A Molecular Dynamics Simulation Study of Defect Production in Vanadium
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length-scales from ballistic binary collisions to collective atomic motion in the thermal spike stage followed by the thermal activation process. The time scale of the radiation effects relevant to the field of nuclear materials ranges widely from 10-15 to 107 seconds, and the length scale ranges, also widely, from 10-9 to 10-3 meters. The displacement cascade [1] is of fundamental importance to microstructural changes since it plays a role as a production source of defects in irradiated materials. Moreover, debris of pointand clustered-defects produced by the cascade has recently been a subject of increased interest since it has been suggested that this debris induces a diffusion bias [2] which acts during subsequent microstructural evolution. Molecular dynamics (MD) simulations can be used to study the space and time development of the cascade process [3-8]. With our MDCASK code [9], we can study systems with up to 106 atoms on massively parallel computers. Vanadium-based alloys have significant advantages for use as structural materials in nuclear fusion devices because of their low activation and high radiation resistance. In the present study, in order to provide a first step towards the comprehensive model simulation of radiation effects in these alloys, we investigate cascade dynamics in pure vanadium at the atomic level by MD simulations with a many-body interatomic potential. First, we evaluated the displacement threshold energies and melting temperature of the model potential system. Second, we simulate the cascade process in vanadium for primary recoils with energy up to 5 keV. We discuss the production efficiencies of point- and clustered-defects produced by cascades in vanadium.
39 Mat. Res. Soc. Symp. Proc. Vol. 396 © 1996 Materials Research Society
Table 1 Comparison of EAM-type many body potentials for vanadium. The activation energies of vacancy formation, E v , vacancy migration, E vm and self diffusion, Qv, and the fcc-bcc energy difference are listed in unit of electron volts. vf Adams & Foiles Johnson & Oh Finnis & Sinclair
2.22 2.02 1.83
Ouyang & Zhang Wang & Boercker
1.58
experiment ab-initio LMTO
2.1
Qv
Efcc - E bcc
Ref.
0.98 0.71 0.72 0.76
3.20 2.73 2.55 2.58
0.035 0.19 0.14 0.046 0.17
[10] [11] [12] [13] [14] [15]
0.5
2.9 0.30
[16] [17]
E
MOLECULAR DYNAMICS MODEL All the simulations in the present study were performed with the molecular dynamics code, MDCASK [91 which runs efficiently on the Cray-2 supercomputer and T3D massively parallel computer at Lawrence Livermore National Laboratory. In order to evaluate existing interatomic potentials for vanadium, we compared the defect properties and fcc-bcc energy difference of several published potentials [10-15] to the experimental data [ 161 and ab-initio LMTO calculation results [17], respectively, as shown in Table 1. Based on the results, we use the Johnson and Oh EAM-potential [11] for our simulations. Since this EAM potential was, however, developed by fitting to just near equilibrium behavior of vanadium, we modified
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