Formation and transformation of metastable LPSO building blocks clusters in Mg-Gd-Y-Zn-Zr alloys by spinodal decomposition and heterogeneous nucleation

To study the formation and transformation mechanism of long-period stacked ordered(LPSO) structures, a systematic atomic scale analysis was conducted for the structural evolution of long-period stacked ordered(LPSO) structures in the Mg-Gd-Y-Zn-Zr alloy annealed at 300 ℃～500 ℃. Various types of metastable LPSO building block clusters were found to exist in alloy structures at different temperatures,which precipitate during the solidification and homogenization process. The stability of Zn/Y clusters is explained by the first principles of density functional theory. The LPSO structure is distinguished by the arrangement of its different Zn/Y enriched LPSO structural units, which comprises local fcc stacking sequences upon a tightly packed plane. The presence of solute atoms causes local lattice distortion, thereby enabling the rearrangement of Mg atoms in the different configurations in the local lattice, and local HCP-FCC transitions occur between Mg and Zn atoms occupying the nearest neighbor positions. This finding indicates that LPSO structures can generate necessary Schockley partial dislocations on specific slip surfaces, providing direct evidence of the transition from 18R to 14H. Growth of the LPSO, devoid of any defects and non-coherent interfaces, was observed separately from other precipitated phases. As a result, the precipitation sequence of LPSO in the solidification stage was as follows: Zn/Ycluster+Mg layers→various metastable LPSO building block clusters→18R/24R LPSO; whereas the precipitation sequence of LPSO during homogenization treatment was observed to be as follows: 18R LPSO→various metastable LPSO building block clusters→14H LPSO. Of these, 14H LPSO was found to be the most thermodynamically stable structure.