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 swelling 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 selfinterstitial 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 and 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%/dpa observed in austenitic steels is proposed

Unlimited damage accumulation in metallic materials under cascade-damage conditions

“…It is proposed in [59] that this is the reason why the void lattice formation is observed at high void and low dislocation density, rather than due to the limitations imposed by equation (9). This also implies that other microstructure features, e.g.…”

“…in a review [58]). In an accompanying paper [59] the conditions for the onset of void ordering in metals under cascade irradiation are analysed by using a mathematical approach of Helbing and Vicsek [60] developed for the description of lane formation in pedestrian crowds. To make the present paper self-consistent, below the method is briefly outlined and the main conclusions are described.…”

“…In [59], the expressions for the void velocity and diffusion coefficient through the microstructure parameters and the basic conditions for destabilisation of homogeneous solution, , of equation (5) are derived. For this the linear stability analysis is applied.…”

“…Equation (9) predicts that, for low enough dislocation density, spatial correlations between voids should always be present, except for very early stages of irradiation, when voids are too small, of the order of ten vacancies (see figure 1 in [59]). …”

“…In the analysis presented above it is assumed that voids are correlated with each other rather than with other elements of microstructure. For a system containing voids and edge dislocations, this assumption is satisfied if the probability that an SIA cluster is generated between two voids is higher than that between a void and a dislocation, that is, if [59] . (10) …”