRN: https://ssrn.com/abstract=3311684 or http://dx.doi.org/10.2139/ssrn.3311684

Suchandrima Das

University of Oxford - Department of Engineering Science

Mansfield Road
Oxford, Oxfordshire OX1 4AU
United Kingdom

Hongbing Yu

University of Oxford - Department of Engineering Science

Mansfield Road
Oxford, Oxfordshire OX1 4AU
United Kingdom

Edmund Tarleton

University of Oxford - Department of Materials

Parks Road
Oxford, OX1 3PH
United Kingdom

Felix Hofmann (Contact Author)

University of Oxford - Department of Engineering Science ( email )

Mansfield Road
Oxford, Oxfordshire OX1 4AU
United Kingdom

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