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AN ATOM-PROBE ANALYSIS OF Al-Li ALLOYS
T. Abe, K. Hono, T. Hashizume, A. Jimbo, G. Carinci, D. Hess, T. Satoh, K.
Hirano, T. Sakurai, H. Pickering
To cite this version:
T. Abe, K. Hono, T. Hashizume, A. Jimbo, G. Carinci, et al.. AN ATOM-PROBE ANAL- YSIS OF Al-Li ALLOYS. Journal de Physique Colloques, 1986, 47 (C2), pp.C2-185-C2-190.
�10.1051/jphyscol:1986227�. �jpa-00225660�
JOURNAL DE PHYSIQUE
Colloque C 2 , supplément au n03, Tome 47, mars 1986 page c2-185
AN ATOM-PROBE ANALYSIS OF A1-Li ALLOYS
T. ABE, K. HONO, T. HASHIZUME', A. JIMBO*, G.M. CARINCI, D.R. HESS, T. SATOH*', K. HIRANO++, T. SAKURAI* and H.W. PICKERING
Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802, U.S.A.
+ T h e Institute for Solid State Physics. The University of Tokyo, Minato-ku, Tokyo, Japan
"Department of Materials Science, Tohoku University, Sendai, Japan
Abstract - The aging process in A1-Li alloys was studied by an atom-probe FIM. Two types of
precipitates were recognized in the FIM image, one is brightly imaged with a high degree of order in its structure and the other appeared dark against the Al matrix. Atom-probe analysis showed that the Li concentration is in a range of 10 to 25 at% in the dark precipitate and approximately 25 at% in the bright precipitate.
1 - INTRODUCTION
A1-Li alloys are very attractive materials in today's energy and performance conscious Society because of their light weight and high modulus/l/. These alloys are always aged in order to develop optimum structure-property characteristics through the process of
age-hardening. It has been demonstrated in previous studies of the A1-Li alloy system that metastable 6 ' phase plays an important role in the age-hardening process/l-3/. The precipitation mechanisms, however, in particular the very early stage of aging before 6' is formed are not well understood.
This is the first attempt to study the microstructures in the early stage of the aging process in Al-x at% Li (x =0.3, 6.1 and 11.5) alloys by atom-probe (field ion microscopy).
II - EXPERIIIENTAL
Ingots of Al-x at8 Li (x = 0.3, 6.2 and 11.5) alloys, supplied by ALCOA, were swaged and drawn to 0.4 mm diameter wires. These wires were annealed at 5 5 0 ' ~ for 30 min. and quenched into O"C water, then aged at three different temperatures, room temperature, 100 C and 150°C in order to study the initial stage of the formation of the metastable phases (Fig. 1). After aging, the wires were
electropolished to a fine needle shape in a solution of perchloric acid (1 part by volume) and methanol (4 parts) at 5 to 10 volts DC or nitric acid with a small addition of water at 4 volts AC.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986227
JOURNAL D E PHYSIQUE
LrTniui <etun m )
-
0 1 2 3 4 5 6 8 IO 15 N
1 I I I . , 8 I
.
10 5 1 0 1 5 2 I . D A 4 3 6 5 0
Llthium ( a t o m l c ~ e r c e n t )
Fig. 1. Phase diaqram of Al-Li alloy suggested by Siqh and Sanchez/4/.
FIFI images were observed at 201Z using pure :Je, hydrogen, or their mixtures as imaginq gas. Atom-probe analyses were perforned at 20K using a pulse ratio of 0.2 to 0.5 and pulse repetition rate of50 to 60 Hz under various ambient conditions, such as 1 x lom4 Torr Fi2 1179 Torr Ne, and UHV of 1 x 1ûl1 Torr. Good quality FIM images can be obtained at 20 K or below using hydroqen imaging gas. FIowever the presence of hydrogen causes continuous evaporation of the imaged surface and also proinotes preferential evapooration of Li atoms.
Recause of a large difference in evaporation field of Al and Li, the results of the atom-probe analyses Vary clepending on the aabient condition. Ne have found that it is only possible by introducing hydrogen gas of 1 x 1 0 - 4 Torr to achieve smooth evaporation and to obtain truc Li concentration profiles.
III - RESULTS
Fig. 2 is a F1P.f image of an Al-6.2 at% Li alloy aged at 150°C for 1 O00 inin. at approximately 15 K using Ne imaqinq gas. This shows several dar!; precipitates of the size of approximately 100 A against the Al matrix background. Their shapes Vary and are somewhat
irregular, departing £rom spherical or ellpsoidal form. Fig. 3 is a mass histogram of the same alloy analysing both the Al matrix and the darkly imaged precipitates in the presence of IO-^ Torr 1%. The ions
detcctecl at ?l/n = 27 are
~ l + and the other ions in the ranqc 05 29 to 33 are sinqly charged ions of
Al-ilyc!rides i ~ p to ~ 1 9 +
.
Theions detected from r 4 / & = 13.5 to 1 6.5 are douhly charged aluainua and its hydrides.
The histograin shov~s that Al evaporates mostly as its hydrides. The ions at ~ / n = 6 and 7 are ~ i + ions. The intensity ratio hetwecn nass 7 and 5 is 92 / 8, and is in good agreement with the natural ahundance ratio.
