Structural and Compositional Modulation in Transformation of LPSO Structure in Mg₉₇Zn₁Y₂ Cast Alloys

Abstract

This study investigated modulation of the long period stacking order (LPSO) structure in aged Mg 97 Zn 1 Y 2 alloys using conventional transmission electron microscopy (TEM) and aberration-corrected high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM). The irregular stacking sequence of a fragment of 24R-type LPSO acts as a catalyst for the transformation from 18R-to 14H-type LPSO. The elementary step of the transformation from 18R-to 24R-type takes place by the ledge-pair movement on different (0001) Mg planes with Shockley partial dislocations. Each ledge has a transition region in front of it. The transition regions are HCP-type stacking sequence with lower Zn and Y concentrations than those of the FCC-type enrichment layer. The solute elements migrate easily in the region, where solute elements produce a kind of diffusion field. Therefore, structural modulation occurs by a mechanism resembling diffusionaldisplacive transformation. Local strain analysis using HAADF-STEM images has elucidated that lattice spacing of (0001) Mg in the FCC-type enrichment layer is shorter than that in the HCP-type transition region. These structural and compositional irregularities are an elementary step in the transformation of LPSO in Mg 97 Zn 1 Y 2 alloys. A diffusionaldisplacive type transformation mechanism in LPSO has been proposed.

Local Strain Fields of LPSO in Mg-Based Ternary Alloys

“…The 24R-type is excluded since it is observed just as a irregular stacking within the quasi-stable 18R-LPSO band. 9) This fact implies that the elastic stability of LPSOs is highest in 14H-type and lowest in 10H-type by the elastic interaction.…”

“…10) We have already investigated that the local strain fields accompanying LPSOs, and revealed that the normal strain field was formed in the [0001] Mg direction in LPSOs. 9) These studies have shown that a local strain field is created in the [0001] Mg direction in LPSOs, and it was suggested that the strain field is affected by the shrinkage of the clusters. In the present study, we have investigated that the relationship between the local strain fields and LPSO polytypes, and the effect of apparent radius of solute elements on the shrinkage of clusters and local strain in LPSOs.…”

“…9) However, as to the formation and growth process prior to the transformation, a number of issues still remains unsolved. For example, Yokobayashi et al and Egusa et al have shown that TM and RE elements form L1 2 -type clusters in the enriched layers of OD structures and LPSOs, respectively.…”

Three-Dimensional Imaging of a Long-Period Stacking Ordered Phase in Mg₉₇Zn₁Gd₂ Using High-Voltage Electron Microscopy

“…The mechanical strength of the materials can be attributed to the characteristic long-period stacking order (LPSO) (synchronized LPSO) formed in the α-Mg matrix 1) . Recent extensive studies based on scanning transmission electron microscopy (STEM) [2][3][4] revealed atomic structures of the LPSO; however, the images obtained by this technique are projections of three-dimensional (3D) objects. To overcome this fundamental limitation, we have studied 3D shapes and spatial distribution of LPSO by electron tomography using a 300 kV-STEM 5) .…”

Microstructure evolution and mechanical properties of the Mg-7Zn-(xY-0.6Zr (x = 6, 9, 12 wt.%) alloys

“…The element Y has strong solution strengthening and ageing strengthening effects because of its large equilibrium solid solubility in Mg matrix (12.5 wt.% [8]). Meanwhile, the ternary equilibrium phases: W-phase (Mg 3 Zn 3 Y 2 , cubic structure) [9], I-phase (Mg 3 Zn 6 Y, icosahedral quasicrystal structure) [10], and X-phase (Mg 12 ZnY, long period stacking ordered structure) [9] have shown outstanding effects on the microstructures and mechanical properties of the Mg-Zn-Y alloys [11][12][13][14][15][16][17].…”