Characterization of High-Purity Nickel Single Crystals by Mechanical Spectroscopy before Irradiation Creep Tests

Mechanical spectroscopy tests of high-purity nickel single crystal with different lattice orientations (100), (110) and (111), were performed in a forced oscillation pendulum, under high vacuum (10 −6 – 10 −5 mbar), at different frequencies. The temperature was varied from room temperature up to 500 °C. The present study focuses on the understanding of the dislocation dynamics responsible for deformation, and their kinetics. The effect of the different crystal lattice orientation is also studied. A general overview of the internal friction (IF) spectrum reveals three mechanical loss peaks, for all the three orientations, namely P0 (transient peak), P1 and P2. P1 an P2 might be related to a motion of dislocations controlled by the migration of jogs of screw and edge type. Activation energies of around 1.5 – 2 eV were found for both the P1 and P2 peaks. These activation energies are comparable to pipe diffusion (1.94 eV) and grain boundary diffusion (1.3 – 2 eV) in nickel. Dislocation pinned in jogs might act as grain boundaries, enhancing the diffusion of vacancy (lower activation energy compared to Ni self-diffusion (2.88 eV)). TEM analyses were performed in parallel and confirmed the presence of dislocation jogs and the absence of sub-grain boundaries and twins. The experiments have demonstrated that mechanical spectroscopy is a powerful tool to better understand the basic dynamics of dislocations in nickel single crystals.