  In metals geometrically necessary dislocations (GNDs) are generated primarily
to accommodate strain gradients and they play a key role in the Bauschinger
effect, strain hardening, micron-scale size effects and fatigue. During bending
large strain gradients naturally emerge which makes this deformation mode
exceptionally suitable to study the evolution of GNDs. Here we present
bi-directional bending experiment of a Cu single crystalline microcantilever
with in situ characterisation of the dislocation microstructure in terms of
high-resolution electron backscatter diffraction (HR-EBSD). The experiments are
complemented with dislocation density modelling to provide physical
understanding of the collective dislocation phenomena. We find that dislocation
pile-ups form around the neutral zone during initial bending, however, these do
not dissolve upon reversed loading, rather they contribute to the development
of a much more complex GND dominated microstructure. This irreversible process
is analysed in detail in terms of the involved Burgers vectors and slip systems
to provide an in-depth explanation of the Bauschinger-effect and strain
hardening at this scale. We conclude that the most dominant role in this
behaviour is played by short-range dislocation interactions.
