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Title: Small scale mechanics applied to nuclear materials
Author: Herring, James
ISNI:       0000 0001 2437 1744
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2015
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Ion implantation is increasingly used as a cost effective and safe alternative to neutron irradiation in the study of materials for future nuclear applications. However, the micron-sized layers of radiation damage necessitate the use of small scale testing techniques to obtain mechanical properties data from these materials. This thesis explores such techniques, highlighting the relative merits and limitations of nanoindentation, and the deflection of FIB-machined specimens, in a variety of applications. As a simple starting point, the effect of crystal orientation on nanoindentation hardness and modulus is explored in a pure Fe polycrystal, with selected indents examined further post-test by HR-EBSD and AFM. Hardness in grains with < 111 > and < 110 > surface normal directions is found to be ~ 20 % higher (~ 1.1 GPa) than those with < 100 > surface normals (~ 0.95 GPa), whilst no discernible trend is seen for modulus. A novel L-shaped micro-cantilever technique is developed and used to extract single crystal elastic constants within a single grain of Fe. A computational routine minimising the difference between experimental data and the response of an FE model is used to extract such constants, with a good agreement between experiment and literature found after two runs. Slip transfer across grain boundaries is studied in unirradiated and ion-irradiated samples using nanoindentation. Grain boundaries in the irradiated specimen show less resistance to slip transfer, and the presence of dislocation bursts associated with the operation of dislocation sources in adjacent grains is reduced compared to the unirradiated sample. Finally, the effects of size and orientation on plastic properties are examined in FIB-machined micro-cantilevers. Two different functions are used to fit flow stresses evaluated at different maximum strains to beam width, and compared. Cantilevers orientated for multiple slip are observed to have lower flow stresses than those aligned for single slip with a Schmid factor of 0.5. The work in this thesis highlights the importance of both types of small scale mechanical test in providing information of relevance to candidate materials for nuclear applications. Nanoindentation is able to quickly probe mechanical characteristics, but at the expense of a well-defined stress state under the indenter tip, whereas FIB-machined specimens can provide a simpler stress state to test, for higher costs of time and instrument usage.
Supervisor: Wilkinson, Angus Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available