Fabrication of Mg Zn intermetallic layer with high hardness and corrosion resistance on AZ31 alloy

Abstract

MgZn intermetallic layer on AZ31 alloy was prepared by solid–state thermal diffusion under 20 MPa pressure. The surface microstructure, microhardness and corrosion resistance of sample were investigated using X–ray diffraction (XRD), optical microscope (OM), scanning electron microscope (SEM), microhardness tester and electrochemical workstation. As a result, the diffusion rate of Zn in AZ31 alloy with pressure was 10–fold higher than that of Zn without pressure, and MgZn intermetallic layer of 5 mm was produced at 390 °C within 2 h. The high–efficiency fabrication of MgZn intermetallic layer could be attributed to the increase of effective collision between Zn and Mg atoms by the application of pressure. During the fabrication procedure, microstructure evolution of Zn diffusion layer could be described as follow: Mg + Zn → Mg7Zn3 → MgZn + MgZn2. And the surface hardness as well as corrosion resistance of the sample first increased and then decreased after Zn diffusion treatment. For the first 1 h, Mg7Zn3 phase formed on AZ31 alloy, which raised the surface hardness to 220.0 HV0.2 and decreased corrosion current of AZ31 alloy about one order of magnitude in 3.5% NaCl solution. When the diffusion time was prolonged to 2 h, more complex phases (e.g., MgZn, MgZn2 and Mg7Zn3) were produced in the sample, and the voids and difference potentials among these MgZn intermetallic compounds led to the deterioration of surface hardness and corrosion resistance.