On the cubic W phase and its relationship to theicosahedral phase in Mg–Zn–Y alloys

Alok Singh *, A.P. Tsai

Materials Engineering Laboratory, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba 305-0047, Japan

Received 19 March 2003; received in revised form 10 April 2003; accepted 10 April 2003

Abstract

Crystallographic relationship is determined between cubic W-Mg2Zn3Y3 and icosahedral phase in Mg–Zn–Y alloys.

The W phase coexists with three h111i axes along three of icosahedral twofold axes, and three h110i axes along fivefoldaxes. The orientation relationship implies that this cubic phase is not an approximant to the icosahedral phase.

� 2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

Keywords: Magnesium alloys; Electron diffraction; W-phase; Quasicrystals; Approximant

1. Introduction

One of the technologically important alloy sys-

tems of magnesium is based on Mg–Zn–Y. It can

be viewed as age hardenable Mg–Zn system in

which yttrium is added to improve casting char-

acteristics, improve higher temperature strength by

elevating the eutectic temperature, and delay theonset of overaging. Several ternary phases occur

on addition of yttrium [1], most prominently a

hexagonal Z phase, a cubic W phase and an

icosahedral quasicrystal phase. The W phase

(Fm3m a ¼ 6:83 �AA) in this alloy system was first

reported by Padezhnova et al. [2]. The icosahedral

phase forms in these alloys as intergranular eu-

tectic phase. Recently the icosahedral phase in thissystem has been utilized as a strengthening phase

[3–5]. Thermomechanical means have been utilized

to distribute icosahedral phase as fine particles in

the matrix.

It has been reported that the cubic W phase also

forms in these alloys as eutectoid morphology on

solidification [6,7]. We have observed a coexistence

of the W phase with the icosahedral phase in a

Mg95Zn4:2Y0:8 alloy after extrusion at 673 K [5].This phase disappears on further heat treatment at

673 K. It can be concluded that the coexistence of

these two phases is a partial state of this trans-

formation. Here we show the close crystallo-

graphic relationship between these two phases,

studied by electron diffraction.

2. Experimental procedure

An alloy of composition Mg95Zn4:2Y0:8 was

prepared by melting high purity elements in anelectric resistance furnace. The as-cast alloy was

*Corresponding author. Tel.: +81-298592346; fax: +81-

298592301.

E-mail address: alok@quasi.nims.go.jp (A. Singh).

1359-6462/03/$ - see front matter � 2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

doi:10.1016/S1359-6462(03)00217-3

Scripta Materialia 49 (2003) 143–148

www.actamat-journals.com

annealed at 673 K for 10 h. The alloy was then

extruded at 673 K with a reduction ratio of 10:1.

Sections from these specimens were cut and

thinned mechanically, followed by ion milling, forobservation in the transmission electron micro-

scope (TEM). A JEOL 2000FX-II microscope

operated at 200 kV was used for observations.

3. Results

The extruded alloy showed existence of inter-

granular icosahedral particles, a few hundred na-

nometers to a micron in size. In addition, there

were nano-sized icosahedral particles in the ma-trix, well facetted and showing a definite ori-

entation relationship with the matrix magnesium

phase, details of which are presented elsewhere [8].

Occasionally the icosahedral phase was found to

coexist with the cubic W phase. In such cases, the

icosahedral phase was not well facetted. Fig. 1

shows one such coexistence and a series of com-

posite diffraction patterns at various angles of tilt-ing. It shows an icosahedral twofold zone axis

parallel to a cubic threefold axis, and mirror zone

axis parallel to cubic axis. A stereogram repre-

senting this orientation relationship is shown in

Fig. 2.

The series of composite diffraction patterns il-

lustrate the lattice match between these two phases

in this orientation relationship. The diffractionspots from the W phase coincide with intense spots

from the icosahedral phase. Fig. 1b illustrates the

match between the mirror zone axis of the icosa-

hedral phase and a cubic axis [0 1 0]. The twofold

vector is along (1 0 1), while the fivefold vector

is along (1 0 �11). The fivefold {2 1 1 1 1 1} and thetwofold {2 2 1 0 0 1} reciprocal spots are almost

coincident with the {2 0 2} spots.Tilting along the twofold vector is arrived at a

threefold and then a twofold zone axis (Fig. 1c).

