Mean Bubble Radius Research Articles

The behaviour of radiation-induced gas in irradiated aluminium-lithium alloys

The behaviour of radiation-induced gas in neutronirradiated aluminium-lithium alloys was investigated in the temperature range 200–600 °C. It was found that gas bubbles nucleate homogeneously during neutron irradiation at 75 °C. These gas bubbles grow by a migration and coalescence mechanism under the influence of a driving force provided by a non-equilibrium of vacancies. Experimental evidence has been obtained for the conservation of lattice strain energy during the growth process so that the internal pressure, P, of the bubble is balanced by the surface tension forces γ such that P = 2 γ/ r. The time dependence of the growth process is highly temperature-dependent, varying from r ∝ t 0.97 at 600 °C to r ∝ t 0.02 at 200 °C, where r is the mean bubble radius and t the annealing time. Small bubbles were observed to persist after annealing at temperatures of 500 °C and below, indicating that not all bubbles take part in the growth process at these temperatures and gas resolution is not an operative process. Under conditions of high vacancy concentration, the bubbles developed major (111) type faces and minor (100) type faces and denuded zones surrounded grain boundaries.

Calculated Size Distributions for Gas Bubble Migration and Coalescence in Solids

Bubble coalescence in solids is assumed to result when collisions occur between bubbles as they migrate by surface diffusion through the solid. The migration of an isolated pore is examined in detail and the results are applied in a finite-difference approach, programmed for a digital computer, to predict the variation of the number of bubbles per unit volume as a function both of bubble size and of post-irradiation annealing time. Two idealized models of bubble coalescence have been treated. The first model considers coalescence resulting from random migration of the bubbles, and the second considers coalescence resulting from unidirectional, biased migration, such as would result in the presence of a thermal gradient. Both models are based on surface diffusion migration of randomly distributed bubbles in a perfect, infinite solid. The results give the predicted variation with time of the entire distribution of bubble sizes, so that various parameters, including the mean bubble radius and volume change, can be predicted. Comparison of the predicted values to parameters measured in appropriate experiments could provide a means of measuring fundamental properties of materials. The results can also be applied to the problem of swelling of reactor fuel materials because of trapped fission gases. Although the present analysis is limited by a number of simplifying assumptions, it is clear that the presence of a thermal gradient can greatly enhance the swelling rate.