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

ZM Hua and SL Wu and M Zha and YL Zhu and ZT Hu and P Chen and XY Xu and S Li and C Wang and SB Jin and U Ramamurty and HY Wang, ACTA MATERIALIA, 301, 121572 (2025).

DOI: 10.1016/j.actamat.2025.121572

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 beta-Mn nanoprecipitates with a high volumetric number density (similar to 1.1 x 10(20) 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 beta-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 beta-Mn nanoprecipitates. These core-shell beta-Mn nanoprecipitates effectively stabilize the fine-grained structure (similar to 13-16 mu m), with negligible degradation of yield strength when extending exposure time from 10 min to 3000 min at 753 K, i.e., similar to 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.

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