First-Principles Investigation of Crystal Structures and Stabilities of Precipitates in GP Zones-Strengthened Mg Alloys

Mg alloys are the lightest of all structural metals, yet they remain underutilized in applications where weight is critical to performance and efficiency, e.g., automotive, rail and aerospace, due to insufficient strength and ductility. It is proposed that Guinier-Preston zones (GP zones, nanometer-sized solute-rich clusters formed coherently on parent lattice in the early stage of precipitations) are excellent means to both high specific strength, ductility, and energy absorption capacity. GP zones have been observed in Mg-Zn, Mg-Ca-X (X = Zn, Al) alloys, which are free of rare-earth elements and, thus are low cost for mass production. However, the crystal structures and stabilities of the precipitates, especially GP zones, are not clear in Mg-Zn and Mg-Ca-X (X = Zn, Al) alloys.
Here we leverage a statistical mechanical method called cluster expansion (CE) parameterized by density functional theory (DFT) calculations to search for low energy structures at ground state and identify potential GP zones in Mg-Zn and Mg-Zn-Ca systems. In Mg-Zn binary alloys, the GP zones on basal plane, {101 ̅0} and (121 ̅0) prismatic planes of the Mg matrix are predicted. The Zig-Zag plates on {101 ̅0} planes are consistent with the observed ones in experiments. Afterwards, Monte Carlo simulation is adopted to determine the stabilities of GP zones at elevated temperatures. The stabilities of peak-aged β_1^' and β_2^' precipitates are also analyzed in addition to GP zones.
In Mg-Zn-Ca ternary alloys, multiple metastable GP zones with various compositions are predicted, which show striking similarities and are all structurally related to monolayer Mg2X with each X atom 6-fold coordinated by Mg atoms on the HCP basal plane. An increased number of Mg atoms in monolayer Mg2X are replaced by Zn atoms to form ordered GP zones at an increased Zn/X ratio due to strong attractive interactions between Zn and X solutes, which ultimately leads to lower formation energies of the GP zones. In addition to monolayer GP zones, the atomic structures of precipitates formed after GP zones during aging are also proposed based on experimental information, i.e., compositions in 3D atomic probe (3DAP), morphologies in high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM), and symmetry in diffraction patterns. To valid the proposed structures, stabilities are verified in DFT, and simulated TEM images and diffraction patterns are compared with experimental observations.
Further, to incorporate effects of large size-mismatch of elements in the above systems, the mixed-space cluster expansion (MSCE) method, which includes the long-ranged coherency strain energy in addition to shorted-ranged chemical interactions in CE, will be generalized to multi-component systems with arbitrary symmetries and applied to the current HCP systems. With MSCE and Monte Carlo, interplay between strain and chemical interactions will be analyzed for coherent precipitates formed in early-stage aging, especially for monolayer GP zones observed in current systems that challenges the capability of traditional calculation methods, e.g., continuum elasticity plus empirical interfacial energies and phase field model. The current work not only searches for potential precipitates in Mg-Zn and Mg-Zn-Ca alloys, the output of MSCE can also guide large-scale simulations of the mechanical behavior of precipitates during deformation.