Achieving high strength above 400 MPa in conventionally extruded Mg-Ca-Zn ternary alloys

In this work, the role of Zn content in modifying the microstructure and mechanical properties of the Mg-1.2Ca-xZn (x=0.6, 2.0 wt.%, named as XZ10-0 and ZX21-0, respectively) based alloys was studied, and it is found that the yield strength (YS) of the present Mg-(Zn)-Ca based alloys increases monotonically with increasing the Zn concentration, from ~339 MPa in low-Zn content XZ10-0 sample to ~406 MPa in high-Zn content ZX21-0 sample. Microstructure characterization shows that the enhanced YS can be attributed to the grain refinement, fine and dispersed nano-phases, a large number of lamellae structures, and the decrease of recrystallization fraction. TEM results show that the formation mechanism for the ultra-fine grains in present Mg-(Zn)-Ca based alloys can be attributed to the co-segregation of Ca and Zn elements at the grain boundary, as well as the dynamic nanoprecipitations. When the Zn content is high, the nano-phases in ZX21-0 sample belong to the Ca2Mg6Zn3 ternary phases, which exert much higher thermal stability than the nano-sized Mg2Ca binary phases formed in the low-Zn content XZ10-0 sample. The finer size and higher number density of the nano-MgZnCa phases lead to the much finer grain size and sub-grain lamellae thickness in ZX21-0 sample, which thus results in the higher YS of ~406 MPa.