Thomas Jourdan

  • Observation and simulation of interstitial dislocation loop coarsening in α-iron

    S. Moll, T. Jourdan, H. Lefaix-Jeuland

    CEA, DEN, Service de Recherches de Métallurgie Physique, F-91191 Gif-sur-Yvette, France

    The kinetics of particle coarsening in materials by Ostwald ripening provides valuable information on physical parameters, such as the diffusion coefficient of monomers in the matrix [1]. Under irradiation, self-defect clusters such as interstitial dislocation loops are created, which can then evolve by Ostwald ripening upon subsequent annealing. In the case of α-iron, thermal vacancies are much more easily created than interstitials by the various elements of the microstructure, owing to the high formation energy of interstitials. In general, interstitial loop coarsening can therefore occur only by vacancy emission. However, in addition to interstitial loops, vacancy clusters are also created by irradiation. These clusters emit vacancies more easily than interstitial loops, which diffuse in the matrix and make interstitial loops shrink. This phenomenon renders the observation of interstitial loop coarsening very difficult.

    In this work we implanted α-iron with 60 keV helium ions at room temperature up to a high fluence of 1016 He/cm2, in order to create bubbles and dislocation loops that can be observed by transmission electron microscopy (TEM). Additional TEM observations during in-situ isochronal annealings revealed that the mean dislocation loop radius sharply grows at around 850 K. To interpret the results, the implantation and annealing phases were modeled using cluster dynamics [2]. These simulations show that bubbles are highly pressurized after implantation, so that the vacancy emission rate by bubbles is very low. Therefore dislocation loops evolve due to a net emission of vacancies by large loops and a net absorption by small loops. As in experiments, this Ostwald ripening by vacancy emission occurs at a precise temperature in our simulations, which greatly depends on the properties of the mono-vacancy. In particular, it is seen that accounting for entropic contributions to the formation and migration free energies of the vacancy permits to significantly improve the agreement between simulations and experiments [3].

    [1] A. J. Ardell, in Phase Transformations ’87, edited by G. W. Lorimer (Institute of Metals, London, 1988), p. 485.
    [2] T. Jourdan, G. Bencteux and G. Adjanor, “Efficient simulation of kinetics of radiation induced defects: A cluster dynamics approach”, J. Nucl. Mater., 444, 298 (2014).
    [3] S. Moll, T. Jourdan and H. Lefaix-Jeuland, “Direct observation of interstitial dislocation loop coarsening in α-iron”, Phys. Rev. Lett., 111, 015503 (2013).