Mechanically Activated State of Nanograins during and Just after In-Situ Deformation Using Synchrotron Radiation

Use of a monochromatic high energy synchrotron x-ray beam allowed the acquisition of x-ray diffraction spectra for nanocrystalline Ni and Cu powders during deformation inside steel jackets in transmission geometry across 2 mm of steel. Good statistics were obtained during 0.2-0.5 seconds and led to two major findings: Firstly, a large elastic (reversible) component contributing 0.2 deg to FWHM indicates the existence of a mechanically activated state of the nanograins during which dislocations, stacking faults and probably locally increased concentrations of vacancies occur under stress. It is shown that with stress release, these defects are expected to relax to the grain boundaries in times much shorter than the acquisition time per frame of 0.5 s here. This mechanically activated state in nano-single-crystals should be representative of the structure of nanograin powders under impact in ball milling. Secondly, in the 10 seconds following the application of stress, a monotonous decrease in FWHM of nanocrystalline copper is observed. The magnitude of the FWHM loss is approx0.17 deg or approx20% of the maximum value and 30% of the initial FWHM and corresponds to thermally activated grain growth immediately after deformation induced grain-size reduction. The very rapid acquisition times seem to have detected these phenomena for the first time.