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Submitted on 1 Jan 1987
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THE EFFECT OF AGEING ON THE FATIGUE BEHAVIOUR OF Al-2.3 wt% Li SINGLE CRYSTALS
Jodi Schneider, H.-J. Gudladt, V. Gerold
To cite this version:
Jodi Schneider, H.-J. Gudladt, V. Gerold. THE EFFECT OF AGEING ON THE FATIGUE BE- HAVIOUR OF Al-2.3 wt% Li SINGLE CRYSTALS. Journal de Physique Colloques, 1987, 48 (C3), pp.C3-745-C3-751. �10.1051/jphyscol:1987387�. �jpa-00226618�
THE EFFECT OF AGEING ON THE FATIGUE BEHAVIOUR OF A1-2.3 w t % Li SINGLE CRYSTALS
J. S C H N E I D E R , H.-J. G U D L A D T and V. GEROLD.
Max-Planck-Institut fur Metallforschung, Institut fur
Werkstoffwissenschaften, Seestrasse 92, 0-7000 Stuttgart 1, F.R.G.
ABSTRACT
I n order t o g e t a b e t t e r i n s i g h t i n t o t h e b a s i c deformation o f f a t i g u e i n p r e c i p i t a - tion-hardened A1-Li, experiments have been undertaken on s i n g l e c r y s t a l s o f a high p u r i t y A1-2.3 w t . % L i a l l o y . The c r y s t a l s were s o l u t i o n h e a t t r e a t e d and aged f o r d i f f e r e n t times and temperatures. I n l a b o r a t o r y a i r symmetrical t u s h - p u l l t e s s were
5
performed w i t h a c o n s t a n t p l a s t i c shear s t r a i n amplitude 10- <ypl< 9.10- . For
n e a r l y a l l ageing c o n d i t i o n s c y c l i c hardening and s o f t e n i n g e f f e c t s were v e r y small compared t o o t h e r p r e c i p i t a t i o n - h a r d e n e d c r y s t a l s such as Cu-Co. For specimens aged a t 160°C (UA) and 200°C (PA), r e s p e c t i v e l y , t h e c y c l i c deformation i s c h a r a c t e r i z e d by p l a n a r s l i p which i s l e s s pronounced f o r PA specimens. UA specimens showed heavy s t r a i n b u r s t e f f e c t s where t h e p l a s t i c s t r a i n reaches up t o 10 times t h e d e s i r e d am- p l i t u d e . These s t r a i n b u r s t s are induced by t h e n u c l e a t i o n o f p l a n a r s l i p bands and a r e t y p i c a l f o r UA specimens which tend t o v e r y extreme p l a n a r s l i p behaviour.
Specimens aged a t 220°C showed no p l a n a r s l i p , b u t some s h o r t PSBs w i t h t h e i r t y p i - c a l e x t r u s i o n s were observed by SEM. The d i m i n u t i o n o f p l a n a r s l i p w i t h i n c r e a s i n g ageing temperature i n d i c a t e s t h a t t h e degree o f s t r a i n l o c a l i z a t i o n i s dependent on t h e average s i z e o f t h e & ' p r e c i p i t a t e s i n t h e i n v e s t i g a t e d A1-Li a l l o y .
1. INTRODUCTION
A1-Li a l l o y s a r e s t i l l of general i n t e r e s t i n r e s p e c t t o t h e mechanical p r o p e r t i e s because these l i g h t - w e i g h t e d a l l o y s a r e age-hardenable by t h e p r e c i p i t a t i o n o f me- t a s t a b l e coherent A13Li ( 6 ' ) - p a r t i c l e s . Concerning t h e A1 - L i phase diagramm t h e 6'-phase i s s t a b l e f o r a l l o y compositions from about 5 t o 25 a t % L i (1,2). Since 1970 basic and extended s t u d i e s have been performed on d i f f e r e n t aspects o f t h e pre- c i p i t a t i o n morphology i n A1 - L i a1 l a y s (3-6). The i n f l u e n c e o f t h e p r e c i p i t a t e s on t h e mechanical behaviour o f A1-Li s i n g l e c r y s t a l s has been i n v e s t i g a t e d i n d e t a i l by monotonic t e s t s o n l y . ( 7 ) . Under c y c l i c l o a d i n g c o n d i t i o n s , a s t r o n g p l a n a r s l i p d i - s t r i b u t i o n was observed f o r p o l y c r y s t a l l i n e specimens c o n t a i n i n g f i n e 6 ' p a r t i c l e s (8,9). I n o r d e r t o s t u d y t h i s p l a n a r s l i p behaviour o f A1-Li based a l l o y s i n more d e t a i l , f a t i g u e t e s t s were c a r r i e d o u t on s i n g l e c r y s t a l s aged a t d i f f e r e n t tempera- t u r e s .
