Thermal Analysis of the Mg-Al Alloys

摘要

The solidification of the mostly used Mg-alloys could be considered after the Mg-Al phase diagram. The equilibrium solubility of Al in Mg at the eutectic temperature of 437°C is 12.6 mass.%. According to the solvus line of the Mg-Al phase diagram the intermetallic compound of Mg{sub}17Al{sub}12 is precipitating on the boundaries of the primary Mg grains after the completed solidification. Solidification of the binary magnesium aluminium alloys has been investigated with a new three thermal analysis at three different cooling rates at the same time. The investigation of solidification has been taking place by evaluation of the cooling curves in connection with metallographic examinations, simultaneous thermal analyses (STA) and thermodynamic calculations. The following characteristic temperatures of the solidification process have been established from the cooling curves and STA curves: the liquidus temperature T{sub}L, solidus T{sub}s, and eutectic temperature T{sub}E, respectively. At the cooling rates smaller then 5 K/s the non equilibrium eutectic (α{sub}(Mg) + Mg{sub}17Al{sub}12) is observed in alloys with concentration higher then 4 mass.% Al which is precipitating on the boundaries of the primary α{sub}(Mg) grains. It is caused by the local Al - microsegregations, This effect was not observed in the case of commercial MgAl5Mn alloy, due to the formation of the Al{sub}4Mn intermetallic compound phase. The consequence of that is the decreased concentration of the dissolved Al in the Mg-Al liquid solution.