Microstructure of die-cast alloys Mg-Zn-Al(-Ca): a study by electron microscopy and small-angle neutron scattering

The microstructure of a magnesium die-cast alloy with 8 wt.% Zn and 5 wt.% Al (ZA85) and of two alloy modifications with 0.3 and 0.9 wt.% Ca (ZACa8503 and ZACa8509) are compared. Optical, scanning electron and transmission electron microscopy (TEM) were used to study the microstructure in as-cast and annealed conditions. Small-angle neutron scattering (SANS) measurements were carried out to characterize the early stages of the precipitation process. The die-cast microstructure of ZA85, ZACa8503 and ZACa8509 consists of dendritic alpha-Mg grains and coarse intermetallic grain-boundary phases. In the Ca-free ZA85 alloy, grain boundaries are decorated with a quasi-crystalline phase with icosahedral point-group symmetry (I phase), a metastable modification of the equilibrium tau phase Mg-32(Al, Zn)(49)). At the grain boundaries of the Ca-containing ZA85 alloys, small amounts of the stable cubic tau phase Mg-32(Al, Zn, Ca)(49) as well as Al-2(Ca, Zn) phase, were detected in addition to the I phase. In the ascast condition, regions supersaturated with Al and Zn were found to surround the alpha-Mg grains and to become tau phase precipitation sites during high-temperature annealing in all alloys investigated. TEM and SANS measurements revealed no influence of Ca on the size distribution and the coarsening rate of the precipitates, which are formed by a continuous precipitation process.