2. EXPERIMENTAL DETAILS
A1-Li s i n g l e c r y s t a l s c o n t a i n i n g 2.3 wt% L i were grown by t h e Bridgman technique.
The c y l i n d r i c a l specimens, o r i e n t e d f o r s i n g l e s l i p , were spark-eroded i n order t o o b t a i n a f r e e gauge l e n g t h o f 16 mm and a specimen diameter o f 3.1 mm. A f t e r a solu- t i o n heat t r e a t m e n t i n a s a l t bath a t 565OC f o r 30 minutes t h e samples were water- quenched and aged a t room temperature f o r about 4 days. F i n a l l y t h e samples were aged a t d i f f e r e n t temperatures and times as shown i n Table 1.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987387
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Table 1 Ageing Conditions for A1-2.3wt.% (8.4at.%) Li Single Crystals
Type of Specimen T f°Cl tach]
RT 20 >lo0
U A 160 3.5,lO
PA 200 16,48
HA 220 60
The Li concentration gradient along the gauge length 1, was calculated after measurements carried out with the AAS method(atomic absorption spectroscopy) in or- der to detect possible losses of Li during sample fabrication and solution treat- ment. Along the gauge length no major deviation (less then 0.15 Wt%) from the expec- ted Li concentration of about 2.2 to 2.3 wt% was found.
Symmetrical push-pull fatigue tests were performed with a servohydraulic machine using frequencies of about 1/3 Hz. For the duration of each test the plast'c shear str in amplitude y 1 was kept constant. The values of ypl ranged from 10-4 up to lo-?. After the faglgue tests, the fracture as well as the outer surface of the spe- cimens were investigated by SEM.
3.0 EXPERIMENTAL RESULTS 3.1 Mechanical Behaviour
The cyclic stress response (CSR) of A1-Li single crystals has been measured fo#- three different ageing temperatures. Fig. 1 shows the CSR curves at different strain amplitudes ypl for the so called 'underaged' UA specimens aged at 160°C for 3.5 and 10 h, respectively. In comparison to these curves the corresponding curve of a cry- stal aged only at room temperature (RT) has been plotted in the same figure. The stress level of this,curve is much lower compared to the others. In additio?, all UA tuprcs start to fail at cycle numbers at and below 1000 after a minor har ening sta-
d .
ge whereas the RT-curve continues to harden until test interruption at 10 cycles.
FIG. 1 Cyclic stress responses (CSR) of A1-Li single crystals. The scatter band represents all data for PA specim ns.
< ypl < 4.10- 4
The highest value of t for all y was reached after ageing the crystals at 200°C .
After ageing for 16 h as well asP+ or 48 h, nearly the same value of E was obtained, showing that the 'peak-aged'condition seems to be maintained over a relatively long period of ageing. The scatter band sketched in Fig. 1 represents all fatigue measu- rements obtained from PA specimens. The individual curves show neither hardening nor softening until beginning failure.
c r y s t a l s aged 60 h.
a t 220°C f o r
I I
lo' lo2 m3 ld
cycle number N
F i g . 2 shows t h e CSR curves f o r HA specimens aged a t 220°C. I n c o n t r a s t t o t h e r e - s u l t s o f UA and PA specimens t h e s t r e s s l e v e l now depends on y p l and increases w i t h i n c r e a s i n g y p l as demonstrated i n F i g . 2. The s t r o n g dependence o f t h e s t r e s s l e v e l on y p l has been a l s o c l e a r l y shown by stepwise increase o f y p l .
