Nano-defects formed under electron irradiation in aluminum

Under irradiation, flows of particles (neutrons, ions, electrons) induce punctual defects (vacancies and interstitials) which generate extended defects (dislocation loops, cavities) that are responsible for materials weakening. Aluminum is a low mass material with a low displacement energy (E_d=16-19 eV) which allows the creation of extended defects directly under electron irradiation in a high resolution transmission electron microscope (HRTEM). This instrument allows not only a fine characterization of nano-defects but also to study their evolution in situ under irradiation, depending on temperature, damage rate and a partial pressure of hydrogen. We noticed an important effect of the hydrogen partial pressure on the kinetic of formation of the cavities and dislocation loops. Simultaneously, we developed a multiscale modelization of finite size effects and structure discretization on equilibrium shapes of cavities based on binding and surface energies determined by ab initio calculation of small vacancy clusters and mixed hydrogen vacancy clusters. Experimental characterization of cavity shapes according to the cavity radius reveals three regular shapes: a cross shape bounded by {100} surfaces, an octahedral non-truncated shape bounded by {111} surfaces and an octahedral truncated shape, bounded by {100} and {111} surfaces as well as the determination of a finite size effect predicted by the model. Also, measurements of cavity shape dispersion during its growth and decrease showed a dominant effect of magic numbers, fixed by geometry and frustration of the structure. Based on a thermodynamic model and a fine experimental characterization of volumic density, we propose a nucleation model of cavities under irradiation. We deduce from this model a critical cluster size and a rate of cavity nucleation depending of temperature and damage, which are compared to the corresponding experimental values extracted from in situ observations. Finally, we discuss the effect of hydrogen on the stability of vacancy-hydrogen clusters and on the kinetic of cavity nucleation. (author)