The twofold zone axis is along the [1 2 �11] axis, inwhich the twofold vectors are along (�11 1 1) and(1 0 1) vectors, while the fivefold vectors are nearly

parallel to {1 1 3} vectors. Tilting further leads to a

threefold axis coinciding with the cubic threefold

axis [1 1 �11]. In Fig. 1d is a fivefold axis coincidentwith [�11 3 1] zone axis, where one of the twofold

vectors is along (1 0 1) while two others are along

{4 2 2}. The {4 2 2} spots lie close to the twofold

vector spots {3 2 2 0 0 2}.

Tilting from the mirror zone axis (Fig. 1b) alongthe fivefold vector, is arrived a twofold zone axis

parallel to [1 3 1] (Fig. 1e). Further tilting brings to

the mirror zone axis at position [1 2 1] (Fig. 1f).

The twofold vector is along (1 �11 1) while the five-fold vector is along (1 0 �11). Finally, the twofoldzone axis is arrived coincident with the cubic

threefold axis [1 1 1]. A twofold axis is along

h110i, and the two fivefold axes are nearly alongh110i of the W phase (Fig. 1g).

The interplanar spacing of the W {2 2 0} planes

is nearly equal to the most intense icosahedral

spots {2 1 1 1 1 1} (along the fivefold reciprocal

direction) and close to the icosahedral spots

{2 2 1 0 0 1} (along the twofold direction). As seen

above, these planes with nearly equal interplanar

spacings are parallel to each other. The quasilat-tice parameter of the Zn–Mg–Y icosahedral phase

is 5.2 �AA [1], which gives the interplanar spacing

for {2 1 1 1 1 1} planes as 2.45 �AA and for the

{2 2 1 0 0 1} planes as 2.33 �AA. It can be comparedwith the interplanar spacing 2.41 �AA of the {2 2 0}planes of W.

4. Discussion

Yttrium has almost no solubility in magnesium.During solidification magnesium-rich grains so-

lidify first, followed by nucleation of the ternary

phases in the solute-enriched liquid in interden-

dritic spaces. The Mg–Zn–Y ternary phase dia-

gram shows that the icosahedral phase as well as

the W phase each is in equilibrium with the melt at

873 K [1]. A fluctuation in the liquid composition

can cause the formation of both the phases. Thesetwo phases can co-exist, because of a two-phase

field between them. In dilute alloys annealing may

transform it to the icosahedral phase, which has

lesser yttrium content.

The orientation relationship between the icosa-

hedral phase and the cubic W phase is similar to

one of the orientation relationship between an

icosahedral phase and a simple cubic phase suchas, for example, elemental lead [9]. It is important

144 A. Singh, A.P. Tsai / Scripta Materialia 49 (2003) 143–148

Fig. 1. (a) A bright field micrograph showing coexisting icosahedral phase and cubic W Zn3Mg2Y3 phase, denoted IQC and W, re-

spectively. (b–g) Electron diffraction patterns from a titling sequence from the following zone axes of the W phase (b) [0 1 0], (c) [1 2 �11],(d) [�11 3 1], (e) [1 3 1], (f) [1 2 1] and (g) [1 1 1].

A. Singh, A.P. Tsai / Scripta Materialia 49 (2003) 143–148 145

to note that this orientation relationship is very

different from the orientation relationship between

an icosahedral phase and its cubic approximant

phase. It follows from the projection formalism

that in case of the cubic approximants, along the

three cubic h100i axes occur icosahedral twofoldaxes [10].

No approximant phases to the quasicrystals are

known to exist in the Mg–Zn–RE (RE@Y or rareearth element) system [11]. Instead, hexagonal

phases occur which are related to the icosahedral

phase [11,12]. Even though the W phase shows a

good crystallographic match with the icosahedral

phase, it is not an approximant phase either. Thisorientation relationship is in some ways similar to

that between the icosahedral phase and the hex-

agonal phases, in the sense that in case of the

hexagonal phases a twofold axis (instead of a

threefold axis) is along the hexagonal axis. Simi-

larly, in case of the icosahedral phase and the cubic

phase, a twofold axis is along a threefold axis.

However, this orientation relationship can also be

viewed with an icosahedral threefold axis parallel

to a cubic threefold axis. Thus this orientation

relationship can be described in two ways, as il-lustrated below.

