ORALS

Recent successful alloy-design showed that Mg alloys with addition of a small amount of Zn and Y (or rare-earth elements) reveal excellent mechanical properties including remarkably improved strength with a reasonable ductility [1]. One of the prominent microstructural features, which are believed to contribute for these excellent properties, is formation of a novel type of long-period structures [2-6]. The structures are fundamentally long-period stacking derivatives of a hexagonal close-packed structure (hcp-Mg), and the resultant stacking polytypes accompany a unique chemical order that occurs to synchronize with the corresponding stacking order; i.e., the synchronized long-period stacking/order (LPSO) structure [3, 6]. We have attempted to evaluate phase stability [7] as well as to construct model structures of the complex LPSO crystals, based on electron microscopy observations and first-principles calculations [4]. Structural characteristics are well represented by the TM₆RE₈ clusters with a L1₂-type short-range order (SRO) configuration, embedded in the local <i>fcc</i>-Mg layer of the LPSO [4, 5]. Interestingly, it turns out that the local electronic structures as well as relaxation behaviors of the SRO clusters significantly contribute to a phase stability. In the talk, I will describe a unique SRO cluster-induced phase stabilization, providing an important clue that leads to a universal concept on how we choose proper elements during alloy design.

References:
[1] Y. Kawamura, M. Yamasaki, Mater. Trans. 48 (2007) 2986.\n[2] E. Abe et al., Acta Mater. 50 (2002) 3845.\n[3] E. Abe et al., Philos. Mag. Lett. 91 (2011) 690.\n[4] D. Egusa, E. Abe, Acta Mater. 60 (2012) 166.\n[5] K. Yamashita et al., J. Alloys and Compounds 788 (2019) 277-282. \n[6] K. Guan et al., Scripta Mater. 207 (2022) 114282.\n[7] M. Egami et al., Materials and Design 188 (2020) 108452.