THE MECHANISM AND KINETICS OF 8 `AND T1 PRECIPITATION

- 19 Soliditication of Meta ls and Al loys, No.28, 1996
}.:rz ep męcie Metali i Stopów, Nr 28. 1996
PAN- Oddztal Katowice: PL ISS N 0208-9386
THE MECHANISM AND KINETICS OF 8 'AND T1
PRECIPITATION IN AILiCu ALLOYS
CIACH R.*, SIMMICH 0.**, DUTKIEWICZ J.*, KABISCH 0.**, KRÓL J.*
*lnstitute of Metallurgy and Materials Science of the Polish Academy of Sciences,
Reymonta 25, 30-059 KRAKÓW, POLAND
** Martin-Luther-University-Halle-Wittemberg, Hoher Weg 7, D-06120 HALLE ,
GERMANY
ABSTRACT
In the present study the effect of capper addition on the mechanical properties, structure and kinetics of the precipitation in the Alli and AILiCu alloys aged at elevated
temperatures was investigated . The hardness changes were measured during ageing
at 200°C . Structure of precipitates was studied using TEM and XSAS methods.
Copf-)er addition increases significantly the strength of aged Alli alloy, due to addi tional disc-likeT 1 precipitates coexisting with spherical particles of 8'. Two types of 8 '
precipitates of size 2 nm and about 8 nm were observed; the larger one grow in size
during ageing . Disc-like T 1 precipitates of the thickness of a few nm form parallel to
{111 } plan es and posses the following crystallographic relationship with the matrix :
[001]a 11 (1ii1]T1 and (110)a 11 (1010)T1
STRESZCZENIE
W przedstawionej pracy przebadano wpływ dodatku miedzi na własności mechaniczne, strukturę i tworzenie wydzieleń w stopie Alli starzonym w podwyższonych temperaturach. Zmiany twardości określano w trakcie starzenia w 2oooc . Badania struktury
wydzieleń prowadzono używając metod transmisyjnej mikroskopii elektronowej i rozpraszania promieni rtg . pod małymi kątami. Dodatek miedzi powoduje znaczny wzrost
twardości starzonego stopu Al li wskutek tworzenia się dodatkowych wydzieleń T 1 o
kształcie
dysków obok obecnych w stopie sferycznych wydz i eleń fazy 8 Obserwowano dwa typy wydzieleń 8 o wielkości 2 nm i około 8 nm; przy czym większe rosną
w czas ie starzenia. Wydzielenia T 1 o kształcie dysków i grubości kilku nm tworzą się
równolegle do płaszczyzn {111} według zależności krystalograficznej z osnową:
[001] a 11 (1211] T1 oraz (110) a 11 (1010) T1 .
1
•
l
INTRODUCTION
The formation of 8 precipitates within a modulated structure due to spinadal decoposition was found in Al-Li alloys during early stages of ageing [1-7]. Sigli and
Sanchez (8] carried out the theoretical calculation of the stable and metastable Al-Li
phase diagram , which was in a good agreement with that determined experimentally.
This diagram predicted a miscibility gap which was metastable with regard to the a-8~
equilibrium. The coarsening of the 8 particles obeyed the LSW model [9]. Gomiero
et. al. [1 O] found, that after ageing of the Al-Li alloy at low temperatures, two different
sizes of the 8 precipitates appear, at the beginning the smali ones ( about 1 nm) and
at further ageing a second type was observed.
The Al-Li alloys attain a moderate strength and ductility. To improve mechanical
properties more complex alloy systems were investigated, in which the property parameters can be beneficially modified. lt is necessary to incorporate elements, which
l
1
l
- 20 -
are able to modify the misfit of o ' particles and matrix or, in particular, provide additional precipitate phases [11, 12]. The early works on the ternary Al-Li-Cu system
[13, 14] s howe d t h at bot h precipitates e ' and o ' occur aft er aging . T he precipitation
of disc shaped, hexagonal T 1 ( AI 2 Culi) phase [15,16,17] was observed in later
works. lt was shown, that at the increased Li- eontent ( above 2 wt.%) the e· ,8 'and
T 1 form at an early stage of aging at zoooc [18,19]. T 1 forms heterogenously on dislocations and subgrain boundaries and inhibit the growth of e '. The T 1 needles are
rarely observed at the beginning of aging at temperatures below 200°C [20]. The
addition of capper increases the strength of aged Alli alloys [21-23] and as results
from the wark of Caponetli et al. [24] capper addition causes an elipsoida! shape od
o ' particles. lt h as not been confirmed in the wark on AILiCuMg [1 O] where it was
suggested that these precipitates always appeared spherical , while the elipsoida!
