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THE ICOSAHEDRAL Al-Li-Cu PHASE
F. Gayle
To cite this version:
F. Gayle. THE ICOSAHEDRAL Al-Li-Cu PHASE. Journal de Physique Colloques, 1987, 48 (C3),
pp.C3-481-C3-488. �10.1051/jphyscol:1987355�. �jpa-00226586�
JOURNAL DE PHYSIQUE
Colloque C3, supplement au n09, Tome 48, septembre 1987
THE ICOSAHEDRAL Al-Li-Cu PHASE F.W. GAYLE
Reynolds Metals Company, Metallurgy Laboratory, Richmond, V A 23219, U.S.A.
The cammon grain boundary precipitate in ccmrmercial Al-Li-Cu-(Mg) alloys which is responsible for detrimental precipitate-free zones is the icosahdral Al-Li-Cu phase designated T2. The present work discusses the grawth of the T2 phase as large, faceted single quasicrystals and reviews results of morphological analyses and high resolution x-ray studies. These results wnfirm the icosahedral point symmetry of the phase and allm carmnent on the atamic structure and the disorder which appears to be inherent in the phase.
Metastable ias&edral inkmetallic phases have been found in several systems since their diswvery in the aluminum-manganese system [I]. Concurrently there has been much developent in the theory of quasiperiodic three-dimensional Penrose tilings and icosahedral glasses which have been used to explain the observed icosahedml diffraction phenomena [2-41. Analysis of iwsahedml structures has been facilitated by the discovery of such an @lib- r i m phase in the aluminum-lithium-copper system w h i c h can be pro- duced through low-solidification-rate casting [5] or by solid-state precipitation [6]. Of particular interest in A1-Li-Cu-X alloys, grain boundary precipitation of T2 leads to 8'-precipitate-free zone formation and grain boundary wedmess.
Recently large single crystals of the T2 phase have been pro- duced [7,8]. These crystals may form with regular facets and are suitable for analytical techniques w h i c h require single crystals or high purity to be effective. This paper describes recent results regarding struc-tural studies of the iwsahedral T2 phase.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987355
J O U R N A L DE PHYSIQUE
Figure 1. Slow solidification of the T2 phase is generally dendritic.
In shrinkage cavities residual liquid is pulled away f m the dendrites revealing sharp facets and a triacontahdral habit. The triacontahedral morphology shows that the point symmetry of the phase is iasahdral.
The T2 phase was described in detail by Hardy and Siloock [9]
thirty years ago. Their x-ray diffraction pattern "appeared fairly simplef1 but could not be kdexed. The phase was described as being close toA16CuLi3, with 10.5 a w c % Cu and 33.3% Li. This ccnrp~osi- tion corresponds very closely with recent analytical results indicat-
ing a oamposition of A15. 5Cu1. 1Li3, [lo]
.
In the present work, alloys of approximately 10 atcnnic % Cu and 30% Li were melted and either allowed to cool slowly in the crucible (-10 C/min) or bottom poured onto an aluminum chill plate. This procedure resulted in solids of 75-99% T2 purity.
In slowly cooled castings the T2 phase solidified denkiticdlly, with mst rapid growth along 5-fold axes [8]. M i t e s were often
obsemed in shrinkage cavities (Figure l), and typically were sharply faceted, as is ccmrmon for solidification of crystalline phases with a high entropy of fusion. It has recently been demnstrated that the equilibrium habit of quasicrystals is also expcted to be faceted [ll]
.
In addition to anisotropy of interfacial energy (equilibrim effects), the kinetics of attachment and ledge growth m y also contribute to the observed grccwth behavior.The most perfectly formed crystals have a rhombic triaconta- hedrdl habit with M i c faces which have an aspect ratio equal to the golden mean, or
Z
[(c +
1)/2]. Thirty such "golden rhcmbuse~~~make up the triacontahedron, with twelve 5-vertices and twenty 3-vertices. By N m l s principle [12] the extend habit of a freely q-rming crystal must contain Me elements of the crystal point spmetry. Thus the triacontahedrdl habit, with an
2 5
symmetry,indicates an i-al point symmetry. In this manner, the earli- est tool of the crystallographer, that of habit analysis [13], has been used to establish the icosahedral synnnetry of the T2 phase.
The T2 phase is brittle and fractures conchoidally when
strained. Flakes of the fractured dendrites have been examined using
transmission electron micmsmpy. The individual flakes show cam- plete icosah- symmetry in seleded area diffraction or convergent beam diffraction using mall (50 rnn) probe diameters (Figure 2).
