Atomistic simulations of temperature and direction dependent threshold displacement energies in [formula omitted]- and [formula omitted]-uranium

Abstract

We performed a systematic study of the threshold displacement energy (Ed) in metallic uranium as a function of both the recoil direction and temperature using Molecular Dynamics simulations. We developed a novel orientation sampling scheme that utilizes crystallographic symmetrical geodesic grids to select directions from the orientation fundamental zone to study the directional dependency. Additionally, we studied the temperature dependency by considering both the α-uranium phase, corresponding to the ground state for temperatures ranging from 0 K to 600 K, and the γ-uranium phase, corresponding to high-temperature state for temperatures above 900 K. In this study, we compared several definitions of the threshold energy: a direction-specific threshold displacement energy (Edθ,ϕ), an angle-averaged threshold energy (Edave), a production probability threshold displacement energy (Edpp), and a defect count threshold displacement energy (Eddc). The direction-specific threshold displacement energies showed large angular anisotropy and variations in accordance with crystallographic considerations. Specifically, preferred defect channeling directions were observed in the [120],[12¯0],[11¯1] directions for the α-uranium, and [001],[111] directions for the γ-uranium. The production probability threshold displacement energy (Edpp) is calculated as approximately 99.2659 eV at 10 K (α-U), 103.4980 eV at 300 K (α-U), 76.0915 eV at 600 K (α-U), and 42.9929 eV at 900 K (γ-U). With exception of those calculated at 10 K, threshold displacement energies decrease with increasing temperature. Analyses of the stable defect structures showed that the most commonly observed interstitial configuration in α-uranium consists of a 〈010〉 dumbbell-like interstitial; while in γ-uranium no preferential defect configuration could be identified due to thermally-induced lattice instabilities at the elevated temperatures.