FIG. 3 C y c l i c s t r e s s s t r a i n (CSS) curves o f A1-Li s i n g l e c r y - s t a l s f o r t h e ageing condi- t i o n s , shown i n Fig. 1 and F i g . 2.
t . . . . I . . ...# . . . . . I . ,...A
6 6 (6' la2 ca'
plastic shear stmin amplituda 7p1 = M .A~p112
The c y c l i c s t r e s s - s t r a i n (CSS) behaviour f o r a l l specimens i s shown i n F i g . 3. For PA specimens a p l a t e a u s t r e s s l e v e l has been found f o r a l a r g e range o f s t r a i n am- p l i t u d e s y 1 from t o 2 - 1 0 - ~ . The CSS data f o r UA specimens show o n l y a weak de- pendence OF r on y p l . For specimens aged a t 220°C, however, a marked d i f f e r e n c e i n t h e CSS behaviour was obser ed. X The hear s t r e s s n e a r l y doubles over a range of 9
s t r a i n amplitudes from 2-10- t o 8.10- . It can be assumed t h a t t h i s s t r o n g d i f f e - rence i n t h e CSS behaviour between t h e HA and t h e other specimens depends on t h e d i f f e r e n c e i n t h e i r m i c r o s t r u c t u r e s .
3.2 Surface Observations
I n order t o understand t h e c y c l i c hardening behaviour o f AL-Li s i n g l e c r y s t a l s i n more d e t a i l . SEM observations have been c a r r i e d o u t on specimens f a t i g u e d up t o a . . . . . . . . - -
c e r t a i n number c f cycles o r u n t i l a crack i n i t i a t e d on t h e surface.
FIG. 4 SEM micrograph o f a UA speci en, FIG. 5 SEM micrograph o f a UA pecimen, ta = 3.5 h, y p l = 1.3-10- J . ta = 10 h, y p l = 7.5-10- 2 .
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F i g . 4 shows a SEM micrograph f o r UA specimens aged f o r 3.5 h. A s t r o n g l y l o c a l i z e d d i s t r i b u t i o n o f s l i p planes i n a dense bundle can be seen on t h e s u r f a c e a t 1000 times m a g n i f i c a t i o n . The s l i p steps are p a r a l l e l t o (111) planes. Beside h e a v i l y de- formed zones, areas w i t h o u t any a c t i v e s l i p plane have been found. With i n c r e a s i n g c y c l e numbers t h e dense bundles o f s l i p planes grow i n t o undeformed areas u n t i l t h e specimen i s t o t a l l y covered w i t h them. T h i s behaviour has been observed d u r i n g t h e f a t i g u e t e s t u s i n g a v i d e o system f o r i n - s i t u examination. For specimens aged a t 10 h, seen i n F i g . 5, t h e s l i p d i s t r i b u t i o n i s l e s s l o c a l i z e d , b u t p l a n a r s l i p has been observed t h e r e , t o o .
FIG. 6 D i s t r i b u t i o n o f s l i p l i n e s and FIG. 7 S l i p l i n e s from F i g . 6 a t h i g h e r s l i p bands on a PA s ecimen, m a g n i f i c a t i o n .
ta = 16 h, y p l = Z-10-9.
F i g . 6 shows t h e specimen s u r f a c e f o r PA specimens aged f o r 16 h a t 200°C. For these c r y s t a l s as w e l l as f o r UA specimens, s t r a i n l o c a l i z a t i o n due t o p l a n a r s l i p has been a l s o observed, b u t t h e number o f s l i p planes i n t h e bundles was much s m a l l e r than f o r UA specimens. I n UA specimens, s l i p i s l o c a l i z e d i n l a r g e and w i d e l y spaced bundles. I n PA specimens, however, t h e bundles a r e s m a l l e r and more evenly d i s t r i b u - t e d , as seen i n F i g . 6. As opposed t o UA specimens, pronounced s l i p steps were n o t observed, b u t small s l i p l i n e s a r e seen i n F i g . 7.
FIG. 8 SEM micrograph of a sample aged FIG. 9 PSBs from F i g . 8 a t h i g h e r mag- a t 220°C f o r 60 h, ypl=l.7.10-3 n i f i c a t i o n .
