Correlation Between Microstructural Features and Corrosion Resistance in a Fine-Grained Severely Deformed Biodegradable Mg‒4Zn Alloy

The corrosion resistance of a biodegradable Mg‒4Zn alloy, severely deformed by equal channel angular pressing (ECAP) and simple shear extrusion (SSE), was investigated and formulated in terms of different microstructural features. The grain size of the extruded alloy decreased after ECAP and SSE. The finest grain size of 3.6 μm was obtained after 4 SSE passes. Second phase particles were refined and distributed more uniformly in the SSE-processed conditions. Electron back-scattered diffraction examinations revealed that 4 ECAP passes transforms the fiber texture of the extruded material to a shear-type texture component. However, a semi-fiber component, with the (0001) planes parallel to the pressing direction, was observed in SSE-processed samples. The corrosion resistance, analyzed via electrochemical testing in a phosphate buffered saline (PBS), was improved by ECAP and SSE processing. Despite the larger grain size of the ECAP-processed conditions, they showed higher corrosion resistances (Rp) than those of the SSE-processed ones. This was attributed to the higher density of basal planes at the surface, vaster dynamically recrystallized regions, larger high-angle grain boundary fractions, lower second phase volume fractions, and more homogeneous grain structures. These microstructural features were quantified and incorporated into a model to formulate the corrosion resistance of the alloy.