| First-Principles Study on The Origin of Structural Stability of Mg-M-Y (M = Ni, Cu, Co, and Zn) Alloys with Long-Period Stacking Ordered Structure Takao Tsumuraya1; Tamio Oguchi2; 1KUMAMOTO UNIVERSITY, Kumamoto, Japan; 2OSAKA UNIVERSITY, Ibaraki, Japan; PAPER: 301/Magnesium/Regular (Oral) SCHEDULED: 16:45/Thu. 24 Oct. 2019/Adonis ABSTRACT: A class of dilute magnesium alloys in which solute atoms (Zn, Y) are enriched in a periodic stacking fault to the (0001) plane of the hcp structure show high-strength and ductility [1]. Much attention has been paid to the studies of materials that are expected to be involved in the strengthening of mechanisms. For the class of Mg - transition-metal (M) - rare-earth (RE) metal alloys, transmission electron microscopy (TEM) found various polytype structures such as 12R, 24R, 10H, 18R, and 14H, which have different numbers of Mg layers between solute atoms (M and RE) concentrated layers [2]. Such a unique atomic structure is referred to as a long-period stacking ordered (LPSO) structure. Furthermore, recent structural analysis using TEM and first-principles calculations reveal that solute elements form M<sub>6</sub>RE<sub></sub>8 (L1<sub>2</sub> type) cluster in the concentrated layer of the LPSO phase [3]. It is experimentally found that the degree of regularity of solute atoms depends on the choice of M and the difference in the amount (composition) of solute atoms, which is correlated to the observed type of LPSO structure [4]. The origin of the phase stabilities and formation mechanisms of the LPSO structure, however, has not been clarified yet. In this study, to understand the microscopic origin of the phase stabilities of the structural polymorphs with different compositions of solute elements, we performed first-principle density-functional theory (DFT) calculations for Mg –M - Y alloys (M = Co, Ni, Cu, Zn) with 12R, 18R and 10H structures. We found that the structural distortion of the L1<sub>2</sub> cluster is uniquely determined by the choice of M atom. We explain how the geometries of the L1<sub>2</sub> solute cluster affects the electronic state near the Fermi level that crucially determines the stabilities of the LPSO phases. References: [1] Y. Kawamura, K. Hayashi, A. Inoue, and T. Masumoto, Mater. Trans. 42, 1172 (2001).<br />[2] E. Abe, Y. Kawamura, K. Hayashi, and A. Inoue, Acta Metall. 50, 3845 (2002).<br />[3] D. Egusa and E. Abe, Acta Mater. 60, 166 (2012). J. Saal and C. Wolverton, Acta Mater. 68, 325 (2014). <br />[4] H. Kimizuka, S. Kurokawa, A. Yamaguchi, A. Sakai, S. Ogata, Sci. Rep. 4 7318 (2014). |
