High temperature in situ tests of Gilsocarbon polygranular nuclear graphite have investigated the microstructure's deformation at two length scales. At the µm-scale, in situ bending tests observed by synchrotron radiation x-ray computed micro-tomography evaluated the bulk mechanical properties of flexural strength and fracture toughness and observed crack propagation at temperatures up to 1000°C; at the atomic-scale, neutron diffraction data correlated the lattice strain with bulk stress at temperatures up to 850°C. Raman scattering observations at temperatures up to 800°C showed the change of micro-scale residual strains. Gilsocarbon graphite was found to have a higher strength and fracture toughness with increased temperature. The mechanism leading to this behaviour has been attributed to the relaxation of residual strains

Barnard, H

Gludovatz, B

Kabra, S

Kuball, M

Liu, D

Marrow, J

Ritchie, RO

ORA - Oxford University Research Archive

In situ mechanical testing of nuclear graphite at elevated temperatures: Synchrotron x-ray tomography, neutron diffraction and Raman scattering

Oxford University Research Archive

14th International Conference on Fracture (ICF 14) June 18-23, 2017, Rhodes, Greece   IN SITU MECHANCIAL TESTING OF NUCLEAR GRAPHITE AT ELEVATED TEMPERATRUES: SYNCHROTRON X-RAY TOMOGRAPHY, NEUTRON DIFFRACTION AND RAMAN SCATTERING Dong Liu1, Bernd Gludovatz2, Harold Barnard 3, Saurabh Kabra4, James Marrow1, Martin Kuball5 and Robert O. Ritchie2 1Department of Materials, University of Oxford, Oxford, UK 2Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, USA  3Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, USA 4ISIS Neutron Source, Rutherford Appleton Laboratory, Oxfordshire, UK 5Center for Device Thermography and Reliability, University of Bristol, Bristol, UK  Abstract High temperature in situ tests of Gilsocarbon polygranular nuclear graphite have investigated the microstructure’s deformation at two length scales. At the µm-scale, in situ bending tests observed by synchrotron radiation x-ray computed micro-tomography evaluated the bulk mechanical properties of flexural strength and fracture toughness and observed crack propagation at temperatures up to 1000°C; at the atomic-scale, neutron diffraction data correlated the lattice strain with bulk stress at temperatures up to 850°C. Raman scattering observations at temperatures up to 800°C showed the change of micro-scale residual strains. Gilsocarbon graphite was found to have a higher strength and fracture toughness with increased temperature. The mechanism leading to this behaviour has been attributed to the relaxation of residual strains. 1. Introduction Gilsocarbon graphite is a structural core component in the advanced gas cooled reactors (AGRs) that supply approximately 15% of UK’s electricity. The polygranular graphite components moderate the fast neutrons for efficient fission, guide the flow of coolant gases and provide channels for the movement of fuel and control rods. Graphite is also a critical material for several designs of future Gen IV reactors that will operate at much higher temperatures; these include the U-battery gas-cooled fast reactor, which is a UK designed micro-reactor (<10 MW) for distributed power generation. For a reliable evaluation of the core integrity, to predict potential fracture and to mitigate premature failure of the graphite components, the mechanical response of graphite should be evaluated at service-relevant temperatures. In situ 3-D tomography can be used to investigate the stress/strain behaviour [1], but work up to date has been performed at ambient temperatures and cannot adequately describe the mechanical behaviour and damage evolution in graphite at realistic temperatures (~650°C for current operating reactors; ~1,000°C outlet temperature for Gen IV reactors). The Gilsocarbon graphite comprises a matrix with coarse filler particles and has porosity ranging in size from nano-scale to several hundred micrometres. In situ tests using diffraction/scattering are also needed to provide a “bridge” between the nano-scale characteristics of graphite, as exemplified by the lattice strains in its network structure of fine crystals, and the macro-scale properties of the heterogeneous microstructure. This allows quantification, under applied load, of the relationships between the elastic strains (i.e., stress) and inelastic deformation (i.e., total strains) [1].  