Abstract
Nanoindentation is a commonly used method to measure the hardness of surfaces with thin layers, and is especially useful in studying the change in mechanical properties of ion irradiated materials. This research compares two different methods of nanoindentation to study the changes in hardness resulting from ion irradiation of SS316 alloy. The samples were irradiated by He2+ ions at beam energies of 1, 2, and 3 MeV, respectively. The first method involves the indentation of the irradiated surface perpendicular to it using the continuous stiffness mode (CSM), while the second applies the indents on an oblique surface, accessing an inclined cross-section of the irradiated material. Finite element modelling has been used to further illuminate the deformation processes below the indents in the two methods. The hardness profiles obtained from the two nanoindentation methods reveal the differences in the outcomes and advantages of the respective procedures, and provide a useful guideline for their applicability to various experimental conditions. It is shown through an in depth analysis of the results that the ‘top-down’ method is preferable in the case when the ion irradiation energy, or, equivalently, the irradiated depth is small, due to its greater spatial resolution. However, the oblique cross section method is more suitable when the ion irradiation energy is >1 MeV, since it allows a more faithful measurement of hardness as a function of dose, as the plastic field is much smaller and more sensitive to local hardness values.
Highlights
The construction of high efficiency, low waste nuclear reactors necessitates the use of materials resistant to severe operating conditions (radiation doses of up to 200 displacements per atom and temperatures of up to 700–1000 ◦ C) [1,2,3]
The authors have endeavoured in this paper to compare two techniques of nanoindentation The authors have endeavoured in this paper to compare two techniques of nanoindentation used for the measurement of hardening due to ion irradiation, and to assess their relative merits used for the measurement of hardening due to ion irradiation, and to assess their relative merits with with respect to the ion irradiation condition
The top-down method is better suited when the ion beam energy is low, resulting in a shallow peak damage depth (
Summary
The construction of high efficiency, low waste nuclear reactors (such as Gen IV reactors) necessitates the use of materials resistant to severe operating conditions (radiation doses of up to 200 displacements per atom (dpa) and temperatures of up to 700–1000 ◦ C) [1,2,3]. A major obstacle in performing research on the effects of extremely high radiation dose on the microstructure and mechanical properties of the relevant structural materials is the extended time period necessary to impart these doses in the available research reactors. The application of such large amounts of neutron flux results in the samples becoming radioactive and, creates problems in handling and further sample processing for characterization and testing. Ion irradiation can be used as a viable surrogate or alternative for achieving high radiation damage doses, while avoiding the problem of radioactivity in the irradiated samples [4]. Most of the radiation affected thickness, starting from the surface, has a very low dose, which increases rapidly close to the Metals 2018, 8, 719; doi:10.3390/met8090719 www.mdpi.com/journal/metals
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