The role of metastable LPSO building block clusters in phase transformations of an Mg-Y-Zn alloy

We present a systematic atomic scale analysis of the structural evolution of long-period-stacking ordered (LPSO) structures in the (i) alpha-Mg matrix and in the (ii) interdendritic LPSO phase of an Mg97Y2Zn1 (at. %) alloy annealed at 500 degrees C, using high resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Various types of metastable LPSO building block clusters have been observed in both regions. The thermodynamic phase stabilities computed by density functional -theory calculations explain the diversity of the LPSO structures which are distinguished by their different arrangements of the Y/Zn enriched LPSO building blocks that have a local fcc stacking sequence on the close packed planes. A direct evidence of the transformation from 18R to 14H is presented. This finding suggests that LPSO structures can change their separation distance quantified by the number of alpha-Mg layers between them at a low energy penalty by generating the necessary Shockley partial dislocation on a specific glide plane. Based on our results the most probable transformation sequence of LPSO precipitate plates in the alpha-Mg matrix is: single building block -> various metastable LPSO building block clusters 14H, and the most probable transformation sequence in the interdendritic LPSO phase is: 18R -> various metastable LPSO building block clusters -> 14H. The thermodynamically most stable structures in both the a-Mg matrix and the interdendritic LPSO phase are a mixture of 14H and alpha-Mg. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.