Void formation during annealing of irradiated molybdenum

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Abstract
Electron microscope observations are presented to show that voids can be produced in molybdenum neutron-irradiated at low temperature (60°c) by post-irradiation annealing up to 900°c. These results contrast with previous extensive work on molybdenum subjected to similar combinations of irradiation and annealing conditions and in which only dislocation networks and loops have been observed. Some of the conditions required for nucleation of voids are discussed and it is suggested that a critical step in void formation during post-irradiation annealing is the simultaneous migration and association of vacancies and gaseous impurity atoms. Thus the presence of gaseous impurities in solution in the specimen is essential. A simple experiment to test the hypothesis is proposed. Some results on accelerator-irradiated molybdenum are also given which show that voids or cavities are produced under similar conditions to those used in the neutron irradiation experiments, i.e. low-temperature irradiation followed by high-temperature annealing.