Role of Yb in enhancing the heat resistance of cast Mg-Sm-Zn alloy

Abstract

The 2%Yb addition into Mg-4Sm-0.6Zn-0.4Zr alloy improves the heat resistance, with a reduction of the minimum creep rate by nearly one order of magnitude, in comparison to Yb-free alloy. And tensile strength at both room and high temperature is also increased by Yb addition and the increment is more than 50 MPa. At the duration of creep, both basal and prismatic plates dynamically precipitated in the Mg matrix of Yb-modified alloy, whereas only basal plate formed in Yb-free alloy. The co-precipitation of prismatic and basal plates to form a closed space goes against dislocation motion more effectively, which is one of the crucial reasons for enhanced heat resistance via Yb. Moreover, basal stacking faults only created in Yb-modified alloy are capable of blocking dislocations motions, contributing to the improved heat resistance. Finally, RE atoms can intrinsically offer stable atomic size-dependent solute hardening at high temperature, while a large atomic radius of Yb is expected to provide a more strengthening effect in favor of the high-temperature strength of Yb-modified alloy. This work sheds a light on developing creep-resistant cast Mg-RE-based alloy containing low RE alloying.