Novel Methods for Recording Stress-Strain Curves in Proton Irradiated Material

Abstract

proton irradiation is often used as a proxy for neutron irradiation but the irradiated layer is typically <50 μm deep; this presents a problem when trying to obtain mechanical test data as a function of irradiation level. two novel methodologies have been developed to record stress-strain curves for thin proton-irradiated surface layers of SA-508-4N ferritic steel. In the first case, in-situ loading experiments are carried out using a combination of X-ray diffraction and digital image correlation on the near surface region in order to measure stress and strain, thereby eliminating the influence of the non-irradiated volume. the second approach is to manufacture small-scale tensile specimens containing only the proton irradiated volume but approaching the smallest representative volume of the material. this is achieved by high-speed focused ion beam (fiB) milling though the application of a Xe + plasma-fiB (PFIB). It is demonstrated that both techniques are capable of recording the early stage of uniaxial flow behaviour of the irradiated material with sufficient accuracy providing a measure of irradiation-induced shift of yield strength, strain hardening and tensile strength.The limited penetration depth of proton irradiation makes measurement of mechanical properties difficult using established techniques 16,17 . Over the last decade, the use of Ga + focussed ion beam (FIB) instruments has allowed for the preparation of small scale samples from proton irradiated layers 18-23 . The mechanical properties of these samples are then tested using a MEMS chip or piezo-actuated test rig without contributions from the non-irradiated volume. This development has allowed studies to take advantage of the increased dose rates and to probe changes in properties previously only attainable using indentation testing [24][25][26][27][28] . However, milling rates are slow because for Ga + FIBs the useable milling currents are limited due to the point source of Ga + ions. Consequently, specimen diameters achievable in a practical time frame are limited to ~10 µm. For most engineering materials, the maximum achievable scale is therefore in the order of single to only a few grains. Hence, the technique is most applicable to single and bi-crystal investigations rather than representing the bulk response of polycrystalline specimens. It has been demonstrated that small scale specimens exhibit an increased hardening inversely proportional to specimen diameter [29][30][31][32][33] . In order to obtain a bulk response, specimens require a minimum length scale sufficient to overcome size reduction effects. The optimum specimen size would have the smallest representative volume (SRV), which would retain the benefits of scale reduction whilst exhibiting bulk behaviour 34 .Recent work by the authors has explored two techniques to increase the sampling volume in low energy proton irradiated samples prepared for mechanical testing 35,36 . The first, an adaptation of the technique outlined by Foecke et al. 37 , combines in-situ X-ray diffraction ...