IN SITU NANO-INDENTATION OF AU CRYSTALS IMAGED BY BRAGG COHERENT X-RAY DIFFRACTION

摘要

The mechanical properties of micro- and nanostructures were demonstrated to vary significantly from their bulk counterparts. Despite numerous studies, plasticity at the nanoscale is, however, not fully understood yet. In situ experiments are perfectly suited for the fundamental understanding of the onset of dislocation nucleation. Recently, we developed a scanning force microscope (SFINX) which is compatible with 3rd generation synchrotron beamlines allowing for in situ nano-mechanical tests in combination with nano-focused X-ray diffraction [1] such as coherent X-ray diffraction imaging (CDI). This novel lensless imaging method retrieves the sample scattering function from a coherent X-ray diffraction data set using computational inversion algorithms, thus determining the phase of the scattered amplitude, which is not directly measured by a detector. In Bragg condition, the retrieved phase is directly related to the displacement field and, hence to the strain within a crystal. Our previous BCDI studies on indented Au crystals demonstrated the capability to imaging a single prismatic loop induced by nano-indentation and trapped inside the crystal [2]. Since any movement of diffractometer motors may induce vibrations that eventually lead to damaging the nano-crystal under load, ordinary rocking scans are not suitable for recording 3D reciprocal space maps in situ. Scanning the energy of the incident X-ray beam instead allows for probing the intensity distribution in reciprocal space without any detrimental vibrations [2]. Here, we report about the in situ nano-indentation of Au crystals with and without containing a twin boundary parallel to the crystal-substrate interface where the evolution of both strain and defects was imaged by multi-wavelength (mw) BCDI. Figure 1(a) shows the electron densities for two parts of a twinned Au nanocrystal reconstructed from mw-BCDPs measured at the Au 200 Bragg peaks. The phase, which is directly related to the displacement field inside the structure, is presented in Fig. 1(b) for a gold crystal during nano-indentation allowing for following the evolution of the morphology, the strain field, and dislocations. With increasing applied mechanical load, defects, probably prismatic dislocation loops, appear at about half-height of the indented crystal, which disappear after unloading [4]. To the best of our knowledge, this is the first time that mw-BCDI has been successfully employed during in situ experiments providing direct insight into the plasticity at the nanoscale and, in particular, the onset of defect nucleation.