2. Experimental set-up and results 2.1 In situ x-ray tomography   A unique in situ ultrahigh temperature tomography rig that permits real-time investigation of damage evolution under load at temperatures up to 2,000°C was adopted [2]. Gilsocarbon specimens, nominally 4x4x20 mm, were loaded in a three-point bending configuration (roller span 16 mm). Both plain and notched specimens were studied for strength and fracture toughness at room temperature (RT), 650°C and 1,000°C, respectively. X-ray tomography scans (25 keV, field of view 4x4 mm and voxel resolution 3.25 µm) was performed as the specimens were incrementally loaded to failure, to reveal for the first time the three-dimensional deformation and fracture of nuclear graphite at temperature. Example images (Fig. 1a) demonstrate several extrinsic toughening mechanisms at 1,000°C including filler particle deflection and crack tip bifurcation. In general, the strength and fracture toughness of Gilsocarbon graphite increased with temperature. 2.2 In situ neutron diffraction and Raman scattering  14th International Conference on Fracture (ICF 14) June 18-23, 2017, Rhodes, Greece   We used neutron diffraction at ENGIN-X, ISIS (Rutherford Appleton Laboratory, UK) [1] to explore the deformation of Gilsocarbon nuclear graphite. . This work is the first such investigation at elevated temperatures that are typical of reactor operation (650°C and above). ‘Brazilian’ disc Gilsocarbon graphite specimens were loaded under diametric compression, to generate a biaxial tension/compression stress state in the observed central volume. A halogen lamp-heated furnace with an Argon gas environment to prevent graphite oxidation was combined with an in situ loading cell and high temperature optical digital image correlation to measure the applied strain. Monotonic loading was applied and we studied the effect of temperature (ambient, 600°C and 850°C) on the damage that occurs as a precursor to tensile fracture, which is observed as a non-linearity in the relationship between lattice strain and tensile stress (Fig. 1b). The compressive region, where there is no microcracking damage expected, is linear.  In tensile region, it is observed that graphite can carry higher tensile strains with increasing temperatures. This is consistent with the tomography observations that the elastic stiffness and flexural strength of polygranular graphite increase with temperature. We suggest that micro-cracking accommodates the applied tensile strain at room temperature, whilst at elevated temperatures there is less micro-cracking, hence higher stresses are measured. This has implications for the accommodation of strain at stress concentrations, whose strength is affected by the interplay between damage and the criterion for unstable fracture.  High temperature Raman spectroscopy mapping (40x40 µm) of a graphite surface were undertaken in a hot cell (with Ar atmosphere) at the Centre for Device Thermography and Reliability, University of Bristol, at room temperature and at 800°C (Fig. 1c). The data reveal that the amplitude of residual strain reduces at high temperature; this could be a contributing factor to an increased resistance to microcracking at temperature that would increase the tensile strength.  Figure. 1 (a) Tomographic images showing the extrinsic toughness mechanism at 1,000°C; (b) neutron diffraction lattice strain change with the local stress at different temperatures; (c) the variation of strains in graphite at RT, measured by Raman spectroscopy mapping. 3. Conclusions The high temperature behaviour of Gilsocarbon nuclear graphite has been investigated at different length-scales by x-ray computed tomography, neutron diffraction and Raman scattering. The graphite exhibits higher flexural/tensile strength and fracture toughness at elevated temperatures, which may be due to changes in micro-cracking behaviour related to residual stress in the microstructure. Acknowledgements DL acknowledges EPSRC Research Fellowship grant (EP/N004493/1) and the Royal Commission for the Exhibition of 1851 Brunel Research Fellowship award. The authors specially acknowledge Dr Claire Acevedo, Dr Dula Parkinson, Dr Yelena Vertyagina, Mr Phil Earp, and Dr James Pomeroy for help with experiments.  Access to Engin-X was through number RB1610238. References [1] T.J. Marrow, D. Liu, S.M. Barhli, L. Saucedo Mora, Ye. Vertyagina, D.M. Collins, et al, In situ measurement of the strains within a mechanically loaded polygranular graphite, Carbon, 96, 285-302, 2016.  [2] H. Bale, A. Haboub, A. A. MacDowell, J. R. Nasiatka, R. O. Ritchie, et al. Real-time quantitative imaging of failure events in materials under load at temperatures above 1,600 °C. Nature Mater., 12, 40–46, 2013. (a) (b) (c) 

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In situ mechanical testing of nuclear graphite at elevated temperatures: Synchrotron x-ray tomography, neutron diffraction and Raman scattering

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