Crystallographic orientations and interfaces of icosahedral quasicrystalline phase growing on cubic W phase in Mg–Zn–Y alloys

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Abstract

Formation of icosahedral quasicrystalline phase on the W cubic phase in α-Mg matrix has been studied in a Mg97Zn2.5Y0.5 alloy. The orientation relationship of the W phase with the matrix is determined to be [0 0 1]W[1¯00]Mg, (1 1 0)W∥(0 0 1)Mg, equivalent to Burgers orientation relationship. The icosahedral phase forms curved interfaces with the W phase, but planar interfaces on five-fold and two-fold planes with the matrix. The icosahedral phase shows its unique orientation relationship to the W phase, which results in two orientation relationships WOR1 and WOR2 with the matrix. WOR1 is a slight deviation of the orientation relation, OR1, of icosahedral phase precipitates in the matrix. In WOR2, one of the five-fold axes occur along the hexagonal axis. The resulting morphology and the interface structures are described here. It is shown that WOR1 and WOR2 are related to each other by twinning of the icosahedral phase.

Introduction

In Mg–Zn–Y alloys, an icosahedral quasicrystalline phase exists in equilibrium with the magnesium matrix [1], [2], [3], due to which stable two phase microstructures can be formed. The icosahedral phase in such alloys can act as a strengthening phase. In dilute alloys, the quasicrystalline phase solidifies as a eutectic phase in the interdendritic spaces. Thermomechanical processing, such as hot rolling [4], [5] and extrusion [6], [7], have been employed to distribute this phase finely in the matrix. The icosahedral phase also precipitates in the matrix [8]. The icosahedral phase has better stability among other phases and pins grain boundaries and dislocations [7].

The icosahedral phase is a quasiperiodic phase with point symmetry m 3¯5¯, which shows a five-fold symmetry and in which the position of successive reciprocal spots is not periodic but scaled by an irrational number known as the Golden mean τ=(1+5)/2=1.618034 [9]. The symmetry in reciprocal space is that of the regular solid icosahedron, which has 20 isosceles triangular faces. Its 12 vertices possess a five-fold symmetry. The major zone axes in the icosahedral phase are 6 five-fold axes, 10 three-fold and 15 two-fold axes. Because of its quasiperiodicity, its diffraction patterns cannot be indexed using schemes for the periodic crystals. Two indexing schemes in use are given by Elser [10] and Cahn et al. [11], both using six integer indices. These two schemes are complimentary to each other [12]. The Elser indices are used in this paper, in which the nearest diffraction spot along a five-fold reciprocal vector in indexed {100000}. Two most intense reciprocal spots are the {211111} which occur along five-fold vectors and {221001} which occur along two-fold vectors (the Mg–Zn–Y icosahedral phase is ordered F type [2] in which the indices are doubled; however, in this paper only primitive indices are used). Diffraction along the two-fold axes is most informative because this zone contains all the important reciprocal vectors—five-fold, three-fold and two-fold. In the two-fold diffraction pattern, a rectangle drawn by joining the five-fold vector spots {211111} is called a Golden rectangle, because the ratio of the length of its sides is the Golden mean τ. The Mg–Zn–Y icosahedral phase shows a pentagonal dodecahedral morphology [2], which suggests that five-fold planes form low energy surfaces. Twinning of the icosahedral phase is observed to occur on five-fold planes, the matrix and the twin sharing a five-fold axis [13], [14].

An orientation relationship (OR) occurs between the icosahedral phase and magnesium, called OR1 here, which is described by Singh et al. [8]. In this orientation relationship, three mutually perpendicular icosahedral two-fold zone axes occur along the hexagonal, [1 0 0] and [1 2 0] axes. A set of icosahedral two-fold planes is parallel to the hexagonal basal planes. A variant of this relationship, called OR2, also occur [7]. Observed occasionally, in this variant too, three of the icosahedral two-fold axes, which are mutually perpendicular, occur parallel to hexagonal, [1 0 0] and [1 2 0] axes, but in this case the icosahedral lattice is rotated 90° with respect to the hexagonal lattice around the hexagonal axis. In both OR1 and OR2, two of the icosahedral two-fold reciprocal vectors are parallel to matrix (0 1 0) and (21¯0), but while in OR1, two of the five-fold vectors correspond to {1 0 0} and in OR2, they correspond to {1 1 0}. Singh et al. [15] have further shown that even more ORs are possible. In an orientation relationship called OR3, a five-fold axis occurs along the matrix hexagonal axis. These three orientation relationships are shown to be related to each other by twinning of the matrix or the icosahedral phase.

A cubic W phase Mg3Zn3Y2 competes strongly with the icosahedral phase in dilute Mg–Zn–Y alloys. It also exists in equilibrium with the matrix, and can also exist in equilibrium with the icosahedral phase. A crystallographic orientation relationship between the icosahedral and the W phase is described by Singh and Tsai [16].

Here, we present studies of interfaces of icosahedral phase formed by solid state reactions in solutionized dilute Mg–Zn–Y alloys on annealing at 400 °C. Formation of the icosahedral phase occur in the matrix and the W–Mg interfaces. The orientation of the icosahedral phase formed on the W phase is dictated by the W phase, rather than the matrix. This leads to two new orientation relationships between the icosahedral phase and the matrix. We have studied the orientation relationships, the resulting morphology and the interfaces.

Section snippets

Experimental

Alloy with composition Mg–2.5 at%Zn–0.5 at%Y was prepared in an electric furnace. Isothermal annealing treatments were performed on the alloy. It was solutionized at 460 °C (the eutectic temperature) for 15 h. The solutionized samples were annealed at 400 °C for 30 min, followed by water quenching. The microstructure was studied by transmission electron microscopy (TEM). For preparing for TEM observations, samples were sliced by a low speed diamond saw, thinned mechanically and finally ion milled. A

Results

On solutionizing at 460 °C, the grain boundary icosahedral phase transformed to a hexagonal phase H with lattice parameters a = 0.918 nm and c = 0.95 nm [17]. Simultaneous with this, started dissolution of the grain boundary phase. The composition of the H phase was determined to be Mg–60 at%Zn–20 at%Y. Rod-like precipitates β1, bearing the structure of MgZn2 and oriented parallel to the matrix hexagonal axis, occurred in the matrix.

On annealing at 400 °C, the grain boundary phase transformed to the W

Discussion

The W phase is a competing phase with the icosahedral phase. Phase diagrams [3] show that both these phases can exist in equilibrium with the matrix. The W phase forms at higher temperatures. According to the ternary phase diagram [19], the W phase can form directly from the melt by a eutectic reactionL=Mg+W;550°CThe W phase and the icosahedral phase I are in equilibrium by the reactionL+W=Mg+I;448°C

The primary eutectic phase has been mentioned in the ternary phase diagrams [19] as

Conclusions

A Mg97Zn2.5Y0.5 alloy containing icosahedral quasicrystalline phase was solutionized at 460 °C. Upon this treatment, the icosahedral phase dissolved or transformed to a hexagonal H phase. On annealing again at 400 °C, icosahedral phase formed by nucleating heterogeneously. The crystallographic orientation, morphology and interfaces with the matrix of the icosahedral phase formed on W phase by the reaction of the W phase and the matrix are studied here.

  • (1)

    The W phase exhibits and orientation

Acknowledgement

This work is partially supported by Japan Science and Technology Corporation.

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