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Lithium depletion in precipitate free zones (PFZ's) in AlLi base alloys
Scripta
METALLURGICA
Vol. 23, Printed
pp. 75-79, 1989 in the U . S . A .
Pergamon
Press
plc
LITHIUM DEPLETION IN PRECIPITATE FREE ZONES (PFZ's) IN AI-Li BASE ALLOYS V. Radmilovic*, A.G. Fox, R.M. Fisher, and G. Thomas Center for Advanced Materials, Materials and Chemical Sciences Division, Lawrence Berkeley Laboratory and Department of Materials Science and Mineral Engineering, University of California. *Permanent address: University of Belgrade, Dept. of Metallurgy, Belgrade, Yugoslavia, P.O. Box 494, Karnegijeva 4. (Received August (Revised October
22, 21,
1988) 1988)
Introduction Since the effects of precipitate free zones (PFZ's) were first examined in high strength AI-Mg-Zn alloys (1,2) much attention has been paid to PFZ's and their deleterious effects, especially with respect to the mechanical behavior, fracture and stress-corrosion cracking, in a wide range of conventional high strength A_l-base alloys (3-9), as well as in the newer high strength AI-Li alloys for aerospace applications (10-14). In terms of complexities and mechanisms of precipitate nucleation and growth, including considerable non-uniformity due to heterogeneous nucleation, conventional AI base alloys are very similar to the AI-Li base alloys. The optimization of properties requires uniformity of precipitation w i t h suitable volume fractions of appropriate precipitates. Heterogeneous nucleation may modify the desired morphology, but this mode can also be utilized to increase nucleation rates (2), e.g., by mechanical deformation as in the complex theromechanical treatments (TMT) currently being pursued. The phenomenon of PFZ's in high strength AI-Li base alloys has been extensively studied (I0, 11, 15). In order to establish the mechanism of formation of PFZ's, it is important to determine w h e t h e r the PFZ's are a result of vacancy or solute depletion (or both) leading to low nucleation rates due to low supersaturations. Possible lithium depletion is indicated from the early analytical work of W i l l i a m s and Edington (15). Such lithium depletion could of course cause a reduction in the volume fraction of fine ~' precipitates near the grain boundaries which would adversely affect the mechanical and stress-corrosion properties. Williams and Edington (15) measured lithium depletion at grain boundaries in an AI-12.9 at %Li alloy by electron energy loss spectroscopy (EELS) using a specially modified Siemens electron microscope. With this device they measured the energy shift of the first plasmon loss peak as a function of lithium content. The difficulties of spectroscopic analysis for Li (mainly because of its low atomic number) are well known. An alternative technique for compositional analysis, although indirect, which can be utilized by high voltage microscopy (HVEM) is that of the so called "critical voltage" effect (16, 17). Specifically Fox and Fisher (18, 19) have detected changes o f ~ 3 at .% lithium in binary AI-Li alloys by the critical voltage (Vc) effect, w h i c h can be measured to within i5kV (17) by appropriate attention to detail ~8). Thus, it should be possible to detect lithium depletion in PFZ's in AI-Li alloys by this method, and such is the object of the present work. The 1.5 MeV HVEM with liquid nitrogen stage available at the National Center for Electron Microscopy is necessary for such analyses. Experimental Procedure Two alloys A (binary AI-L~ and B (ternary Ai-Li-Cu) have been investigated (for analysis details see Table i). These alloys were heat treated as follows: solution treatment at 823K for two hours, ice brine quenched and aged 5 minutes at 573K to produce overaged
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PFZ's
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material with relatively large PFZ's (~2p m across). Alloy B was also deformed 6% prior to aging. 0.125mm slices were cut with a slow-speed diamond saw from both the as-quenched and overaged alloys from which 3 m m discs were punched. These samples were jet electropolished with a 3% perchloric acid/35% n-butoxy ethanol/62% ethanol solution below 248Kat 40V.In the case of alloy B which contains Cu, severe copper redeposltion during electropolishing was e n c o u n t e r e d w h i c h r e s u l t e d in poor K i k u c h i p a t t e r n s (essential for a c c u r a t e critical voltage measurements) and so a two-stage technique was adopted where the final thinning was performed by ion milling at 77K. TABLE 1 Chemical Composition
(wt.%)
Alloying Elements Alloy A B AI* A2*
Li 2.45 2.37 1.40 2.20
Cu 0.02 2.49 ---
Zr 0.ii 0.13 0.12 0.12
Al bal. bal. hal. bal.
