Simultaneous refinement of α-Mg grains and β-Mg17Al12 in Mg-Al based alloys via heterogeneous nucleation on Al8Mn4Sm

Abstract

Due to the significant differences in the formation temperature and crystal structure between the primary α-Mg and eutectic β-Mg17Al12, it is a great challenge to achieve simultaneous refinement of the primary and eutectic phases in Mg-Al based alloys via heterogeneous nucleation. Surprisingly, we found that the α-Mg and β-Mg17Al12 in the AZ80 alloy can be simultaneously refined after 0.2 wt.% Sm addition, with the grain size decreasing from ∼217 ± 15 μm to ∼170 ± 10 μm and the β-Mg17Al12 morphology changing from a typical continuous network to a nod-like or spherical structure. The simultaneous refinement mechanism is investigated through solidification simulation, transmission electron microscopy (TEM), and differential thermal analysis (DTA). In the AZ80-0.2Sm alloy, many Al8Mn4Sm particles can be observed near the center of the α-Mg grains or inside the β-Mg17Al12. Crystallographic calculations further reveal that the Al8Mn4Sm has good crystallographic matching with both the α-Mg and β-Mg17Al12, so it possesses the potency to serve as heterogeneous nucleation sites for both phases. The promoted heterogeneous nucleation on the Al8Mn4Sm decreases the undercooling required by the nucleation of the primary and eutectic phases, which enhances the heterogeneous nucleation rate, thus causing the simultaneous refinement of the α-Mg and β-Mg17Al12. The orientation relationships between the Al8Mn4Sm and Mg/Mg17Al12 are identified, which are [1¯21¯0]Mg//[010]Al8Mn4Sm,(101¯0)Mg//(301)Al8Mn4Sm and [1¯12]Mg17Al12//[010]Al8Mn4Sm,(110)Mg17Al12//(301)Al8Mn4Sm, respectively. Furthermore, the refinement of the β-Mg17Al12 accelerates its dissolution during the solution treatment, which is beneficial for cost saving in industrial applications. Other Al8Mn4RE compounds such as Al8Mn4Y might have the same positive effect on the simultaneous refinement due to the similar physicochemical properties of rare earth elements. This work not only proves the possibility of simultaneously refining the primary and eutectic phases in Mg-Al based alloys via heterogeneous nucleation, but also provides new insights into the development of refiners for cast Mg alloys.