PRODUCTION BIAS DUE TO CLUSTERING OF POINT-DEFECTS IN IRRADIATION-INDUCED CASCADES

In the conventional rate-theory treatment of irradiation-induced dimensional changes based on the concept of dislocation bias, it is implicitly assumed that interstitials and vacancies are produced continuously and uniformly in the form of Frenkel pairs. This implies that all produced point defects are free and are available for annihilation at sinks. This approach may not be appropriate, however, for cases where the damage is produced in the form of cascades. In the cascades both vacancies and interstitials are produced in a highly segregated fashion in that the distributions of vacancies and interstitials are separated from each other in space. This spatial segregation is maintained even after the end of the thermal spike phase during which vacancies condense in the central region of the cascades whereas the high concentration of highly mobile interstitials form clusters at a certain distance from the cascade centre. The consideration of interstitial clustering within the cascade region yields a production bias. A model is presented which takes into account the formation and lifetime of vacancy and interstitial clusters in the cascades. As a first application of the model, the temperature dependence of the steady-state swelling rate is calculated. It is shown that the steady-state swelling rate in the peak-swelling temperature regime is controlled by the production bias. Predictions of the production bias model are found to be consistent with the available experimental observations. The general implications of the present theoretical approach is discussed.