*Alloys used by Fox and Fisher (16) C a l c u l a t i o n s b a s e d on the w o r k of Fox and F i s h e r (19) and Shirley a n % ~ s h e r (%~nwere performed to produce theoretical 222 and 400 critical voltage values (Vc ~ and Vc~VV) for the as-quenched alloys A and B. These are shown in Table 2 together with the experimental results and the results of Fox and Fisher. Large PFZ's were selected, after the usual examination by dark field and diffraction, which were about 2pm across at the grain boundariesjwith a high density of 6' + T 1 phases for the a n a l y s e s in the c e n t e r of the grains. (An e x a m p l e is s h o w n in fig. I (at B). B e c a u s e of the difficulties of obtaining sharp Kikuchi patterns in the TMT treated alloys due to diffuse scattering from the high density of dislocations, the results to be reported here w e r e o b t a i n e d from m e a s u r e m e n t s of the c r i t i c a l voltages, in the PFZ's in alloy B, and c r i t i c a l v o l t a g e s in t h e u + 6e p h a s e regions in the c e n t e r of the g r a i n s in alloy A. The critical voltage effect measures or depends on the structure factors of the systematic row concerned. H e n c e a c r i t i c a l v o l t a g e in as q u e n c h e d 6 ' - f r e e A I - L i solid solution w i l l be the s a m e as that in an a g e d e +6' t w o p h a s e alloy of the same l i t h i u m content (6' is c o h e r e n t w i t h e) p r o v i d e d the 6' p r e c i p i t a t e s are s m a l l c o m p a r e d w i t h the e l e c t r o n b e a m probe size, which they are in the present work. This has been confirmed experimentally by Fox and Fisher. (19). Both Vc 222 and Vc400 were measured at 293K and below room temperature in the center of the grains of alloy A. Vc222 was measured at 293K iD alloy B in both the center of the grains and in the PFZ s h o w n in figure I. A t t e m p t s w e r e m a d e to m e a s u r e Vc400 in alloy B, but it was immediately evident that because of the poor diffraction patterns (particularly in the center of the grains) sufficient accuracy would not be obtained with those measurements to search for possible lithium depletion. The e x p e r i m e n t s w e r e p e r f o r m e d using c o n v e r g e n t beam diffraction at a beam spot size oft 0.7~m at 400 to 500 kV reducing to about 0.5 ~m at 900 to 1000kV in the Kratos EM 1500 HVEM. The results thus have these spatial resolution limitations. It should be noted that the low temperature measurements made on alloy A were performed by maintaining the microscope voltage constant and reducing the temperature until the a p p r o p r i a t e B ragg r e f l e c t i o n disappeared, i.e., a "critical t e m p e r a t u r e " technique, originally adopted by Sellar et al. (22). This approach lead to improved accuracy. Results and Discussion The chemical analyses of alloys A and B shown in Table 2 indicate that the lithium contents
I
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23,
No.
1
PFZ's
IN A 1 - L i
ALLOYS
77
ol the center of the grains in both alloys are very nearly the same in both the as-quenched and o v e r a g e d conditions. This a l l o w s a c o m p a r i s o n b e t w e e n the c r i t i c a l v o l t a g e m e a s u r e m e n t s to be m a d e on the t w o a l l o y s so that p o s s i b l e Li d e p l e t i o n at PFZ's can be confirmed. In spite of the u n a v o i d a b l y large errors, the V 222 m e a s u r e m e n t s on alloy B suggest that there is lithium depletion at PFZ's (see Table 3, which is in agreement with W i l l i a m s and E d i n g t o n (15). It is very s a t i s f y i n g to note tht all the critical v o l t a g e s measured in this work agree in a consistent way with those made by Fox and Fisher (16).
Calculated
TABLE 2 and Experimental Critical Voltages Temp.
