Enhancing the mechanical properties of Mg-Al alloy by introducing Al2Y nanoparticles via rapid-cooling solidification and optimized heat treatments

Coarse grain and poor thermal stability of Mg17Al12 are usually present in the as-cast Mg-Al alloys, which are detrimental to the mechanical properties. In this work, the microstructure evolution, and mechanical properties of as-cast AZ31-1Y alloy with different cooling rates (4.0-29.9 degree celsius/s) were systematically investigated by thermal analysis and microstructure characterization. The results show that the addition of Y to AZ31 alloy results in the formation of primary Al2Y and eutectic Al2Y, while suppressing the formation of beta-Mg17Al12. The AZ31-1Y alloy with a cooling rate of 29.9 degree celsius/s exhibits the finest grain, with a grain refinement efficiency 40.9 % higher than that of AZ31 alloy with a cooling rate of 4.0 degree celsius/s. After heat treatment, the acicular eutectic Al2Y is modified into a granular shape and numerous nano-sized Al2Y precipitates are observed within the alpha-Mg matrix. The addition of Y can effectively improve the mechanical properties of AZ31 alloy under high cooling rates, but the mechanical properties deteriorate at slower cooling rates due to stress concentration caused by the agglomeration of large-sized Al2Y particles. The optimal mechanical properties of the AZ31-1Y alloy are achieved at a cooling rate of 29.9 degree celsius/s combined with subsequent heat treatment, and its YS, UTS, and EL reached up to 69 MPa, 293 MPa, and 14.5 %, respectively. Moreover, the related strengthening mechanisms are quantified. It is believed that this work can provide guidance for controlling the microstructure and optimizing the mechanical properties of as-cast Mg-Al alloys.