Probing nanoscale damage gradients in ion-irradiated metals using spherical nanoindentation

Abstract

We discuss and demonstrate the application of recently developed spherical nanoindentation stressstrain protocols in characterizing the mechanical behavior of tungsten polycrystalline samples with ionirradiated surfaces. It is demonstrated that a simple variation of the indenter size (radius) can provide valuable insights into heterogeneous characteristics of the radiation-induced-damage zone. We have also studied the effect of irradiation for the different grain orientations in the same sample.Materials with modified surfaces -either as a consequence of a graded microstructure or due to an intentional alteration of the surface such that its physical, chemical or biological characteristics are different from the bulk of the material -are of increasing interest for a variety of applications such as enhanced wear and corrosion resistance, superior thermal and biomedical properties, and higher fracture toughness 1,2 . In some cases such gradations at the surface may also be caused unintentionally as a consequence of the service life of the material, such as in wear applications 3 or irradiated materials which show varying degrees of radiation damage that change with depth, location of radiation source, etc.4 . Quantifying the resulting property gradations poses a significant challenge, especially when the changes occur over small (sub-micrometer) depths. In this communication, we present a novel indentation approach, which together with the corresponding local structure information obtained from electron back-scattered diffraction (EBSD), allows us to probe nanoscale surface modifications in solid materials and quantify the resulting changes in its mechanical response.The study of mechanical degradation in the surface layers of ion-irradiated materials is an example of one such outstanding challenge for which few practically viable solutions 4-7 exist. In materials undergoing irradiation in reactor or spacecraft applications, the resulting damage is often highly heterogeneous (with strong gradients normal to the surface) depending on component location as well as the nature of the irradiation source itself. In nuclear materials research, reactor conditions can be mimicked using ion beams where large amounts of radiation damage (several displacements per atom (dpa)) are imparted in relatively short time spans of hours or days that would require months or years to achieve in reactor conditions [8][9][10] . However, the volume of ion-irradiated material is limited by the beam energy to depths of fractions of a micron to several microns, making the investigation of bulk mechanical properties very difficult. A key challenge then becomes: "How can we study the mechanical response of materials with varying degrees of damage over scales of only a few hundreds of nanometers in such a way that the data can be related to bulk values?" The very small thickness of irradiated material, high level of damage heterogeneity, sensitivity to sample preparation techniques, and the time and effort needed for sample preparation and testi...

Three mechanisms of hydrogen-induced dislocation pinning in tungsten

“…the indentation stress and strain measures to convert the corrected P − h response into an indentation stress-strain curve (see reference [43] for more details). The obtained indentation stress-strain curves from multiple grain interiors on the reference sample were found to be continuous and similar, as displayed in figure 2(c), which are consistent with previous studies on recrystallized tungsten [43,73]. It should be noted that the stress measure we use here is called indentation stress, which is different from the yield stress used in a uniaxial tensile/compression test.…”

Deformation Behaviour of Ion-Irradiated FeCr: A Nanoindentation Study

“…The origins of this, for tests all performed at the same temperature, is solid solution hardening, which increases with the square root of the solute concentration [60,61]. The yield strength also increases monotonically with irradiation level for all FeCr materials measured, which has been previously observed in other irradiated materials [54,62].…”

“…Using the protocols described in Section 2.3 [54,55], indentation stress-strain (ISS) curves were calculated for the indents made with the 5.12 µm spherical tip (Figure 8). The effects of pop-ins are clearly seen, with large elastic stresses in the ISS response before the flow portion proceeds at much lower stress levels.…”

Comparative Study of Two Nanoindentation Approaches for Assessing Mechanical Properties of Ion-Irradiated Stainless Steel 316

“…Furthermore, it is necessary to compute the ISE for the CSM top-down method, but this effect is reduced in the OCS method by virtue of the increase in the irradiation-induced defects for indents in the peak damage region, while the ISE remains an important factor in the low or zero dose regions. Other studies have shown the ISE pop-ins are the result of difficulty in activating dislocations sources (e.g., Frank-Read sources) [26,27], but for the inclined nanoindentation studies presented herein (i.e., the indentations in the regions in and adjacent to the peak of the hardness curve), there is greater activation due to the higher number of irradiation induced dislocation sources [28]. This diminishes the ISE in the highly irradiated regions and provides for greater correlation between the model and experiments after the use of ISE corrections in the virgin material.…”