Sample
[K]
(Vc) for AI-Li Alloys
Vc222
V 400
%LI
[kV]
~kV]
[at] (wt)
Alloy A as quenched
(calculated)
293
478
994.5
[ 8 . 9 ] (2.45)
Alloy B as quenched
(calculated)
293
477
990
[8.7] (2.37)
Alloy B (overaged;
in PFZ)
293
465+-15
[5.3+_4.5] (i.~ 1.2)
Alloy B (overaged;
in grain center)
293
475+-30
[8.3 +6.0] (2.30+_1.66)
Alloy A (overaged,
in grain center)
293
480+- i0
1000+_25 [9.8~_3]
Alloy A (overaged,
in grain center)
253
500-+ 7.5
---
293
465+_ I0
975 +_15
(as quenched)*
92
535+-10
1110+15115.3+_3]
A2 (as quenched)*
293
475+_ I0
990-+ 15 1
A2 (as quenched)*
92
550+_10
AI (as quenched)* A1
* Data from ref.
1140+_15
(2.7(~_0.83)
[9.6+_2.5] (2.6f~_0.69)
I18"i±3]
(1.4_+0.8)
(2.20_+0.34)
[16]
The most striking confirmation of the lowering of V 222 in the PFZ's is the comparison of the iii s y s t e m a t i c c o n v e r g e n t b e a m p a t t e r n s (CBDp)Ctaken at 460kV in both the c e n t e r of g r a i n s in alloy A and in the PFZ's in alloy B. E x a m p l e s are shown in figures 2 and 3, respectively. It is clear from these figures that the CBDP taken from the PFZ in alloy B is close to V c 222, since the 222 r e f l e c t i o n and K i k u c h i lines are absent in figure 3, w h e r e a s that taken from the c e n t e r of the grain in alloy A is o b v i o u s l y w e l l b e l o w V c. From these results it can be estimated that the lithium content of the PFZ's in alloy B is about 5..25 at.% (l.4wt.%) and just a little h i g h e r the the s o l u b i l i t y limit at 300K. On the other h a n d the l i t h i u m content of the grain centers is close to that of the overall a n a l y s i s s h o w n in Table I. However, it should be noted that the errors in the l i t h i u m c o n t e n t s m e a s u r e d in the c e n t e r of g r a i n s and in the PFZ's overlap, s h o w i n g that it is difficult but just about possible to detect lithium depletion in the PFZ's by this method. In a d d i t i o n to these studies on grain boundaries, a s i m i l a r e x e r c i s e w a s p e r f o r m e d on subgrain boundaries in overaged alloy A and a dark field micrograph is shown in figure 4. It is clear that small PFZ's are also formed next to these subgrain boundaries. However, because they are so narrow it was not possible to perform V c measurements there and hence, no conclusion can yet be drawn about the Li content. In addition, precipitates which are e n c o u n t e r e d at the subgrain b o u n d a r y d i s l o c a t i o n s a r e ~ ' (AI 3Li)' rather than ~ (AI Li)
78
PFZ's
IN A I - L i
ALLOYS
Vol.
23,
No.
w h i c h occur on grain boundaries. This is clearly shown in figure 4 w h e r e the 6' precipitates in the center of the grains are of the usual spherical morphology, whereas those at the subgrain boundary dislocations are constrained in shape by the presence of the dislocations as has been explained by Cassada et al. (23). It is likely that these PFZ's at subgrain boundaries also contribute to non-uniform mechanical and corrosion properties in these alloys. Conclusions I. The critical voltage effect provides a unique method for estimating the lithium content in Al-alloys with a spatial resolution of ~0.5~m and accuracy of about ± 3 at.%Li. 2. Such critical voltage measurements confirm that there is lithium depletion near grain boundaries in the investigated alloys. 