Microstructural Evolution of Long-Period Stacking Ordered Structures in Mg 97 Y 2 Zn 1 Alloys Examined by In - Situ Smal
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I.
INTRODUCTION
LONG-PERIOD stacking ordered (LPSO) structures having periodic modulation in composition synchronized with periodic stacking fault have drawn attention due to their mechanical performance as a new light metal alloy system,[1,2] unique structure, and mechanical properties,[3–5] in particular, their characteristic deformation mechanism.[6,7] Mg-Y-Zn ternary alloy was the first alloy system reported for this group of materials. In particular, Mg97Y2Zn1 alloy is a two-phase alloy found to contain such structures at the grain boundaries of primary a-Mg matrix and to exhibit potential usefulness for lightweight structural materials.[1–3,6–9] The improved mechanical properties of forged/extruded samples having a small volume fraction of LPSO structures drew attention to the structures, and a series of intensive studies reported on the structures and properties of single-phase LPSO samples with higher compositions.[10–14] Close examination of the LPSO structure in well-annealed MgAlGd alloys revealed that the structure of segregated layers in LPSO structures is not a simple segregation at the stacking faults but formation of a 23 9 23 two-dimensional superstructure of Al6Gd8 molecular-like L12 clusters in each layer, as schematically shown in Figure 1.[5] On the other hand, the temporal evolution of the microstructures examined by the HIROSHI OKUDA, Associate Professor, TOSHIKI HORIUCHI and SHOKI HIFUMI, Graduate Students, are with the Department of Materials Science and Engineering, Graduate School of Engineering, Kyoto University, Kyoto 606-8501 Japan. Contact e-mail: okuda@ materials.mbox.media.kyoto-u.ac.jp MICHIAKI YAMASAKI, Associate Professor, and YOSHIHITO KAWAMURA, Professor, are with the Department of Materials Science and Engineering, Kumamoto University, Kurokami, Kumamoto 860-8555 Japan. SHIGERU KIMURA, Group Leader, Associate Chief Scientist, is with the Synchrotron Radiation Instrumentation/measurements, Japan Synchrotron Radiation Research Institute (JASRI), SPring-8, Sayo 679-5198 Japan. Manuscript submitted February 14, 2014. Article published online June 25, 2014 4780—VOLUME 45A, OCTOBER 2014
scattering technique suggests[12] that the structure of LPSO samples may change with time. Competitive growth of a metastable periodicity along the c-axis such as from 10H to 18R observed in Mg85Y9Zn6 alloys can be understood as just competition between two phases, but changes in the average two-dimensional superstructure at the segregation layers are rather anomalous. Such results bring back into question whether the LPSO structures of the samples annealed for exceptionally long times in a supposedly single-phase region and examined precisely by electron microscopy[5,6] may also apply for those in dilute samples, such as the two-phase Mg97Y2Zn1 alloys examined in the present work. Another important viewpoint is that the phase diagram and the formation kinetics of LPSO structures are not well understood yet. In particular, the stability of cluster distribution in the segregation layers in the LPS
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