Variations of microstructure in large strain cold-rolled pure aluminium

Scripta METALLURGICA

Vol. 19, pp. 505-510, 1985 Printed in the U.S.A.

Pergamon Press Ltd. All rights reserved

V A R I A T I O N S OF M I C R O S T R U C T U R E IN L A R G E STRAIN C O L D - R O L L E D PURE A L U M I N I U M W. Pfeiler, M. Zehetbauer and J. Schrank* Institut f~r F e s t k 6 r p e r p h y s i k der U n i v e r s i t ~ t Wien,

A-1090

Vienna,

Austria

(Received December'f8, 1984) (Revised January I0, 1985) Introduction D e f o r m a t i o n in a l u m i n i u m has b e e n s t u d i e d in the p a s t b y m a n y w o r k e r s (i). The m i c r o s t r u c t u r e p r o d u c e d and the way it forms are well u n d e r s t o o d for stages I and II of w o r k - h a r d e n i n g . Because of m a n i f o l d structural evolutions, the u n d e r s t a n d i n g of m e c h a n i c a l b e h a v i o u r in stage III p r o v e s more difficult. Lately, the m a r k e d o n s e t of d i s l o c a t i o n a n n i h i l a t i o n has b e e n a c c e p t e d as the main feature of stage III (2,3). U n d e r certain conditions, dynamic a n n i h i l a t i o n can b e c o m e so p r e v a l e n t that steady-state d e f o r m a t i o n sets in, which m e a n s p r i m a r i l y a d y n a m i c e q u i l i b r i u m b e tween p r o d u c t i o n and a n n i h i l a t i o n of dislocations. This u s u a l l y is o b s e r v e d in h i g h t e m p e r a t u r e creep (4), but also can o c c u r at t e m p e r a t u r e s T << 0.5 T m (Tm ... m e l t i n g t e m p e r a t u r e in K), p r o v i d e d s u f f i c i e n t l y large strains have b e e n achieved. In accordance with o t h e r large strain coldw o r k i n g e x p e r i m e n t s in f.c.c, m e t a l s (5), a recent i n v e s t i g a t i o n in a l u m i n i u m (6) has r e v e a l e d an a d d i t i o n a l r e - s t r e n @ t h e n i n g , in c o n t r a s t to p r e d i c t i o n s of c u r r e n t large strain d e f o r m a t i o n t h e o r i e s (7). Moreover, it even s h o w e d a c o n t i n u o u s decrease of stress (6) i n s t e a d of an a s y m p totic increase to steady state. As b o t h features cannot be a s c r i b e d to e s s e n t i a l changes in d i s location p r o d u c t i o n / a n n i h i l a t i o n rate (8), the r e - s t r e n g t h e n i n g as well as the drop of stress must be c o n s e q u e n c e s of c h a n @ e s in d i s l o c a t i o n arrays. The p r e s e n t i n v e s t i g a t i o n g i v e s i n f o r m a t i o n about these important m i c r o s t r u c t u r a l processes: d i r e c t o b s e r v a t i o n s b y T E M were p e r f o r m e d to study the v a r i a t i o n s in d e f o r m a t i o n structure of a l u m i n i u m in the course of c o l d - r o l l i n g to u l t r a h i g h strains. Experimental Sheets were c o l d - r o l l e d from 99.99% r e f i n e d and a n n e a l e d a l u m i n i u m to various initial thickn e s s e s and r e - a n n e a l e d at 600 K for 20 hours. By s u b s e q u e n t r o l l i n g deformation, true strains 0 < ~ ~ 5.0 were achieved, the final sample thickness b e i n g roughly c o n s t a n t a n d not larger t h a n 0.22 m~, thus well suited for T E M - p r e p a r a t i o n s . Special care was taken to k e e p r e d u c t i o n steps small, in order to a v o i d local h e a t i n g of the m a t e r i a l and to a p p r o x i m a t e c o n d i t i o n s of u n i d i rectional compression. The average rolling step a p p l i e d was AE = 0.I, g i v i n g a m e a n strain rate of ~ = 22 sec -I, its v a r i a t i o n s b e i n g n e g l i g i b l e w i t h i n the error of all subsequent m e a s u r e m e n t s a p p l i e d (for closer details see (6)). T E M - p i c t u r e s were taken with a Philips EM 300 from samples o r i g i n a t i n g from the same sheets as those u s e d for m e c h a n i c a l - (6) and r e s i s t i v i t y m e a s u r e m e n t s (8). The p r e f e r e n t i a l e v o l u t i o n of m i c r o s t r u c t u r e in the rolling plane was observed. Sample p r e p a r a t i o n was done b y a Struers d o u b l e - j e t equipment, using an e l e c t r o l y t e c o n s i s t i n g of ethanol, g l y c e r o l and p e r c h l o r i c a c i d at a r a t i o 7:2:1. E l e c t r o p o l i s h i n g was p e r f o r m e d at a t e m p e r a t u r e of 15°C with a voltage of 14 V and a c u r r e n t d e n s i t y of 2x103 A m -2. Results In a p r e v i o u s p a p e r (8), the e l e c t r i c a l r e s i s t i v i t y p r o v e d to b e a useful tool for m e a s u r i n g d i s l o c a t i o n d e n s i t i e s in the large strain ranges where T E M - m e t h o d s u s u a l l y fail. This way, values of d i s l o c a t i o n d e n s i t y N c o u l d b e g a i n e d for the actual iterative rolling d e f o r m a t i o n up to e = 5.0. With respect to e, three c h a r a c t e r i s t i c ranges of N can be distinguished: range A (e~ 0.3): steep increase of N range B (0.3 < ~ ~ 4.0): linear increase of N (but m a r k e d l y smaller than in A) range C (E > 4.0): s a t u r a t i o n and c o n s t a n c y of N * I n s t i t u t ffir T e c h n i s c h e Physik, T e c h n i s c h e U n i v e r s i t ~ t W i e n and B u n d e s a m t ffir Eich- u n d V e r m e s s u n g s w e s e n , Wien. N o w w i t h the Erich S c h m i d I n s t i t u t ffir F e s t k 6 r p e r p h y s i k der 0 s t e r r e i c h i s c h e n A k a d e m i e der W i s s e n s c h a f t e n , Leoben, Austria.

