STRUCTURE OF STABLE INTERMETALLIC COMPOUNDS OF THE Al-Li-Cu-(Mg) AND Al-Li-Zn-(Cu) SYSTEMS

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STRUCTURE OF STABLE INTERMETALLIC COMPOUNDS OF THE Al-Li-Cu-(Mg) AND

Al-Li-Zn-(Cu) SYSTEMS

B. Dubost, M. Audier, P. Jeanmart, J.-M. Lang, P. Sainfort

To cite this version:

B. Dubost, M. Audier, P. Jeanmart, J.-M. Lang, P. Sainfort. STRUCTURE OF STABLE INTER-

METALLIC COMPOUNDS OF THE Al-Li-Cu-(Mg) AND Al-Li-Zn-(Cu) SYSTEMS. Journal de

Physique Colloques, 1987, 48 (C3), pp.C3-497-C3-504. �10.1051/jphyscol:1987357�. �jpa-00226588�

S T R U C T U R E OF S T A B L E I N T E R M E T A L L I C C O M P O U N D S OF T H E Al-Li-Cu-(Mg) AND Al-Li-Zn-(Cu) S Y S T E M S

B. DUBOST, M. & U D I E R ( ~ ) , P. J E A N M A R T , J.-M. L A N G and P. SAINFORT Ckgedur-PBchiney, Centre de Recherche et Developpement,

B.P. 27, F-38340 Voreppe, France

ABSTRACT

The results of a thorough structural study of crystalline intermetallic compounds within the Al-Li-(Cu,Mg,Zn) system are given. Characteristic morphologies of these ternary or quaternary phases are exhibited by concentrated alloys or large single crystals. Typical X-ray powder diffraction data, selected area electron diffraction patterns and high resolution electron microscopy images are shown, allowing unambiguous structure identification of two new intermetallic compounds designated Z and z stable respectively in the Al-Li-Cu-Mg and Al-Li-Zn-(Cu) systems. The latter are found to have common structural features with the R-AJsCuLi3 and T-(AI,Zn)2Li isomorphous phases.

INTRODUCTION

Many engineering properties (e.g. ductility, damage tolerance and corrosion resistance) of the low density aluminium-lithium alloys currently developed for the aerospace industry can be strongly influenced by the distribution, mechanical properties and electrochemical behaviour of constituent particles or matrix and grain boundary precipitates through the effects of alloying, processing and heat treating. Despite its importance for metallurgical optimization, no systematic structural study of intermetallic compounds within the Al-Li-Cu-Mg system has been reported since the extensive experimental work of Hardy and Silcock (1) on the constitution of the Al-Li-Cu system and the identification of the ternary compounds TB-A17,5Cu4Li, T-A12CuLi, T2-A16CuLi3 and R-A15CuLi3.

The AI-Li-CU-(Mg) system has nevertheless been the focus of renewed interest since the discovery of the existence of the "quasi-crystalline" structure exhibited by the stable T2-A16C u ( L i , M g ) 3 compound (2-5) and the fabrication of large single-quasi-crystals ( 6 ) allowing direct crystallographic approaches of the study of the structure of the T2 phase (mainly by neutron and X-ray diffraction) and characterization of its specific physical and chemical properties.

("permanent address : Laboratoire de Thermodynamique et de Physico Chimie N&tallurgiques, E.N. S. E .E. G., B.P. 7 5 . F-38402 Saint-Martin-d'HBres Cedex, France

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987357

C3-498 JOURNAL DE PHYSIQUE

However, an alternate route for the determination of the structure of quasi-crystals, namely the derivation of its icosahedral atomic clusters or units, could be the study of intermetallic periodic compounds with a composition close to that of T2, since the existence of an orientation relationship between the b.c.c. compound R-A15CuLi3 and the icosahedral T2-A16CuLi3 phase has been reported (7). In addition, as R-A15CuLi3 is isomorphous with the T-Mgx(AI,Zn)49 and T-(AI,Zn)2Li compounds (8), zinc containing alloys have also to b e considered due to the possible formation of icosahedral compounds in AI-Mg-Zn and Al-Li-Zn systems (9,lO). The recent results published by Ma et al. (11) on the hyperfine structure of T2 and R confirm the interest of such an indirect approach.

