The Young's modulus of carbon and graphite artefacts

Measurements of dynamic Young's modulus have been made over the temperature range −196 to 1000°C on specimens of British nuclear-grade carbon heat treated at temperatures between 1200°C and 3000°C. The results are discussed in terms of the uniform stress model for graphite, whereby the elastic modulus depends essentially on the effective C44 shear stiffness of the single crystal, on the orientation of crystallites, and on the artefact porosity. Dislocations lying in the basal plane reduce the effective C44 to values below that expected for the lattice alone. This effect is dependent on temperature because of the thermal unpinning of dislocations, and on crystallite size. It is deduced that the lattice value of C44 is 4.5 times higher for material treated at 3000°C than for that treated only at 1200°C. The rise of modulus with temperature, occurring above 300°C, is attributed to a process of void-filling determined by the difference between the volume thermal expansion coefficients of artefact and constituent crystallites. The effect of these voids is to reduce the modulus measured at room temperature by a factor of about five for material treated at 3000°C.