Spanning Time and Length Scales in Simulations of Radiation Damage
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Spanning Time and Length Scales in Simulations of Radiation Damage Blas P. Uberuaga Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87545 ABSTRACT Simulating radiation damage production and evolution in materials is a problem spanning many orders of magnitude in both time and length scales. Therefore, any one simulation method will be inadequate for studying radiation damage. We apply several modeling techniques to study radiation damage in oxides, specifically MgO, Al-doped MgO and MgAl2 O4 spinel. We use molecular dynamics to simulate damage production in collision cascades, where energies between 400 eV and 5 keV were investigated. The kinetic behavior of the resulting defects was probed on long time scales using temperature accelerated dynamics. Molecular statics was used to calculate fundamental properties of these defects, and density functional theory calculations support the basic results of the empirical potential studies. Finally, a chemical rate theory model is developed to understand the impact of the atomic scale behavior on dislocation loop size. INTRODUCTION The evolution of radiation damage in materials is a classic example of a problem that spans many time and length scales. Damage is initially produced on the atomic scale via collision cascades which occur on the picosecond time scale. However, this damage ultimately manifests itself macroscopically in the form of swelling or cracking, phenomena which can take years to develop. In between, there is a wide range of phenomena which bridge these two extremes, including defect diffusion, annihilation and aggregation, the formation of interstitial loops and voids, and the development of more complex microstructure which can lead to amorphization or swelling. As a result, there is no one simulation method that can be employed to study the problem of radiation damage in its entirety. Rather, a combination of many techniques must be used to address this problem. In this work, we use several computational modeling techniques to examine the initial generation and subsequent evolution of radiation damage in oxide ceramics on the atomic scale. However, instead of following the more standard approach of studying larger and larger systems, we focus on extending the time scales which we explore. We compliment standard molecular dynamics (MD) simulations, which are well suited for the collisional phase of a typical radiation damage-inducing event [1], with accelerated dynamics methods, in particular temperature accelerated dynamics (TAD). TAD allows us to follow the dynamics of the atomic-scale system out to much longer times than possible with MD, often to time scales of seconds. We use both molecular statics and density functional theory (DFT) to calculate fundamental properties, such as binding energy, of the defects seen in the MD and TAD simulations as well as to verify key results with higher quality descriptions of the material. Finally, we incorporate our atomic scale simulations into a chemical rate theor
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