The kinetics of structural and phase transformations in polycrystalline AL layers in the course of nitrogen implantation to the concentrations close to the equilibrium limit of nitrogen solubility, has been studied by transmission electron microscopy and electron diffraction techniques. A model is suggested according to which the nucleation of the ALN phase takes place on radiation-induced stacking faults ({111} habitus) and occurs by means of migration of nitrogen to the defect plane and the filling in the octahedral pores in the region of hexagonal packing of the layers. This process leads to a sequential formation of layers having the wurtzite structure.