Thermal behavior of radiation damage cascades via the binary collision approximation: Comparison with molecular dynamics
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One of the problems involved in the design of a fusion reactor and in predicting the lifetime of conventional fission reactors is the degradation of the engineering properties of structural components under irradiation.1 The understanding of the microscopic mechanisms of damage is also relevant in the field of ion implantation.2 The starting point of the damage, the displacement cascade, cannot be treated analytically because it drives the material into a highly nonequilibrium state, where many-body interactions, electronphonon coupling, thermodynamic and kinetics effects simultaneously contribute and compete. Thus, a complete theoretical model is not yet available; computer simulations are the main method for the analysis of the initial stages of radiation damage, over a time scale that is inaccessible to experimentation. Some of the most relevant parameters for the prediction of the material's behavior under irradiation are the number of freely migrating defects (FMD); their tendency to cluster, forming voids, dislocation loops, or stacking faults; the rate of atomic mixing; and the interaction with the impurities created by nuclear transmutations. All of them depend on the interaction between ions within the crystalline lattice, and therefore a full many-body ion-ion interaction calculation in the computer simulation is strictly necessary. This is satisfactorily achieved in the molecular dynamics simulations (MD),3 where the space-time trajectories of a large ensemble of atoms is constructed by solving the classical equations of motion generated by a model interaction potential. However, present-day computational restrictions impose severe limitations in the number of ions that can be followed, typically a hundred thousand, and therefore only low-energy cascades, up to 5 KeV, can be simulated.4"7 a)Fellow
of Consejo Nacional de Investigaciones Cientificas y Tecnicas, Argentina.
2652
http://journals.cambridge.org
J. Mater. Res., Vol. 5, No. 11, Nov 1990
Downloaded: 14 Mar 2015
MD simulations have been very successful in giving a clear representation of the different stages occurring in a cascade: the collisional phase, the thermal spike, the cascade quenching, and collapse. Quantitative information related to the above-mentioned parameters has also been obtained. One of the most important results of MD simulations is the prediction of the cascade core melting in Cu and Ni.4"7 This result is obtained by comparison between (a) pair correlation functions calculated in the liquid and in the cascade, (b) the diffusion coefficient calculated in the core of the cascade and experimental values for the liquid, and (c) atomic densities in the cascade core and experimental value for the liquid. This result came from MD simulations on Cu and Ni; therefore it cannot be concluded that cascade cores melt in all materials, and also that there is not direct experimental verification of melting. In this work we assume that the melting picture for Cu and Ni is correct and apply simple thermodynamics to predict the behavior of
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