In Situ Microscale Tensile Testing in Irradiated Materials

Materials used in power generation applications are often required to perform in harsh conditions of intense stress, high temperature, aggressive environments, and under irradiation. The principal motivation for conducting microscale tensile tests is that quantitative mechanical properties can be gleaned from small volumes of material under tensile loading conditions which are often very similar to the loading states and environments experienced by a material in service. In the case of studies on radiation damage effects in ion-irradiated structural materials, microtensile testing takes on the added significance of being the only viable means of directly accessing the tensile properties in the thin surface layer affected by the irradiation process. Even in the case of neutron irradiated materials, microtensile testing may be extremely valuable, as it allows the extraction and testing of small volumes of active material, thus minimizing the exposure risks for the personnel involved. Microtensile test specimens are geometrically symmetric and exhibit the same simple stress distributions that have been analyzed at the macroscopic level for over a century. The small volume can contain all the microstructural features of the material in the bulk state or may be composed of some local assembly of features and phases. With highly localized, site-specific testing, these experiments can provide insight into mechanisms that control deformation and thus aid in developing models to describe plastic flow and failure in materials loaded under a state of tension. The implementation of in situ microtensile tests provides visualization to better develop models and understand the deformations mechanisms at play. Conducting experiments while imaging using light microscopy, scanning electron microscopy, focused ion beam imaging or X-ray microscopy relies heavily on specimen and test methodology development. Selection of specimen production methods that are appropriate for each given topic of study and the material under investigation are key. A number of the most important general considerations related to in situ microtensile experiments will be described in the present chapter. Specific examples are given to illustrate the implementation of test design criteria that are intended to measure material properties or to investigate the underlying microstructural mechanisms that govern deformation behavior. These topics of experiment design include specimen fabrication approaches, testing apparatus construction and performance, grip designs, loading options, imaging techniques, characterization methodologies, environmental conditions and information goals, and the interplay between these themes.