The orientation relationship between the W

phase and icosahedral phase is illustrated by in-

scribing an icosahedron inside a cube in Fig. 3a

and b. These two figures show the same relation-

ship viewed along two different h111i axes of thecube. When viewed along the [1 1 1] axis of the

cube (Fig. 3a), the icosahedron is seen along one ofits twofold axes. In this orientation, three of the

icosahedral twofold axes are seen to be coincident

with cubic h110i axes (along �c� and �c0� andanother pointing upwards). Two of the fivefold

axes are also nearly parallel to two other h110i

Fig. 2. A stereogram showing the orientation relationship between the icosahedral phase and the cubic W phase.

146 A. Singh, A.P. Tsai / Scripta Materialia 49 (2003) 143–148

cubic axes. A yet another icosahedral twofold axis

is coincident with a cubic h11�22i direction.When viewed along another cubic threefold di-

rection [1 1 �11] (Fig. 3b), occurs the icosahedronalong one of its threefold axes. Thus one each of

threefold axes of both the phases coincide. In this

orientation it is immediately observed that three of

the icosahedral fivefold axes are nearly along cubic

twofold (h110i) axes (at b1, b2 and b3). The coin-cidence of three of the icosahedral twofold axeswith cubic h110i axes is also readily seen. Theclose fitting of the outer envelopes of the two solids

brings out the close match between the two lattices

in this orientation.

A close relationship is shown by the fact that

similar planes/axes of one phase match with more

of similar planes/axes of the other phase. Here

three of the six fivefold zone axes are close toh110i axes of the W phase and the other three to

h113i axes. Three of the twofold axes are along orclose to h110i, another three close to h111i, threemore to h211i and the rest six are close to h113i.Thus all the fifteen twofold axes are also along low

index directions. The h113i axes emerge out tobe the most prominent ones matching with the

icosahedral axes. Another important axis is themirror zone axis. Six of these are along h112i, five

along h221i and three along h100i. This is sum-marized in Table 1.Luo et al. [6] also reported a superstructure W0

with three times the unit cell of the W. A similar

phase in Zn–Mg–Er system has been considered

to be an approximant phase to the icosahedral

quasicrystal [13]. The correspondence considered

between this cubic phase and the icosahedral phase

conforms to the orientation relationship deter-

mined here. However, for this same reason the W0

phase cannot be considered an approximant struc-

ture to the icosahedral phase. Through structural

Table 1

Zone axes correspondence between the W phase and the

icosahedral phase. Same correspondence between planes of

same indices holds

Icosahedral Cubic W phase Number

Twofold h1 1 0i 3

h1 1 1i 3

h1 1 2i 3

h1 1 3i 6

Fivefold h1 0 1i 3

h1 1 3i 3

Mirror h1 0 0i 3

h1 1 2i 6

h1 2 2i 5

Fig. 3. Solid figures illustrating the orientation relationship between the cubic W phase and the icosahedral phase. (a) When viewed

along the cubic axis [1 1 1], the icosahedron is in a twofold orientation. (b) Viewed along another cube axis [1 1 �11], the icosahedron is inoriented along one of its threefold axes.

A. Singh, A.P. Tsai / Scripta Materialia 49 (2003) 143–148 147

model it has been further shown that the space

group of this phase is F�443m, which is not a sub-group of the icosahedral space group [14]. Thus

the orientation relationship presented here givesfurther evidence to the argument [15] that this

phase is not an approximant phase.

5. Conclusions

Coexistence of the cubic W phase Mg2Zn3Y3with icosahedral phase in an extruded Mg95-Zn4:2Y0:8 alloy has been studied by electron dif-

fraction. Diffraction spots from the W phase

coincide with intense spots from the icosahedral

phase, and a good correspondence of major axes

and planes is observed. The orientation relation-

ship between these two phases can be described in

two different ways (i) a twofold axis of the icosa-

hedral phase coincident with a {1 1 1} zone axis ofthe W phase and/or (ii) a threefold axis of the

icosahedral phase coincident with a {1 1 1} axis of

the W phase. In this relationship three W h111iaxes occur along three of the icosahedral twofold

axes, three of the h110i axes occur nearly parallelto icosahedral fivefold axes, and the three h100icubic axes occur along icosahedral hs10i zone axeswith square-like appearances. This orientationrelationship implies that this phase, or its super-

structure W0, cannot be considered as an approx-

imant to the icosahedral quasicrystal.

Acknowledgement

This work is partially supported by Japan Sci-

ence and Technology Corporation.

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