Synthetic polygranular graphites have a wide range of current and potential applications. Indeed, some are being considered as candidate moderator materials for the next generation of nuclear power plants, which are designed to operate at temperatures as high as 1000 °C. Detailed experimental work has been performed to investigate the electrical resistivity and mechanical behaviour of a synthetic graphite over a range of temperatures. An electrothermomechanical testing rig has been employed to conduct tensile tests to evaluate elastic modulus and fracture strength over the temperature range from room temperature to 700 °C for a near-isotropic synthetic polygranular graphite. A series of mm length scale ‘dogbone’ geometry specimens, containing varying levels of porosity between 8% and 18%, were used for testing. Acquired data revealed an average of 30% gain in material stiffness, occurring at temperatures above 400 °C. Tensile strength decreased linearly with increasing porosity at higher temperatures (700 °C). The accumulated percentage reduction of resistivity during the transition from room temperature to 700 °C reduced linearly with increasing amounts of porosity. The resistivity and mechanical property measurements are discussed, with particular attention given to the porosity of the synthetic graphite.

The contiguity of a hardmetal is a measure of the proportion of the carbide grain boundaries that are in direct contact with other carbide grain boundaries. Recent analysis of data available in the literature shows a large scatter in results and a significant difference in values measured from scanning electron microscope (SEM) images and from electron backscatter diffraction (EBSD) mapping. An interlaboratory exercise has been carried out with the measurement of a range of WC-Co hardmetal grades. For each grade, SEM images were acquired from both an etched surface and an ion beam polished surface and EBSD maps with two different processing routes. These maps and images were provided to the participants for measurement to eliminate variability from sample preparation and image acquisition. It was shown that measurement of contiguity from EBSD maps is likely to lead to an overestimation of contiguity, largely because EBSD maps do not have the resolution of SEM images to identify small binder phase regions between WC grains. Ion beam polishing combined with backscattered electron imaging was found to provide the best images of the microstructure to underpin a confident measurement of contiguity. However, high resolution SEM images of etched surfaces gave values close to those from ion beam polished samples so it is recommended that, as etching is much more widely available, high-resolution imaging of a lightly etched WC surface should be promoted as the preferred method for measurement of contiguity, in combination with backscattered imaging where possible. Even with good images, variation between operators can give uncertainties of approximately ±10%.

International audienceThis document outlines good measurement practice to be adopted for the determination of flow stress at high temperature in torsion tests on solid metallic materials. The recommendations are applicable to hot (isothermal) tests at medium to high rates of Strain (10(-3)-10(2) s(-1)) at temperatures relevant to metal working practice. Guidance is provided on appropriate testpiece geometries and methods of verifying the temperature distribution along the length of the testpiece. Flow diagrams are given showing all the steps that are necessary, including the correction factors that need to be applied for deformational heating and for the determination of true stress. Details are given of the calibration procedures that should be followed to provide traceability to the National Measurement System. Technical input to the document has been provided by the steering group of a European SMT funded project on High Temperature Testing, TESTIFY, that contained representatives of industrial users and producers of a wide range of engineering materials as well as academic modellers. An experimental programme was conducted during the preparation of this document to underpin the procedures in this guide

Good measurement infrastructure is vital for providing essential data for process model development that is used to provide better control during production. Recently a consortium of research technology organisations within the UK have developed “good practice guides” for hot deformation tests, such as axisymmetric compression, plane strain compression and solid torsion. Taking this activity forward a key objective now is to develop a “Good Practice Guide” for measuring the local microstructures in terms of grain and phase size, shape, spatial distribution and volume fraction of second phase (or recrystallised fraction). Measurement issues are being addressed with new characterisation methods, for example, orientation and hardness mapping, predominantly in these representative hot deformation compression tests.