Theoretical models are suggested which describe micromechanisms for stress- induced nucleation of nanoscale grains (nanograins) and nanoscale amorphization in deformed nanocrystalline materials. Within the models, nucleation of nanograins and nanoscale amorphization are initiated by plastic deformation and represent processes related to transformations of dipoles and quadrupoles of disclinations (rotational defects) at grain boundaries. The nanograin nucleation occurs through splitting and migration of grain boundaries containing disclination dipoles. The nanoscale amorphization is realized through transformations of disclination quadrupoles. It is shown that processes of nanograin nucleation in deformed nanocrystalline materials (in particular, nanocrystalline Al and nanoceramic α-Al2O3) as well as nanoscale amorphization in deformed nanocrystalline silicon are energetically favorable in certain ranges of their parameters and the external stress level.