MS-4-O-2935 Structure and formation of novel LPSO structures in Mg-Co-Y alloy

Recently, dilute Mg-Zn-Y alloys have attracted great attention because of their excellent mechanical properties, and their key microstructural feature strongly depends on a long period stacking/order (LPSO) phase. So far, four types of LPSO structures, 10H, 18R, 14H and 24R have been reported for Mg-Zn-Y alloy, and all of them are systematically described by two common structural units; AB stacking of hcp structure and AB’C’A stacking where B’ and C’ layer have local fcc environment [1]. The chemical order occurs to synchronize with stacking order. That is, Zn/Y atoms distribute at the particular four layers at AB’C’A.
The LPSO structures are also observed in several Mg-transition metal-rare earth systems. In Mg-Co-Y alloys, novel types of LPSO structures are observed [2] as well as those described previously (i.e., 10H, 18R, 14H and 24R). We investigate the details of these novel LPSO structures in Mg-Co-Y alloys, based on scanning electron microscopy observations and first principles calculations.
Figure 1 shows three novel LPSO structures, 15R, 12H and 21R. They are described by two structural units composed of AB stacking and AB’C stacking, and Co/Y atoms distribute at three particular layers AB’C. For the previous LPSO structures, the local AB’C’A stacking is attributed to intrinsic-2 (I₂)-type stacking fault (SF) with respect to the original 2H stacking. However, the AB’C stacking represents intrinsic-1 (I₁)-type SF; therefore, the present LPSO structures are systematically described as periodic introduction of I₁-type SFs into 2H stacking and solute segregations at the SFs. Hereafter, we denote these LPSO structures as I₂-LPSO and I₁-LPSO structures.
Figure 2 shows the interfaces between the LPSO and 2H crystals. Generally, I₂-SF is introduced into 2H by an <a> dislocation and I₁-SF is introduced by an <a+c> dislocation. Fig. 2a shows the partial <a> dislocations at the end of LPSO, but in fig. 2b no dislocations with <c> component could be observed. At the interface, phase inversion between AB structural unit of LPSO and 2H periodically appear because AB’C stacking invert ABAB… into BABA..., forming I₁-LPSO without <a+c> dislocation motion.

[1] E. Abe et al, Philos. Mag. Lett. 91 (2011)
[2] S. B. Mi and Q. Q. Jin, Scr. Mater. 68 (2013)