505 0036-9748/85 $3.00 + .00 Copyright (c) 1985 Pergamon Press Ltd.

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This classification is used for the presentation of the TEM-pictures; values of dislocation density N are correlated with corresponding TEM-micrographs, thus serving as a basis for the microstructural analysis.

I0 " N

b 0

I

2

3

4

5

FIG. 1 Dislocation tangling and cell development at small deformations

(range A)

For e ~ 0.3 dislocations start tangling and then form cells, as arrays of dipoles arrange into cell walls; numerous single dislocations in the interior of the cells can be recognized. With continued deformation, their number decreases gradually and the mean cell size shrinks from about 2.0 ~m to 1 . 5 ~ m (see also Fig. 4). The cell size strongly varies within the sample area being investigated, which is reflected by more diverging values in size/strain dependence (Fig.4) as well as by a raised mean error of ± 0.1 ~m. The mean difference in orientation of cells amounts to 1.5 ° .

Here, cell refining as well as cleaning out continues up to e ~ 1.6. Average misorientation increases to about 2.5 ° . In Fig. 2(c) (representing e = 2.6) a new type of cells can be recognized, which consists Of entirely clean areas surrounded by very thin, line shaped and regular walls (low angle boundaries, see white arrow in Fig. 2(c)) : they shall be called "subgrains" here, although this term has not been always used in the literature in this sense (9). In the following pictures (Fig. 2, (d) to (e)), the volume fraction of such subgrains rises, whilst their diameter keeps constant at 1.3 ± 0.05 ~m , even up to the highest strains achieved of e = 5.0 (Fig. 4). Misorientation of subgrains markedly exceeds that of prior cells, being now typically between 3 ° and 4 ° .

In this range, structural features are similar to that of range B: sharp subgrain walls resulting from single dislocation contrast coexist with more irregular cell walls consisting of numerous both-signed edge dislocations. The volume fraction of subgrains has reached i/3 of the total sample volume and remains constant up to the highest strains. Subgrain-dominated regions

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of about i00 ~m alternate with others exhibiting highly deformed, distinct dislocation density in their interior.

