Orientation-dependent indentation response of helium-implanted tungsten

TL;DR

This study investigates how crystallographic orientation affects nano-indentation responses in helium-implanted tungsten, revealing orientation-dependent deformation behaviors and emphasizing the importance of considering orientation in such analyses.

Contribution

It introduces a comprehensive analysis of orientation effects on nano-indentation in helium-implanted tungsten, combining experimental AFM observations with a crystal plasticity model.

Findings

Larger, localized pile-up observed in <001> grains after helium implantation.

The indentation response is strongly dependent on crystallographic orientation.

The proposed model accurately reproduces the orientation-dependent indentation morphology.

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 pile-up 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 helium-induced 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.