  During plastic deformation of metals and alloys, dislocations self-organise
in cells, which subsequently continuously decrease in size. How and when these
processes take place has remained elusive, because observations of the
structural dynamics in the bulk have not been feasible. We here present X-ray
diffraction microscopy movies of the structural evolution during tensile
deformation of a mm-sized aluminium (111) single crystal. The formation and
subsequent development of 40,000 cells are visualised. We reveal that cells
form in a stochastic and isotropic manner already at 1% strain. We show that
the cell size and dislocation density distributions are log-normal and bi-modal
Gaussian distributions, respectively, throughout. This insight leads to an
interpretation of the formation and evolution steps in terms of universal
stochastic multiplicative processes. This work will guide dislocation dynamics
modelling, as it provides unique results on cell formation.