;0

507

irregularly walled cells with

N

x I0 I0 c m ' 2 ~ / ~ ' ~

0

I

I

I

I

I

I

2

3

4

5

FIG. 2 Cell refining and subgrain formation at linear increase of dislocation density N (range B) Discussion

Similar to the stress-strain behaviour of heavily cold-rolled A1 (6), the corresponding microstructure (Fig. I, (a)-(c)) reveals the dislocation structure as known from usual stage II work-hardening, consisting of dislocation tangles which form networks and finally distinct dislocation cells. The decrease of cell size d (Fig. 4) fits to d ~ i/o (o... external stress), hinting at a substructural mechanism being operative for strengthening. This behaviour agrees with findings of Schuh and v. Heimendahl (I0) and Hansen (ii) and appears therefore to be independent of the special deformation mode applied (Fig. 4).

508

MICROSTRUCTURE

N

10

"

IN A1

Vol.

19, No. 4

°r

7 0

I

|

I

I

I

1

2

3

4

5

FIG. 3 C o n s t a n t volume fractions of cells and subgrains a t s a t u r a t i o n of d i s l o c a t i o n d e n s i t y N (range C)

reduction 20 40 3 Ii I , I

r

60 !

90

80

1

I

97

95

I

I

I

I

99

I

I

99,5 %

I

I

o rolling, this work o rolling, Schuh 8 v.Heimondshl •

wire- drawing, Hanson

0

N

-

N

o

00

[]

0

n

0

o--

I

1

I

I

!

1

2

3

~

5

true

equivalent

strain

FIG. 4 V a r i a t i o n of cell- (subgrain) size with d e f o r m a t i o n of h e a v i l y c o l d - w o r k e d aluminium. For comparison, results from literature (Schuh and v. H e i m e n d a h l (i0), H a n s e n (ii)) have b e e n added.

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The investigation by electrical resistivity (8) indicates that for true strains E > 0.3 the increase of dislocation density N is markedly reduced. With the assumption that the mechanism of dislocation production does not alter, annihilation must have been activated. This should lead to a strong decrease of work-hardening coefficient which in fact is observed at a strain of £ = 0.3 (6), being comparable to that of well-known stage II - stage III transition in conventional tensile test. Nevertheless, cell size becomes smaller up to e = 1.6, as has also been observed by other authors (i0, Ii, Fig. 4). An interesting feature of strengthening behaviour observed in (6) is the reincrease in strengthening following stage III work-hardening: as has been repeatedly found in large strain cold-working (5), this "stage IV" preferentially occurs for strain rates ~ ~ I0 -I sec -I. According to Mecking (12), its basic structural event should be the formation of shear- and/or microbands which, however, does not occur in pure A1 (5), probably bacause of its high stacking fault energy. Our microstructural analysis provides an alternative explanation: the drastic reincrease of stress takes place at a strain of £ ~ 1.6, just where cell refinement ends. However, there is no change in the growth rate of dislocation density N (all Figs. and (8)). Thus it follows that additionally created dislocations have to be incorporated into already existing cell walls, requiring an increase in external stress i) because of repulsive forces of equally signed wall dislocations; ii) because of reactions of wall dislocations during the continuous increase of dislocation density i__n_nthe cell wall; this may lead in average to a shortening of dislocation segments AI, which implies an increase of external stress A~, according to A o ~ (Gb)/AI, (G ... shear modulus, b ... Burgers length). For ~ ~ 2.6, microhardness (as relevant magnitude for in-situ rolling stress (13)) exhibits a peak stress followed by a continuous stress decrease with increased deformation (6). Because the disloca~on density N increases also within this deformation range (8), the actual stress decrease must be caused by rearrangement rather than by loss of dislocations. The strain where softening starts coincides with that of the beginning of subgrain formation, ~ = 2.6,Fig. 2(c). Obviously, in some cell walls which have been pumped with dislocations during the preceding work-hardening, a threshold is reached for the dislocation rearrangement to configurations of much lower energy. This structural reorganization is thought to be basically a change from dislocation pile-ups to a regular low-angle boundary and to occur preferentially during the stress releases in the actual stepwise rolling deformation (6, 13).

