Orientation-dependent indentation response of helium-implanted tungsten

Abstract

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...

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“…Surprisingly however, while 500 nm deep spherical nano-indents in 〈001〉-oriented W-0.3wt%. He showed large pile-up and slip steps [58], similar indents made in 〈011〉 and 〈111〉 grains of the sample helium-implanted tungsten sample showed negligible pile-up [90]. Figure 9j-l shows this experimentally observed orientationdependence of pile-up pattern in helium-implanted tungsten.…”

“…Consequently a CPFE simulation of indentation in 〈001〉-oriented W-0.3wt%. He was developed based on the theory of irradiation-induced strain-softening [90]. With only one fitting parameter the formulation could capture the localised large pile-up and predict confined fields of lattice distortions beneath indents in quantitative agreement with experimental measurements, confirming the accuracy of the proposed theory [90].…”

“…9g-i. Importantly, this suggests that the underlying strain localisation is orientation-independent and that changes in pile-up arise due to the relative orientations of slip systems, sample surface, and the indenter [90].…”

“…Scientific understanding obtained from experimental characterisation can be used to develop a numerical model of the irradiated material. Such material models can be used to numerically simulate the mechanical behaviour of similarly irradiated engineering components through for example crystal plasticity finite element simulations (CPFE) [89][90][91][92][93]. By constraining dislocation-mediated plasticity to specific slip directions, consistent with the crystallographic geometry of the material, CPFE renders a physical basis to predictions and makes them directly comparable to experimental observations [94][95][96].…”

“…The AFM micrographs have been rotated to match the in-plane orientations as labelled on the CPFE plots. The colour scale and 5 µm scale bar are the same for all plots [90] swelling. Unlike hydrogen, helium forms a stable bond with itself and helium clusters are strong enough to create lattice defects.…”