Magnesium Alloys Strengthened by Nanosaucer Precipitates with Confined New Topologically Close-Packed Structure

The γ″ nanosaucer precipitates in many Mg-RE-Zn(Ag) alloys, also named the G. P. zones in some cases such as in the Mg–Ca–Al(Zn) alloys, play the critical role in strengthening the α-Mg matrix and enhancing their creep resistance. However, the previous reports on the crystal structure of γ″ phase are still controversial at present, and thus it is hard to correlate the γ″ phase with the mechanical properties of Mg alloys. In this study, we confirmed a new topological close-packed (TCP) structure for the γ″ precipitate in a typical peak-aged Mg–Gd–Zn alloy using Cs-corrected high-angle annular dark-field-scanning transmission electron microscopy (HAADF-STEM) and computational simulation. The new structure is totally different from the commonly accepted structure for the γ″ phase that consisted of three atomic layers. In contrast, this TCP nanosaucer precipitate is composed of the monolayer atomic icosahedral clusters with five (0001)γ″ atomic layers (hexagon structure, space group: P6/mmm, a = 5.56 Å, c = 5.21 Å; stacking sequence: ABCBA; grid structure: A → 63(Mg), B → 36(Gd), C → 3636(Zn)). Moreover, the chemical formula of the γ″ nanosaucer precipitate is also identified as Mg2Gd2Zn3 (A2B2C3), consistent with previous 3D atom probe results. The orientation relationship between the γ″ precipitate and α-Mg matrix is also determined as (0001)γ″ // (0001)α; [011̅0]γ″ // [112̅0]α. The finding would not only shed light on deeper understanding of the confined existence of monolayer icosahedral in the field of crystallography, and of the γ″ nanosaucer precipitates strengthening mechanism in Mg alloys, but also guides the further design of new high-strength and creep resistant Mg alloys.