Effects of alloying elements on the microstructure and high-temperature mechanical properties of Mg-Al alloys

The effects of alloying elements on the microstructure and high temperature mechanical properties of Mg-Al alloys were examined in this study. In order to improve the high temperature mechanical properties, Sr or Mm (Misch metal) was added to the Mg-9Al alloy. The effect of Sn on the Mg-9AI alloy was also examined as Sn was expected to improve the high temperature mechanical properties by forming the thermally stable Mg(2)Sn phase. All specimens were die-cast on a 320 ton high-pressure die casting (HPDC) machine. The microstructures of the specimens were examined by optical and scanning electron microscopy (SEM). Tensile tests were carried out at room temperature and 150 degrees C using an initial strain rate of 2x10(-4)/sec. In addition, tensile creep tests were conducted at the stress level of 70 MPa. Microstructure analyses of the specimens after heat treatment at 400 degrees C for 12 hours revealed that most of the Mg(17)Al(12) precipitates dissolved into the matrix, while the thermally stable phases continued to exist. In addition, these thermally stable phases appeared to suppress the precipitation of the discontinuous Mg(17)Al(12) phase at grain boundaries. The high temperature mechanical properties of the Mg-9Al alloys were found to improve significantly with the addition of alloying elements due to the formation of the thermally stable precipitates.