Rare earth redistribution governs competitive surface oxidation and grain refinement equilibrium in magnesium alloys

Abstract

WE43 magnesium alloy, mainly alloyed with rare earth (RE) elements including Y, Nd and Gd, possesses important application prospects in lightweight structural components and biodegradable biomedical fields. Its corrosion resistance and mechanical integrity are highly dependent on microstructure evolution regulated by RE elements. During high temperature processes, the RE solutes in WE43 can redistribute between the alloy surface and the substrate, which makes passivation at the surface and microstructure evolution in the bulk mutually influential, and significantly determines the biodegradable and mechanical responses. Here, laser powder bed fusion-fabricated WE43 alloy was used as a model system to clarify how RE redistribution governs the coupled evolution of surface oxide layer, transition zone, and substrate microstructure. The results show that the intermediate redistribution produces the most favorable architecture, combining a continuous RE enriched oxide layer with a spatially uniform fine-grained substrate. In contrast, prolonged high-temperature oxidation produces a well-developed oxide layer but depletes RE in the transition zone and promotes grain coarsening and microstructural heterogeneity through continued outward RE diffusion. Such unbalanced microstructure shows poorer mass loss resistance and mechanical retention, demonstrating that the development of surface oxide layer alone does not determine corrosion resistance. These findings reveal a competitive surface oxidation and grain refinement equilibrium, termed CORE, in which surface passivation and microstructural stabilization compete due to the RE redistribution during high temperature processes. This framework shifts the design focus of thermally processed RE containing magnesium alloys from maximizing the protection of surface oxide layer alone toward balancing oxide layer growth and grain refinement stability.