Dr. Maxime Dupraz (Paul Scherrer Institute, Switzerland)

Characterization of the Microstructure of Small Crystals using Coherent X-ray Diffraction and Atomistic Simulations
When Jan 16, 2017
from 02:00 PM to 03:00 PM
Where LR8
Contact Name
Contact Phone 01865-283446
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Physical properties in small length scale objects, typically below the micrometer, deviate strongly from their bulk
counterpart. For instance, mechanical strength increases with reducing size and large residual strain due to
processing are present in nanostructures. Thus a better understanding of the physical properties in relationship with the
microstructure is needed for sub-micrometer materials. Coherent X-ray Diffraction (CXD) is an emerging synchrotron
technique highly sensitive to strain fields and structural defects that allows the detailed measurement of the crystal
structure, including strain field and defects, of micro/nano-objects.
The calculation of CXD patterns from atomic structures modelled with interatomic potentials allows to evidence the
unique character of the signature induced by a single crystal defect at the vicinity of a Bragg reflection. The study of
CXD patterns at several chosen Bragg reflections enables to determine all the defect characteristics (edge or screw type,
slip plane, dissociation in partials, position in the particle, …). This approach is illustrated on th e case of a gold
nanocrystal undergoing simulated indentation. It is shown that the defects generated in the early stages of indentation
can in principle be identified, provided that their number remains low. The interpretation of CXD patterns from more
complex systems with multiple defects nucleated on several slip planes proves to be far more challenging. In this case,
the reconstruction of the displacement field of the sample provides a more comprehensive picture.
To characterize the first stages of plastic deformation in a realistic structure, the in-situ nanoindentation of a submicrometer
gold island grown on a sapphire substrate is carried out in a synchrotron facility. The in-situ loading is
performed by a recently developed compact scanning force microscope for in situ nanofocused X-ray diffraction studies
(SFINX), designed to be mounted on a diffractometer, while the particle is illuminated in Bragg conditions with a
coherent X-ray beam. Using 3D reconstructions at different stages of the mechanical loading, nucleation of a prismatic
dislocation loop is clearly identified. In addition, a strain relaxation equivalent to a “mechanical annealing” process is
observed during the successive indentation steps, as well as an evolution of the particle shape and strain distribution
after 24h ageing under the X-ray beam.