Helium-implantation-induced lattice strains and defects in tungsten probed by X-ray micro-diffraction

Applied Physics Letters

Tarleton

et al. 2019

Self Cite

A literature review of studies investigating the topography of nano-indents in ion-implanted materials reveals seemingly inconsistent observations, with report of both pile-up and sink-in. This may be due to the crystallographic orientation of the measured sample point, which is often not considered when evaluating implantation-induced changes in the deformation response. Here we explore the orientation dependence of spherical nano-indentation in pure and helium-implanted tungsten, considering grains with <001>, <110> and <111> out-of-plane orientations. Atomic force microscopy (AFM) of indents in unimplanted tungsten shows little orientation dependence. However, in the implanted material a much larger, more localised pileup is observed for <001> grains than for <110> and <111> orientations. Based on the observations for <001> grains, we hypothesise that a large initial hardening due to heliuminduced defects is followed by localised defect removal and subsequent strain softening. A crystal plasticity finite element model of the indentation process, formulated based on this hypothesis, accurately reproduces the experimentally-observed orientation-dependence of indent morphology. The results suggest that the mechanism governing the interaction of helium-induced defects with glide dislocations is orientation independent. Rather, differences in pile-up morphology are due to the relative orientations of the crystal slip systems, sample surface and spherical indenter. This highlights the importance of accounting for crystallographic orientation when probing the deformation behaviour of ion-implanted materials using nano-indentation. Main textIon-implantation is commonly used to mimic irradiation damage in materials. This costeffective technique allows examination of specific irradiation factors. For example heliumimplantation introduces irradiation-like defects, while enabling examination of the interaction of these defects with the implanted helium [1]- [3]. Similarly self-ion implantation has been extensively employed to emulate the cascade damage caused by neutron irradiation [4], [5]. Nano-indentation is often used to gain insight into the mechanical properties of the few-micronthick ion damaged layers. Marked changes in pile-up morphology around nano-indents have been observed in irradiated materials, even for low damage levels. A review of several studies reveals seemingly contradictory observations. For example, large pile-up around 250 nm deep Berkovich nano-indents was observed in 0.3 at.% helium-implanted W-1 at. % Re [6]. But for HT9 ferritic/martensitic steel, implanted with both helium and protons, there was no noticeable difference in indent surface profile [7]. A suppression of pile-up was noticed around indents in 2 MeV W + ion implanted W-5 wt%Ta (0.04 dpa) [8]. On the other hand Fe + implantation of Fe-12 wt%Cr lead to a distinct increase in pile-up [9]. This raises the question why implantation with helium or self-ions causes pile-up increase in some materials and suppression in others. Generally, pi...

mentioning

confidence: 99%

Orientation-dependent indentation response of helium-implanted tungsten

Applied Physics Letters

Tarleton

et al. 2019

Self Cite

“…Of the two additional parameters introduced for helium-implanted tungsten, only is fitted to the experimental result of the implanted sample. The fitted load-displacement curves from CPFE are superimposed as dotted lines on the experimentally obtained curves in Supplementary Material) 35 . It is interesting to compare the CPFE predicted load-displacement curves for the implanted material with and without these stress boundary conditions, keeping all other UMAT parameters the same.…”

Section: Developing the Cpfe Modelmentioning

confidence: 99%

Hardening and Strain Localisation in Helium-Ion-Implanted Tungsten

Tarleton

et al. 2019

SSRN Journal

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 heliumimplanted 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 crosssection lift-outs from centre of nano-indents in W-3000He.

“…This change in defect retention as a function of irradiation dose or presence of impurities, is particularly important in order to accurately estimate irradiationinduced residual stresses that can affect the fatigue performance of the components. Analysis of helium-implanted W-1wt% Re alloy samples, of different crystal orientations and with varying implantation doses, in a similar manner (through strain measurements from diffraction) showed that both the presence of rhenium, and crystal orientation, have a relatively small influence on defect retention [69].…”

Section: The Effect Of Helium On the Plasma-facing Componentmentioning

confidence: 97%

“…Further complexities may include distinction of vacancies from nano-voids or identification of defect combinations of vacancies and selfinterstitials. Alternatively, the entire defect population (irrespective of size) can be probed by measuring the lattice strain induced by them using synchrotron X-ray micro-diffraction which uniquely permits the determination of lattice strain with sub-micron 3D spatial resolution and ~ 10 −4 strain sensitivity [48,58,[67][68][69]. Using a combination of this approach and electronic structure calculations, it is possible to deduce the nature of the defect (vacancy or self-interstitial), morphology and its depth-resolved distribution in the implanted layer [48,70].…”

Section: Techniques To Study Irradiated Materialsmentioning

confidence: 99%

“…For example, Laue diffraction measurements performed on 300 and 3000 appm heliumimplanted tungsten samples showed an increase volumetric strain by only ~ 2.4 times, suggesting that defect retention per injected helium atom is ~ 3 times higher at low doses. [69]. This implies that retention of defects may not be a simple function of implanted helium dose, but can strongly depend on material composition and presence of impurities.…”

Section: The Effect Of Helium On the Plasma-facing Componentmentioning

confidence: 99%

See 1 more Smart Citation

Recent advances in characterising irradiation damage in tungsten for fusion power

2019

SN Appl. Sci.

Tungsten is the front-runner candidate for building the plasma-facing armour components for future fusion reactors. However, in-service irradiation by fusion-neutrons and helium will create lattice-defects in the material, compromising its properties and lifetime. Improving the component's resilience to radiation damage and accurately predicting the lifetime of irradiated components is key for commercial feasibility of the reactor. For this purpose, understanding the creation and evolution of radiation damage is essential. This paper reviews recent advances in characterising radiation damage through experimental and modelling techniques. Tungsten-ion-and helium-ion-implantation are commonly used to mimic the damage created by neutron-and helium-irradiation respectively. Defects (> 1.5 nm) can be directly imaged using transmission electron microscopy while all defects (size-independent), may be indirectly probed by measuring lattice strains induced by them (using diffraction techniques; synchrotron X-rays or high-resolution electron-backscatter). Neutron-irradiation produces mainly ½〈111〉 prismatic loops. Loop-interaction and structural organisation evolves with changing implantation dose and temperature. Helium-irradiation, < 573 K, induces formation of small helium-vacancy clusters, which evolve into bubbles, blisters and "fuzz" structure with changing temperature and dose. Nano-indentation or micro-cantilever bending tests can be used to examine mechanical properties of ion-implanted layers. Both heliumand neutron-implantation defects induce increased hardening often followed by subsequent strain-softening and localised deformation. Such irradiation-induced alterations are detrimental to material ductility and long-term structural integrity of tungsten-based components. Development of physically-based material models that capture the physics of underlying irradiation-induced changes, inspire confidence of reliably using simulations to predict mechanical behaviour and in-service performance of irradiated engineering components in future.