Microstructural investigation and thermodynamic calculations on the precipitation of Mg–Al–Zn–Sr alloys

Microstructural investigation and thermodynamic simulation were carried out to study precipitation during the solidification of AZ31 Mg alloy containing up to 1wt% Sr. Increasing Sr content from 0.01 to 1 wt% led to the formation of an Al–Sr line compound (Al4Sr) and to the suppression of Al–Mg precipitate (β-Mg17Al12). Transmission electron microscopic (TEM) investigation and energy dispersive spectroscopic analysis on extracted precipitates revealed Mg and Zn solubility in the Al4Sr particles. It is shown that Sr content also affects the precipitation of Al–Mn precipitates. Thermodynamic calculations predict that the increase in Sr content limits the Al–Mn reaction and the precipitation of Al–Mn precipitates with low Al/Mn ratio. Microstructural investigations determined the presence of two Al–Mn precipitates (Al8Mn5and AlMn), either in the form of large dendritic plates or small nano-scale particles in the Mg matrix. It has been calculated by the thermodynamic model and confirmed by TEM that by increasing the Sr content, solubility of Al solid decreases whereas the level of Mn increases slightly.