Neutron irradiation damage in molybdenum

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Abstract
The technique of transmission electron microscopy has been used to study neutron irradiation damage and its behaviour on annealing in molybdenum. Micro-hardness tests were carried out on all specimens before thinning to obtain some indication of the relation between structural effects and the mechanical properties. The results show that part of the damage in molybdenum neutron irradiated to a dose of 2 × 1022 n cm−2 (fission) is in the form of point defect clusters which are deduced to be interstitial. The interstitial point defects must therefore be sufficiently mobile during irradiation at 60°c to form into these clusters. However, the experimentally estimated point defect density only accounts for a small proportion of the theoretical number suggesting that most of the interstitials are present singly or as sub-microscopic clusters. On annealing there is little change in the observable damage up to 800°c but the micro-hardness shows a peak at 170°c followed by steady recovery. Above 800°c the loops grow rapidly and finally join up to form dislocation networks. This stage is accompanied by an increase in the rate of recovery of the micro-hardness. The loops are shown to be interstitial in nature with Burgers vector a/2〈111〉, though during the early stages of network formation some 〈100〉 type dislocations form through the reaction:a/2[111]+a/2[11 1]→a[100]. These results are discussed in conjunction with other more indirect recovery data and a model is proposed for the annealing of irradiation damage in molybdenum above room temperature. The general conclusion is that the initial damage structure and its subsequent behaviour on annealing are largely governed by the distribution and mobility of interstitial impurity atoms during irradiation.