Jean-Marc Delaye

  • Ballistic effects on structure and mechanical properties of borosilicate glasses: a molecular dynamics study
    J.-M. Delaye 1*, D. Kilymis1, L.-H. Kieu1, S. Peuget1
     
    1CEA Marcoule, DEN, Laboratoire d’Etudes des Matériaux et Procédés Actifs, 30207 Bagnols-sur-Cèze, France

    In France, the long lived radionuclides are isolated in a glassy matrix called R7T7 glass. This is a complex aluminoborosilicate glass containing more than 30 different oxides. This matrix is aimed to be stored in a deep geological repository and, in order to guarantee its long term behaviour, it is important to certify in particular its resistance to internal irradiation.

    Recently, it has been shown experimentally using complex and simplified borosilicate glasses that the ballistic damage associated to the recoil nuclei stemming from α disintegrations is responsible for the evolution of some structural characteristics (swelling, decrease of the glass polymerization, etc.) that can explain some mechanical property changes (hardness and elastic moduli decrease, fracture toughness increase) [1,2]. Moreover, it appears that the fictive temperature of a borosilicate glass submitted to ballistic damage increases. All these effects present analogies with what happens when a glass is fast quenched.

    To have a deeper insight into the atomic processes induced by recoil nuclei, we have simulated series of displacement cascades by classical molecular dynamics in several simplified nuclear glasses prepared at different quenching rates [3]. Seven initial structures (containing SiO2, B2O3 and Na2O) were prepared with quenching rates ranging from 2 1012 K/s to instantaneous quench to dispose of structures characterized by a set of initial disorder and polymerization levels. Then, each glass is subjected to a series of 100 displacement cascades (600eV) to accumulate ballistic damage and the final structures are analyzed. We find effectively analogies between the structural changes observed in the irradiated structures and those induced by accelerating the quenching rate even if some differences remain.

    To complete our understanding of the ballistic effects in borosilicate glasses, we have simulated by classical molecular dynamics in pristine and in pseudo irradiated glasses (i.e. glasses quenched quickly to induce the ballistic effects) fracture propagation and nanoindentation. The dynamical processes, in particular the plastic flows, are different between the pristine and the pseudo irradiated glasses due to differences in density and polymerization level. These modifications can explain why there is an increase of the fracture toughness and a decrease of the hardness in glasses subjected to ballistic effects, in agreement with what is observed experimentally.

    [1] S. Peuget, P.-Y. Noël, J.-L. Loubet, S. Pavan, P. Nivet, A. Chenet, “Effects of deposited nuclear and electronic energy on the hardness of R7T7-type containment glass”, Nuclear Instruments and Methods in Physics Research B, 246, 379 (2006).

    [2] J. de Bonfils, S. Peuget, G. Panczer, D. de Ligny, S. Henry, P.-Y. Noël, A. Chenet, B. Champagnon, “Effect of chemical composition on borosilicate glass behavior under irradiation”, Journal of Non-Crystalline Solids, 356, 388 (2010).

    [3] J.-M. Delaye, S. Peuget, G. Bureau, G. Calas, “Molecular dynamics simulation of radiation damage in glasses”, Journal of Non-Crystalline Solids, 357, 2763 (2011).