Evolution of long-period stacking order (LPSO) in Mg₉₇Zn₁Gd₂ cast alloys viewed by HAADF-STEM multi-scale electron tomography

Abstract

We have studied three-dimensional (3D) structures and growth processes of 14H-type long-period stacking order (LPSO) formed in Mg₉₇Zn₁Gd₂ cast alloys by single tilt-axis electron tomography (ET) using high-angle annular dark-field scanning transmission electron microscopy. Evolution of the solute-enriched stacking faults (SFs) and the 14H LPSO by ageing were visualised in 3D with a high spatial resolution in multi-scale fields of views from a few nanometres to ~10 μm. Lateral growth of the solute-enriched SFs and the LPSO in the (0 0 0 1)_Mg plane is notable compared to the out-of-plane growth in the [0 0 0 1]_Mg direction. The 14H LPSO grows at the cost of decomposition of the (Mg, Zn)₃Gd-type precipitates, and accompany a change of in-plane edge angles from 30 to 60°. We have updated the Time–Temperature–Transformation diagram for precipitation in Mg₉₇Zn₁Gd₂ alloys: starting temperatures of both solute-enriched SFs and LPSO formation shifted to a shorter time side than those in the previous diagram.

Keywords:

• Long-period stacking order (LPSO)
• magnesium alloys
• electron tomography
• scanning transmission electron microscopy (STEM)
• lateral morphology

Acknowledgements

The authors wish to thank Prof. Y. Kawamura and Dr. M. Yamasaki in Kumamoto University for providing us Mg–Zn–Gd alloys, and Ms. K. Ito for 3D reconstruction.