For HA specimens t h e f a t i g u e behaviour represented by t h e s l i p d i s t r i b u t i o n i s q u i t e d i f f e r e n t . Fig. 8 shows t h e s u r f a c e o f a specimen f a t i g u e d up t o crack i n i t i a t i o n a t uplT1.8-10-3 . One p a r t o f t h e crack propagates p a r a l l e l t o a (111) s l i p plane i n d i - c a t i n g t h a t s t r a i n l o c a l i z a t i o n i n a s l i p band leads t o crack i n i t i a t i o n . As opposed t o UA as w e l l as t o PA specimens no p l a n a r s l i p bands were observed, b u t p e r s i s t e n t s l i p bands (PSBs) were formed d u r i n g c y c l i n g . The h i g h e r m a g n i f i c a t i o n o f PSBs, seen i n F i g . 9 shows small e x t r u s i o n s which a r e t y p i c a l f o r PSB formation. I n c o n t r a s t t o hardenable AlZnMg (10) and CuCo s i n g l e c r y s t a l s ( l l ) , t h e PSBs do n o t cover t h e whole c r o s s s e c t i o n o f t h e c r y s t a l b u t a r e r e s t r i c t e d t o s m a l l e r r e g i o n s o f t h e o r -
der o f 200 vm.
The r e s u l t s o f t h e presented i n v e s t i g a t i o n s demonstrate t h a t t h e f a t i g u e behaviour o f t h e hardened A1-Li s i n g l e c r y s t a l s i s s e n s i t i v e on t h e ageing treatment and t h e r e f o r e on t h e m i c r o s t r u c t u r e . I n UA specimens v e r y small s p h e r i c a l 6 ' p a r t i c l e s ( R < 6 nm) a r e homogeneously d i s t r i b u t e d i n t h e m a t r i x ( 2 ) . During c y c l i n g these p r e c i p i t a t e s are sheared by p a i r - d i s l o c a t i o n s moving p r e f e r a b l y on (111) s l i p planes. T h i s behaviour i s q u i t e s i m i l a r t o t h e behaviour o f s o l i d soluti'ons such as Cu-A1 (12). It was r e c e n t l y r e p o r t e d t h a t i n t h a t case p l a n a r s l i p i s caused by pro- nounced s h o r t range o r d e r i n t h e s o l i d s o l u t i o n and n o t by low s t a c k i n g f a u l t ener- g i e s (13,14). I n c o n t r a s t t o AlZnMg-alloys, which tend t o form p e r s i s t e n t s l i p bands ( l o ) , t h e p l a n a r s l i p i s c h a r a c t e r i z e d by s i n g l e a c t i v e s l i p planes which a r e w e l l separated from each o t h e r by more than 50 nm b u t may s t a y i n l a r g e r groups forming a p l a n a r band ( F i g . 4 and 5 ) . The 6 ' p a r t i c l e s a r e c u t by moving p a i r - d i s l o c a t i o n s l e a d i n g t o a l o c a l s o f t e n i n g e f f e c t . For A1-Li s i n g l e c r y s t a l s i t i s p o s s i b l e t h a t a s i n g l e p l a n a r s l i p band nucleates and spraeds o u t through t h e c r y s t a l d u r i n g one c y c l e . The n u c l e a t i o n o f these bands depends on a c r i t i c a l s t r e s s s which i s necessary f o r t h e d i s l o c a t i o n s t o become mobile and t o s t a r t t o c u t tRe p r e c i p i - t a t e s . The s t r e s s sp i s governed by t h e p r e c i p i t a t i o n morphology and increases w i t h growing 6 ' p a r t i c l e s .
The l a t t e r i s shown ( F i g . 1 ) f o r specimens aged a t 20°C (more than 100 h ) and f o r UA specimens, aged f o r 3.5 and 10 h, r e s p e c t i v e l y . For each t e s t t h e s t r e s s response s t a y s constant f o r many c y c l e s i n d i c a t i n g a constant n u c l e a t i o n r a t e o f new p l a n a r s l i p bands which develop i n t h e undeformed m a t r i x t a k i n g over t h e c y c l i c p l a s t i c de- f o r m a t i o n a t t h e same s t r e s s l e v e l as t h e bands b e f o r e .
