Bragg Coherent Imaging of nanoprecipitates: role of superstructure reflections
Imagerie par diffraction des rayons X cohérents de nanoprécipités: rôle des réflexions de surstructure
Résumé
Coherent precipitation of ordered phases is responsible for providing exceptional high
temperature mechanical properties in a wide range of compositionally complex alloys (CCAs).
Ordered phases are also essential to enhance the magnetic or catalytic properties of alloyed
nanoparticles. The present work aims at demonstrating the relevance of Bragg coherent
diffraction imaging (BCDI) to study bulk and thin film samples or isolated nanoparticles
containing coherent nanoprecipitates / ordered phases. Crystals of a few tens of nanometres are
modelled with realistic interatomic potentials and relaxed after introduction of coherent ordered
nanoprecipitates. Diffraction patterns from fundamental and superstructure reflections are
calculated in the kinematic approximation and used as input to retrieve the strain fields using
algorithmic inversion. We first tackle the case of single nanoprecipitates and show that the strain
field distribution from the ordered phase is retrieved very accurately. Then, we investigate the
influence of the order parameter S on the strain field retrieved from the superstructure reflections
and evidence that a very accurate strain distribution can be retrieved for partially ordered phases
with large and inhomogeneous strains. In a subsequent section, we evaluate the relevance of
BCDI for the study of systems containing many precipitates and demonstrate that the technique
is relevant for such systems. Finally, we discuss the experimental feasibility of using BCDI to
image ordered phases, in the light of the new possibilities offered by the 4 th generation
synchrotron sources.
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