Concurrent Annealing Of Vacancy-Type And Interstitial-Type Damage In Neutron Irradiated Stainless Steel
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Concurrent Annealing Of Vacancy-Type And Interstitial-Type Damage In Neutron Irradiated Stainless Steel E. P. Simonen, D. J. Edwards and S. M. Bruemmer Pacific Northwest National Laboratory, P. O. BOX 999/ P8-15, Richland, WA 99352 ABSTRACT Analysis of the effect of annealing on change in void and loop size distributions provided insights that complement microstructural characterization using transmission electron microscopy (TEM). The predictions of concurrent vacancy-type and interstitial-type damage annealing are applied to measurements in neutron-irradiated austenitic stainless steels. Irradiation at 330Û& SURGXFHV YRLG DQG ORRS PLFURVWUXFWXUHV IRU ZKLFK WKH PHDVXUHG DQQHDOLQJ response is in accord with predictions. Irradiation at 275Û& SURGuces only Frank loop microstructure for which the annealing response cannot be predicted from measured microstructures. The model predictions are based on an assumed vacancy source not detected using TEM. The measured loop microstructure is typically reported to be interstitial in character but this analysis suggests that a significant component of the loop population is vacancy-type damage based on defect inventory and kinetic arguments. INTRODUCTION Irradiation of alloys results in the production of vacancy and interstitial components of microstructure. The component size distributions are affected by annealing similar to annealing of precipitate size distributions during heat treatment of alloys [1,2]. The irradiation microstructure can either coarsen or annihilate depending on the relative proportion of vacancytype versus interstitial-type damage. Post-irradiation characterization of microstructural distributions after annealing can reveal the vacancy or interstitial character of defect aggregates by observing whether coarsening or annihilation occurs. At low temperatures and low doses, significant microstructure develops at and below the resolution of TEM. This low-resolution limit makes experimental assignment of defect clusters difficult. During post-irradiation annealing, the size distributions of aggregates change in response to vacancy production by thermal emission. Comparison of measured changes with calculated changes in wellcharacterized size distributions reveals important characteristics of damage microstructure at and below the TEM resolution. MECHANISTIC APPROACH Precipitate coarsening theory describes the effect of annealing on the change in precipitate size distributions [3,4]. Conservation of solute requires an increase in average radius concurrent with a decrease in number density. The annealing of radiation-induced defect aggregates occurs by a similar process but is complicated by the fact that vacancies and interstitials can be annihilated by mutual recombination. Therefore the process of coarsening occurs in competition with annihilation of the damage. The partial differential equation that describes coarsening is •
∂(f r ) ∂f =− ∂t ∂r
(1)
R2.5.1
The distribution function, f, depends on radius, r, and time, t. The rate of change in r
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