For PA specimens t h e h i g h e r sp-values are caused by g r e a t e r 6 ' p a r t i c l e s being more r e s i s t a n t t o d i s l o c a t i o n shearing than t h e smaller p a r t i c l e s i n UA specimens. The growing o f s l i p bands may be s i m i l a r t o t h a t o f UA specimens and a constant s t r e s s l e v e l has been observed, demonstrated by t h e s c a t t e r band i n F i g . 1. Compared t o t h e UA t e s t s t h e p l a n a r s l i p bands c o n s i s t now o f l e s s a c t i v e s l i p planes ( F i g . 6) sepa- r a t e d by l a r g e r distances from each o t h e r ( F i g . 7 ) .
For t h e HA specimens aged a t 220°C t h e p l a t e a u s t r e s s depends on yp i n c o n t r a s t t o UA and PA specimens. The growth o f p r e c i p i t a t e s a t 220°C i s more e i f e c t i v e than a t lower temperatures and v e r y l a r g e r 6 ' p r e c i p i t a t e s can be expected. T h i s has an i n - f l u e n c e on t h e i n t e r a c t i o n between moving d i s l o c a t i o n s and these p r e c i p i t a t e s . The s h o r t PSBs, seen i n F i g . 9, i n d i c a t e t h a t t h e p l a n a r s l i p i s suppressed by t h e presence o f t h e coarse p a r t i c l e s which p i n d i s 1 o c a t i o . i ~ . S i m i l a r t o AlZnMg-single c r y s t a l s i n t h e overaged c o n d i t i o n (15) PSBs o n l y n u c l e a t e on t h e specimen s u r f a c e b u t t h e i r propagation i s blocked by l a r g e p a r t i c l e s , t h e r e a f t e r .
I n order t o compare t h e CSS behaviour f o r s i n g l e c r y s t a l s w i t h d a t a o b t a i n e d from p o l y c r y s t a l l i n e specimens t h e s t r e s s and s t r a i n amp1 i tudes o and Acpl a r e c a l c u l a t e d as f o l l o w s :
T = o / M and y p l = M - A F ~ ~ / ~
The f a c t o r M represents e i t h e r t h e Sachs-factor (2.24) f o r s i n g l e s l i p o r t h e T a y l o r - f a c t o r (3.06) f o r m u l t i p l e s l i p , r e s p e c t i v e l y . The d o t t e d l i n e i n F i g . 3 i s obtained f o r p o l y c r y s t a l l i n e specimens aged a t 200°C f o r 1 h ( 9 ) using M=3.06. It should be mentioned, however, t h a t t h e Sachs-factor M=2.24 would be more a p p r o p r i a t e i n t h i s case. T h i s would s h i f t t h e dashed curve t o much h i g h e r s t r e s s values which a r e no more compatible t o t h e s i n g l e c r y s t a l data. I t appears t h a t t h e i n t e r a c t i o n between g r a i n boundaries and d i s l o c a t i o n s make a c o n t r i b u t i o n t o t h e t o t a l s t r e s s . Therefore, t h e CSS curves obtained f o r p o l y c r y s t a l l i n e specimens may be s h i f t e d t o h i g h e r s t r e s s values.
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4.2 S t r a i n B u r s t E f f e c t s
The occurrence o f p l a n a r s l i p along (111) planes i s t y p i c a l f o r underaged and psak- aged A1-Li c r y s t a l s . I n order t o study t h i s p l a n a r s l i p behaviour, several UA speci- mens aged f o r 3.5 and 10 h were examined by SEM i n more d e t a i l . F i g . 10 shows a SEM micrograph from a specimen s u r f a c e p r o f i l e p a r a l l e l t o t h e l o a d a x i s . For specimens o r i e n t e d f o r s i n g l e s l i p t h e (111) planes made an angle o f 45 deg t o t h e l o a d a x i s . The rough steps due t o t h e bundles o f p l a n a r s l i p planes a r e c l e a r l y v i s i b l e i n t h i s p i c t u r e . A t h i g h e r m a g n i f i c a t i o n t h e f i n e s t r u c t u r e i n s i d e t h i s p l a n a r band i s shown on F i g . 11. A s c r a t c h crosses t h e s l i p steps a t t h e s u r f a c e and marks t h e d i s p l a c e - ment due t o t h e s l i p on i n d i v i d u a l a c t i v e s l i p planes. The maximum s t e p h e i g h t i s o f t h e o r d e r o f 2 pm w h i l e t h e minimum i s found t o be about 300 nm on t h e f i n e c o n t r a s t on t h e l e f t - h a n d s i d e o f t h e micrograph.
