Anisotropy of point defect diffusion in alpha-zirconium

Abstract

Two types of intrinsic defect, i.e., vacancy and self-interstitial atom (SIA), are formed in metals during irradiation with energetic particles. The evolution of defect population leads to significant changes in microstructure and causes a number of radiation-induced property changes. Some phenomena, such as radiation growth of anisotropic materials, are due to anisotropy in the atomic mass transport by point defects. Detailed information on atomic-scale mechanisms is, therefore, necessary to understand such phenomena. In this article, we present results of a computer simulation study of mass transport via point defects in alpha-zirconium. The matrix of self-diffusion coefficients and activation energies for vacancy and SIA defects have been obtained, and different methods of treatment of diffusion have been tested. Molecular dynamics (MD) shows that vacancy diffusion is approximately isotropic in the temperature range studied (1050 to 1650 K), although some preference for basalplane diffusion was observed at the lower end of the range. The mechanism of interstitial diffusion changes from one-dimensional (1-D) in a ͗1120͘ direction at low temperature (Ͻ300 K) to twodimensional (2-D) in the basal plane and, then, three-dimensional (3-D) at higher temperatures.

Influence of self-interstitial mobility on damage accumulation in zirconium under fission irradiation conditions

“…On the other hand, MD simulations have also been used to study the mobility of defects in a-Zr [3]. These simulations show that single interstitials move in one dimension in the basal plane.…”

Neutron irradiation performance of Zircaloy-4 under research reactor operating conditions

“…Holt and Gilbert (1986) examined the hci type dislocation loop of annealed zircaloys under a high fluence with 8.6 Â 10 25 n/m 2 between 553 and 584 K. In these results, hci-loops were observed for annealed zircaloy-2 and zircaloy-4 at greater than 3.0 Â 10 25 n/m 2 , while hai-loops were only shown for specimens irradiated with fluence below 3.0 Â 10 25 n/m 2 . Further work by many authors confirmed that hai-loops nucleate from the beginning of the irradiation, while hci-loops form above a threshold dose only, indicating that the kinetics of the formation of hai and hci-type loops differ (Bacon, 1993;Carpenter et al, 1981Carpenter et al, , 1988Frank, 1988;Holt and Gilbert, 1986;Hood, 1988;Jostsons et al, 1977aJostsons et al, , 1977bNorthwood et al, 1979;Osetsky et al, 2002;Pasianot and Monti, 1999;Was, 2007;Woo, 1979;Woo, 1988Woo, , 2000Woo and Liu, 2009). …”

Temperature dependence of damage accumulation in α-zirconium

“…Molecular dynamics simulations of formation energies of interstitial clusters show high stability of these clusters, with binding energies on the order of 2 eV. Jump frequencies for self-interstitials were taken from the function [5,6] …”

“…The migration energy for single vacancies is 0.93 eV [5] with an attempt frequency of 1.22 · 10 14 s À1 . Further details of the parameters used are given in Refs.…”

“…Further details of the parameters used are given in Refs. [5][6][7][8]. The kinetic Monte Carlo code called BIGMAC was used and modified for these calculations [9].…”