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Submitted on 1 Jan 1981
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PRECIPITATION STAGES IN Al-Ag ALLOYS
DETERMINED BY THE EVOLUTION OF THE
INTERNAL FRICTION SPECTRUM
R. Schaller, W. Benoit
To cite this version:
JOURNAL DE PHYSIQUE
CoZZoque C5, suppZ6ment au nOIO, Tome 42, octobre 1981 page C5-881
PRECIPITATION STAGES I N A1-Ag ALLOYS DETERMINED BY THE EVOLUTION OF THE
INTERNAL FRICTION SPECTRUM R. Schaller and W. Benoit
I n s t i t u t de Ge'nie Atomique, Swiss Federal I n s t i t u t e o f TeehnoZogy, EcubZens CH-1015 Lausanne,
Swi
t z e r ZandAbstract.- The e v o l u t i o n o f t h e i n t e r n a l f r i c t i o n spectrum d u r i n g p r e c i p i t - a t i o n i n Al-Ag a l l o y s i s c h a r a c t e r i z e d by t h e appearance o f t h r e e atomic r e l a x a t i o n peaks and an e v o l u t i o n o f the background due t o t h e GP-zones e v o l u t i o n and t o t h e p r e c i p i t a t i o n hardening phenomena. Two o f these peaks are Zener peaks due t o t h e quenched s o l i d s o l u t i o n and t o t h e p a r t i a l l y r e v e r t e d one r e s p e c t i v e l y , when t h e GP-zones a r e d e s t a b i l i z e d by t h e appear- ance o f new metastable p r e c i p i t a t e s , the y " p r e c i p i t a t e s . The t h i r d peak i s due t o atomic r e l a x a t i o n s i n s i d e t h e y phase p r e c i p i t a t e s ( y ' and y ) . I t s thermodynamical c h a r a c t e r i s t i c s show t h a t the y " p r e c i p i t a t e s are d i f f e r e n t from the y ' p r e c i p i t a t e s .
I n t r o d u c t i o n . - Numerous s t u d i e s (1 t o 14) devoted t o t h e p r e c i p i t a t i o n i n t h e A1-Ag a1 l o y s have l e d t o d e f i n e t h e f o l l o w i n g sequence: ass -f GP-zones + y ' + y, where a
S s i s t h e quenched supersaturated s o l i d s o l u t i o n , t h e GP-zones s p h e r i c a l c l u s t e r s o f h i g h s i l v e r c o n c e n t r a t i o n and the y ' metastable semicoherent p r e c i p i t a t e s o f t h e e q u i l i b r i u m y phase (Ag2A1
-
hcp).During t h i s p r e c i p i t a t i o n , hardness shows two maxima a t t r i b u t e d r e s p e c t i v e l y t o the GP-zones ( c o l d hardening stage) and t o the y ' p r e c i p i t a t e s (warm hardening stage) (1,2)
.
The c o l d hardening stage developed by isochronal ageings, i s i n t e r r u p t - ed a t %450K by a sudden hardness drop a t t r i b u t e d t o t h e GP-zones r e v e r s i o n .Yet t h i s r e v e r s i o n stage cannot be supported by t h e small angles X-ray s c a t t e r - i n g r e s u l t s which show t h a t the GP-zones are growing c o n t i n u o u s l y when the hardness goes down (3-7). On t h e o t h e r hand, c a l o r i m e t r i c measurements (10) present a heat a b s o r p t i o n g i v i n g an account f o r a zones d i s s o l u t i o n a t t h i s stage. B u t a t o t a l r e v e r s i o n has n o t been observed by transmission e l e c t r o n microscopy (8,9)
.
The apparent disagreement o f t h e r e s u l t s obtained by t h e c l a s s i c a l technics concerning t h e t r a n s f o r m a t i o n stage "GP-zones + y ' p r e c i p i t a t e s " can g i v e an account o f t h e d i f f i c u l t y t o e x p l a i n the i n t e r n a l f r i c t i o n e v o l u t i o n d u r i n g p r e c i p i t a t i o n i n the A1-Ag system.
As i n the case o f c l a s s i c a l technics, t h e i n t e r n a l f r i c t i o n r e s u l t s obtained by d i f f e r e n t authors (11-14) a r e o n l y i n agreement among each o t h e r a f t e r t h e appear- ance o f t h e y ' p r e c i p i t a t e s , which g i v e r i s e t o a s t a b l e r e l a x a t i o n peak. But, d u r i n g t h e f i r s t p r e c i p i t a t i o n stages, the r e s u l t s seem t o be c o n t r a d i c t o r y .
JOURNAL DE PHYSIQUE
So depending on the experimental conditions i t i s possible t o obtain one peak (11, 14)
,
two peaks (12) or no peak (13) during the f i r s t increase of temperature a f t e r quenching. This complex evolution of the spectrum i s due t o the g r e a t s e n s i t i - v i t y of internal f r i c t i o n to a l l changes of the microstructure.In the present work the discovery of the origin of the relaxation peak asso- ciated with the y ' p r e c i p i t a t e s has led us t o a new i n t e r p r e t a t i o n of the evolution of t h e internal f r i c t i o n spectrum during precipitation. This i n t e r p r e t a t i o n has been supported by r e s u l t s obtained by hardness and e l e c t r i c a l r e s i s t i v i t y measurements, and by electron microscopy observations (1 5, 16, 17)
.
