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A TEM STUDY OF ICOSAHEDRAL AND NEAR ICOSAHEDRAL PHASES IN 8090 ALLOY
G. Lapasset, A. Loiseau
To cite this version:
G. Lapasset, A. Loiseau. A TEM STUDY OF ICOSAHEDRAL AND NEAR ICOSAHEDRAL PHASES IN 8090 ALLOY. Journal de Physique Colloques, 1987, 48 (C3), pp.C3-489-C3-495.
�10.1051/jphyscol:1987356�. �jpa-00226587�
JOURNAL DE PHYSIQUE
Colloque C3, suppl6ment au n o 9, Tome 48, septembre 1987
A TEM STUDY OF ICOSAHEDRAL AND NEAR ICOSAHEDRAL PHASES IN 8090 ALLOY
G. LAPASSET and A. LOISEAU
ONERA, B.P. 72, F - 9 2 3 2 2 C h a t i l l o n C e d e x , F r a n c e
SUMMARY
An icosahedral phase (i-phase) and a crystalline tetragonal phase (c-phase).
which are formed during annealing of 8090 alloy, exhibit interesting properties:
both phases obey definite orientation relationships with A1 matrix. Moreover, as shown by a systematic study of c-phase electron diffraction patterns, the motif of this phase appears to be near icosahedral. The c-phase can thus be considered as an approximant structure of the icosahedral phase.
In Al-Li-Cu-(Mg) alloys, an icosahedral phase can form by solid state reaction in as-equilibrium conditions /I-5/. Moreover, the quasi-crystalline precipitates exhibit typical morphologies and develop systematic orientation relationships (OR.) with the Al f.c.c. matrix /2,3/. In the case of Al-Li-Cu and Al-Li-Cu-Mg alloys with a low k4g content ( 0.2 2 ) . the icosahedral phase, T2 (AlgCuLi3). can coexist with a related periodic phase, R (A15CuLi?). OR. between T2 and both R and A1 cubic phases fit the natural orientation between a cube and an icosahedron : 3-fold axis are in correspondence so that a set of three orthogonal 2-fold axis are parallel to the <loo, axis of the cube /2/. In 8090 alloy with a 1% Mg content, the icosahedral phase (i-phase) exhibits quite different OR. with A1 and coexists with a tetragonal phase (c-phase). Furthermore, a strong connection can be expected between i- and c- phases /3/. The objective of this paper is to continue the description of orientation relationships between i-phase. c-phase and A1 and to investigate the correspondence between i - and c-phases as deduced from electron diffraction patterns comparison.
EXPERIMENTAL
The composition of the alloy studied here is : 2.5 % Li, 1.3 % Cu. 1 % Mg. 0.09 %
Zr (wght 2 ) . After the solution heat treatment (Ih. at 540.C). the specimens were cold wafer quenched and then annealed. Main work was devoted to examinations of 400'C annealing practice (during lh., 4h., 24h.. 32 days) but another annealing condition (4h. at 330.C) was also studied. Thin foils for transmission electron microscopy (TEM) investigations were double-jet electrothinned in a 1/3 H N q - 2/3 methanol electrolyte at -40'C.
Previous examinations of as-quenched specimens showed no presence of i- and c-phases / 3 / Therefore, these phases grow during the annealing treatment. The i-phase nucleates both within grain (or subgrain) boundaries and within the matrix.
The c-phase precipitates within grain (or subgrain) boundaries during a 400' or 330'C annealing but it can also formwithin the matrix during the 330'C annealing.
ORIENTATION RELATIONSHIPS BETWEEN I-PHASE, C-PHASE AND AL MATRIX OR. between i-ohase and A1 matrix
Four OR, were observed. All of them exhibit a common property : a basic 2-fold symmetry axis of the i-phase is parallel to an (110, A1 axis. This axis coincides with
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987356
C3-490 J O U R N A L D E PHYSIQUE
the growth axis of quasi-crystals which generally show a pencil-like morphology.
Three of these O.R. have been previously described and illustrated in /3/ and are labelled O.R. 1.2 and 3. The fourth one (labelled O.R. 4 ) was observed i n the 46. at 330.C annealed specimen but it seemed to occur rarely.
Typical features of these O.R. are determined by considering the i-phase axis lying i n the plane perpendicular to the basic 2-fold axis (see Fig 1 where the basic 2-fold axis is parallel to IOlll All:
- O.R. 1 : the 2-fold axis a r e respectively parallel to l l l i l and to [Zlil A1 axis.
- OR. 2 : the 2-fold and the J-fold axis roughly coincide with I1001, I01i1, 11111 and [ i ~ i ~ A{ directions.
- OR. 3 : one of the 5-fold axis is parallel to [I l f 1 A1 direction and a 3-fold axis is close to I0111 A1 axis.
