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MIGRATION OF THE INTERSTITIAL-IMPURITY
MIXED DUMBBELL CONFIGURATION IN Al-Zn
P. Wallace, K. Hultman, J. Holder, A. Granato
To cite this version:
JOURNAL D E PHYSIQUE
Colloque C10, suppldment
au 11-12, Tome 46,
ddcembre 1985
page C10-59
MIGRATION O F
THE INTERSTITIAL-IMPURITY MIXED <loo> DUMBBELL
CONFIGURATION I N Al-Zn
P.W. WALLACE, K.L. HULTMAN, J. HOLDER AND A.V. GRANATO
University
of
Illinois a t Urbana-Champaign, Urbana,
Illinois
61801,
U.S.A
Abstract: From measurements of ultrasonic attenuation and velocity in alloys
with different atomic Zn concentrations (.01%, 0.1%, and 0.5%),
we obtain
positive evidence that the Al-Zn dumbbell migrates as a unit.
From
measurements in specimens containing 1000 ppm Zn and 70 ppm of Fe it is found
that only 3% of the dumbbells dissociate at the annealing temperature near
140
K for
Al-0.1% Zn. In the present measurements it is found that the
annealing temperature decreases with increasing zinc concentration.
A
possible migration mechanism involving looping of the interstitial which
leads to long range migration of the dumbbell is developed, and dumbbell
migration and interstitial escape energies of Um
=0.36 eV and
UE =0.46 eV
are obtained.
We have made measurements of ultrasonic attenuation and velocity changes at
10 and 30 MHz on a number of electron irradiated aluminum alloys, looking for
systematic trends and relationships. The surprising result is that there is
little relationship between systems; rather each system seems to have its own
characteristic pattern of relaxation peaks. The most useful information
obtained from the measurements is symmetry information, but the hope one may
have had originally from resistivity, channeling and Mossbauer measurements
of finding single characteristic defects is largely urealized. In general,
several relaxation peaks are obtained, which usually correlate with
resistivity stages, but which correspond to different defect configurations,
as determined by their annealing behavior.
Our most detailed studies of interstitial-impurity dipolar defects are on
Al-Zn (11. We have irradiated specimens containing 0.5,
0.1,and 0.01 atomic
percent of zinc in aluminum at
65 K. Adefect with <Ill> symmetry is found
at 103 K which anneals during the measurement, while another with
<loo>
symmetry is found at 5 K which anneals at
135K. The measurements for the 5 K
peak provide the first strong experimental evidence for the theoretically much
discussed and expected caged mixed dumbbell configuration
[Z]. In addition,
the surprising result is found that the defect tunnels between the equivalent
states. The evidence for the mixed caged configuration arises mainly from the
polarization dependence of the dielastic and paraelastic ultrasonic response of
the defect. A large diaelastic effect in C44 anneals with this defect, while
the paraelastic response occurs entirely in the C' elastic constant, as would
be expected for this symmetry. We will have nothing further to say about the
configuration, or the tunneling, of the defect here. Instead, we describe
evidence that this defect migrates as a unit through the lattice. The evidence
is obtained by examining the annealing behavior of Al-Zn for different
concentrations of Zn and in the presence of
Feimpurities at lower
concentrations.
C10-60
JOURNAL DE
PHYSIQUE
I n Al -100 ppm Fe specimens i r r a d i a t e d t o -2 ppm FP, t h r e e peaks were found [ 3 ] i n t h e C44 mode a t T = 19, 43, and 97 K a t 10 MHz. A peak i n C' i s a l s o found a t 20 K. The C' peak was n o t s e e n i n e a r l i e r lower f r e q u e n c y measurements by Rehn, Robrock and Jacques [ 4 ] , b u t t h e C44 peaks correspond w e l l w i t h t h e lower f r e q u e n c y measurements. A l l t h e peaks d i s a p p e a r i n S t a g e 111. The d i e l a s t i c e f f e c t i s -36 f o r C' and -5 f o r C44. Although Fe and Mn have s i m i l a r mass and s i z e d i f f e r e n c e s w i t h Al, t h e r e s u l t s a r e v e r y d i f f e r e n t , c o n t r a r y t o
e x p e c t a t i o n s from e x i s t i n g t h e o r y [ 2 ] based on s i z e d i f f e r e n c e c o n s i d e r a t i o n s . A d d i t i o n a l symmetry i n f o r m a t i o n f o r t h e s y s t e m comes from Mossbauer
measurements by P e t r y , Vogl and Manse1 [ 5 ] , but t h e r e i s n o t y e t good agreement on a d e t a i l e d model [ 3 ] . N e v e r t h e l e s s , t h e prominent A1-Fe peak can be
e x p l o i t e d a s a deep t r a p which s h o u l d hold i n t e r s t i t i a l s l e a v i n g weaker t r a p s a t t e m p e r a t u r e s below 200 K.