The same Lind of the dark precipitates were also
observed in two other Al-Li alloys both aged at room temperature for a few days and at 150 T for 100 to 1,000 min. The rnorphology of those dar!c precipitates in terms of size, nunber density, shape and spatial distribution agrees with the morphology of the 6 ' precipitate
Fig. 2. Ne FIX image of Al-6.2 at%
alloy showing the dark precipitates
that the ~ i - concentration may MS$/CWRGE range hetween 10 to 25 at%.
In the case of an A1-ll - 5 Fig. 3 . Mass histogram of the at% Li alloy, brightly imaged dark precipitate, shovring the Li precipitates were ohserved enrichment.
dacumentod by means of TE:I/I /.
quite often together with the
dark ones. Fig. 5 is a series of photographs of the FIM images of the hright precipitate formed in the Al- 11.5 at% Li alloy upon aging at 100 C for 30 min. As the surface layerç were developed by slow field evaporation, the originally-subsurface precipitate emerged and became larye (a-5-c-d). Its maximun size in the direction parallel to
Pig. 4 is an 2xample of 8000 the atom-probe analysis of a
darkly imaged precipitate on
-
the (220) plane aged at 150°C in the Al-6.2 at% Li alloy. An
-
average concentration of Li
within thc precipitate is
-
ar>proxiinately 1 0 at"onr7 is
-
much lower than that of the
well-known b ' precipitate
-
(Al Li). TTowever this + U>
precipitate does have an 3 ,
ordered structure in the <110> 8 direction as is seen in the
-
ladder-lilce layer-hy-layer
evaporation of alternating
-
pure Al and Li-rich layers.
In other analyses we have
-
found a somewhat higher Li concentration using an Al-11.5 O at% Li alloy, and it appears O
LI*
6b+
I
10 20
AL*
I J
I I I
~ 2 - 1 8 8 JOURNAL DE PHYSIQUE
the irnaged plane is approxiniately 500 A in diameter. In these photographs some netplanes are clearly recognized inside the
200.
precipitate and the V3
positions of the Z netplanes coincide well O u
with those of the matrix image. Fig. 6 shows LL
that the composition of O
ix
the hright precipitate W is nearly 25 at% in Li. in
These two 2E 2
observations,in terrnç of z structure and chemical
composition agree srith
the well-known features
O
of ô ' phase: a L I 2 ordered structure witi
high coharency with t\e
O
NUMBER OFPULSES
260000Al rnatrix. Therefore, it seems natural to
assume Fiq. 4 . atoin-proSe analysis of the that the bright dark region in the < 1 1 0 > direction precipitates correspond showing layer-by-layer evaporation to 6 ' phase excopt for
Fig. 5. FI micrographs showing the evolution of the bright
' precipitate.
the facts that its numher
density may be 700 -
too low and its
size is too large
-
compared with the
-
estahlished TEM
values.
The
relationship is not clear, at 2
this noment, Ci O
-
betrgeen the dar!cly imaged precipitates and the brightly imaged ones tentatively identified as the 6 ' phase.
One possibility
we suggest is O
that the dark one is a precursor,
with a lower Fig. 6 . Compositional depth profile of degree of order, to Li atoms in the brightly imaged region the 6 ' phase at of the Al-11.5 at% Li alloy aged at room the initial staae temperature for two days.
of precipitation.
IV. - CONCLUSION
The aging process in Al-Li alloys was investigated making use of an atom-probe technique which enables one to obtain the information on both the atomic arrangement and chemical composition. Although it is preliminary, we conclude:
( 1 ) There are two kinds of precipitates on a basis of FIM image
observation. One is imaged bright and its size is approximately
2 0 0 to 5 0 0 A, and itç shape appears to be shperical. The other
is darkly imaged and small in a range of 1 0 0 to 2 0 0 A with irregular shapes.
( 2 ) The Li concentration is approximately 25 at%, very close to
the 6 ' phase for the bright precipitate and is in a range of
1 0 to 25 at% for the darkly imaged precipitate.
( 3 ) Ue assume that the brightly imaged precipitates correspond to
6' phase and that the smaller darkly imaged precipitates may he a precursor with a lesser degree of order to the fully ordered
6 ' phase in the early stage of precipitation.
More detailed study is warranted to fully understand the aging process of the A l - L ï alloy systern.
ACRNOIVLEDGEIIENTS
The authors thank R. Schmidt and A. P. Divecha for their suyport of this work and the Naval Surface Weapons Center for Contract
CZ-190 JOURNAL DE PHYSIQUE
/ 1 / 3. X o b l e anr? G. E. Thornpson, Net. Sci. J. 5 , 1 1 4 ( 1 9 7 1 ) . / 2 / E. A . Çtars!ce and T. 13. S a n d e r s J r . a n d 1. G . P a l m e r , J . M e t a l s
3 3 , 2 4 ( 1 9 8 1 ) .
/ 3 / P. Baunann a n d D. B. W i l l i a m s , Acta Clet. 1 0 6 9 ( 1 9 0 5 ) . / 4 / C. S i g h a n d J . X. S a n c h e z , s u b n i t t e d t o Acta ?.kt.
