Microstructure and mechanical properties of extruded and caliber rolled biodegradable Zn-0.8Mn-0.4Ag alloy with high ductility

Caliber rolling can be a processing step of biodegradable Zn alloys for making implant devices such as stents and bone screws. However, effects of caliber rolling on microstructures and mechanical properties of biodegradable Zn alloys are seldom investigated. In the present study, it is employed in order to strengthen extruded Zn-0.8Mn-0.4Ag alloy. The as-rolled alloy has yield strength (YS) of 173.4 MPa, ultimate tensile strength (UTS) of 262.2 MPa and elongation to failure (EL) of 46.0%. Before the rolling, the as-extruded alloy has a bimodal Zn grain structure featured by very large Zn grains longer than 87 mu m and fine Zn grains of 5.5 mu m, which endows it a high EL of 62.6%. After the rolling, the bimodal structure is replaced by a uniform grain structure with finer Zn grains of 4.0 mu m. MnZn13 grains are also refined from 6.7 mu m to 4.2 mu m. Twinning happens not only in Zn grains, but also in MnZn13 grains. The large Zn grains are more prone to be twinned, while multiple narrow twins with width as small as about 22 nm form in a MnZn13 particle. The grain refinement strengthens the alloy, while the bimodal grain structure toughens the alloy. The rolling achieves a more thorough grain refinement, meanwhile it annihilates the bimodal grain structure, resulting in a strengthening effect in sacrifice of ductility. However, the ductility of the as-rolled alloy is still higher than most biodegradable Zn alloys.