3. PFZ's oft 0.2 to 0.5~m w i d t h can also occur at subgrain boundaries in AI-Li base allo~s, and the precipitates w h i c h form on the subgrain boundary dislocations are of 6'(AI Li rather than 6(AI Li) which occur mainly on the grain boundaries. Acknowledgements This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division, U.S. Dept. of Energy, under contract DE-AC376SF00098. The authors gratefully acknowledge critical discussions with Ms. J. Glazer. The alloys were suppled by ALCOA. G. Thomas and V. Radmilovic also gratefully acknowledge a grant from Allied Signal, Inc., and the latter also acknowedges partial financial support from the Council for Basic Sciences of Serbia, (RZNS), Belgrade, Yugoslavia. References 1. G. Thomas and J. Nutting, J. Inst. Metals, 88, 81, (1959). 2. G. Thomas, J. Inst. Metals, 89, 287, (1960). 3. P.N.T. Unwin, G.W. Lorimer, and R.B. Nicholson, Acta Met., 17, 1363, (1969). 4. J.D. Embury and R.B. Nicholson, Acta Met., 13, 403, (1965). 5. G. W. L o r i m e r and R.B. Nicholson, Acta Met., 14, 1009, (1966). 6. N. Ryum, Acta Met., 16, 327, (1968). 7. G.W. L o r i m e r and R.B. Nicholson, The Mechanism of Phase Transformation in Crystalline Solids, Institute of Metals, London, 36, (1969). 8. M. Abe, K. Asano, and A. Fujiwara, J. Japan Inst. Metals, 36, 597, (1972). 9. M. Abe, K. Asano, and A. Fujiwara, Met. Trans., AI4, 1499, (1973). i0. O. Jensrud and N. Ryum, Mater. Sci. Eng., 64, 229, (1984). ii. T. Sanders, E.A. Ludwiczak, and R. R. Sawtell, Materi. Sci. Eng., 43, 247, (1980). 12. S. Suresh, A.K. Vasudevan, M. Tosten, and P. R. Howell, Acta Met., 35, 25, (1987). 13. A. K. Vasudevan and R. D. Doherty, Acta Met., 35, 1193, (1987). 14. M.H. Tosten, A. K. Vasudevan and P. R. Howell, Met. Trans., 19A, 51, (1988). 15. D. B. W i l l i a m s and J.W. Edington, Philos. Mag., 30, 1147, (1974). 16. D. Watanabe, R. Uyeda, and M. Kogiso, Acta Cryst.. A24, 249, 1968. 17. W.L. Bell and G. Thomas, "Electron Microscopy & Structure of Materials", eds. G. Thomas, P~M. Fulrath, R.M. Fisher, University of California Press, 45, (1971). 18. G. Thomas, and M.J. Goringe, "Transmission Electron Microscopy of Materials", John Wiley & Sons, New York, 129, (1979). 19. A.G. Fox and R.M. Fisher, Acta Cryst., A43, 260, (1987). 20. A.G. Fox and R.M. Fisher, J. Mat. Sci. Lefts., 7, 301, (1988). 21. C.G. Shirley and R.M. Fisher, Philos. Mag. A, 3_~9, 91, (1979). 22. J.R. Sellar, D. Imeson and C.J. Humphreys, Acta Cryst., A36, 686, (1980). 23. W.A. Cassada, G.J. Shiflet, and W.A. Jesser, submitted to Acta Met. (1988).
1
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No.
1
PFZ's
IN A I - L i
ALLOYS
79
Fig. 2 Fig. 1
Fig. I - T E M b r i g h t f i e l d i m a g e of PFZ at a g r a i n a l l o y p r o d u c e d a f t e r a g i n g 5 m i n at 573K.
boundary
in an A 1 - L i - C u
Fig. 2 - C B D P i m a g e u s i n g Iii s y s t e m a t i c r e f l e c t i o n s , f r o m the c e n t e r of the g r a i n :in A I - L i b i n a r y a l l o y . N o t e the 222 r e f l e c t i o n a n d c o n t r a s t at the Kikuchi lines. S i n c e the b r i g h t c o n t r a s t f r o m the K i k u c h i line in the Iii r e f l e c t i o n lies to the 222 side of the o r i g i n t h e n the v o l t a g e is b e l o w t h a t of the c r i t i c a l v a l u e .
Fig. 3
Fig. 4
Fig. 3 - C B D P i m a g e u s i n g iii s y s t e m a t i c r e f l e c t i o n s f r o m a PFZ in A I - L i - C u a l l o y . The a b s e n c e of K i k u c h i line i n t e n s i t y at the 222 r e f l e c t i o n i n d i c a t e s the v o l t a g e is at the c r i t i c a l v a l u e . Fig. 4 - T E M c e n t e r e d d a r k f i e l d i m a g e t a k e n u s i n g a [iii] . Heterogeneous p r e c i p i t a t i o n of 5' produced a f t e r a g i n g 5 m i n at 573K. N o t e PFZ and n o n s p h e r i c a l m o r p h o l o g y of 5' p r e c i p i t a t e s a l o n g s u b g r a i n b o u n d a r y .
