Structural-phase transformations in the Zn–Li–Mg alloy exposed to the severe plastic torsion deformation

Abstract

In this paper, using the X-ray scattering method, the authors found the similaritues and differences in the structural-phase transformations in a Zn–Li–Mg alloy under the artificial and dynamic aging. The artificial aging (AA) of the alloy was implemented at a temperature of 300 ºС for 24 h, while the dynamic aging (DA) was performed through high-pressure torsion at room temperature for a few minutes. For the first time, using X-ray phase analysis, the authors identified the type and parameters of the LiZn2 phase crystal lattice (Pmmm, a=0.48635 nm, b=1.11021 nm, c=0.43719 nm, α=β=γ=90º) and the β-LiZn4 phase (P63/mmc, a=b=0.279868 nm, c=0.438598 nm, α=β=90º, γ=120º) to the eutectics in specified conditions. The study found that SPD leads to intensive precipitation of Zn particles in the primary β-LiZn4 phase, and β-LiZn4 particles precipitation in the Zn eutectics phase. While analyzing the diffraction patterns, the authors estimated the lattice parameter, the size distribution of coherent scattering regions, the averaged dislocation density, and the fraction of edge and screw dislocations after AA and DA. For the first time, by small-angle X-ray scattering, the authors identified the quantitative characteristics of the size, shape, and nature of the bimodal precipitate distribution in the above-mentioned conditions. In particular, it was found that fine Zn precipitates in the form of needles of 8 nm in diameter and up to 27 nm in length and coarse Zn precipitates in the form of rods of 460 nm in diameter and up to 1000 nm in length are produced in the alloy after AA. In the case of DA, fine Zn precipitates of a primarily spherical shape with an average diameter of 20 nm and coarse Zn precipitates, which formed in the primary β-LiZn4 phase a network with a cell diameter of 200–300 nm and wall thickness of 62 nm are produced in the Zn–Li–Mg alloy.