The room‐temperature internal friction of pure (99.989%) annealed polycrystalline aluminum has been studied as a function of fatigue and neutron irradiation. Fatigue was found to reduce the internal friction of unirradiated aluminum by more than 50% within the first 1% of the fatigue life. This effect is attributed to rapid strain hardening in the pure aluminum. Neutron irradiation causes a large reduction in internal friction, presumably because of dislocation pinning by irradiation‐generated vacancies. It appears that these are multiple vacancies or vacancy clusters that are relatively stable at room temperature. Fatigue causes the internal friction of irradiated aluminum to converge to the same value as unirradiated aluminum near the fatigue limit. This result indicates that irradiation‐generated vacancies may be readily absorbed by moving dislocations and explains the observation that the fatigue strength of aluminum is relatively unchanged by irradiation.
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