Unlimited damage accumulation in metallic materials under cascade-damage conditions

Abstract

Most experiments on neutron or heavy-ion cascade-produced irradiation of pure metals and metallic alloys demonstrate unlimited void growth as well as development of the dislocation structure. In contrast, the theory of radiation damage predicts saturation of void size at sufficiently high irradiation doses and, accordingly, termination of accumulation of interstitial-type defects. It is shown in the present paper that, under conditions of steady production of one-dimensionally (1-D) mobile clusters of self-interstitial atoms (SIAs) in displacement cascades, any one of the following three conditions can result in indefinite damage accumulation. First, if the fraction of SIAs generated in the clustered form is smaller than some finite value of the order of the dislocation bias factor. Second, if solute, impurity or transmuted atoms form atmospheres around voids and repel the SIA clusters. Third, if spatial correlations between voids and other defects, such as second-phase precipitates or dislocations, exist that provide shadowing of voids from the SIA clusters. The driving force for the development of such correlations is the same as for void lattice formation and is argued to be always present under cascade-damage conditions. It is emphasised that the mean-free path of 1-D migrating SIA clusters is typically at least an order of magnitude longer than the average distance between microstructural defects; hence, spatial correlations on the same scale should be taken into consideration. A way of developing a predictive theory is discussed. An interpretation of the steady-state swelling rate of ∼1%/displacement per atom (dpa) observed in austenitic steels is proposed.

Keywords:

• neutron irradiation
• displacement cascades
• void swelling
• self-interstitial atoms

Acknowledgements

The authors express their gratitude to Dr. B.N. Singh (Risø National Laboratory, Denmark), Dr. H. Trinkaus (Forschungscentrum Jülich, Germany), Dr. S.J. Zinkle (Oak Ridge National Laboratory, USA) and Professor D.J. Bacon (The University of Liverpool, UK) for careful reading and useful discussions of the manuscript. The research received partial funding from the EURATOM 7th framework programme (2007–2011) under grant agreement number 212175 (GetMat project) and was sponsored by a research grant from the UK Engineering and Physical Sciences Research Council and (AVB) and by the Office of Fusion Energy Sciences US Department of Energy, under contract DE-AC05-00OR22725 with UT-Battelle, LLC (SIG).