THEORETICAL ASSESSMENT OF IRRADIATION SWELLING IN BERYLLIUM

Current models of irradiation swelling in beryllium are based on the a ssumption that the swelling arises from bubble growth rather than void growth. However, the factors governing cavity formation in beryllium are more complex than those in cubic metals which are based on elastic interaction difference (EID) between vacancies and self-interstitial atoms. Beryllium belongs to the set of hexagonal close-packed metals w here diffusion has been shown to be anisotropic. Diffusional anisotrop y difference (DAD) between point defects changes the cavity bias for t heir absorption and leads to dependence of the dislocation bias on the distribution of dislocations over crystallographic directions. This r esults in new critical quantities for the bubble-void transition that controls transition from low-dose to high-dose irradiation effects. In the present paper, we show how the critical parameters controlling ir radiation swelling in beryllium depend on the distribution of dislocat ions over crystallographic directions taking into account an anomalous ly high anisotropy of self-diffusion in beryllium. The latter is shown to be in agreement with experimentally observed resistance of berylli um to void swelling at early stages of irradiation when a-type disloca tions prevail over c-type dislocations. However, a mechanism of radiat ion-induced production of c-type dislocation loops is proposed that ca n lead to a subsequent transition of bubbles to voids.