METALLURGICA
Vol. 23, Printed
pp. 75-79, 1989 in the U . S . A .
Pergamon
Press
plc
LITHIUM DEPLETION IN PRECIPITATE FREE ZONES (PFZ's) IN AI-Li BASE ALLOYS V. Radmilovic*, A.G. Fox, R.M. Fisher, and G. Thomas Center for Advanced Materials, Materials and Chemical Sciences Division, Lawrence Berkeley Laboratory and Department of Materials Science and Mineral Engineering, University of California. *Permanent address: University of Belgrade, Dept. of Metallurgy, Belgrade, Yugoslavia, P.O. Box 494, Karnegijeva 4. (Received August (Revised October
22, 21,
1988) 1988)
Introduction Since the effects of precipitate free zones (PFZ's) were first examined in high strength AI-Mg-Zn alloys (1,2) much attention has been paid to PFZ's and their deleterious effects, especially with respect to the mechanical behavior, fracture and stress-corrosion cracking, in a wide range of conventional high strength A_l-base alloys (3-9), as well as in the newer high strength AI-Li alloys for aerospace applications (10-14). In terms of complexities and mechanisms of precipitate nucleation and growth, including considerable non-uniformity due to heterogeneous nucleation, conventional AI base alloys are very similar to the AI-Li base alloys. The optimization of properties requires uniformity of precipitation w i t h suitable volume fractions of appropriate precipitates. Heterogeneous nucleation may modify the desired morphology, but this mode can also be utilized to increase nucleation rates (2), e.g., by mechanical deformation as in the complex theromechanical treatments (TMT) currently being pursued. The phenomenon of PFZ's in high strength AI-Li base alloys has been extensively studied (I0, 11, 15). In order to establish the mechanism of formation of PFZ's, it is important to determine w h e t h e r the PFZ's are a result of vacancy or solute depletion (or both) leading to low nucleation rates due to low supersaturations. Possible lithium depletion is indicated from the early analytical work of W i l l i a m s and Edington (15). Such lithium depletion could of course cause a reduction in the volume fraction of fine ~' precipitates near the grain boundaries which would adversely affect the mechanical and stress-corrosion properties. Williams and Edington (15) measured lithium depletion at grain boundaries in an AI-12.9 at %Li alloy by electron energy loss spectroscopy (EELS) using a specially modified Siemens electron microscope. With this device they measured the energy shift of the first plasmon loss peak as a function of lithium content. The difficulties of spectroscopic analysis for Li (mainly because of its low atomic number) are well known. An alternative technique for compositional analysis, although indirect, which can be utilized by high voltage microscopy (HVEM) is that of the so called "critical voltage" effect (16, 17). Specifically Fox and Fisher (18, 19) have detected changes o f ~ 3 at .% lithium in binary AI-Li alloys by the critical voltage (Vc) effect, w h i c h can be measured to within i5kV (17) by appropriate attention to detail ~8). Thus, it should be possible to detect lithium depletion in PFZ's in AI-Li alloys by this method, and such is the object of the present work. The 1.5 MeV HVEM with liquid nitrogen stage available at the National Center for Electron Microscopy is necessary for such analyses. Experimental Procedure Two alloys A (binary AI-L~ and B (ternary Ai-Li-Cu) have been investigated (for analysis details see Table i). These alloys were heat treated as follows: solution treatment at 823K for two hours, ice brine quenched and aged 5 minutes at 573K to produce overaged
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+ .00
76
PFZ's
IN AI-Li ALLOYS
Vol.
23, No.
material with relatively large PFZ's (~2p m across). Alloy B was also deformed 6% prior to aging. 0.125mm slices were cut with a slow-speed diamond saw from both the as-quenched and overaged alloys from which 3 m m discs were punched. These samples were jet electropolished with a 3% perchloric acid/35% n-butoxy ethanol/62% ethanol solution below 248Kat 40V.In the case of alloy B which contains Cu, severe copper redeposltion during electropolishing was e n c o u n t e r e d w h i c h r e s u l t e d in poor K i k u c h i p a t t e r n s (essential for a c c u r a t e critical voltage measurements) and so a two-stage technique was adopted where the final thinning was performed by ion milling at 77K. TABLE 1 Chemical Composition
(wt.%)
Alloying Elements Alloy A B AI* A2*
Li 2.45 2.37 1.40 2.20
Cu 0.02 2.49 ---
Zr 0.ii 0.13 0.12 0.12
Al bal. bal. hal. bal.