- OR. 4 : as in OR. 1. a ?fold axis coincides with [ l l i l A1 (Fig 2a and 2b). A
?-fold axis is nearly parallel to I01 11 A1 axis (Fig 2c). Note that in this O.R. a 3-fold symmetry axis is preserved.
4,
F I G E I: The /bur orienlalja r n l a ~ ~ o n s h ~ ) ~ , labelled l,Z,3, <
observed between the i-p4a.w and h e matrix : they ar6
defined by the reIaCive orientation between the stereographicprojecbbn o f tht i-pbme aloag a 2-foldm~s a r r ~ that of the matrix along a /Of 1, axis parsllel to the 2-fold mis.
Only the FEpoles f y ~ h g in the (Ulllplane are sketched
FIGURF2 : D!Y[racrion patterns related to the orientalton rnldionsh~ps I and a/
OX I : the (ZljlAl zone axis ispara/lel to a i-2-fo/d mis. bd) and cl OR. 4 : bd) the (TfI'lAl zone axis is parallel to a i-2-fold axis Note the rotalrbd b y 97' of the i-symmetry axis between this pattern andthat in Pig Za, cl the (21 0d)Al zone axis is parallel to a i-3-foldaxis. This featurn isparticulw to the OR. 4
Orientation relationshivs between c-phase and A1 matrix Two definite O.R. were observed :
- i n the firstone, (110)c//(1_11)Al and [liolc / / [ I I O ~ A I - i n the second one, ( 1 1 0 ) ~ // (211) A1 and [0011c // [I101 A1
T h i ~ l e t O.R. is illustrated in Fig 3 showing a characteristic ( i l i ) A1 diffraction pattern ( [2111 A1 is roughly parallel to [1101c 1.
FIGL4W3: al Diffracbun psCCBrns typical of t&e scond Oh? between t&e c - p h m andAk t&e (I IIIAlzone a8isispadIeI to a (IfUlc &ris
bl Bright fiefdimage of a c-prec~pitssfe in t&e orien/ation sho wedin al.
EXAMINATIONS OF C-PHASE DIFFRACTION PATERNS
It must be recalled that the tetragonal c-phase can be described as a superlattice of a fundamental f.c.c. latiice ( a = 2 nm.) with .an irregular streaking along the [0011c axis which is due to a long period ( = 7nm.I modulation of planar defects /3/. The f.c.c. notations of the basic lattice are kept along thereafter.
Therefore a diffraction paUern can contain, besides fundamental spots, additional spots introduced by the intersection between this streaking and the Ewald sphere. their relative intensities being related to the nature of the motif. This study has shown that in different c-phase diffraction patterns the distribution of the most intense spots is very close to the distribution of the most intense spots i n the true 5-fold 3-fold and 2-fold symmetry patterns of the i-phase. As f a r as they are concerned. c-phase diffraction patterns and corresponding axis are denoted pseudo 5-fold, ffold, 2-fold symmetry patterns and axis. Some of these diffraction p a t e r n s a r e shown in Fig 4, 5 , 6. Certain of these axis correspond to genuine rows of the reciprocal space, for instance to (1 1 0 ~ and (1 1 l )c rows. Other pseudo-axis are in fact
"rough rows" and can only be defined as "mean rows". Let us consider for instance, the pseudo 3-fold pattern labelled 3d i n Fig 5 : it contains three pseudo 2-fold axis at SO'? 1' each other enhanced by white rows. The horizontal one is a true row, being a I I l l l c row, and along it. a11 reflections are perfectly aligned. Along the two others, intense reflections are not aligned but slightly oscillate around a mean direction ,
thus defined as the direction of the pseudo axis. As a consequence, planes perpendicular to such a pseudo axis are also "rough planes". It is worth mentioning that two among the pseudo 2-fold axis are exactly parallel to the 11101 c and IllOIc directions and consequently a r e true 2-fold symmetry patterns. Of course, the whole set of pseudo axis is submitted to the symmetry elements of the tetragonal structure.
I t is the reason why two different pseudo symmetry axis (for instance a pseudo 2-fold and a pseudo 5-fold) can be found lying within afew degrees.
C3-492 JOURNAL DE PHYSIQUE
FIGCrRFf: The diffemnt pseudo 5-fold symmetty p-rns observed in t h e c-phm.Their noL&on and t h e k locaCion in the c-miprocaf space is defined ia Irfg 7 Pos2~on of the pseudo 2-foldaxis conCsinedin th8seD.P areindic&dbymws.
PlGCrRFJ: The diffemnt pmudo 3-fod symmetty pBUBrns observed in the c-phm.TheiraoL&on and .!heir location fn the c-~~~cf;Pmcafspace b d e f n e d h lFig 7 &si~on of thepmudo 2-foldaxis cor~~whedia thesD.P are~hdicatedbymws.