Results and i n t e r p r e t a t i o n : During the f i r s t increase of temperature a f t e r quenching, the internal f r i c t i o n spectrum shows two peaks (PI and P 2 ) i n concen- t r a t e d A1-Ag alloys (more than 10 a t % Ag) ( f i g . 1 a ) . For a lower amount of Ag, these peaks do not appear. ( f i g . 1 b ) . One can see only a saturation of the i n t e r - nal f r i c t i o n increase a t a450 K .
A
a-I
.
lo3 A I - ~ g 30% F(HZ)A 14 - I :After quenching - .85 300 350 400 450 500 1 :After quenching-
.80 2 - 300 350 400 450 500Fig. 1 : Characteristic internal f r i c t i o n spectrum of an A1-30
w t
% Ag alloy ( a ) and A1-5w t
% Ag (b)After annealing a t a520
K ,
the internal f r i c t i o n has decreased and a t h i r d peak P 3 appears associated with the y ' - p r e c i p i t a t e s . Once i t has appeared, t h i s peak remains s t a b l e during a l l treatments of overageing. This means t h a t P 3 peak i s insensitive t o the shape modifications of t h e y ' p r e c i p i t a t e s . I t i s easy t o under- stand the s t a b i l i t y of P,, when i t i s known t h a t the a n e l a s t i c relaxation takes place i n theY
phase. P 3 peak appears i n a l l i t s magnitude i n a y monophase alloy i . e . A1-
85w t
% Ag ( f i g . 2 ) . In the two-phase a l l o y s , the height ofP 3
i s d i r e c t l y proportional t o the volume f r a c t i o n of the precipitated y phase.A t
the equilibrium,C
I
a-I .
lo2 Q - I ( c ) =-
Q - I
where c and c a r e respec-P3 peak max cy y Y 1 : A[ 85% ~g t i v e l y t h e concentrations o f s i l v e r i n t h e 2 : A1 30% Ag 3 : AI 20% ~g a l l o y and i n t h e p r e c i p i t a t e d y phase. 4 : A1 10% A g (cy 2 59.5 a t % Ag). I n a d d i t i o n ,the anel a s t i c c h a r a c t e r i s t i c s o f the P 3 - peak are those o f an atomic r e l a x a - t i o n . Therefore, one can imagine t h a t t h i s r e l a x a t i o n i s due t o t h e s t r e s s induced o r i e n t a t i o n o f e l a s t i c d i p o l e s i n the hexago- 0
~UA\,
, T(K)*
n a l y phase.400 500 I n t h e A1 -Ag a1 1 oys, t h e y phase p r e c i p i - t a t e s present an e q u i l i b r i um concentration, F i g . 2: P 3 peak, a f t e r back-
ground s u b s t r a c t i o n , as i t g i v e n by t h e phase diagram (181, o f
. .
59.5 a t appears i n the Y phase (A1-
Ag ( 4 5 wt % l g ) . 85 w t % Ag) and i n the two-
phase a l l o y s (A1
-
10,20,30 w t % Ag)This value i s lower than t h e s t o i c h i o m e t r i c composition AgzAl (66.6 a t %Ag). Some A1 atoms a r e i n excess w i t h regard t o t h e stoi.chiometry.
So on t h e compact planes, these A1 atoms can c r e a t e e l a s t i c d i p o l e s around them ( f i g . 3)
F i g . 3: Two p o s s i b l e c o n f i g u r a t i o n s o f d i p o l e s
D l , D,, D3
created by an excess A1 atom i n the y phase.C5-884 JOURNAL DE PHYSIQUE
Without e x t e r n a l stresses, t h e d i p o l e s
D l , D2,
D3 ( f i g . 3 a ) c r e a t e an i s o t r o p i c deformation o f t h e l a t t i c e . B u t t h e deformation can become a n i s o t r o p i c by a s t r e s s induced o r i e n t a t i o n o f one d i p o l e ( f i g . 3 b ) .The i n t e n s i t y o f such a r e l a x a t i o n gives a reasonable account f o r t h e h e i g h t o f t h e P3 peak (1 5)
.
The new i n t e r p r e t a t i o n o f t h e P3 peak, which r e f u t e s t h e models based on t h e i n t e r f a c e s r e l a x a t i o n (13), a l l o w s one t o c l a i m t h a t t h e A1-Ag a l l o y s e x h i b i t more than one peak d u r i n g p r e c i p i t a t i o n : t h e P1 peak cannot be t h e P3 peak because t h e
y
phase i s absent i n t h e quenched specimen.So t h e e v o l u t i o n o f t h e i n t e r n a l f r i c t i o n spectrum can be f o l l o w e d d u r i n g a programme o f isochronal ageings by a s e r i e s o f measurements made d u r i n g heatings (Mi) and c o o l i n g s ( D i ) ( f i g . 4 ) .
t
Q-I.
