Atomic-scale cocoonlike interfacial segregation stabilizes the nanoprecipitates in a dilute Mg-Mn-Al-Ca-Ce alloy

Abstract

Achieving densely distributed and thermally stable nanoprecipitates is critical in the development of Mg alloys suitable for long-time thermal exposure at elevated temperatures. Herein, highly stable β-Mn nanoprecipitates with a high volumetric number density (∼1.1 × 10²⁰ m^-3) are created in a dilute Mg-0.6Mn-0.5Al-0.2Ca-0.3Ce (wt.%) alloy, which stem from the formation of a unique core-shell structure characterized by a Ca-Ce cocoonlike segregation layer. Through detailed multi-scale characterizations and Molecular Dynamics/Monte Carlo simulations, we reveal that the segregation layer forms through a complex dynamic solute repartitioning behavior, wherein Ca and Ce, attracted into the β-Mn nanoprecipitates during hot rolling at 598 K, are repelled toward precipitate interface upon subsequent thermal exposure at 753 K. This process is driven by the changes in the local strain fields that arise from the increased Mn/Al ratio in the β-Mn nanoprecipitates. These core-shell β-Mn nanoprecipitates effectively stabilize the fine-grained structure (∼13-16 μm), with negligible degradation of yield strength when extending exposure time from 10 min to 3000 min at 753 K, i.e., ∼82% of the melting point of pure Mg. Our work illustrates how solute atoms with large differences in diffusivities can work synergistically to form densely distributed and highly stable nanoprecipitates even in a dilute Mg alloy, and the results could provide new insights into the development of high-performance alloys with superior microstructure stability.