precipitates were d ue to capper rich on es. In the overaged ternary alloys the T 6 phase ( AI 7 5 Cu 4 Li) could form [25] . The aim of the present paper is to show the effect
of capper on the mechanical properties , structure and evolution of the precipitates in
Alli alloys aged at elevated temperatures.
2. EXPERIMENTAL PROCEDURE
T he alloys were cast in a special chamber under pure argon ( under 1 ppm N2 ) . They
were of nominal compositions Alli2.5 and Alli2.5Cu2 .0 ( all compositions in wt.%)
The purity of elements was : Al and Cu 4N and Li 3N, respectively. The alloys were
homogenised in argon atmosphere at 550°C and quenched into RT water. The ageing was performed in argon atmosphere at 200°C up to 192 hours . Hardness was
measured using Vickers method at 5 kg load . The TEM examinations were carried
out using PHILIPS CM -20 transmission electron microscope operating at 200 kV.
Thin foils for TEM observations were obtained by jet electopolishing in a solution of
perchiarie acid in methyl alcehol at temperatures below 0°C .
For the evaluation of the amount and size of coherent precipitates by XSAS the
scaterring function of a certain sample volume was as follows [26] :
(1 )
w ith notation as in [26]. from which by Guiner approximation a scaterring function
from a single particie radiusof gyration can be obtained :
(2)
jP(s) ""exp [-Rg 2 h2/3] where
h = 2n/A. 2 sin
(E
/2)
wh ere E is the scaterring angle , A.- the wavelength of Cu Ka radiation and Rg the
G uinier radius of gyration , which is a measure for the particie size . Rg is calculated
in a graphic way from the plot of log J(s) vs . E2 utilising relation (2) . The total amount
of precipitates in the alloy can be calculated from the integral intensity:
S;
(3)
Q=
K
f
sJ(s) ds =f (1-f) (L'lp) 2
v.
SD
where f is the volume fraction of the precipitates. Q is thus a measure of f. The tests
were conducted using Rigaku - Denki small-angle-camera with vertical slits , using a
scintillation counter for Cu-Ka radiation . The X-ray intensity was standarised using
Lupolen 1811 M sample . Rg and Q were calculated using computer programs .
- 2 1-
3.RESULTS AND DISCUSSION
The changes of hardness of Alli and AILiCu during ageing at 200°C are shown in
Fig .1. One can see a very large difference of the hardness curves for both alloys .
Copper addition increases distinctly hardness of the aged alloy and shifts its
maximum towards longer aging times, in comparison with the binary alloy. The
highest hardness value of the binary alfoy is 67 f-N after 1O hours ag ing but in the
ternary alloy this quantities are: 131 HV after 24 hours , respectively . In both alloys
140
- - - - --
l
AIUCu
'
Fig . 1. Changes of hardness during
ageing at 200°C of Alli and AILiCu
alloys.
120 ~
1~
li)
:f:
Rys . 1. Zmiany twardości w czasie
starzenia w·2ooac stopów Alli oraz
AILiCu .
80
AlU
,•- ·- - -+- -- -- -·-- ---------- ...
60 -' •
l
l
40 -·---------~--------~~
o
40
aota (h)120
160
200
the hardness only slightly decrease after longest aging times.
The experimental results of XSAS method are generally represented in the form of
diagrams . The precipitate size ( as Guinier radius - Rg) versus aging time of the both
alloys are shown on Figs. 2 and 3 for the binary and ternary alloys and the integral
intensity Q - (in respect to the volume fraction of the precipitates) versus ageing time,
on Figs. 4 and 5, respectively.
