Extraordinary hardening-by-annealing in bulk ultrafine grained magnesium with ultra-low yttrium addition

RX Zheng and MW Liu and JP Du and HB Xie and W Gong and YY Cheng and S Ogata and N Tsuji, ACTA MATERIALIA, 293, 121098 (2025).

DOI: 10.1016/j.actamat.2025.121098

Hall-Petch law fails when grains smaller than a critical size (e.g., 10 similar to 30 nm for copper and iron), due to grain boundary (GB) kinetics-dominated plasticity. To enhance strength, improving GB stability is a consideration. However, this often requires a significant amount of alloying elements, posing resource challenges. Additionally, practical fabrication of extremely fine grains is still an issue. In our study, we firstly demonstrate a remarkable hardening-by-annealing phenomenon in magnesium (Mg) with relatively large grain sizes of 0.2 similar to 0.5 mu m, even with ultra-low yttrium (Y) addition (<0.3 at.%). We reveal that annealing induces GB segregation/relaxation, effectively limiting the GB kinetics and promoting dislocation-dominated plasticity. Furthermore, the accompanying dislocation annihilation hinders deformation due to dislocation scarcity. As a result, we discovered extraordinary hardening (247 % increase in yield strength) in bulk ultrafine grained Mg-Y ultra-dilute alloy. This work offers a promising avenue for developing energy- and resource-efficient sustainable Mg alloys with superior mechanical properties.

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