On the corrosion of ZK60 magnesium alloy after severe plastic deformation

Magnesium and its alloys are promising materials for the surgical implants due to their exceptional mechanical properties, biocompatibility and biodegradability. Binary Mg-Zn and ternary Mg-Zn-Zr alloys are the most obvious candidates for further design of biomaterials. However, they have to meet many requirements, including corrosion performance. In this work, we demonstrate that the corrosion resistance of Mg-6Zn-0.5Zr alloy ZK60 can be controlled to a large extent by the thermomechanical treatment involving hot severe plastic deformation (SPD). The multi-axial isothermal forging (MIF) is employed to deform the alloy ZK60 to different strains at 400°C and 300°C. The influence of the microstructure, its heterogeneity, grain size and distribution of second phases on the corrosion rate is demonstrated. It was found, that microstructure refinement by hot SPD leads to the increasing fraction of grain boundaries, promotes the formation of a reasonably uniform protective layer, reduces the inhomogeneity of the second phases and increases the overall corrosion resistance of the investigated ZK60 alloy. The homogeneous microstructure after multi-axial isothermal forging plays an important role in the corrosion performance since bimodal grain boundary structure can lead to large differences in the driving force for oxidation at different points of the material, and, as a consequence, to the difference in the spatial properties and the heterogeneity of the protective oxide film. With the reasonable corrosion performance and excellent mechanical properties, the fine-grained alloy ZK60 manufactured by hot two-step MIF processing has a great potential for bio-medical applications as a material for bio-resorbable implants or vascular stents.