Optimizing long-term corrosion behavior of the Mg-4Zn-xY-0.5Nd alloy by minor Y regulation and multi-pass hot extrusion

• A slightly increased Y content in the Mg-4Zn- x Y-0.5Nd alloy refines equiaxed grains of as-cast alloy and greatly decreases the average grain size to 2.5 μm in the two-pass extruded alloy. • Hot extrusion promotes the formation of texture, especially the non-basal texture and the alloy with higher Y content exhibits a strong tendency. • A minor increase in Y content from 0.5% to 0.8% enhances the corrosion resistance of both ascast and extruded alloys. • Hot extrusion increases the short-term corrosion rate of the alloy, but decreases it during long-term immersion, particularly for the two-pass extruded alloy. . In the present research, biodegradable Mg-4Zn- x Y-0.5Nd alloys were designed by minor modification of Y content and prepared through multi-pass hot extrusion, with the aim of optimizing their microstructure and corrosion performance. Their microstructure, secondary phase morphology and corrosion behaviors were characterized to explore the underlying mechanism. Results indicate that a minor increase in Y content decreases the average grain size of the as-cast alloy from 53.0 μm to 38.3 μm and enhances the basal crystallographic plane preference. Furthermore, higher Y content further refines the grain structure in the two-pass extruded alloy, reaching a minimal average grain size of 2.5 μm, but reduces the recrystallization ratio, particularly in the one-pass extruded alloy. Compared to the as-cast alloy, hot extrusion promotes the formation of texture, especially the non-basal texture. The alloy with higher Y content exhibits a strong tendency, which should be attributed to the elongated grains induced by secondary phases. The minor increase in Y content from 0.5% to 0.8% raises corrosion potentials and reduces corrosion current densities and rates in both the as-cast and extruded alloys simultaneously. Moreover, the extruded alloys demonstrate more uniform corrosion. As a result, the two-pass extruded Mg-4Zn-0.8Y-0.5Nd alloy achieves the minimal corrosion rate of 0.5353 mm/a, only 30% of that of the as-cast Mg-4Zn-0.5Y-0.5Nd alloy. The improved corrosion resistance of the high-Y doped alloy with two-pass extrusion can be attributed to the well-refined grain structure and secondary phases, which mitigate the obvious localized corrosion and promote the formation of a homogeneous corrosion film, preventing the rapid infiltration of the corrosion medium and contributing to long-term corrosion resistance.