FIG. 10 SEM micrograph o f a FIG. 11 SEM micrograph o f t h e f i n e s t r u c - s u r f a c e p r o f i l e from t u r e o f t h e bundles (see Fig.10).
a f a t i g u e d UA speci- men ( t a = 10 h ) .
I n connection w i t h t h e n u c l e a t i o n o f such a p l a n a r s l i p band ( F i g . 10) a discon- t i n u i t y i n t h e p l a s t i c s t r a i n r a t e i s observed which r e s u l t e d i n a heavy s t r a i n bur*st. These b u r s t s were observed i n r e c o r d i n g t h e h y s t e r e s i s loops. They s t a r t du- r i n g t h e t e n s i l e h a l f - c y c l e and continue i n compression and d u r i n g a few subsequent c y c l e s . The maximum p l a s t i c shear s t r a i n observed d u r i n g a b u r s t was about 10 f o l d t h e c o n t r o l l e d s t r a i n v a l u e which was made p o s s i b l e by t h e unusual method we used t o c o n t r o l t h i s s t r a i n .
I n one experiment we were a b l e t o use a microscope connected t o a video system. Ac- c o r d i n g t o these observations t h e s t r a i n b u r s t s can be c o r r e l a t e d t o t h e n u c l e a t i o n o f one o r more p l a n a r s l i p bands which propagate through t h e whole c r o s s s e c t i o n o f t h e specimen w i t h i n a f r a c t i o n o f a second which i s much smaller than t h e c y c l e t i m e . Again, a comparison can be made t o t h e case o f Cu-A1 s o l i d s o l u t i o n s where t h e occurrence o f s i m i l a r b u r s t s has been r e p o r t e d (12).
5. CONCLUSIONS
The r e s u l t s o f these i n v e s t i g a t i o n s demonstrate t h a t t h e f a t i g u e behaviour o f p r e c i - p i t a t i o n hardened A1-Li s i n g l e c r y s t a l s s t r o n g l y depends on t h e ageing t r e a t m e n t and t h e r e f o r e on t h e m i c r o s t r u c t u r e . For t h e UA and PA specimens which c o n t a i n coherent 6 ' p a r t i c l e s p l a n a r s l i p was observed i n a l l cases. I t can be concluded t h a t p l a n a r s l i p i s i n i t i a t e d by m o b i l e d i s l o c a t i o n s which shear t h e 6 ' p r e c i p i t a t e s . As soon as a c r i t i c a l s t r e s s r i s reached, some d i s l o c a t i o n s become mobile and induce an ava- lanche o f h i g h l y mogile d i s l o c a t i o n s . The avalanche i s accompanied by t h e develop- ment o f heavy s t r a i n b u r s t s which r e s u l t i n p l a s t i c s t r a i n s o f ap t o t e n times t h e c o n t r o l l e d y p l . These s t r a i n b u r s t s obtained f o r UA specimens resemble those found t o occur i n s o l i d s o l u t i o n s such as Cu-A1, where a w e l l e s t a b l i s h e d s h o r t range o r - der seems t o be r e s p o n s i b l e f o r p l a n a r s l i p . For A1-Li s i n g l e c r y s t a l s small 6 ' par- t i c l e s a r e s u s c e p t i b l e t o t h e development o f s t r a i n b u r s t s and t h e s t r a i n b u r s t e f f e c t decreases w i t h growing p a r t i c l e s i z e , as demonstrated f o r PA specimens. For HA specimens l a r g e p a r t i c l e s do n o t promote p l a n a r s l i p . Instead, evidence f o r t h e occurance o f PSBs i s found - a t l e a s t i n areas c l o s e t o t h e s u r f a c e - which i s t y p i - c a l f o r non-planar s l i p .
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