New polytypes of LPSO structures in an Mg–Co–Y alloy

Abstract

The magnesium alloys containing long-period stacking ordered (LPSO) structures exhibit excellent mechanical properties. Each LPSO structure is known to contain either AB′C′A or AB′C building block and feature its own stacking sequences. By atomic-scale high-angle annular dark field scanning transmission electron microscopy, we find the co-existence of AB′C′A and AB′C building block in a single LPSO structure of the as-cast Mg₉₂Co₂Y₆ (at.%) alloy, leading to the formation of six new polytypes of the LPSO structures determined as 29H, 51R, 60H, 72R, 102R and 192R. The lattice parameter of each LPSO structure is derived as and (n presents the number of basal layers in a unit cell). The stacking sequences and the space groups of these newly identified LPSO structures are proposed based on the electron diffraction and atomic-scale aberration-corrected high-resolution images. A random distribution of Co/Y elements in the basal planes of AB′C′A and AB′C structural units is also observed and discussed.

Keywords:

• Magnesium alloys
• long-period stacking ordered (LPSO) structure
• scanning transmission electron microscopy
• electron diffraction

Acknowledgements

The authors are grateful to Prof S. J. Zheng and Dr Y. T. Zhou for fruitful discussion. We gratefully acknowledge Dr J. H. You and Prof K. Q. Qiu in Shenyang University of Technology for sample preparation.