Examination of the Mg-Zn Phase Formation in Hot-Dip Galvanized Steel Sheet

Zinc-magnesium layers have been produced by a combination of conventional hot-dip galvanizing and PVD magnesium coating. A subsequent short heat treatment causes the diffusion of magnesium into the zinc layer. The influence of the maximum temperature of the heat treatment on the diffusion of magnesium and on the type and distribution of the formed intermetallic Mg-Zn phases has been examined in detail in a temperature range from 300 to 400 degrees C by means of X-ray phase analyses and metallographic cross sections. With increasing temperature a sequential intermetallic Mg-Zn phase formation has been detected. At 300 degrees C, an initial magnesium rich, intermetallic MgZn2 surface layer is formed, which, at higher temperatures from 335 to 379 degrees C, is progressively transformed into a zinc rich Mg2Zn11 layer by interdiffusion. In this context, the nucleation of the Mg2Zn11 phase takes place on the interface between the MgZn2 and the Zn phase. Above the peritectic temperature of 381 degrees C, the microstructure is composed of a zinc matrix in which precipitated MgZn2 is dispersed.