Precipitation of stable icosahedral quasicrystal phase in a Mg-Zn-Al alloy

Precipitation of a stable quasicrystalline i-phase has been discovered in a Mg-6Zn-3Al (wt%, ZA63) magnesium alloy, possibly a first involving a stable i-phase. Dissolution of i-phase solidified at grain boundaries leads to precipitation in the Mg-matrix during cooling due to a solvus boundary observed by phase diagram calculation. Phase transformation, nucleation characteristics, morphology and interfaces with the matrix have been studied. Differential scanning calorimetry (DSC) and in-situ X-ray diffraction (XRD) showed dissolution and precipitation of i-phase during heating and cooling cycles, forming a hysteresis between about 250 and 325 degrees C. The quasilattice parameter a(R) of i-phase was calculated to be 5.17 angstrom at room temperature, close to the c-parameter 5.18 angstrom of the Mg-matrix. At dissolution/nucleation, the diffraction peaks of i-phase were sharper, (quasi)lattice constants a(R )and C-Mg become equal to each other (5.22 angstrom ) and the lattice strain in the matrix was minimized. Precipitation occurred with a fivefold plane parallel to a matrix pyramidal {01 (1) over bar1} plane, which gave rise to two possible orientation relationships (OR): 5f parallel to[0001] (OR3') and 2f parallel to[0001] (OR1). Presence of both ORs in the same precipitates generated icosahedral twinning. The precipitates were faceted sharply on fivefold, twofold and threefold planes which were on basal, prismatic and pyramidal planes of the Mg-matrix. Results suggest the OR selection to be nucleation temperature dependent. Based on the crystallographic relationships, 3-dimensional tiling model of icosahedral lattice has been used to model lattice match with the matrix. (C) 2021 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Automated summary

The precipitation of a stable quasicrystalline i-phase has been observed in a Mg-6Zn-3Al magnesium alloy, which may be the first discovery of a stable i-phase in this alloy. The dissolution of i-phase at grain boundaries leads to precipitation in the Mg-matrix during cooling. Nucleation characteristics, phase transformation, and interface morphology were investigated.