10
12 ,----------------------~
~
Rg 1
o
l
l
J
l
l
Rg1
8 _Jl
l
i
l
~6
E
.s
Ol
6
0:::4
J.
o
Rg2
?
l
10
20
~
l
30
40
l
50
ta [h]
Fig. 2. The Guinier radii vs. ageing time of
the investigated Alli alloy
Rys.2 . Zależność wielkośc i promieni Guiniera od czasu starzenia stopu Alli
l
-t-+ ---- +-- -- -? ------ ,.
2-r·
Rg 3
o -_. __ -"--- - "";
o ,_--,_-.--~-,--.---,--.-41
O
20
40 ta [h) 60
80
Fig . 3. The Guinier radii vs. ageing
time of the Al LiCu alloy
Rys . 3 . Zależność wielkości promieni
Guiniera od czasu starzenia
stopu AILiCu
- 22 -
In the investigated binary alloy AlU a smali amount of twa types of 8' precipitates
(Rg = 2.54 nm and 7.5 nm) is observed immediately after quenching . The smaller
particles decrease their size ( to 2.26 nm after 50 hours of ageing ) but the large
ones, grow very fast up to 11.48 nm after the maximum ageing time applied in the
investigation ( Fig .2).
In the investigated AILiCu alloy (Fig .3) a smali amount of precipitates of the 8 ' is
observed immediately after quenching in the form of spherical particles of two sizes
(1 .92 and 5.68 nm). The larger particles grow very fast ( after 1 hr. of ageing time up
to 7.04 nm) but the smali ones slightly diminish ( after 6 hr. to 1.63 nm). Larger
particles grow with the aging time and reach the size of 9.37 nm after 61 hours . After
1 hour ageing the appearance of particles of size of about 3 nm is observed .After
ageing time of 61 hours this value increases up to 3.7 nm . These particles are
estimated from the cross section of the disc-like AIUCu type precipitates . The
changes of the integral intensity (Q) of the binary alloy during ageing is shown in
Fig.4. 0 2 refers to smaller particles. Their amount is smali at the beginning, and
decrease slightly after 45 min . ageing . 0 1 refers to large particles which amount is
very smali after quenching but increase strongly after 4 hours ageing . After 25 hours
in the the range between 25 to 50 hours the increase is slower.
1000 -------- - - - ··-·-- ----
--
1200 -- -
- -- - -- - - - - - - -------:=r--,---
~
QT
800 ~ 800 --
!
/-
---o ,
/
l"
200 Q -o . _ _
o
_ -
- - --
10
- - - - -- - · - - - - -- - - -.'
20
30
40
50
ta [h]
Fig . 4.The integral intensity vs . ageing time
the AlU alloy aged at 200°C
Rys.4 . Zależność całkowej intensywności
od czasu starzenia w 200°C stopu
AlU
80
Fig . 5. The integral intensity vs. ageing time of the Al liCu alloy
at 200°C
Rys.5 . Zależność całkowej intensywności od czasu starzenia w
200°C stopu AIUCu
The integral intensity (Q) changes of aged ternary alloy are shown in Fig .5. Q values designate the amount of the precipitates, where 0 1 refers to large particles,
0 2 to these with smallest Rg 2 and 0 3 to AILiCu precipitates with Rg 3 . The amount of
smallest precipitates is relatively large at the beginning and slightly decrease after 1
hour of ageing . The amount of the large precipitates is relatively smali at the
beg inning and increase after 20 hours with a lower rate after the longer aging time .
The integral intensity of the AIUCu particles is smali from the begining and increases
after 20 hours, similarly to the large 8' precipitates . The course of the integral
- 23 -
ageing time of the o ' precipitates (particularly the larger ones) is very similar for the
binary and ternary alloys but the size of the large o ' particles in the Alli alloy is
much larger than in the AILiCu. In the ternary alloy T1 precipitates act as nucleation
sites for the o ' precipitates. One can assume that the volume fraction of the o ' partides in ternary alloy is smaller (T, contain Li) and the nucleation sites are more numerous than in the binary alloy. The increase of the integral intensity is in a good
agreement with the changes of the hardness data .