~_~ For true strains e ~ 3.8, a saturation of strength occurs up to the highest strains achieved (6). Hence it seems that the rate of recovery becomes equal to that of work-hardening, resulting in a constant strength. Almost parallel to strength, resistivity stays constant beyond e = 4.0 (8). Therefore, the balance of work-hardening and recovery can be described in terms of a dynamic equilibrium between production and annihilation of dislocations, typical of a steady state deformation. The present TEM-investigation (Fig. 3, (a)-(c)) illustrates well the constancy of strength level: i) The average size of subgrains as well as that of cells remains equal to that of the previous range B of deformation, at 1.6 ~ £ ~ 4.0. It does not change up to the highest strains achieved (Fig. 4). ii) The volume fraction of subgrains with respect to cells reaches a value of i/3 at approximately e ~ 4.0, and then remains constant. The dislocation rearrangement into subgrains still operates here as in range B, in addition to dynamic production and annihilation of dislocations; therefore the situation is not directly comparable to conventional steady state. The finite volume fraction of subgrains indicates that the threshold for dislocation rearrangement has been reached only for %1/3 of the total sample volume. The question of whether a cell or a subgrain forms may be considered a question of the local internal stress governing the mutual change of cells and subgrains into each other. Conclusions i) The re-strengthening subsequent to stage III deformation ("stage IV") is ascribed to the incorporation of newly generated dislocations into already existing cell walls. This process is suggested to cause stage IV in pure metals of high stacking fault energy. ii) The stress decrease can be related to the formation of subgrains, which preferentially takes place during the stress releases of stepwise rolling deformation. iii) The constancy in strength at ultrahigh strains is correlated with an equilibrium between

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dynamic production and annihilation of dislocations and their static rearrangement. Because of the finite fraction of subgrains, deformation differs from that in the conventional steady state. Acknowledgements The continued interest of Prof. Dr. K. Lintner, Institut far Festk~rperphysik der Universit~t Wien, is gratefully acknowledged. The authors thank Dr. A. Korner and Dr. W. H~llerbauer for providing much help in performing the TEM-micrographs. References i. see, f. e. : J. A. Dorn, P. Pietrokowsky and T. E. Tietz, Trans. AIME 188, 933 (1950); K. L0cke and B. Lange, Z. Metallk. 43, 55 (1952); W. Staubwasser, Acta Met. 7, 43 (1955) 2. U. F. Kocks, H. S. Chen, D. A. Rigney and R. J. Schaefer, Work Hardening in Stage III, in: Work Hardening, ed. J. Hirth and J. Weertman, p. 151, Gordon & Breach, New York (1968) 3. H. Mecking, in: Work Hardening in Tension and Fatigue, ed. A. W. Thompson, p. 67, AIME New York (1977) 4. B. Ilschner, Hochtemperaturplastizit~t, Springer, Berlin (1973) 5. J. Gil Sevillano, P. van Houtte and E. Aernoudt, Progr. Mat. Sci. 25, 2-4 (1980) 6. M. Zehetbauer, W. Pfeiler and J. Schrank, Scripta Met. 17, 221 (1983) 7. U. F. Kocks, Trans. ASME (ser. H) J. Eng. Mater. Tech. 97, 76 (1976) 8. J. Schrank, M. Zehetbauer, W. Pfeiler and L. Trieb, Scripta Met. 14, 1125 (1980) 9. see, f. e. : J. Weertman, High Temperature Creep Produced by Dislocation Motion, in: Rate Processes in Plastic Deformation of Materials, Proc. J. E. Dorn Symposium, ed. J. C. M. Li and A. K. Mukherjee, p. 315, ASM, Metals Park, Ohio (1975) Io. F. Schuh and H. v. Heimendahl, Z. Metallk. 65, 346 (1974) ii. N. Hansen, Trans. AIME 245, 2061 (1969) 12. H. Mecking, Strain Hardening and Dynamic Recovery, in: Dislocation Modelling of Physical Systems, Proc. Int. Conf. Gainesville, Florida, U. S. A., ed. M. F. Ashby et al., p. 197 (1980) 13. M. Zehetbauer and D. Trattner, to be published