Self-interstitial Diffusion in a-Zirconium
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Self-LQWHUVWLWLDO'LIIXVLRQLQ -Zirconium W.J. Zhu, C.H. Woo @, Hanchen Huang Department of Mechanical Engineering, The Hong Kong Polytechnic University, Kowloon, Hong Kong
ABSTRACT Self-LQWHUVWLWLDOVGLIIXVLRQLQ -Zr is studied using Molecular Dynamic (MD) and molecular static (MS) simulation using Ackland’s many-body inter-atomic potential. The basal crowdion configuration is found to be the ground state. The diffusion process in Zr is complex. Four types of diffusion jumps can be identified, two in-plane and two out-of plane. The in-plane migration mechanism is dominated by one-dimensional crowdion motion along the 1120 directions, interrupted by occasional out-of-plane and on-line or off-line jumps. The mean lifetime before rotation of the crowdion is reported as a function of temperature. The activation energies for the diffusion processes are obtained. The diffusional anisotropy factor Dc/Da is also obtained, and compares well with experiment results. INTRODUCTION Zirconium and its alloys are important due to their extensive application in water-cooled and water-moderated reactors. Because of the anisotropic crystallographic structure of Zr, which is hexagonal close pack (HCP), irradiation damage accumulation in Zr produces effects that are different from those in cubic metals. Thus, in addition to void swelling and irradiation creep, the anisotropy of the evolving dislocation structure produces a deviatoric straining even in the absence of an external stress. Indeed, irradiation growth is the name given to the volumeconserved shape deformation that occurs in non-cubic crystalline materials under irradiation in the absence of an applied stress. Besides zirconium and its alloys [1-4], examples of irradiation growth are also found in graphite [5-8], and uranium [9]. To take into account the anisotropy of the diffusion of point defects in the kinetics of their reaction with other crystal defects, Woo and Goesele [10] generalized the rate theory for application to HCP metals like Zr, using the reaction kinetic theory of anisotropically diffusing reactants. The difference in diffusional anisotropy (DAD) [11,12] between the vacancies and interstitials was found to produce a large bias in their reaction rates with sinks. The resulting large variability of the biases for sinks adds a new dimension to the complexity of irradiation damage behaviour of the HCPs. It has been shown that DAD explains, qualitatively or semiqualitatively, many of the anomalous behaviour of irradiation damage accumulation in zirconium [10-14]. A major difficulty in modeling irradiation damage in Zr is the lack of conclusive information on the anisotropy of the migration of both the vacancies and the interstitials [15]. Much work has focused on the configuration and associated energies of the self-interstitials. Early work by Fuse [16] and Monti [17-18] used central pair potential in their investigations of the intrinsic defect configuration and their diffusion. Since pair potential is not considered to be adequate in describing the HCPs
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