Microstructure evolution of a high-strength low-alloy Zn-Mn-Ca alloy through casting, hot extrusion and warm caliber rolling

Through casting, hot extrusion and then warm caliber rolling, biodegradable Zn-0.8Mn-0.4Ca alloy exhibits yield strength (YS) of 245 MPa, ultimate tensile strength (UTS) of 323 MPa, and elongation to failure (EL) of 12%. Its strengths are better than most of the existing biodegradable Zn alloys with ELs of 10-15%. So the alloy can be considered as a more competitive candidate for making bone screws or intravascular stents. Ca has a high efficiency of forming CaZn13 phase, the equilibrium volume fraction of which reaches 10.23 vol% with the minor addition of 0.43 wt% Ca. The CaZn13 particles not only impede dislocation motion, but also stimulate recrystallization of Zn grains, resulting in a considerable strengthening effect. The average size of Zn dendrites in the as-cast alloy reaches 289 mu m, while that of Zn grains in the as-rolled alloy is 5.0 mu m. On the other hand, the minor Ca addition severely decreases the ductility of the Zn-0.8Mn base alloy due to the large size of CaZn13 particles inherited from the as-cast microstructure. Although their average size is reduced to 12.9 mu m after the caliber rolling, it is about five times of that of MnZn13 particles. For design and fabrication of Ca-containing Zn alloys in future, refinement of CaZn13 phase should be a matter of vital concern.