This paper reports the first results of an extensive study of the structural characterization of stable crystalline compounds with composition close to that of T2, including namely two new intermetallic compounds designated Z and z and identified respectively in the Al-Li-Cu-Mg and Al-Li-Zn-(Cu) systems. These compounds were first observed as equilibrium constituent particles in experimental aluminium-lithium alloys in a preliminary step. Following the determination of their chemical composition by electron probe and nuclear microanalysis (12), they were synthesized as large single crystals by a slow solidification process (13), thereby allowing unambiguous phase identification by X-ray powder diffraction, selected area electron diffraction (SAED) and high resolution electron microscopy (HREM).

RESULTS AND DISCUSSION

J 3 - ~ h a s e : This b.c.c. compound is in stable equilibrum with the f.c.c. aluminium matrix (a Al) at high temperature in dilute AI-Li-CU-Mg alloys with a composition range 1,5 5 Li 5 2,5 wt% ; 1,s I Cu I 3,5 wt% and 1 5 Mg 5 3 wt%, including the composition range of the. AA. alloy 2091. Alloys with composition close to AIsCuLis generally exhibit primary R crystals with polyhedral morphology. The latter appear to by surrounded by T2 when the alloy composition is close to AlsCuLi3 (Fig. I-a).

Millimeter sized single crystals of R-A15CuLi3 obtained by slow solidification exhibit a typical { I I O ) truncated cubic morphology (Fig. I - b ) . The cell parameter of the ternary R-A15CuLi3 phase has been checked to be equal to aR = 13,89

A

by X-ray powder diffraction (Table I). In Fig. 1-c a HREM image of [ I 1 I ] zone axis obtained from crushed single crystals shows a number of structural { I I O ) planar faults. On the basis of previous crystallographic studies (8,14), the atomic structure of this compound was shown to consist of a packing of triacontahedra partially overlapped by the 8 neighbouring others along the <Ill> directions (7). Such {110) structural faults of R can then be identified as stacking faults due to a change of coordination number from 8 to 7 between triacontahedra (fig. 1 -d-e).

Fig 1 : (a) Backscattered SEM image, showing a slight atomic number contrast between the R and T2 (AICuLi) phases, embedded in finer eutectic type structures (b) R-A15CuLi3 facetted single crystals (c) high resolution electron microscopy image and corresponding diffraction pattern of [ I l l ] zone axis showing [I101 stacking faults exhibited by the R phase (d) modelling of a (110) stacking fault (e) triacontahedral atomic units forming the R structure. I)

C3-500 JOURNAL DE PHYSIQUE

Z ~ h a s e : This new phase was discovered to be in equilibrum with the matrix and or the liquid at high temperature in alloys with composition range

(0,2 5 Cu 5 1 wt% ; Li 2 2 wt% ; 5 5 Li

+

Mg

<

8 wt%), forming acicular constituent particles or polyhedral laths. Large polyhedral Z particles can be observed on a cbncentrated alloy in Fig 2-a. Single crystals with polyhedral or lath shaped morphology were synthesized (Fig 2-b). The hexagonal structure of this Z-(AI,Li,Mg,Cu) phase was established by SAED and X-ray powder diffraction (Fig 2-c ; Table I) : a particular value of the ratio cia = 2 was deduced from SAED patterns of [OOI] and [I101 zone axes, whereas a layered structure at c/2 is revealed by HREM [fig. 2 d

-

e). The value of the cell parameters for Z (a = 14,03

A)

has been determined from X-ray powder diffraction data (Table 1). As it is found to be very close to the parameter of the R-A15CuLi3 phase, one may think about a possible similarity between the atomic arrangements of both phases. For example a hexagonal layer of cell parameter aR could be effectively considered by connecting triacontahedra by faces (see inset of fig. 2-e).

5 has^ : This new phase was first found to form as a stable compound in equilibrium with the a Al solid solution in dilute alloys containing 2 - 3 wt% Li and

4-6 wt% Zn. In concentrated alloys, this phase also crystallizes within a relatively narrow composition range (2 < ZnILi < 4) and exhibits a polyhedral shape similar to that of the stable b.c.c. compound designated T-(AI,Zn)nLi, which could explain why it has remained unreported in a previous study of the Al-Li-Zn system (15). The composition of the z compound, is very close to AlsZnLis (12) in the ternary Al-Li-Zn system and is thought to be close to Als(Zn,Cu)Lis i n quaternary alloys. Large single crystals of z-(AI,Li,Zn,Cu) obtained by slow solidification exhibit a bipyramidal shape with square basis (Fig 3-b). The growth 4-fold axis of pyramids corresponds to the c axis of the quadratic structure of this phase.Such a quadratic structure was established for this z phase by careful SAED analysis and HREM (fig. 3-c-d) and checked by X-ray diffraction (Table I), showing interesting characteristics : the square cell parameter (a = 13,91

A)

is again very close to that of the isomorphous R-A15CuLi3 and T-(A1,Zn)nLi b.c.c. phases (which therefore suggests some common atomic arrangement) and the other cell parameter c is found to be very large (c = 82,05

A)

No icosahedral compound was found to crystallize from the liquid phase in the Al-Li-Zn system. It has to be noticed that previous work on AA alloy 8090 (AI-Li-CU-Mg) in annealed condition reported the identification of orthorhombic and monoclinic compounds (15) and that of a compound (C- phase) of roughly cubic structure with a modulation of planar defects along a cube axis and pseudo 5-fold symmetry (16).

