The present work reports the creep behavior and microstructural evolution of the sand-cast Mg–14Gd–0.4Zr alloy (wt.%) prepared by the differential pressure casting machine. Their compressive creep tests at 250 ℃ were performed under various applied stresses (i.e., 60, 80 and 100 MPa). Among them, the sand-cast Mg–14Gd–0.4Zr samples examined under 250 ℃/80 MPa for 39 and 95 h, respectively, were chosen to systemically analyze their creep mechanisms using high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM). The obtained results showed that the enhancement of creep resistance can be mainly attributed to the coherent β’ and β’_F phases with an alternate distribution, effectively impeding the basal <a> dislocations movement. However, with the creep time increasing, the fine β’+β’_F precipitate chains coarsened and transformed to semi-coherent β₁ phase and even to large incoherent β phase (surrounded by precipitate-free areas) in grain interiors. The precipitate-free zones (PFZs) at grain boundaries (GBs) were formed, and they could expand during creep deformation. Apart from the main cross-slip of basal and prismatic <a> dislocations, <c + a> type dislocations were activated and tended to distribute near the GBs. The aforementioned phenomena induced the stress concentrations, consequently leading to the increment of the creep strain.

Microstructural evolution and strengthening mechanisms of Mg-3Sn-1Ca based alloys with additions of different amounts of AlN nano-particles were investigated. It was found that with increasing the amount of AlN nano-particles the grain size decreases obviously. The existence of AlN nano-particles could refine the primary crystal phases CaMgSn, which provided more heterogeneous nucleation sites for the formation of magnesium. Moreover, such nano-particles could also restrict the grain growth during solidification. After adding AlN nano-particles, both the tensile properties at room temperature and high temperature 250 °C and the hardness are largely improved. The improvement of strength is attributed to grain refinement and second phase refinement.