103
AI
-
ACJ
20~10
lsochrones
15mn
F i g . 4: E v o l u t i o n o f t h e i n t e r n a l f r i c t i o n spectrum o f an A1-20 wt % Ag d u r i n g the f o l l o w i n g programme: quenching, MI, 15 mn a t 370K,
D,,
M3, 15 mn a t 400K,D,,,
M 5 , 15 mn a t 430K,D6
and so on every 30K.One can observe t h r e e main stages i n t h e e v o l u t i o n o f t h e curves: t h e d i s - appearance o f the PI peak i n the temperature range where t h e GP-zones are growing ( " c o l d hardening stage"), the appearance o f an h y s t e r e s i s i n t h e i n t e r n a l f r i c t i o n curves associated w i t h t h e Pp peak (hardness r e v e r s i o n stage) and f i n a l l y t h e d i s - appearance o f t h e h y s t e r e s i s w i t h t h e c r e a t i o n o f t h e
P ,
peak by the p r e c i p i t a t i o n o f the y phase ( y ' p r e c i p i t a t e s ) .t h e s o l i d s o l u t i o n . So t h e
P I
peak i s a Zener (19) peak due t o the quenched s o l i d s o l u t i o n . I t s disappearance allows one t o f o l l o w t h e GP-zones e v o l u t i o n as i n the case o f r e s i d u a l e l e c t r i c a l r e s i s t i v i t y measurements (15,16).
The t r a n s i t i o n GP-zones -t y ' p r e c i p i t a t e s gives r i s e t o i n t e r n a l f r i c t i o n
curves which present an h y s t e r e s i s phenomenon: one peak i s p r e s e n t on t h e curves obtained d u r i n g c o o l i n g and t h i s peak i s absent on t h e curves measured d u r i n g heat- i n g . This means t h a t t h e r e a r e two s t a t e s o f metastable thermodynamical e q u i l i b r i u m : one s t a t e connected w i t h t h e c r e a t i o n o f t h e peak and another one w i t h i t s d e s t r u c t - i o n .
This i n t e r n a l f r i c t i o n h y s t e r e s i s i s associated w i t h an h y s t e r e s i s o f t h e e l e c t r i c a l r e s i s t i v i t y , which increases d u r i n g t h e c r e a t i o n o f t h e i n t e r n a l f r i c t i o n peak and decreases d u r i n g i t s d e s t r u c t i o n (16).
I n a d d i t i o n , transmission e l e c t r o n microscopy shows t h a t new small p r e c i p i t a t e s appear a t t h e temperature o f c r e a t i o n o f the peak, i . e . a t %450K ( f i g . 5 ) . I n t h e
Fig.5: A t %450K, new p r e c i p i t a t e s ( y " )
appear
which a r e surrounded by zones depleted o f GP.
neighbourhood o f these p r e c i p i t a t e s , which we can c a l l y" p r e c i p i t a t e s , t h e GP-zones are s m a l l e r than i n t h e m a t r i x .
This would mean t h a t a t 450K t h e a l l o y i s composed o f GP-zones, new y" p r e c i p i t a t e s and a s o l i d s o l u t i o n enriched by a p a r t i a l d i s s o l u t i o n o f t h e GP-zones.
The i n t e r n a l f r i c t i o n curves obtained d u r i n g c o o l i n g s show t h a t P2 peak i s formed
Fig.6: Composition o f P2 peak:
-
a hump ( A ) due t o GP zones e v o l u t i o n and a r e l a x a t i o n peak ( B ) observed a f t e r p a r t i a1 r e v e r s i o n .
by two c o n t r i b u t i o n s ( f i g
.6):
an e v o l u t i o n o f t h e i n t e r n a l f r i c t i o n which c o u l d be connected w i t h t h e e v o l u t i o n of t h e GP- zones, and one r e l a x a t i o n peak which i s o n l y observed d u r i n g c o o l i n g a f t e r a p a r t i a l r e v e r s i o n o f t h e GP-zones.C5-886 JOURNAL DE PHYSIQUE
The growing and fusion of the y" p r e c i p i t a t e s lead t o the appearance of the
y ' p r e c i p i t a t e s which are responsible f o r the s t a b l e
P ,peak.
Conclusions
.-
The internal f r i c t i o n spectrum associated with precipitation in the
A1-Ag alloys i s composed of three relaxation peaks:
PIand P2 due t o the quenched
and reverted s o l i d solutions respectively, and P g due t o the y phase p r e c i p i t a t e s .
The evolution of t h i s spectrum allows one t o define the following sequence:
CL +
GP-zones
+ y " +y '
+y
S Swith the following phenomena:
ass
9 GP:disappearance of
P I
peak
GP -t
y": hysteresis phenomenon, with creation of P p peak due t o reversion
y" 9
