NOVEL METHODS OF RECORDING FLOW CURVES IN PROTON IRRADIATED MATERIAL

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Abstract

Observing the physical effects of neutrons is logistically complicated due to their poor displacement efficiency, which requires irradiation durations extending from months to years to attain a relevant level of damage. Specimen activation leads to the requirement of a cooling off period to perform post irradiation examination outside of specialised facilities. Protons can produce defects with a similar structure and a density comparable to years of irradiation in a reactor in a few hours or days. Furthermore, at low energies their positive charge is repelled by nuclei, which reduces the probability of specimen activation and allows for off-site examination. The high dose rates and limited residual activity makes protons attractive as a surrogate for the study of irradiation damage. However, the limited penetration of protons, in the order of a few tens of microns, complicates studies probing standard mechanical properties beyond hardness measurement. In terms of the uniaxial tensile test the difficulty lies in the global nature of data acquisition, where the hardened surface properties are convoluted with that of the non-irradiated volume. The percentage of non-irradiated material in a 1 mm thick specimen irradiated with 3 MeV protons is upwards of 95%, which would lead to results heavily biased towards the non-irradiated properties. Therefore, the solution is to either employ a novel technique to directly record the surface properties or extract the desired layer. This project explored both solutions to the problem: 1) Directly probing the irradiated layer using a combination laboratory based X-ray diffraction (sin2ψ) & digital image correlation (DIC) and 2) Small scale tensile specimens were prepared using Xe+ plasma focused ion beam (PFIB) to increase specimen scale and attain a smallest representative volume. Both techniques were initially applied to non-irradiated SA508-4N for the purposes of validation. The combination of XRD and DIC accurately described both elastic and plastic regimes, that were recorded using the standardised technique on bulk specimens. Specimens prepared using PFIB exhibited a bulk representative yield stress, however specimen dimension and scale diminished the plastic response. Finite element analysis was applied to distinguish between the two limiting factors. It was demonstrated that the specimen geometry had a pronounced effect on the reduction of tensile strength and the reduced scale is thought to restrict strain hardening. The techniques were applied to SA508-4N irradiated with 3 MeV protons to 10 mdpa at the University of Manchester’s Dalton Cumbrian Facility. Both techniques recorded a positive yield shift in the same order estimated using indentation testing, with a reduction in strain hardening and an invariant tensile strength, which is consistent with literature reported behaviour. High resolution digital image correlation was applied to 316L in the non-irradiated state and irradiated to 100 mpda. A host of analysis techniques provided insight into deformation mechanisms just beyond the critical dose for dislocation channelling (100 mdpa). The work highlighted the differences in strain localisation and strain hardening behaviour of the two states and provided the opportunity to construct a flow curve based on the principal of equivalent pre-strain.

Keyword(s)

Digital image correlation; HRDIC; Mechanical Testing; PFIB; Proton irradiation; X-ray stress measurement

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