Crystal structure, disorder and composition of the 2/1 approximant in the Al–Mg–Zn system revisited
Graphical abstract
Introduction
Complex metallic alloys (CMA's) are a fascinating class of materials. They comprise intermetallic compounds with giant unit cells, incommensurate modulated crystal structures and quasicrystalline phases. Famous examples can be found in the Al–Mg–Zn system, which has been examined frequently due to its importance for the development of light and high strength Al and Mg-based alloys. Four ternary complex intermetallic phases, called τ1 [1], τ2, q [2] and Φ [3] are known so far. The τ1 and τ2 phases are cubic crystalline 1/1 and 2/1 approximants of the quasicrystalline q phase. Their crystal structures can be described as packings of Bergman clusters, which decorate nodes of canonical cell tilings [4]. Recently, we have reported on the phase equilibria, the homogeneity range, the crystal and the electronic structure of the Φ phase. The Φ phase has an extended homogeneity range with a wedge-like shape of about 13 at.% for Al and Zn and maximal 2–3 at.% Mg. The homogeneity range is shown in Fig. 1 on top of the isothermal sections at 330 °C and 360 °C, which were calculated with the Pandat 8.1 software [5] based on the data of Petrov et al. [6] and using the COST507 alloy database [7].
The heterogeneous equilibria of the Al-rich Φ phase with (Mg), Mg17Al12 (γ) and τ1 are in agreement with our experimental data, whereas contradictory results have been found for phase equilibria of Φ, q and τ2. A reason for the discrepancies could be the phase stabilities of the τ2 and the q phase, because the stability ranges of the q and the τ2 phases are known approximately only [6]. Therefore, their heterogeneous equilibria are still tentative. Al15Mg43Zn42 and Al15Mg44Zn41 were assumed to be the compositions of the τ2 phase and the q phase in the Calphad modeling, respectively [6].
The formation of the τ2 phase at Al15Mg43Zn42 with a lattice parameter a = 22.9 Å was first reported by Takeuchi [2]. The crystal structure was then described by Spiekermann [8] with a = 23.03 Å, cP676 and space group . Later Sugiyama [9] and Lin [10] both reported 23.06 Å for the lattice parameter a. In all studies the samples were annealed at 360 °C with the same nominal composition Al15Mg43Zn42. The composition of the τ2 phase was reported by Takeuchi to be Al15Mg43Zn42 and by Spiekermann to be Al14Mg43Zn43, thus exactly on or very close to the nominal composition. However, Sugiyama reports for the composition of the τ2 phase Al17Mg46Zn37 and Lin Al13Mg32Zn55. They are thus, due to phase separation, reported to be substantially different in Mg content, although the lattice parameter is reported to be nearly equal. The latter two compositions are points in the three-phase fields Φ + τ1 + q and τ1 + MgZn2 + (Al) according to the phase diagram of Petrov et al. [6]. None of the authors has reported a perceptible homogeneity range for the τ2 phase. In addition, the chemical disorder of the τ2 phase has been treated by the various authors in a different way.
The main objective of this work is to obtain reliable information on the stability range of the τ2 phase for an improved Calphad modeling. The outline of this paper is as follows. Firstly, we will show that the compositions Al17Mg46Zn37 and Al13Mg32Zn55 do not correspond to single phase τ2 material. Secondly, the heterogeneous equilibria of the τ2 phase will be discussed in detail based on multi-phase, equilibrated samples. Finally, we will discuss the chemical and configurational disorder in the crystal structure of the τ2 phase, based on an X-ray single crystal and neutron powder refinements, as well as first principles calculations.
Section snippets
Alloy preparation and characterization
In total, 29 Al–Mg–Zn samples were prepared from granules of the elements in this work. A detailed description of the alloy preparation and the characterization methods have been given already in Ref. [3]. Therefore, this section provides only additional information. All samples listed in Table 1, Table 2 were quenched in water after melting, to obtain a fine matrix, except sample No. 22. Samples No. 1–14 have been then annealed at 330 °C and No. 15–29 at 360 °C for 14 days and finally they
Phase analysis
To obtain information about the homogeneity range of the τ2 phase, a number of samples equilibrated at 330 °C and 360 °C has been investigated. At 330 °C the isothermal section is below the solidus surface, whereas at 360 °C the liquid phase appears already. The results are listed in Table 1, Table 2. Table 3, Table 4 provide information about the composition (WDXS), the lattice parameter a, the lattice parameter obtained by a linear fit (see below and Equation (1)) and the phases in
Conclusions
The homogeneity range of the τ2 phase or 2/1 approximant in the Al–Mg–Zn system has been determined at 330 °C and 360 °C. At 330 °C the Al content can vary about 5 at.%, the Mg content about 2 at.% and the Zn content about 6 at.%. The τ2 phase field extends from 14.1 to 19.3 at.% Al, 41.0 to 42.6 at.% Mg and 38.7 to 44.9 at.% Zn. Five neighboring phases have been observed at 330 °C: Φ, τ1, β, Mg21Zn25 and (Mg). The τ1 phase in equilibrium with the τ2 phase have both nearly the same composition.
Acknowledgments
We would like to thank Jens Hunger for the neutron diffraction measurements, Steffen Hückmann for the measurement of the powder diffraction data, Karin Reinhardt and Sylvia Kostmann for specimen preparation for the microstructural examination, Monika Eckert and Petra Scheppan for SEM pictures, EDX and WDXS measurements and Gudrun Auffermann, Ulrike Schmidt, Anja Völzke and Sebastian Schwinger for chemical analyses. Marek Mihalkovic was supported by the Slovak National Grants VEGA 2/0189/14,
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