Probing Nanoscale Damage of Nuclear Irradiated Metals using Spherical Nanoindentation

Understanding how ion-irradiated materials, as seen in nuclear reactors, behave mechanically is critical to the design of future energy generation facilities.

(University of Nevada, Reno) – 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. Of special concern are cases where such gradations are caused unintentionally as a consequence of the service life of the material, such as in wear applications or irradiated materials which show varying degrees of radiation damage that change with depth, location of radiation source, etc. Quantifying the resulting property gradations poses a significant challenge, especially when the changes occur over small (sub-micrometer) depths. We have been working on a novel indentation approach, which together with the corresponding local structure information obtained from electron back-scattered diffraction (EBSD) and transmission electron microscopy (TEM), 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 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. 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?”

Among the experimental techniques available at these length scales, nanoindentation, with its high resolution load and depth sensing capabilities, shows the greatest promise due to its non-destructive nature, ease of experimentation (only a polished surface prior to ion irradiation is needed) and versatility. In particular, using spherical indenters, our recent work has demonstrated the feasibility of transforming the raw load-displacement data into meaningful indentation stress-strain curves. These indentation data analysis methods have captured successfully the local loading and unloading elastic moduli, the local indentation yield strengths, and certain aspects of post-yield strain hardening behavior in various polycrystalline metal samples. More specifically, the use of these indentation stress-strain curves makes it possible to analyze the initial loading segments of spherical indentation – before the indentation itself imposes additional local plastic deformation and alters the local microstructure and its properties. Coupling the mechanical data obtained from nanoindentation with the structure information obtained from EBSD has also provided new insights into the local elastic-plastic properties of interest. This has enabled the measurement of the local indentation yield strengths in individual grains of deformed polycrystalline metallic samples, and across their grain boundaries, which in turn can be related to percentage increases in the local slip resistances from their fully annealed conditions. Moving forward, we apply these methods to indentations on ion-irradiated metallic materials, and compare their relative mechanical behavior to the unirradiated state. For full details see http://rdcu.be/v11u

These measurements (Figure below) show our first attempts at utilizing spherical indentation stress-strain curves to investigate the changes in the mechanical response of tungsten with ion-irradiation induced surface damages. These methods are cost-effective in extracting huge amounts of reliable and reproducible information from very small nanometer sample volumes. By simply varying the indenter size, this technique can be used to provide new insights into the mechanical response of the irradiated layers in these samples, and correlate those effects with the local material structure obtained from EBSD. As such, the ideas presented in this communication are applicable to all polycrystalline material systems (including metals and ceramics) with a modified surface layer. They can also be extended to a broad range of complex material systems where the local structure information is obtained by other materials characterization techniques (e.g., Raman-spectroscopy maps on bone, back-scattered electron images).

Figure. (a) Causes for the change in Yind. In annealed electro-polished tungsten, the defect density is low across all grains. Here Yind varies from one grain to another mainly due to the differences in the activities of the different slip systems in the different grains and their orientation with the indentation direction. Upon ion-irradiation, the metal surface is modified by a damaged layer, which causes a change in its mechanical response as compared to the bulk of the sample. The Yind in irradiated samples therefore depends on both the grain orientation and the interaction of the indentation zone with the radiation damaged layer at the indentation site.
Typical (b) load-displacement and (c) indentation stress-strain responses for a near (001) grain in annealed electro-polished tungsten using 4 different indenter sizes of radii 1, 5, 10 and 100 µm before and after He irradiation.

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Siddhartha Pathak is an Assistant Professor in the Department of Chemical and Materials Engineering at the University of Nevada, Reno.

You can contact him regarding his research at spathak@unr.edu, (775) 784-7098, 1664 N Virginia St, Mail Stop 0388, University of Nevada, Reno, Reno, NV 89557-0388 USA

https://wolfweb.unr.edu/homepage/spathak/

Feature Image Credit/GettyImages

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