*Alloys used by Fox and Fisher (16) C a l c u l a t i o n s b a s e d on the w o r k of Fox and F i s h e r (19) and Shirley a n % ~ s h e r (%~nwere performed to produce theoretical 222 and 400 critical voltage values (Vc ~ and Vc~VV) for the as-quenched alloys A and B. These are shown in Table 2 together with the experimental results and the results of Fox and Fisher. Large PFZ's were selected, after the usual examination by dark field and diffraction, which were about 2pm across at the grain boundariesjwith a high density of 6' + T 1 phases for the a n a l y s e s in the c e n t e r of the grains. (An e x a m p l e is s h o w n in fig. I (at B). B e c a u s e of the difficulties of obtaining sharp Kikuchi patterns in the TMT treated alloys due to diffuse scattering from the high density of dislocations, the results to be reported here w e r e o b t a i n e d from m e a s u r e m e n t s of the c r i t i c a l voltages, in the PFZ's in alloy B, and c r i t i c a l v o l t a g e s in t h e u + 6e p h a s e regions in the c e n t e r of the g r a i n s in alloy A. The critical voltage effect measures or depends on the structure factors of the systematic row concerned. H e n c e a c r i t i c a l v o l t a g e in as q u e n c h e d 6 ' - f r e e A I - L i solid solution w i l l be the s a m e as that in an a g e d e +6' t w o p h a s e alloy of the same l i t h i u m content (6' is c o h e r e n t w i t h e) p r o v i d e d the 6' p r e c i p i t a t e s are s m a l l c o m p a r e d w i t h the e l e c t r o n b e a m probe size, which they are in the present work. This has been confirmed experimentally by Fox and Fisher. (19). Both Vc 222 and Vc400 were measured at 293K and below room temperature in the center of the grains of alloy A. Vc222 was measured at 293K iD alloy B in both the center of the grains and in the PFZ s h o w n in figure I. A t t e m p t s w e r e m a d e to m e a s u r e Vc400 in alloy B, but it was immediately evident that because of the poor diffraction patterns (particularly in the center of the grains) sufficient accuracy would not be obtained with those measurements to search for possible lithium depletion. The e x p e r i m e n t s w e r e p e r f o r m e d using c o n v e r g e n t beam diffraction at a beam spot size oft 0.7~m at 400 to 500 kV reducing to about 0.5 ~m at 900 to 1000kV in the Kratos EM 1500 HVEM. The results thus have these spatial resolution limitations. It should be noted that the low temperature measurements made on alloy A were performed by maintaining the microscope voltage constant and reducing the temperature until the a p p r o p r i a t e B ragg r e f l e c t i o n disappeared, i.e., a "critical t e m p e r a t u r e " technique, originally adopted by Sellar et al. (22). This approach lead to improved accuracy. Results and Discussion The chemical analyses of alloys A and B shown in Table 2 indicate that the lithium contents
I
Vol.
23,
No.
1
PFZ's
IN A 1 - L i
ALLOYS
77
ol the center of the grains in both alloys are very nearly the same in both the as-quenched and o v e r a g e d conditions. This a l l o w s a c o m p a r i s o n b e t w e e n the c r i t i c a l v o l t a g e m e a s u r e m e n t s to be m a d e on the t w o a l l o y s so that p o s s i b l e Li d e p l e t i o n at PFZ's can be confirmed. In spite of the u n a v o i d a b l y large errors, the V 222 m e a s u r e m e n t s on alloy B suggest that there is lithium depletion at PFZ's (see Table 3, which is in agreement with W i l l i a m s and E d i n g t o n (15). It is very s a t i s f y i n g to note tht all the critical v o l t a g e s measured in this work agree in a consistent way with those made by Fox and Fisher (16).
Calculated
TABLE 2 and Experimental Critical Voltages Temp.
