Atomistic Study of Helium Bubbles in Fe: Equilibrium State
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Atomistic Study of Helium Bubbles in Fe: Equilibrium State. David M. Stewart1, 2, Yury N. Osetskiy1, Roger E. Stoller1 Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6138, United States. 2 Center for Materials Processing, The University of Tennessee, Knoxville, TN 37996-0750, United States. 1
ABSTRACT In the fusion irradiation environment, helium created by transmutation will play an important role in the response of structural materials to neutron radiation damage. Recently we have developed a new 3-body potential to describe the Fe–He interaction in an Fe matrix. We have used this potential to investigate the equilibrium state of He bubbles embedded into the bcc Fe matrix. We have investigated bubble size, He content and temperature effects. It was found that the equilibrium He content is rather low and at a room temperature it is ~0.38 to 0.5 He per vacancy for bubble diameters from 1 to 6 nm. At constant bubble size, the equilibrium He/vacancy ratio decreases with temperature increase. For bubbles of 6 nm diameter it goes down as low as ~0.25 at 900K. The results are compared with the capillarity model often used for estimating the equilibrium pressure of He bubbles. INTRODUCTION Reduced-activation ferritic/martensitic (RAFM) steels are candidate materials for fusion applications. Neutron radiation damage in these materials leads to the formation of bubbles and voids. The nucleation and growth of these cavities is strongly influenced by transmutationformed helium [1]. The cavities can cause swelling and contribute to mechanical property change [2]. In order to estimate the amount of helium present from an observed size distribution, it is necessary to understand the relationships between bubble size, pressure and helium content. METHOD Molecular dynamics was used to simulate helium bubbles of diameters 1, 1.5, 2, 4 and 6 nm. They consist of helium atoms in an approximately spherical void containing 59, 169, 339, 2741 or 9577 vacancies respectively. Depending on the size of the bubble the simulated crystal size varied from 16,000 to 432,000 atoms. First, a void of the corresponding size was created and then a certain number of He atoms was homogenously distributed inside. Periodic boundaries were applied on {100} faces of the cubic crystal, and NVE dynamics was used. The lattice parameter was adjusted to zero pressure at each temperature and the velocity Verlet algorithm with a timestep of 0.3 fs was used. The ORNL Fe–He potential, the Aziz He–He potential [6] and the Ackland Fe–Fe potential [7] were the primary potentials used in this work. The ORNL potential was fitted [4,5] to DFT
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calculations of single-He defects [3] and small He–V clusters [4], and verified in refs [5,8,13]. Its implementation is discussed in more detail in ref [14]. RESULTS Equilibrium He/V ratio In a void, the surface matrix atoms relax inwards slightly. For a helium bubble, the pressure from the helium pushes them back outwards again. We define the mechanical equilibrium condition as t
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