Molecular Dynamics as a Tool to Interpret Macroscopic Amorphization-Induced Swelling in Silicon Carbide
Résumé
The evolution of structural and mechanical properties of materials in high-radiation environment is a significant issue in nuclear applications. In particular, irradiation-induced swelling has important consequences which can affect considerably the material performance. In this paper irradiation-induced swelling of SiC is investigated using classical molecular dynamics (CMD) simulations. Heavy ion irradiation has been assumed to affect the material in two steps (a) creation of local atomic disorder, (b) induced swelling. To mimic this irradiation extended amorphous areas with various sizes and shapes were first introduced in a crystalline SiC sample at constant volume. The resulting configurations were then relaxed using molecular dynamics at constant pressure. The induced swelling has been determined as a function of the amorphous fraction introduced. Two different definitions of the amorphous fraction were introduced to enable meaningful comparisons of our calculations with experiments and elastic modeling. One definition based on the displacements relative to the ideal lattice positions was used to compare the CMD results with data from experiments combining ion implantations and channeled Rutherford Backscattering analyses. A second definition based on atomic coordination was used to compare the CMD results to those yielded by an simplified elastic model. --The simulation results using the lattice-based definition of the amorphous fraction compare very well with the experimental results. This proves that the modeling in two steps chosen for the creation of the amorphous regions is reasonable. Moreover, the results show very clearly that SiC swelling does not scale linearly with the amorphous fraction introduced. Two swelling regimes are observed relatively to the size of the amorphous area. Comparison of the elastic model with the CMD results using the coordination-based definition of the amorphous fraction has also enabled us to shed light on the swelling mechanisms and has shown that amorphization-induced swelling exhibits an elastic behavior.--The results yielded by the use of two different definitions for the amorphous fraction introduced underlines the crucial importance of the definition of the amorphous state at the atomic scale. This definition must be precise and adapted to the phenomena investigated. Furthermore, scalings for the swelling as a function of the two amorphous fractions considered, which can be used by larger scale models, have been determined. Finally, our study shows that classical molecular dynamics calculations enable one to connect the results of the available experiments with the elastic calculations and to get further insight into the swelling mechanisms.