Formation and transformation of metastable LPSO building blocks clusters in Mg-Gd-Y-Zn-Zr alloys by spinodal decomposition and heterogeneous nucleation
Available online 12 October 2023
Full Length Article
Xin Zhao, Zhong Yang, Jiachen Zhang, Minxian Liang, Liying Wang
Abstract
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
°C∼500 °C. 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.