Structural relationships among MgZn₂and Mg₄Zn₇phases and transition structures in Mg-Zn-Y alloys

Abstract

Isothermal ageing of plastically deformed Mg-Zn-Y alloys resulted in precipitation along {1012} twin boundaries. The bulky precipitates formed had structures similar to those recently reported for the rod-like β ′ 1 precipitates, but afforded a more detailed study by high resolution TEM due to their larger size. The core of the precipitates often had the structure of monoclinic Mg 4 Zn 7 phase, and had the orientation [0001]Mg [010]; {1010}Mg (201) with either the matrix or the twin. On this Mg 4 Zn 7 phase, hexagonal MgZn 2 phase grew in two orientations, both with [010]Mg 4 Zn 7 [1120]MgZn 2 . One of these orientations formed a known orientation relationship [0001]Mg [1120]MgZn 2 ; {1120}Mg (0001)MgZn 2 with the matrix. The part of the precipitate with the MgZn 2 structure was usually in direct contact with the twin boundary. Both Mg 4 Zn 7 and MgZn 2 phases have layered structures that can be described with similar building blocks of icosahedrally coordinated atoms. The atomic positions of zinc atoms comprise the vertices of these icosahedra and form "thick" rhombic tiles. Orientation of these rhombuses remain unchanged across the interfaces between the two phases. Near the interface with MgZn 2 , transition structures formed in Mg 4 Zn 7 phase, with Zn:Mg atom ratio between those of Mg 4 Zn 7 and MgZn 2 phases. In these transition structures, the unit cell of Mg 4 Zn 7 phase is extended along [100] or [001] by half a unit cell length by continuation of the rhombic tiling. Structures of these extended unit cells are proposed.Mg-Zn alloys since rare earth elements and yttrium segregate to ternary phase precipitates at the grain boundaries [11] and have not been detected within the β ′ 1 and β ′ 2 precipitates [9, 11]. Among the ternary phases also exists the quasi-crystalline icosahedral phase (I-phase, Mg 3 ZnY 6 ) [12,13]. This phase exists in equilibrium with α-Mg phase, forming a two phase field when the atomic ratio of Zn:Y is near 6, as in the alloy studied here.The monoclinic Mg 4 Zn 7 and Laves phase MgZn 2 phases include zinc lattice sites which possess icosahedral symmetry and are thus are structurally related. The monoclinic Mg 4 Zn 7 phase has been observed to co-exist with a decagonal quasi-crystalline approximant phase [14] and recent work has shown that Mg 4 Zn 7 and MgZn 2 phases co-exist within the rod-like β ′ 1 precipitates in Mg-Zn-Y alloys [15]. There appear to be structural similarities between the binary phases and the icosahedral phase and the monoclinic phase provides a model for the icosahedral structure [16]. A proper understanding of the relationships between the monoclinic and hexagonal phases would assist in clarifying their structural relationship to the i-phase and resolve the uncertainty regarding the phase present in the rod-shaped β ′ 1 precipitates. In the present work, extrusion followed by slight compression has been used to produce twins. Precipitation occurred along twin boundaries upon ageing. Twin boundary precipitates have been identified as belonging to Mg 4 Zn 7 and ...

Lattice Correspondence and Growth Structures of Monoclinic Mg4Zn7 Phase Growing on an Icosahedral Quasicrystal

“…The length of this unit cell A along a has shortened to about 22.3 Å. This unit is similar to that shown by Rosalie et al [23]. The other type of defect, B, shown in Figure 8c, can be described as one in which the hexagonal units along the line of fault have changed direction, and are now oriented along the fault line.…”

“…An interesting question is about the stability of these structures. Similar modifications of the Mg 4 Zn 7 structure shown by us previously were tied to chemistry (local composition) [23]. In the present study, however, they seem to arise out of the lattice constraint of a periodic crystal growing on a quasiperiodic lattice.…”

“…Table 1 reports the various structures observed as variations of Mg 4 Zn 7 . The structures Tr(A) and Tr(B) reported by Rosalie et al [23] are called transitional structures, referring to a transition to the MgZn 2 phase. Defects on plane (201) can be understood from Rosalie et al [26], where it is shown that an icosahedron can be defined at hexagonal sites of the Mg 4 Zn 7 unit cell, whose one of the twofold axis is along this plane.…”

On the scaling factor in Debye–Grüneisen model: A case study of the Mg–Zn binary system

“…Note that the exact stoichiometry for MgZn is not known, and Mg 51 Zn 20 is not of interest as it exists over a minuscule temperature range (598-614 K). Additionally, the stoichiometry for Mg 2 Zn 3 is debated with Mg 4 Zn 7 being accepted as the best model [41,42]. We have analyzed Mg 4 Zn 7 , MgZn 2 , and Mg 2 Zn 11 in the same fashion as Mg and Zn.…”

Precipitation and Hardening in Magnesium Alloys

“…The proposed crystal structure and the orientation relationship are subsequently confirmed in separate studies on precipitates in Mg-Zn-Y alloys. [66][67][68] In these latest electron microscopy studies, it is revealed that the Mg 4 Zn 7 phase has a complex substructure and planar defects elongated along the long axis of b 0 1 rods ( Figure 5(d)). In a very recent study, [69] it was reported that the b .…”