James Marrow

3D Studies of Indentation by Combined X-Ray Tomography and Digital Volume Correlation

Hardness testing obtains material properties from small specimens via measurement of load-displacement response to an imposed indentation; it is a surface characterisation technique so, except in optically transparent materials, there is no direct observation of the assumed damage and deformation processes within the material. Three-dimensional digital image correlation (digital volume correlation) is applied to study deformation beneath indentations, mapping the relative displacements between high-resolution synchrotron X-ray computed tomographs (0.9 μm voxel size). Two classes of material are examined: ductile aluminium-silicon carbide composite (Al-SiC) and brittle alumina (Al2O3). The measured displacements for Hertzian indentation in Al-SiC are in good agreement with an elastic-plastic finite element simulation. In alumina, radial cracking is observed beneath a Vickers indentation and the crack opening displacements are measured, in situ under load, for the first time. Potential applications are discussed of this characterization technique, which does not require resolution of microstructural features.

Observations of intergranular stress corrosion cracking in a grain-mapped polycrystal.

Nondestructive three-dimensional mapping of grain shape, crystallographic orientation, and grain boundary geometry by diffraction contrast tomography (DCT) provides opportunities for the study of the interaction between intergranular stress corrosion cracking and microstructure. A stress corrosion crack was grown through a volume of sensitized austenitic stainless steel mapped with DCT and observed in situ by synchrotron tomography. Several sensitization-resistant crack-bridging boundaries were identified, and although they have special geometric properties, they are not the twin variant boundaries usually maximized during grain boundary engineering.

FAFNIR: Strategy and risk reduction in accelerator driven neutron sources for fusion materials irradiation data

Abstract The need to populate the fusion materials engineering data base has long been recognized, the IFMIF facility being the present proposed neutron source for this purpose. Re-evaluation of the regulatory approach for the EU proposed DEMO device shows that the specification of the neutron source can be reduced with respect to IFMIF, allowing lower risk technology solutions to be considered. The justification for this approach is presented and a description of a proposed facility, FAFNIR, is presented with more detailed discussion of the accelerator and target designs.

Damage nucleation in nuclear graphite

Abstract This paper reports the use of advanced materials characterisation techniques, X-ray microtomography and surface strain mapping by electronic speckle pattern interferometry (ESPI), to study the mechanisms of damage nucleation in polygranular nuclear graphite. It is found that strain localisation occurs owing to the heterogeneous microstructure, giving rise to microcrack nucleation and coalescence before tensile failure. This produces a substantial damage zone at stress concentrations, such as notches and crack tips, which can be observed directly and in situ. Crack propagation occurs by the coalescence of microcracks in the damage zone. The measured R curve is a consequence of both frictional bridging in the crack wake and energy dissipation in the microcrack process zone.

Reducing risk and accelerating delivery of a neutron source for fusion materials research

The materials engineering database relevant to fusion irradiation is poorly populated and it has long been recognized that a fusion spectrum neutron source will be required, the facility IFMIF being the present proposal. Re-evaluation of the regulatory approach for the EU proposed DEMO device shows that the purpose of the source can be changed from lifetime equivalent irradiation exposure to data generation at lower levels of exposure by adopting a defence in depth strategy and regular component surveillance. This reduces the specification of the source with respect to IFMIF allowing lower risk technology solutions to be considered. A description of such a source, the Facility for Fusion Neutron Irradiation Research, FAFNIR, is presented here along with project timescales and costs.

Application of accelerator based neutron sources in fusion materials research

The materials engineering data base relevant to fusion irradiation is poorly populated and it has long been recognized that a fusion spectrum neutron source will be required, the facility IFMIF being the present proposal. Reevaluation of the regulatory approach for the EU proposed DEMO device shows that the purpose of the source can be changed from lifetime equivalent irradiation exposure to data generation at lower levels of exposure by adopting a defence in depth strategy and regular component surveillance. This reduces the specification of the source with respect to IFMIF allowing lower risk technology solutions to be considered. A description of such a source, FAFNIR, is presented here along with project timescales and costs.

Towards understanding the development of grain boundary clusters in austenitic stainless steel

The development of global microstructure characteristics has been compared to the local distribution and extent of Σ3 and Σ3n (1 n 3) grain boundary clusters as a function of thermo-mechanical processing in Type 304 stainless steel. A cold reduction of 5% produced GBE modified microstructures on annealing at 1050°C, containing almost one order of magnitude longer maximum cluster lengths than the corresponding annealing treatments for a reduction of 15%. Differences in the development of the distributions of cluster length scales were observed between the thermo-mechanical treatments. A re-conversion of the longest cluster obtained after GBE processing was observed with long annealing times. The local distribution of Σ3n boundary clusters was assessed, and regions with a low density of clusters are indicative of the onset of GBE conversion of microstructure.

Nanostructures of carbon in nuclear graphite

A variety of nanostructures of carbon in nuclear graphite were revealed by transmission electron microscopy (TEM) and high resolution TEM. Theses nanostructures include nanosized graphite particles, quinoline insoluble particles, a chaotic structure and a non-graphitizable structure of carbon. The basic structure of these nanostructures was observed to be nanosized packets of graphitic sheets or nanosized graphene.

Studying SiC/SiC Composites by X-Ray Tomography

Continuous SiC fiber reinforced SiC matrix composites (SiC/SiC) have been studied and developed for high temperature applications and nuclear applications. In this study, SiC/SiC composites were fabricated via polymer impregnation and pyrolysis (PIP) process and studied by X-ray tomography. The SiC/SiC composites were first scanned using a Metris X-tek 320 kV source at the Henry Moseley X-ray Imaging Facility at the University of Manchester, the closed porosities were investigated after three dimensional (3D) imaging of the samples. Furthermore, high-resolution synchrotron X-ray tomography was applied to the SiC/SiC composite at Diamond Light Source. Digital volume correlation was employed for Hertzian indentation testing of the SiC/SiC composite, quantifying damage by measurement of the displacement fields within the material. A Cellular Automata integrated with Finite Elements (CAFE) method was developed to account for the effect of microstructure on the fracture behavior of the SiC/SiC composite. Graded microstructures, textures and multiple phases were simulated and a mesh-free framework was developed to compute the damage development through the microstructure. The results indicated that we could study the development of discontinuous cracking and damage coalescence, and its sensitivity to microstructure with this method.

Crack Propagation Resistance and Damage Mechanisms in Nuclear Graphite

Sharply rising resistance to crack propagation in polygranular nuclear graphite, immediately following crack initiation from a stress concentrator has been suggested to be due to bridging by filler particles in the wake of the crack. In other brittle materials in which such rising resistance (R-curve) is observed, the length of the bridging zone is linked to the crack extension over which rising resistance occurs. This is termed Stage I of the R-curve. Following this stage II of the Rcurve, a plateau, is commonly observed (Fig. 1).