  (Abridged, full abstract available in the text)
  The main goal of the study presented in this thesis was to perform in-situ
investigations on deformation structures in plastically deformed
polycrystalline copper at low degrees (<5%) of tensile deformation. Copper is
taken as a model system for pure fcc metals.
  A novel synchrotron-radiation based technique has been developed. The
technique allows for in-situ 3D reciprocal space mapping of reflections from
deeply-embedded individual grains in polycrystalline samples during tensile
deformation. We have shown that the resulting 3D maps from copper, consists of
bright sharp peaks superimposed on a cloud of enhanced intensity. It is
concluded that the individual peaks arise from the individual dislocation-free
regions (the subgrains) in the dislocation structure.
  Based on data taken under static load, the strain distribution in and between
the individual subgrains is analyzed and compared to composite type models. The
consequences of stress relaxation and unloading is moreover discussed. The
dynamics of the dislocation structure are investigated by continuous and
stepwise deformation experiments. On the basis of such data the unset of
subgrain formation and the subgrain refinement process is analyzed.