Sample
[K]
(Vc) for AI-Li Alloys
Vc222
V 400
%LI
[kV]
~kV]
[at] (wt)
Alloy A as quenched
(calculated)
293
478
994.5
[ 8 . 9 ] (2.45)
Alloy B as quenched
(calculated)
293
477
990
[8.7] (2.37)
Alloy B (overaged;
in PFZ)
293
465+-15
[5.3+_4.5] (i.~ 1.2)
Alloy B (overaged;
in grain center)
293
475+-30
[8.3 +6.0] (2.30+_1.66)
Alloy A (overaged,
in grain center)
293
480+- i0
1000+_25 [9.8~_3]
Alloy A (overaged,
in grain center)
253
500-+ 7.5
---
293
465+_ I0
975 +_15
(as quenched)*
92
535+-10
1110+15115.3+_3]
A2 (as quenched)*
293
475+_ I0
990-+ 15 1
A2 (as quenched)*
92
550+_10
AI (as quenched)* A1
* Data from ref.
1140+_15
(2.7(~_0.83)
[9.6+_2.5] (2.6f~_0.69)
I18"i±3]
(1.4_+0.8)
(2.20_+0.34)
[16]
The most striking confirmation of the lowering of V 222 in the PFZ's is the comparison of the iii s y s t e m a t i c c o n v e r g e n t b e a m p a t t e r n s (CBDp)Ctaken at 460kV in both the c e n t e r of g r a i n s in alloy A and in the PFZ's in alloy B. E x a m p l e s are shown in figures 2 and 3, respectively. It is clear from these figures that the CBDP taken from the PFZ in alloy B is close to V c 222, since the 222 r e f l e c t i o n and K i k u c h i lines are absent in figure 3, w h e r e a s that taken from the c e n t e r of the grain in alloy A is o b v i o u s l y w e l l b e l o w V c. From these results it can be estimated that the lithium content of the PFZ's in alloy B is about 5..25 at.% (l.4wt.%) and just a little h i g h e r the the s o l u b i l i t y limit at 300K. On the other h a n d the l i t h i u m content of the grain centers is close to that of the overall a n a l y s i s s h o w n in Table I. However, it should be noted that the errors in the l i t h i u m c o n t e n t s m e a s u r e d in the c e n t e r of g r a i n s and in the PFZ's overlap, s h o w i n g that it is difficult but just about possible to detect lithium depletion in the PFZ's by this method. In a d d i t i o n to these studies on grain boundaries, a s i m i l a r e x e r c i s e w a s p e r f o r m e d on subgrain boundaries in overaged alloy A and a dark field micrograph is shown in figure 4. It is clear that small PFZ's are also formed next to these subgrain boundaries. However, because they are so narrow it was not possible to perform V c measurements there and hence, no conclusion can yet be drawn about the Li content. In addition, precipitates which are e n c o u n t e r e d at the subgrain b o u n d a r y d i s l o c a t i o n s a r e ~ ' (AI 3Li)' rather than ~ (AI Li)
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w h i c h occur on grain boundaries. This is clearly shown in figure 4 w h e r e the 6' precipitates in the center of the grains are of the usual spherical morphology, whereas those at the subgrain boundary dislocations are constrained in shape by the presence of the dislocations as has been explained by Cassada et al. (23). It is likely that these PFZ's at subgrain boundaries also contribute to non-uniform mechanical and corrosion properties in these alloys. Conclusions I. The critical voltage effect provides a unique method for estimating the lithium content in Al-alloys with a spatial resolution of ~0.5~m and accuracy of about ± 3 at.%Li. 2. Such critical voltage measurements confirm that there is lithium depletion near grain boundaries in the investigated alloys. 