A molecular dynamics study in combination with experimental research is applied for investigation of diffusion-zone formation on the phase interface between aluminium-based alloy Д16 (2024) and Fe-alloyed layer on its surface formed in the process of ultrasonic impact treatment (UIT) of Д16 alloy by Armco-iron pin. The formation of surface-layer structure, its thickening and diffusion zone formation between base material and alloyed layer is studied by scanning electron microscopy and energy-dispersive x-ray spectroscopic analysis of cross-section of treated samples. In the UIT process, the microstructure in the surface layer becomes finely fragmented and the diffusion zone becomes thicker with increasing of UIT duration. The molecular dynamics simulation is applied to investigate atom behaviour on the phase interface between Al- and Fe-layers, to observe defect formation and its migration in the process of impact loading, which contributes to the formation of diffusion zone and restructuring of near-phase interface layers.