Structures of Al2Sm phase in a high-pressure die-cast Mg–4Al–4Sm–0.3Mn alloy

The structures of a petaloid phase, which acts as the dominant intermetallic phase in a high-pressure die-cast Mg–4Al–4Sm–0.3Mn alloy with outstanding ductility and strength, were thoroughly studied in this work. The results indicate that the petaloid phase is Al2Sm phase (face-centered cubic structure: a=0.7945nm), not Al11Sm3, and composed of multiple (111) twins with the (111) plane serving as the twinning plane. However, most (111) twins are extraordinarily different from the conventional one, featuring a sandwiched structure with the filling being Al12Sm2Mn5 phase. Nevertheless, the appearance of Al12Sm2Mn5 bands is not always accompanied with twins. Finally, the underlying mechanisms for the peculiar Al2Sm structures were carefully discussed and revealed as a random heterogeneous nucleation of the Al2Sm phase on the pre-existed Al12Sm2Mn5 sites during solidification.