In situ Bragg coherent x-ray diffraction imaging of nanoscale strain evolution
Keywords: in situ measurments
high resolution
lattice strain
crystal orientation

The properties of crystalline materials are dictated by their lattice structure. The periodic arrangement of a perfect lattice is often impossible to achieve. Lattice defects such as dislocations induce lattice strains that can alter the behaviour of materials in certain environments. These defects could potentially be engineered to optimise desirable material properties if the relationship between these defects and material properties is well understood.
The study of how defects influence material properties requires the ability to image them during dynamic processes. A promising technique is Bragg coherent X-ray diffraction imaging (BCDI), which allows for the high resolution probing of 3D lattice strain in sub-micron volumes, making it an ideal technique to study crystal defects. BCDI leverages the coherence properties of X-rays to produce a 3D coherent X-ray diffraction pattern (CXDP) in the Fraunhofer regime for a specific Bragg reflection. This pattern can be inverted using phase retrieval algorithms to reconstruct the crystal morphology and phase, which provides direct information about internal atomic displacements along the scattering vector. Importantly, BCDI can be performed under ambient conditions, allowing for a variety of sample environments to be explored to study lattice strain evolution.
This thesis reports the study of defects and their evolution in annealing and corrosion using in situ BCDI. Prior to the in situ work, some challenges in the BCDI measurements are addressed. The first is to correctly transform data from the detector coordinate frame in which data is acquired to an orthogonal sample frame. This allows for the correct interpretation of the result in the sample frame for multi-reflection BCDI measurements. While this was done for one BCDI instrument, the framework is completely general and coordinate transformations for any other instrument can be easily carried out. To measure CXDPs of multiple reflections from the same crystal, detailed knowledge of the crystal orientation is essential. This is not generally known a priori. Here, EBSD is used to produce an accurate orientation matrix for BCDI alignment, allowing the orientation to be determined without the use of dedicated synchrotron time or beamlines. Finally, a refined strain computation approach for multi-reflection BCDI is also presented. By using a complex exponential to calculate the strain, a more faithful reconstruction is obtained by interpolating and finding phase derivatives in data with phase jumps.
Using these refinements, in situ BCDI studies are carried out. In situ BCDI annealing of focused-ion beam (FIB) damaged Au microcrystals was performed to study the annihilation of defects through thermal diffusion. A change in morphology, average lattice strain, displacement field, and facet areas is observed throughout the annealing cycle, and suggest that self-diffusion of Au is the primary cause for the evolution of lattice structure, rather than Ga impurities caused by FIB milling. Furthermore, the study of separate reflections shows quite different behaviour depending on which reflection is analysed. These new insights open up opportunities for managing and removing FIB and more generally irradiation damage at the nanoscale.
In situ BCDI corrosion of a Co-Fe microcrystal shows how corrosion can affect the lattice strain inside a crystal. Here, the more strained surface layers of the crystal corrode first, and an increase in average tensile strain accumulates during corrosion. It also enabled the computation of the local, 3D-resolved dissolution rate. It is revealed that the preferential dissolution of facets is not necessarily dictated by localised strain, but by the geometry associated with the measurement. This opens the door to more detailed studies to identify the most critical corrosion sites and mechanisms, which is important for designing next generation materials for harsh environments.