Lowering creep rate in Mg-Zn-Ca magnesium alloy with hierarchical distribution of phases

The improvement of creep resistance remains an important challenge for engineering applications of magnesium (Mg) alloys at elevated temperatures. Based on the experimental investigations and crystal plastic finite element method (CPFEM), the creep deformation mechanisms of the AC3 (Mg-5Al-3Ca) and ZC1 (Mg-5Zn-1Ca) alloys (both in wt.%) were proposed separately in this study. The results indicate that the ZC1 alloy with lower content of Ca and hierarchical distribution of strengthening phases exhibits superior creep resistance compared to the AC3 alloy. This superiority of ZC1 alloy is attributed to the low mechanical incompatibility between the skeleton Ca₂Mg₆Zn₃ phases and the α-Mg matrix, as well as the presence of small dispersed particles in the grain interior surrounded by stable skeleton phases. The interconnectedness of the skeleton intermetallic phase affects the creep resistance of Mg alloys. During the creep process of the AC3 alloy, local stress concentration led to the cracking of the hard skeleton Al₂Ca phase, grain boundaries (GBs) sliding, and grain coarsening/rotating, resulting in large creep rate. In the ZC1 alloy, the skeleton Ca₂Mg₆Zn₃ phases distributed along the GBs act as barriers to GB sliding. In addition, the particle precipitates inside the grains which have an orientation relationship with the α-Mg matrix can additionally strengthen the matrix, effectively preventing the motion of basal 〈a〉 dislocations. The findings of this study provide a strategy to design high creep-resistant Mg alloys by synergistic effect of the stable skeleton phase and dispersed particle phase.