Microstructure and Creep Behavior of High-Pressure Die-Cast Magnesium Alloy AE44

The microstructure and creep behavior of a high-pressure die-cast AE44 (Mg-4Al-4RE) alloy have been studied. The creep properties were evaluated at 423 K and 448 K (150 °C and 175 °C) under stresses in the range 90 to 110 MPa. The microstructures before and after creep were examined by transmission electron microscopy (TEM). After creep, AE44 exhibits anomalously high stress exponents (n  = 67 at 423 K [150 °C] and n  = 41 at 448 K [175 °C]) and stress-dependant activation energies ranging from 221 to 286 kJ/mol. The dislocation substructure developed during creep is characterized by extensive nonbasal slip and isolated but well-defined subgrain boundaries. It is shown that the anomalously high stress exponents cannot be rationalized by the threshold stress approach that is commonly adopted in analyzing the creep behavior of dispersion-strengthened alloys or metal matrix composites. A comparison in creep resistance is also made between AE44 and AE42 (Mg-4Al-2RE).