I n Fig. 1, t h e decrement a t a f u n c t i o n of t e m p e r a t u r e i s shown f o r an
i r r a d i a t e d specimen c o n t a i n i n g 1000 ppm of Zn and 70 ppm of Fe. Both t h e Al-Zn peak a t 5 K and t h e Al-Fe peaks n e a r 20 K a r e found but t h e Al-Fe peaks a r e much s m a l l e r ( a b o u t 14 times s m a l l e r ) t h a n t h e y would be w i t h o u t t h e Zn. On a n n e a l i n g , a s i n d i c a t e d by t h e dashed l i n e s , t h e Al-Zn peak d e c r e a s e s n e a r 140 K, b u t t h e Al-Fe peaks do n o t i n c r e a s e by a f a c t o r of 14, as could be expected i f t h e i n t e r s t i t i a l s d i s s o c i a t e d a t 140 K, w i t h t h e i n t e r s t i a l m i g r a t i n g t o t h e s t r o n g e r Fe t r a p s . I n s t e a d , t h e Al-Fe peak grows by a b o u t 40%. This shows t h a t dumbbell d i s s o c i a t i o n is o n l y a s m a l l component of t h e a n n e a l i n g of t h i s d e f e c t .
For Al-Zn specimens of d i f f e r e n t Zn c o n c e n t r a t i o n s , it was found t h a t t h e a n n e a l i n g t e m p e r a t u r e depends upon t h e Zn c o n c e n t r a t i o n , w i t h t h e h i g h e s t a n n e a l i n g t e m p e r a t u r e o c c u r r i n g f o r t h e l o w e s t c o n c e n t r a t i o n . This is expected f o r a n a n n e a l i n g mechanism where m i g r a t i n g mixed dumbbells a r e t r a p p e d a t s u b s t i t u t i o n a l z i n c atoms, s o t h a t more m i g r a t i o n jumps a r e r e q u i r e d i n more d i l u t e a l l o y s .
We have developed a s i m p l e m i g r a t i o n model, i l l u s t r a t e d i n F i g s . 2 and 3, i n which t h e i n t e r s t i t i a l can f o l l o w two p a t h s . One l e a d s t o an a p p a r e n t 1800 r o t a t i o n of t h e mixed dumbbell which allows t h e Zn t o m i g r a t e o u t of t h e cage by a l o o p i n g p r o c e s s [ 2 ] , The o t h e r l e a d s t o dumbbell d i s s o c i a t i o n . Rate e q u a t i o n s a l l o w i n g f o r b o t h t h e s e p r o c e s s e s a r e e a s i l y w r i t t e n and by f i t t i n g t h e r e s u l t s t o t h e d a t a , one f i n d s Urn = 0.36 eV f o r t h e m i g r a t i o n energy and UE = 0.46 eV f o r t h e e s c a p e energy. These r e s u l t s a r e i n f a i r agreement w i t h c a l c u l a t i o n s by Lam, e t a 1 [ 6 ] .
I n a d d i t i o n , measurements of changes of t h e h e i g h t of t h e Al-Zn peak i n a 100 ppm Zn a l l o y a f t e r r e p e a t e d i r r a d i a t i o n and a n n e a l i n g experiments gave r e s u l t s s u p p o r t i n g a model of d e p l e t i o n of Zn t r a p s above t h e Zn a n n e a l i n g
t e m p e r a t u r e .
This work was s u p p o r t e d by t h e Department of Energy, D i v i s i o n of M a t e r i a l s S c i e n c e , under C o n t r a c t DE-AC)2-76ER01198.
P r e s e n t a d d r e s s e s :
P. W. Wallace: Dupont, Research T r i a n g l e Park, N. C a r o l i n a 27709 K. L. Hultman: Geomation Corp., Wheat Ridge, Colorado 80033 J. Holder: CGS. A u s t i n , Texas 78704
REFERENCES
1. K.
L.
Hultman, T h e s i s , U n i v e r s i t y of I l l i n o i s (1979).K. L. Hultman, J. Holder, and A. V. Granato, J. de Physique, C5-753 (1981).
2. P. H. Dederichs, C. Lehmann, H. Z. Schober, A. Scholz, and R. Z e l l e r , 3 . Nucl. Mat.
69
& 7 0 , 176 (1978).4.
L.E.
Rehn, K.-H. Robrock, andH.
Jacques, J. Phys.s,
1835 (1978).5. W.
Petry, G. Vogl andW.
Mansel, Phys. Rev. Lett.65,
1862 (1980). 6. N. M. Lam,N.
V. Doan andY.
Adda, 3 . Phys.E ,
2359 (1980).Fig. 1. Decrement as a function of temperature in A1-1000 ppm
---
211-70 ppm Fe alloy. The dashedline indicates the annealing behavior of the Al-Zn peak at
5 K
0 8
Q and the A1-Fe peaks near 20 K.
heffsctivs Fig. 2. Schematic diagram for a
PI
return mixed Al-Zn caged mixed (100)@
( 0 ) dumbbell migrating by a looping process and dissociating.(a)
(b)
x Zinc lntrastitial Atom
('I
@o,s$AM
o Lottice tom
