Zn Segregation at Precipitate/Matrix Interface in Mg–Sn–Zn Alloys

Magnesium-tin based alloys have received considerable attention in the past 15 years for developing high strength alloys. Mg–Sn binary alloys are precipitation hardenable, but their age-hardening response is moderate. Additions of Zn can significantly improve the age-hardening response of binary Mg–Sn alloys by refining the distribution of Mg₂Sn precipitates. To understand the role of Zn in the precipitation process, Mg₂Sn precipitates with different morphologies and orientations in under-, peak- and over-aged samples of a Mg–9.8Sn–1.2Zn (wt%) alloy are characterized by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and STEM X-ray mapping. It is found that Zn atoms always segregate to the precipitate-matrix interface, irrespective of the interfacial structures and orientation relationships of the Mg₂Sn precipitates. This finding provides an insightful clue to the understanding of the enhanced nucleation and thermal stability of Mg₂Sn precipitates in the Mg–Sn–Zn alloys.