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

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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.

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