However, the comparison of those results with ours leads u s to think that, instead of these different structures, the quadratic structure established in this study should be considered.

Fig 2 : (a) SEM image showing the Z (AILiCuMg) phase surrounded by eutectic phase (b) morphology of Z single crystals (c) electron diffraction patterns of [Ool], [A101 and [I101 zone axes revealing the hexagonal structure of Z phase and that its particular ratio cla = 2 (d) and (c) HREM images of [I101 and [001] zone axes. The cell parameter az neatly equal to aR could be due to an arrangement of

triacontahedra as shown in inset. I)

C3-502 J O U R N A L D E PHYSIQUE

This identification of several crystalline intermetallic compounds in the AI-Li-CU-(Mg) and Al-Li-Zn-(Cu) systems has revealed some common structural features between the b.c.c. compound R-AI5CuLi3 (a = 13,09 A) and the hexagonal and quadratic compounds respectively designated Z (a = 14,03

A

; c = 28,06 A) and z ( a = 13,91

A

; c = 82,05 A).

On one hand, these results are expected to be helpful for the crystallographic study of these phases and the fundamental understanding of the "quasi-crystalline" structure of T2-AlsCu(Li,Mg)s. On the other hand, the characterization of the physical and chemical properties of all these compounds, which should contribute to the metallurgical understanding of the fracture and corrosion .behaviours of industrial aluminium-lithium alloys, is in progress.

Acknowledaernents

The authors wish to thank the D.R.E.T. for financial support. The contribution of P. Le Barz to this work is acknowledged.

References

1 H.K. Hardy and J.M. Silcock, J. Inst. Met., 24 (1955-56), 423

2 P. Sainfort, B. Dubost, A. Dubus, C.R. Acad. Sc. Paris, t.301, Serie 11, 10 (1985), 689 3 M.P. Ball and D.J. Lloyd, Scripta Metall. 19, (1985), 1065

4 W.A. Cassada, G.D. Shiflet and E.A. Starke, Jr, Scripta Metall., 20 (1986),p 751 5 P. Sainfort and B. Dubost,J. de Phys., Paris Coll C 3, 47 (1986), 321

6 B. Dubost, J.M. Lang, M. Tanaka, P. Sainfort, M. Audier Nature, 324 (1986), 48

7 M. Audier, P. Sainfort and B. Dubost, Phil. Mag. B, 54 (1986), 4, L105 8 E.E. Cherkashin, P.I. Kripyakevich and G.I. Oleksiv, Soviet Phys. Crystallogr.,

8 (1964), 681

9 P. Ramachandrarao and G.V.S. Sastry, Pramana, 25 (1985), L 225 ;

1 0 W.A. Cassada , Y. Shen, S.J. Poon and G.J. Shiftet, Phys. Rev. 834 (1986) 7413 11 Y. Ma, E.A.Stern and F.W. Gayle, Phys. Rev. Letters, Vol 58 (1987), p1956 1 2 F. Degreve, B. Dubost, A. Dubus, N. Thorne, F. Bodart and F. Dernortier

Paper 19 in these proceedings

1 3 J.M. Lang, M. Audier, B. Dubost and P. Sainfort, J. of Crystal Growth, in the press 1 4 G. Bergman, J.L.T. Waugh and L. Pauling, Acta Crystallogr., 10 (1957), 254

15 M.D. Ball and H. Lagace, Aluminium-Lithium Alloys Ill, ed. by C. Baker, P.J. Gregson, S.J. Harris and C. Peel, The Institute of Metals, London (1986), 555

1 6 A. Loiseau and G. Lapasset, J. de Phys., Paris, Coll. C3, 47 (1986) 331, also Paper 53

~n these proceedings

Fig 3 : (a) SEM image showing the z-Als(Zn,Cu)Lia phase surrounded of eutectic phase (b) morphology of z single crystals (c) electron diffraction patterns of [OOI], [OIO] and [I101 zone axis allowing the quadratic structure identification. (d) HREM image of [OIO] zone axis confirming the characteristic of a large

- r ~parameter value (= 82 A).

