Mobility of interstitial clusters in HCP zirconium

Clusters of self-interstitial atoms (SIAs) formed in displacement cascades in metals irradiated with energetic particles play on important role in microstructure evolution under irradiation. They have been studied in the fcc and bcc metals by atomic scale computer simulation and in this paper we present the first results of a similar study of SIA clusters in an hcp crystal. Clusters of 4 to 30 SIAs were modelled over a wide temperature range using molecular dynamics and a many-body Finnis-Sinclair type interatomic potential for Zr. The results show a qualitative similarity of the dynamic properties of clusters to those for cubic metals. In particular, all clusters larger than 4 SIAs exhibit fast thermally-activated one-dimensional glide, which is in a <112̄0> direction in the hcp case. Due to the crystallographic features of the hcp lattice, this mechanism leads to two-dimensional mass transport in basal planes. Smaller clusters (≤ 4 SIAs) exhibit behaviour peculiar to the hcp structure, however, for they can migrate two-dimensionally in the basal plane. The jump frequency, activation energy and correlation factors of clusters have been estimated and comparison drawn between the behaviour of SIA clusters in different structures.