This work delves into the fundamental aspects of plastic deformation in metals, focusing on dislocations - the fundamental unit that carries plastic deformation. These dislocations organise into patterns, forming walls and cells, which are essential in defining the macroscopic properties of materials. Traditionally these dislocation microstructures have been characterised using Transmission Electron Microscopy (TEM). However these studies are limited to thin foils and may not accurately represent the dynamics of bulk samples. Here, the limitations of TEM are addressed through the use of Dark Field X-ray Microscopy (DFXM) at the ESRF beamlines ID06 and ID03. 

DFXM is a diffraction-based method, analogous to dark field TEM, that offers the unique capability of visualising deeply embedded layers in a sample, creating 3D movies of the local orientation and strain in samples up to millimetre sizes. With a spatial resolution of about 100 nm, an angular resolution of 0.001 degrees, and a strain resolution of $10^{-4}$, DFXM provides detailed intragranular information. As a result, the different length scales on which dislocation pattering occurs can be simultaneously probed using a single experiment.

Here the investigation of pure aluminium single crystals during in-situ tensile deformation is presented. This includes a study of the birth, growth and refinement of 80,000 cells during tensile deformation. The cell shapes and the local dislocation densities are mapped. Using statistical physics concepts we formulate and discuss the validity of models of the underlying mechanisms, driving the patterning processes.  

Based on this example some of the potential and limitations of DFXM for multiscale modelling of hierarchical structures will be highlighted.