Microstructures and creep properties of high-performance heat-resistant magnesium-alkaline earth alloys

The microstructures of Mg-4Al alloys containing alkaline earth Sr and Ca were investigated by optical microscope (OM), scanning electronic microscope (SEM) and transmission electron microscope (TEM), and creep properties were also tested. The as-cast microstructure of the studied alloys consists of dendritic a-Mg and grain boundary second-phases. Divorced eutectic and lamellar eutectic Al₄Sr, and bulky ternary τ phase are observed along grain boundary with 2% of Sr addition to the based alloy. 2% of Ca addition results in the formation of lamellar eutectic Mg₂Ca at grain boundary and Al₂Ca particles in grains. In the Mg-4Al-2Sr-ICa alloy, grain boundary phases are r phase and lamellar eutectic Mg₂Ca, and Al₂Ca particles are also precipitated in grains. With the increase of Al content in the Mg-4Al-2Sr-ICa, the coarse irregular-shaped (Mg, Al)₂Ca eutectic forms along grain boundary and its volume fraction gradually increases, meanwhile Mg₂Ca and τ phase gradually decrease. The new fine lamellar Al₄Sr appears when the Al content reaches to 7%. The additions of Sr and Ca improve the creep resistance of Mg-Al alloys significantly, and the Mg-5Al-2Sr-ICa and Mg-6Al-2Sr -1Ca alloys indicate the best creep properties in all studied alloys. According to the power-law equation, under conditions of 175℃/50~80 MPa and 70 MPa/150~200℃, the creep behavior of the Mg-4Al-2Sr alloy is diffusion controlled dislocation climb at lower stresses (〈 60 MPa) and shows the breakdown of power-law at higher stresses. The creep mechanism of the Mg-4Al-2Sr-1Ca alloy seems controlled dislocation movement and grain boundary sliding.