3. PFZ's oft 0.2 to 0.5~m w i d t h can also occur at subgrain boundaries in AI-Li base allo~s, and the precipitates w h i c h form on the subgrain boundary dislocations are of 6'(AI Li rather than 6(AI Li) which occur mainly on the grain boundaries. Acknowledgements This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division, U.S. Dept. of Energy, under contract DE-AC376SF00098. The authors gratefully acknowledge critical discussions with Ms. J. Glazer. The alloys were suppled by ALCOA. G. Thomas and V. Radmilovic also gratefully acknowledge a grant from Allied Signal, Inc., and the latter also acknowedges partial financial support from the Council for Basic Sciences of Serbia, (RZNS), Belgrade, Yugoslavia. References 1. G. Thomas and J. Nutting, J. Inst. Metals, 88, 81, (1959). 2. G. Thomas, J. Inst. Metals, 89, 287, (1960). 3. P.N.T. Unwin, G.W. Lorimer, and R.B. Nicholson, Acta Met., 17, 1363, (1969). 4. J.D. Embury and R.B. Nicholson, Acta Met., 13, 403, (1965). 5. G. W. L o r i m e r and R.B. Nicholson, Acta Met., 14, 1009, (1966). 6. N. Ryum, Acta Met., 16, 327, (1968). 7. G.W. L o r i m e r and R.B. Nicholson, The Mechanism of Phase Transformation in Crystalline Solids, Institute of Metals, London, 36, (1969). 8. M. Abe, K. Asano, and A. Fujiwara, J. Japan Inst. Metals, 36, 597, (1972). 9. M. Abe, K. Asano, and A. Fujiwara, Met. Trans., AI4, 1499, (1973). i0. O. Jensrud and N. Ryum, Mater. Sci. Eng., 64, 229, (1984). ii. T. Sanders, E.A. Ludwiczak, and R. R. Sawtell, Materi. Sci. Eng., 43, 247, (1980). 12. S. Suresh, A.K. Vasudevan, M. Tosten, and P. R. Howell, Acta Met., 35, 25, (1987). 13. A. K. Vasudevan and R. D. Doherty, Acta Met., 35, 1193, (1987). 14. M.H. Tosten, A. K. Vasudevan and P. R. Howell, Met. Trans., 19A, 51, (1988). 15. D. B. W i l l i a m s and J.W. Edington, Philos. Mag., 30, 1147, (1974). 16. D. Watanabe, R. Uyeda, and M. Kogiso, Acta Cryst.. A24, 249, 1968. 17. W.L. Bell and G. Thomas, "Electron Microscopy & Structure of Materials", eds. G. Thomas, P~M. Fulrath, R.M. Fisher, University of California Press, 45, (1971). 18. G. Thomas, and M.J. Goringe, "Transmission Electron Microscopy of Materials", John Wiley & Sons, New York, 129, (1979). 19. A.G. Fox and R.M. Fisher, Acta Cryst., A43, 260, (1987). 20. A.G. Fox and R.M. Fisher, J. Mat. Sci. Lefts., 7, 301, (1988). 21. C.G. Shirley and R.M. Fisher, Philos. Mag. A, 3_~9, 91, (1979). 22. J.R. Sellar, D. Imeson and C.J. Humphreys, Acta Cryst., A36, 686, (1980). 23. W.A. Cassada, G.J. Shiflet, and W.A. Jesser, submitted to Acta Met. (1988).
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Fig. 2 Fig. 1
Fig. I - T E M b r i g h t f i e l d i m a g e of PFZ at a g r a i n a l l o y p r o d u c e d a f t e r a g i n g 5 m i n at 573K.
boundary
in an A 1 - L i - C u
Fig. 2 - C B D P i m a g e u s i n g Iii s y s t e m a t i c r e f l e c t i o n s , f r o m the c e n t e r of the g r a i n :in A I - L i b i n a r y a l l o y . N o t e the 222 r e f l e c t i o n a n d c o n t r a s t at the Kikuchi lines. S i n c e the b r i g h t c o n t r a s t f r o m the K i k u c h i line in the Iii r e f l e c t i o n lies to the 222 side of the o r i g i n t h e n the v o l t a g e is b e l o w t h a t of the c r i t i c a l v a l u e .
Fig. 3
Fig. 4
Fig. 3 - C B D P i m a g e u s i n g iii s y s t e m a t i c r e f l e c t i o n s f r o m a PFZ in A I - L i - C u a l l o y . The a b s e n c e of K i k u c h i line i n t e n s i t y at the 222 r e f l e c t i o n i n d i c a t e s the v o l t a g e is at the c r i t i c a l v a l u e . Fig. 4 - T E M c e n t e r e d d a r k f i e l d i m a g e t a k e n u s i n g a [iii] . Heterogeneous p r e c i p i t a t i o n of 5' produced a f t e r a g i n g 5 m i n at 573K. N o t e PFZ and n o n s p h e r i c a l m o r p h o l o g y of 5' p r e c i p i t a t e s a l o n g s u b g r a i n b o u n d a r y .