Scaling in the diffraction pattern of this phase is by
T.
C3-484 J O U R N A L DE PHYSIQUE
Figure 2. Conchoidally fractured chip of T2. Icosahdml symmetry is reflected i n comergent-beam electron diffraction patterns:
b) 5-fold pattern, B = [100000], c) 2-fold symmetry [110000].
Figure 3 . Blocky morpholcgy obsewed for T2. Facet edges remain angularly correlated throughout the f i e l d of view, suggesting t h a t the entire area is a "single q u a s i c r y ~ t a l . ~ ~ Such regions may extend for areas up t o a square centimeter.
Solidification mrphology can take various forms depending upon cooling corslitions. These forms vary frcnn blocky faceted structures (Figure 3) to a wtannenbaumll morphology which may not be faceted (Figure 4). In each case, elements of iaosahedral symmetry are preserved.
X-RAY STUDIES
X-ray absorption fine strudxre (XAFS) techniques were used to probe the environment of the Cu atoms in the T2 phase and in the closely related crystalline (cubic) R phase (Figure 5 ) [14]. The results indicate that Cu has nearly identical enviromts in the two
phases. This leads to the conclusion that the two phases are pro- duced fm the same structural units. By analogy with c3erkashi.n et dl. [15], the unit is concluded to be the 105-am truncated icosa- hedron given for the R-phase. CoincidentdLly, this unit is also part of a recent twinning model given for icosahedral phases by Paul*
[161.
High resolution scattering experiments indicate that there is considerable disorder in the T2 phase [lo], as indicated by the large peak widths in Figure 6. Figure 6c shcws that the disorder, measured by HWiM of the peaks, is not due to elastic stxains, which would scale with scattering vector, g. There is a linear relationship, however, between HWM and phason mmentum, or gg. Such disorder in the phason variable is inherent in the icosahedral glass models of the phase, and can also be intrcciuced in the Penrase tiling mcdels
when matching rules are not completely follawed. The magnitudes of
HWM of slowly solidified T2 ard rapidly solidified i - M i are similar, suggesting that a specific degree of disorder m y be a universal characteristic of i-phases [lo].
Fram the present study, it can be concluded that the T2 phase is an equilibrium phase which can be produced by slcw solidification ( - 1 0 s )
.
Disorder appears to be inherent, and it is suggested that the degree of disorder is not a function o f pmcessing condi- tions. The structual unit is the 105-at.qn1 truncated icwahdmn similar to that of the R-phase.C3-486 J O U R N A L DE PHYSIQUE
Figure 4. Another morphology f o r t h e T2 phase is a non-faceted lltaUnenbaumll habit. The grcwth direction is inmriably
slow axes of five-fold q m ~ t r y . The large plate-like phase is TI, or A12CuLi.
Figure 5. a) XAFS results 1141 for the Cu K edge for the T phase solid) and the cubic R phase (dashed). The structural unit ?s pro- posed to be the same for the two phases: b) the 105-atam( truncated
~cosahedral unit of the R phase (arranged on a BCC lattlce) deter- mined by QIerkashin, et 61. [15].
a ) z
3000 VERTEX A A
N
p
2000z
2
1000 OTHER V0
2 62 2.63 2.64 2.65
Q (A-')
b
5
60002 Y
4000
\ cn
I- Z
2
2000 00
2 7 4 2.76 2.78 2 80 0 4 8 12 16
o(A-l) G ,
(A-')
Figure 6. Single crystal high resolution x-ray scattering results for the T phase [lo]
.
Scattered intensity versus momentum transfer, Q, for3)
(100000) (vertex) and b) (110000) (edge) l3ragg peaks.(Interplanar spacing d = 2 /Q). m e Bragg peaks have considerable width. In addition, vertex p a k s have bimodal structure. c) Half width at half m a x h for various peaks plotted as a function of phason momentum. Inset shows that INHM is not a function of scatter- ing vector (g) magnitude.
JOURNAL DE PHYSIQUE
As indicated by the references, this paper has drawn on the work of a mmber of people. In addition, discussions with P. M. Horn, D.
L. Kaiser, P. A. Bancel, P. A. Heiney, M. H. Skillhg-keq, H. M.
Edwards, Y. Ma, E. A. Stern, ard B.
J.
Wuensch have been valuablethroughout this study. The XAFS and X-ray scatterbig studies were confiucted at the National Synchrotron Light Source, Brookhaven National Iabratory.
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