Abstract
Tungsten is the main candidate material for plasma-facing armour components in future fusion reactors. In-service, fusion neutron irradiation creates lattice defects through collision cascades. Helium, injected from plasma, aggravates damage by increasing defect retention. Both can be mimicked using helium-ion-implantation. In a recent study on 3000 appm helium-implanted tungsten (W-3000He), we hypothesized helium-induced irradiation hardening, followed by softening during deformation. The hypothesis was founded on observations of large increase in hardness, substantial pile-up and slip-step formation around nano-indents and Laue diffraction measurements of localised deformation underlying indents. Here we test this hypothesis by implementing it in a crystal plasticity finite element (CPFE) formulation, simulating nano-indentation in W-3000He at 300 K. The model considers thermally-activated dislocation glide through helium-defect obstacles, whose barrier strength is derived as a function of defect concentration and morphology. Only one fitting parameter is used for the simulated helium-implanted tungsten; defect removal rate. The simulation captures the localised large pile-up remarkably well and predicts confined fields of lattice distortions and geometrically necessary dislocation underlying indents which agree quantitatively with previous Laue measurements. Strain localisation is further confirmed through high resolution electron backscatter diffraction and transmission electron microscopy measurements on cross-section lift-outs from centre of nano-indents in W-3000He.
Highlights
Tungsten is the main candidate material for plasma-facing armour components in future fusion reactors
Armour components will be exposed to high temperatures (>1200 K) and irradiated by high energy neutrons (14.2 MeV, ~2 dpa per year for tungsten) which will generate lattice defects[3,4]
We have presented a comprehensive analysis of the effect of helium-implantation damage on the mechanical properties of tungsten
Summary
Tungsten is the main candidate material for plasma-facing armour components in future fusion reactors. Armour components will be exposed to high temperatures (>1200 K) and irradiated by high energy neutrons (14.2 MeV, ~2 dpa per year for tungsten) which will generate lattice defects[3,4]. Helium-ion implantation can mimic nuclear fusion induced radiation damage by creating irradiation-damage-like lattice defects, while allowing examination of the interaction of the injected helium with these lattice defects As such helium-implanted tungsten has been widely studied through both simulations and experiments for varying conditions of temperature and helium fluence[7,8,9,10]. High-resolution Transmission Electron Microscopy (TEM) micrographs of 0.3 at.% helium-implanted tungsten samples (exposed at room temperature) showed no visible damage[11] This implies that the induced defects are smaller than 1.5 nm, the sensitivity limit of TEM to lattice defects[12]. 3D resolved micro-beam Laue diffraction measurement of residual lattice distortions beneath the indents showed a significantly smaller deformation zone in the implanted material[14]
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