C3-504 JOURNAL DE PHYSIQUE

,Li,Mg,Cu) TABLE I : X-ray

R-A15CuLi3

2.734 2 3 5 2

C

_{1 3 4 6} ¹

^r10

powder diffraction data of R-AISCuLi3,Z-(A1 and 'GAlS(Zn,Cu)Li3 phases

82.05h) h k 1 0 1 1 0 1 2 0 0 7 0 1 4 1 1

o

0 1 7 0 1 9 1 1 7 1 1 8 0 2 0 1 1 9 1 2 0 1 2 3 0 0 14 I 2 6 0 2 9 1 2 7 2 2 1 0 3 0 1 2 13 1 3 0 1 3 5 0 3 8 1 3 6 1 1 18 0 3 9 1 3 8 0 3 10

1 3 9 0 0 21 2 2 13 2 3 0 2 3 1 2 3 2 1 1 2 0 2 2 14 2 3 5 0 3 13 0 0 22 1 3 12 1 2 18 1 1 21 2 3 8 1 2 2 0 0 23 r-

(az= 13.91

dmes (A) 13.753 13.182 11.602 9.88 8.923 7.582 7,126 6.99 6,661 6.245 6.01 1 5.84 5.693 5.518 4.904 4.635 4.416 4.242 4.15

4.061 4.009 3.962 3.890 3.855

3.771 3.735

3.687 3.626 3,571 (az=

dmes (A) 7.031 6.298 6.09 5.58 5.106 4.968 4.594 4.557 4.388

4.144 4.07

3.900 3.86 3.73 3.515

3.417 3.358

3.29 3.16 3.054 h k I

1 1 0 2 0 0 2 1 1 2 2 0 3 1

o

2 2 2 3 2 1 4 0 0

4 2 0 3 3 2 4 2 2

{i: A

5 2 1 4 4 0 4 3 3 (5 3 0 4 4 2 1 6 0 0

12

_{6 2 0}

:

5 4 1 6 2 2 6 3 1 4 4 4 (aR

Imes

(A)

not.mes.

-

5.673 4.912 4.395 4.012 3.716 3.48 3.275 3.107 2.962 2.836 2.724 2.536 2.455 2.382 2.315 2.252 2.196 2.143 2.094 2.048

-

A15(Zn,Cu)Li3

A ,

CF dcale

A

13.718 13.177 11.722 11.515 9.84 8.965 7.626 7.536 7.100 6.957 6.687 6.223 6.067 5.86 5.664 5.531 5.496 4.91 1 4.638 4.431 4.400 4.25 4.226 4.189 4.136 4.134 4.044 4.038 3.963 3.907 3.88 3.859 3.855 3.842 3.787 3.768 3.757 3.737 3.730 3.700 3.677 3.631 3.612 3.603 3.568

= 13,89A)

&ale

A

9.82 6.945

5.67 4.911 4.392 4.01 3.712 3.472 3.274 3.106 2.961 2.835 2.724 2.536 2.455 2.382 2.315 2.253 2.196 2.143 2.094 2.048 2.005

Z-(A1,Li ,Mg,Cu)

7 1 0

14.03h

&ale

A

7.014 6.274 6.074 5.574 5.094 4.96 4.592 4.531 4.382 4.364 4.122 4.05 4.01 3,891 3.842 3.716 3.507

3.402 3.369

1.926 1.89

1.855 1.824 1.764

-

,Cz=2az) h k l 0 0 0 4 1 1

12

0 1 1

2

2 0 2

2

0

l o i i

⁴

0 2

2

2 0 2

2

3 0 1 7 5 ' 1 1 1 4 0 2 2 4 [ I 2 2 0 1 2

3

⁰

1 1 _2 5 1 2 3 2 10 1 7 6 0 2 2 5 11-23 3

0 3

3

³

0 0 8 7 f 3 0 3 1 1 3 0 J 2 1 1 2 6 1 2 3_ 4 03: 3 224-0 l 0 3 ? 4

0 0 0 - 8 2 2 4_ 2

1.926 1.89

1.856 1.824 1.764 1.736

6 4 0 5 5 2 i 6 3 3 7 2 1 6 4 2 7 3 0

{;

8 0 0

[A i:

3.284 3.17 3.061

2 2 $ 3 1 0 3 3 5 1 3 4 3 0 3

3

6