Fig .6. Alloy 2 aged at 4 hours at 200°C . (a).Transmission electron micro.9_raph (b)
(DF) image taken using 01 O o' spot (c) DF image taken using (2020) spot.
(d) SAD pattern from the area shown in (a).
Rys .6.Stop 2 starzony 4 godz. w 200°C. (a) Transmisyjn2 mikrofotografria (b) Mikrostruktura w ciemnym polu 01 O o' (c) Mikrostruktura w ciemnym polu (2020)
T, (d) Dyfrakcja elektronowa obszaru widocznego na (a) .
" 24"
Transmission electron microscopy studies of the as quenched AILiCu alloy revealed
superlatlice L 12 spots already in the as queched state due to the presence of very
fine o' precipitates of size of a few nm. During ageing a rapid growth of o ' precipitates occured and after 4 hours of ageing at 200°C they attain the size of 20 nm
(Fig .6). Spherical o ' precipitates are visible in a very good contrast as the white circles in the dark field image taken using 01 O eS' spot. The plate like T, phase plates
running along the <11 O> and <1 00> directions are presented in Fig.6c in the dark
field image taken using 2020 T, reflection . The existence of the fringe like contrast
indicates that plates are inclined in the foil under the angle less than 90°. From the
electron diffraction pattern one can find the following crystallographic relationship
between the a matrix and the T,: [001] a 11 [1211] T, and (110) a 11 (1 of o) T,. The
same relationship was reported by Huang and Ardell [27] .
Rys .?. Stop Alli2 .5Cu2
starzony 4 godz. w 200°C
(a)Transmisyjna mikros' truktura elektronowa w (b)
' ciemnym polu
w rogu
przy użyc i u refleksu 2020
T,
At the micrographs (Fig .?) taken at [110] zone axis orientation one can see narrow
needles of T, running in <110> or <111> directions . From their contrast and
- 25 -
thickness one can conclude that at this orientation they are perpendicular to the foil.
Their thickness is very smali and can be estimated from this and other micrographs
as 2 - 3 nm. D~rk field image was taken using 2020 T 1 spot and plates running along
the traces of (211) piane can be seen. In the diffraction pattern T 1 spots coming
from other orientation can be seen with streaks along the 0002 reflection . One can
see that similarly like in Fig .6 plates grow in (1 01 O] T 1 direction . At other plate orientation 0002 T 1 planes lie parallel to the habit planes of plates and to the {111} a
planes . After longer ageing time of 48 hours after the maximum of hardness 81 partides grow without changing their shape. They attain the size of 30 nm .The length of
T 1 plates also increases with ageing time. The 8 particles often nucleate at the T/a
interfacel since several half-spheres can be seen at the plate interfaces. The diffraction pattern is very similar to that in Fig .6d , however the intensity of reflections from
precipitates is much higher.
1
CONCLUSIONS
1. The addition of capper causes significant strengthening of Alli alloys during ageing at 200°C d ue to additional to the 8 formation of the plate-like T 1 (AI 2 LiCu) precipitates
2. Two main types of precipitates were found during ageing; the spherical 8 precip
itates existing already after quenching and growing steadily during ageing and T1
discs of the thickness of a few nm forming parallel to {111} matrix planes . The following crtsta~ographic re~tionship _9f the a matrix and T 1 phase was observed :
[001]a 11 [1211]T 1 and (110)a 11 (1010)T 1
3. Twa types of 8 l precipitates were found after ageing using the XSAS method .
The smaller ones decrease their radius and amount during ageing . lt may indicate
dissolution of mod ulation and coarsening of spherical particles . T 1 act as nucleation
sites of 8 l particles.
4.1 n the binary alloy the Guinier radiusof large 8 l particles and their amount ( integral intensity) is larger than in the ternary one. In the Al LiCu alloy the nucleation sites
a re mo re numerous than in the Al li and same lithium appears in the T 1 phase what
decrease the lithium amount which could precipitate within 8.
1
1
ACKNOWLEDGMENTS
Authors are grateful to the Foundation of the Polish-German Cooperation for the fi nancial support No 1185/93/LN of the mutual cooperation
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