Microstructure of die-cast alloys Mg–Zn–Al(–Ca): a study by electron microscopy and small-angle neutron scattering : Mikrostruktur von Druckgusslegierungen Mg–Zn–Al(–Ca): Eine Untersuchung mittels Elektronenmikroskopie und Kleinwinkel-Neutronenstreuung

Abstract

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 α-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 τ phase (Mg32(Al, Zn)49). At the grain boundaries of the Ca-containing ZA85 alloys, small amounts of the stable cubic τ phase Mg32(Al, Zn, Ca)49 as well as Al2(Ca, Zn) phase, were detected in addition to the I phase. In the as-cast condition, regions supersaturated with Al and Zn were found to surround the α-Mg grains and to become τ 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.