On the corrosion of ZK60 magnesium alloy after severe plastic deformation

Magnesium 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 high strength and corrosion performance. In this work, we demonstrate that the corrosion resistance of the alloy Mg-6Zn-0.5Zr (ZK60) can be controlled to a large extent by its microstructure and distribution of phases, formed by thermomechanical treatment involving hot severe plastic deformation (SPD). The two-step multi-axial isothermal forging (MIF) is employed to deform the alloy ZK60 to different strains at 400 degrees C and 300 degrees C. The influence of microstructural parameters on the corrosion rate is demonstrated. It was found that MIF leads to the microstructure refinement, increases the fraction of high-angle grain boundaries, and reduces the size and distribution inhomogeneity of the second phases, thus promotes the formation of a reasonably uniform protective layer at the surface, and increases the overall corrosion resistance of the investigated ZK60 alloy. The homogeneous microstructure fabricated by MIF plays an important role for corrosion resistance since any heterogeneity or bi-modality of grain boundary structure can lead to large gradients of the driving force for oxidation within the material, and, as a consequence, to the difference in the spatial properties and the heterogeneity of the protective oxide film. With the observed 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.