FIGURF6 : 3 hrnpwimn between the &rue i-2-foldpattern, denotedi-Z, a n d the p ~ u d o c-2-fold paltern labelled Za ((l0I)c zone1 Posi&ons of Clle Crue andpseudo 5-,3-,2--fold&1isare indicztedid both paUerns by r o ws.
6176-e other pseudo 2-foldpatferns o f t h e c-phase. Their dotatfun rroo fheirlocation i n the c-reciprocuf space isdefinedin Fig il
In fact, taking into account the symmetry elements of the c-phase, it is possible to divide the set of pseudo axis into four equivalent families. One family can be deduced from another through a nm/4 rotation around the I001lc axis. The location, in the c-reciprocal space, of the pseudo axis belonging to one family is given in Fig 7a. In a given family, diffraction patterns exhibiting a given pseudo symmetry are not all equivalent (Fig 7a). There are four pseudo 5-fold patterns (labelled 5a to Jd) which are shown in Fig 4 : two of them are 1110)~ and (101)~ zones (respectively DP 5c and 5b in Fig 4) and the two other are near ( 1 1 3 ) ~ and ( 3 1 1 ) ~ zones (respectively DP 5a and 5d in Fig 4). There are six pseudo 3-fold patterns (labelled 3a to 3f) which are shown in Fig 5; the DP 3a is a (1ll)c zone. There are seven pseudo &fold patterns labelled 2a to 2%. Four of them are shown i n Fig 6. Note that ( 1 1 0 ~ and <101,c axis are decoded at the same time as a pseudo 5- and as a pseudo 2-fold axis : this is illustrated by patterns 56 and 2a which correspond to the (101)~
zone (Fig 4 and 6 ) . In each family, the total number of pseudo axis is compared in Fig.
7b with that of the m33 point group symmetry axis, assuming ths6 a (1 10lc axis. which is a true 2-fold axis, exactly corresponds to a 2-fold axis of the m35 point group. The fit is almost perfect, angular deviations being less than five degrees. The remarkable quality of the fit is to be related to the small roughness of pseudo axis in D.P..
This comparison clearly shows that the-ephase motif has a near icosahedraJ symmetry. As it will be demonstrated elsewhere / Loiseau A.. Laoasset G. submitted to Phil. ~ a g . / , taking into account this near icosahedral symmetry allows to find a consistency between the c-phase/ A1 OR. and the &phase/ Al 02.. This result reinforces the idea of a structural connection between i- and c-phases.
Unfortunately, direct transformation of one phase into another has not been observed up to now; moreover. the precise structure of the c-phase is not yet elucidated. Nevertheless. in a next paper / Loiseau A,, Lapssset G. submitted to Phil.
Mag./, two temptative approaches for describing this structural connection will be proposed and discussed within the framework of crystalline approximant structure of the quasi-crystalline structure.
JOURNAL DE PHYSIQUE
FIGURFP: al S&mogmp&ic pmjeclion dong a (llOlc M o l d axis ; black symbols and lioes mpmsent the true symmetry elements of the c - p b e and open symbols m p m ~ n t the psudo 5-fold Cia Co 5 4 psudo .?-fold (3s to 3f), pseudo 2-fold (2s &
Zglgvnmet2yaxfk.
b l bmparimn betmen the posi~ons of the pseudo wmmetry axis (oped symbolsland &ese of &he symmet2y elements of the m3poidtgroup (blacbsymboh andlinesl.
CONCLUSION
Four orientation relationships between i-phase and A1 were observed. as well as two orientation relationships between c-phase and Al.
The most important property of the c-phase (tetragonal -f.c.c. based- periodic structure) lies in the structural arrangement of atoms which appears to have a pseudo icosahedral symmetry, very close to that of the i-structure. This result clearly emerges from the striking similarity in the spatial distribution of the most intense reflections -which are modulated by the nature of the motif- in both structures.
Therefore, the c-phase can be considered as an approximant structure of the quasi-crystal.
ACKNOWLEDGEMENTS
PECHINEY Voreppe Research Center is acknowledged for supplying the alloy.
The authors are indebted to F. DUCASTELLE for his continuous interest to this work and for helpful discussions.
/ I / Sainfort P.. Dubost B.. Dubus A. (1985) CR.A.S. 301.11, n'10.689 /2/ Sainfort P.. Dubost B. (1986) J. de Physique Colloq. 47,C3-321 /3/ Loiseau A.. Lapasset G. (1986) J. de Physique Colloq. 47.C3-331 / 4 / Cassada W.A., Shiflet G.J. (1986) Phys. Rev. Lett. 56,2276 151 Cassada W.A., Shiflet G.J.. Starke E.A. (1986) Scripta Met. 20.751
