The influence of material purity and irradiation temperature on self-ion damage in molybdenum

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Abstract
An electron microscope study has been made of vacancy loops formed as a result of displacement cascade collapse in molybdenum irradiated with 60 keV self-ions. The experiments were directed towards exposing two aspects of this process. Firstly, the influence of material purity was investigated by comparing the structures generated in three crystals, one being of high purity, the second containing a significantly higher level of substitutional impurities and the third doped specifically with nitrogen. The results demonstrated that nitrogen in solution was the most effective in reducing both the number of visible vacancy loops formed by cascade collapse and the fraction of vacancies surviving in the cascades to form loops. The second major aspect studied was the influence of irradiation temperature on vacancy loop formation in the high purity crystal. In this case the number of visible loops decreased sharply with temperature over the range studied, i.e. up to 535°C. The decrease is shown to follow a different form to that observed in copper and possible mechanisms to account for this are considered. Comparisons of the experimentally observed images in the high purity crystal with those predicted theoretically for pure edge and non-edge loops showed that all of the loops are perfect with b = ½〈111〉. Moreover, the loops are mostly non-edge, lying on {110} planes in the room temperature irradiated foils, but they rotate towards the pure edge configuration as irradiation temperature increases.