On the Structure, Transformation and Deformation of Long-Period Stacking Ordered Phases in Mg-Y-Zn Alloys
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INTRODUCTION
LONG-PERIOD stacking-ordered (LPSO) structures in Mg-Y-Zn alloys have received considerable interest in recent years due to their unique crystallographic features and promising mechanical properties. In general, such LPSO phases form in Mg-RE-X systems, where RE represents Y, Gd, Dy, Ho, Er, Tm, and Tb, and X represents Zn, Cu, Ni, or Co.[1–8] In most of these systems, the LPSO phase has an 18R structure in the as-cast condition and a 14H structure in the heat-treated condition. LPSO phases with 24R or 10H structures have also been reported, even though they are much less frequently observed in the alloy microstructures. While significant progress has been made over the past 10 years, there are still some controversial issues on the structure, thermal stability, structural transformation, and deformation of the LPSO phases. It is the purpose of the present paper to provide an overview of the crystal structures, transformations, and deformation of the LPSO phases, with the assistance of recent results made from atomic-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Some unsolved issues that require further research are highlighted and discussed in this overview. We hope that such knowledge will provide the basis for searching for new Mg-based alloy systems that are free of RE elements and still have LPSO phases and for developing magnesium alloys with desirable microstructures for achieving improved mechanical properties. J.F. NIE, Professor, and Y.M. ZHU, Postdoctoral Research Fellow, are with the Department of Materials Engineering, Monash University, Clayton, VIC 3800, Australia. Contact e-mail: [email protected] A.J. MORTON, Honorary Senior Research Fellow, is with the Department of Materials Engineering, Monash University, and also with the CSIRO Process Science and Engineering, Clayton, VIC 3168, Australia. A.J. Morton has been a retired person, and work covered by this paper was done at Monash. Manuscript submitted November 8, 2013. Article published online April 29, 2014 3338—VOLUME 45A, JULY 2014
II.
STRUCTURES OF LPSO PHASES
A. 18R and 14H Figure 1 shows intermetallic particles of LPSO phases that are typically observed in Mg-Y-Zn alloys. The formation of the LPSO phases in magnesium alloys was first reported by Luo et al.[1] They reported that the intermetallic particles formed in grain boundaries of their alloy, Mg-7.2 wt pct Y-1.2 wt pct Zn-0.48 wt pct Zr, are X phase and that this X phase has an 18R modulated structure. They further suggested that the 18R modulated structure could be described as a face-centered cubic structure with a = 0.45 nm. While having not mentioned specifically in their paper, they took these intermetallic particles as the equilibrium phase X, with an atomic composition of Mg12ZnY, which has been reported to exist in the Mg-Y-Zn system.[9] About 6 years later, Luo and Zhang[10] provided, in a separate paper, a more detailed characterization and description of the so-called X phase. Based on selected area electro
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