Characterisation of structural similarities of precipitates in Mg–Zn and Al–Zn–Mg alloys systems

Abstract

High angle annular dark field scanning transmission electron microscopy has been employed to observe precipitate structures in Al-Zn-Mg and Mg-Zn alloys. h 1 precipitate structures in Al-Zn-Mg are commonly formed by MgZn 2 Penrose bricks, but also frequently observed to incorporate Mg 6 Zn 7 elongated hexagons via two different modes. Tilings of MgZn 2 and Mg 6 Zn 7 building blocks in both b ′ 1 in Mg-Zn and h 1 in Al-Zn-Mg alloys, create overall patterns which deviate from the chemical and structural configuration of solely monoclinic Mg 4 Zn 7 or MgZn 2 unit cells. Precipitate morphologies were found to be correlated to their internal sub-unit cell arrangements, especially to Mg 6 Zn 7 elongated hexagons. Systematic or random arrangements of Mg 6 Zn 7 elongated hexagons inside precipitates and therefore compositional and structural patterns, were found to be strongly related to the aspect ratio of the precipitates and altering of the precipitate/matrix interfaces.

Density functional simulations of pressurized Mg-Zn and Al-Zn alloys

“…The stable phases with hexagonal symmetries in experiment are Mg 2 Zn 3 , Mg 2 Zn 11 , MgZn 2 , Mg 4 Zn 7 , and MgZn. [37][38][39][40][41][42][43] The last case was also explored and confirmed in our results above. The ratios of the other two compositions, 1:2 and 4:7, do not match exactly with the 4×4×4 structures considered in this study, but the predicted phase Mg 21 Zn 43 lends support also for these experimental observations.…”

“…The ratios of the other two compositions, 1:2 and 4:7, do not match exactly with the 4×4×4 structures considered in this study, but the predicted phase Mg 21 Zn 43 lends support also for these experimental observations. [37][38][39][40][41][42][43] Another limitation considers the (possible) structural transition upon alloying or applying pressure. In this case, the searchable configuration space would considerably increases and include numerous new possibilities.…”

“…We note here that according to the experimental observations, large enough precipitates develop hexagonal lattice symmetry under ambient conditions. 37,38,[41][42][43] B. Electronic band structure and density of states DFT calculations include explicit information of the electronic structure for all configurations sampled in this study.…”

“…Our findings are in a good agreement with experimental observations where MgZn 2 , Mg 4 Zn 7 , and MgZn compounds with hexago- nal symmetry are found to be the most stable phases of larger precipitates. 37,38,[41][42][43] For the HCP lattice, the increase of external pressure to P≈10 GPa results in that Mg 19 Zn 45 becomes unstable and Mg 34 Zn 30 is less favorable. At pressure values of the order of 20 GPa, a new compound, Mg 3 Zn, shows stability and becomes more stable at higher pressure values, while Mg 34 Zn 30 turns unstable at pressure values larger than ≈30 GPa.…”

“…The advantages of this method compared to conventional temperature-aging approaches are (i) a shorter processing time and (ii) a more uniformly distributed microstructure with no precipitate-free zones 36 . Also, it has been experimentally observed that several phases of precipitates can develop in Mg-Zn and Al-Zn-Mg alloys, including MgZn, monoclinic Mg 4 Zn 7 , η-MgZn 2 , β-MgZn 2 with hexagonal symmetry, Mg 2 Zn 3 , and Mg 2 Zn 11 [37][38][39][40][41][42][43] . Recent systematic investigations have demonstrated that the morphology and structural evolution of precipitates in Mg alloys are closely linked to their internal sub-unit cell arrangements, the aspect ratio of precipitates, misfit strains, and their interaction with matrix interfaces 37,[44][45][46] .…”