Diffusion mechanism of immiscible Fe-Mg system induced by high-density defects at the steel/Mg composite interface

•Through the severe plastic deformation of the 20#steel/Mg interface during the large deformation rolling, high-density crystal defects were constructed in the 20#steel/Mg composite interface area.•The high-density crystal defects in 20#steel of the interface area provided diffusion channels for Mg element under the effect of heat-force coupling.•The higher internal energy of 20#steel in the interface area was able to overcome the positive mixing heat of the immiscible Fe-Mg system and provide the driving force for the diffusion of the Mg elements.