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

Abstract

Due to positive mixing heat between Fe and Mg, it is difficult to diffuse for Fe-Mg at the interface of steel/Mg laminated composites, resulting in the inability to achieve high-strength metallurgical bonding. In this paper, 20#steel/Mg laminated composites were prepared by large deformation rolling and subsequent diffusion heat treatment process. The interfacial bonding strength was improved by constructing high-density crystal defects at the interface to promote element diffusion. The mechanisms of interface morphology evolution and element diffusion were analyzed by finite element simulation and theoretical calculation. The results show after diffusion heat treatment, the bond strength of the large deformation rolled interface was increased from 14 MPa to 30 MPa. Fe-Mg transition layer with about 80 nm thickness as well as high-density vacancies, dislocations and grain boundaries were formed in the large deformation rolled interface region. During diffusion heat treatment, Mg elements diffused into grain interior and grain boundary regions of 20#steel under the effect of heat-force coupling, and the thickness of Fe-Mg transition layer increased to 150 nm, forming an Fe-based supersaturated solid solution. The interface with high-density defects constituted a non-equilibrium interface. The 20#steel internal energy in the non-equilibrium interface is able to overcome positive mixing heat of immiscible Fe-Mg system and provide the driving force for Mg elements diffusion. Promoting elemental diffusion by constructing high-density defects can be a new concept to achieve metallurgical bonding at the interface of immiscible metal laminated composites.