Elastic Constants and Electron-Microscope Observations of Neutron-Irradiated Compression-Annealed Pyrolytic and Single-Crystal Graphite

Abstract

Compression-annealed pyrolytic graphite and single-crystal graphite specimens were irradiated with neutrons for total exposures of 3 to 71×1017 neutrons/cm2 (nvt) at 50°C, 3 to 8×1019 nvt at 650°C and 1.2×1020 nvt at 1000°C. The elastic constants of the materials were determined before and after irradiation by ultrasonic-, sonic-resonance-, and static-test methods. The only elastic constant that changed significantly with irradiation was the shear modulus c44 although a small change was also seen in c33. The natural crystals after irradiation had shear moduli in the range of 0.16–0.46×1011 dyn/cm2, the modulus after irradiation showing no correlation with irradiation temperature, exposure, or modulus before irradiation. The average values of the shear modulus of the pyrolytic graphite were 0.22−0.40×1011 dyn/cm2 after irradiation at 50°C, 0.030−0.033×1011 dyn/cm2 after irradiation at 650°C, and 0.042×1011 dyn/cm2 after irradiation at 1000°C; the shear modulus c44 of graphite when all dislocations are pinned is thus equal to or greater than 0.40×1011 dyn/cm2. The results can be correlated with electron-microscope observations and interpreted in terms of nucleation theory of radiation damage in analogy to the interpretation of the electron-microscope observations of W. N. Reynolds and P. A. Thrower [Phil. Mag. 12, 573 (1965)] or in terms of a difference in the availability of sinks for the displaced atoms.