Cyclic hardening of aluminium at various test frequencies

Scripta

METALLURGICA

CYCLIC

Vol. 12, pp. 1047-1050, 1978 Printed in the United States

HARDENING

OF A L U M I N I U M

AT VARIOUS

Pergamon

Press,

Inc.

TEST F R E Q U E N C I E S

W. H o f f e l n e r +) r B. Weiss fur P h y s i k a l i s c h e Chemie III, U n i v e r s i t y of Vienna, W ~ h r i n g e r s t r a s s e 42, A-1090 Vienna, A u s t r i a and II. P h y s i k a l i s c h e s Institut, U n i v e r s i t y of Vienna, S t r u d l h o f g a s s e 4, A-1090 Vienna, A u s t r i a

Institut

(Received

September

1, 1978]

Introduction In a previous paper (i) the influence of the frequency on the fatigue h a r d e n i n g b e h a v i o u r of A1 p o l y e r y s t a l s at room t e m p e r a t u r e (RT) w i t h i n a range of 2 Hz 2.104 Hz was discussed. With i n c r e a s i n g frequency increasing fatigue h a r d e n i n g was found. B e t w e e n 200 Hz and 2.104 Hz this d e p e n d e n c e of the fatigue h a r d e n i n g seems to run into a s a t u r a t i o n stage. In FIG. 1 the cyclic h a r d e n i n g is shown as a function of the f r e q u e n c y for two strain amplitudes. The number of cycles N was 1.105. In the present work these curves will be d i s c u s s e d on the basis of T E M - i n v e s t i g a tions (TEM = t r a n s m i s s i o n e l e c t r o n microscope) Experimental As spec i m e n m a t e r i a l h e a t - t r e a t e d 99,8 p.c. A l - p o l y c r y s t a l s were used. Heat treatment was p e r f o r m e d in v a c u u m at 450°C for one hour. A d e t a i l e d d e s c r i p t i o n of the various m a c h i n e s used for d e f o r m a t i o n is given in (i). During a period of 105 cycles the specimens were d e f o r m e d at two d i f f e r e n t amplitudes of total strain i.e. £ t 2'8"10-4 and 7.10-4 respectively. S i g n i f i c a n t changes in the defect arr a n g e m e n t could only be d e t e c t e d b e t w e e n 200 Hz and 2.104 as shown e l s e w h e r e (2). Therefore, in the present study only this frequency range was investigated. Results

and D i s c u s s i o n

The d i f f e r e n c e s in the defect a r r a n g e m e n t a c c o r d i n g to frequency and strain amplitude can be seen in FIG. 2. At the lower strain amplitude d i s l o c a t i o n bundles were predominant. At the higher strain amplitude well developed d i s l o c a t i o n cells could be found. This d e p e n d e n c e of the d i s l o c a t i o n a r r a n g e m e n t on the strain amplitude is in accordance with other i n v e s t i g a t i o n s of fcc metals (3, 4, 5). It can be seen from FIG. 2 that a l t h o u g h the d i s l o c a t i o n a r r a n g e m e n t and the d i s l o c a t i o n density are not a f f e c t e d by the frequency there is a strong influence of the freq u e n c y on the loop density. This loop density increases r e m a r k a b l y with i n c r e a s i n g frequency. These loops are p r o b a b l y p r i s m a t i c d i s l o c a t i o n loops with a Burgers vector a/2 ~ii0} w h i c h would be in a g r e e m e n t with (6). The densities of dislocations and loops are given with an a c c u r a n c y of approx. 15 p.c. in tables 1 a and 1 b respectively. +)

Now with

BBC Brown

Boveri

Research

Center

1047

CH-5513

D~ttwil/Switzerland

1048

CYCLIC

HARDENING

OF A L U M I N I U M

TABLE Densities

of Defects in A1 after Cyclic a) ~ t = 2'8"10-4 200 Hz

2.104

Deformation

at 200 Hz and 2.104 b) ~ t = 7"10-4 200 Hz

~,l.1010/cm 2

Loop density (in bunHies)

5,5.1014/cm 3

12, No.

II

1

Hz

Dislocation density (in 9 . 1 0 9 / c m 2 bundles)

1,8.1014/cm 3

Vol.

Loop density (in cells)

1,2.1014/cm3

Hz at RT

2.104

Hz

6,7.1014/cm 3

On the basis of these o b s e r v a t i o n s the following model for the e f f e c t of frequency on the cyclic h a r d e n i n g of A1 at RT is proposed. As already known (7) during fatigue d e f o r m a t i o n the main source of point defects is the n o n - c o n s e r v a t i v e motion of jogs in screw dislocations. The p r o b a b i l i t y of the f o r m a t i o n of these jogs is closely related to cross slip. From the high stacking fault e n e r g y of 0.135 J/m 2 of A1 (8) a high density of d i s l o c a t i o n jogs can be assumed, and consequently, d u r i n g fatigue of A1 a high density of point defects can be expected. The p o i n t defects p r o d u c e d during each cycle of d e f o r m a t i o n can migrate to sinks, recombine or form clusters. A c c o r d i n g to (9) will be assumed that the very mobile i n t e r s t i t i a l s p r e d o m i n a n t l y migrate to sinks and therefore m a i n l y the slower v a c a n c i e s (mean jumping f r e q u e n c y of a m o n o v a c a n c y at RT = i.i0 sec -I (i0)) are respons i b l e for the cluster formation. This formation of clusters is clearly a function of the d e f o r m a t i o n frequency. With i n c r e a s i n g frequency the vacancies can not migrate as fast as new v a c a n c i e s are created and therefore a high density of v a c a n c i e s can be built up w h i c h favours the formation of clusters. The mechanism r e s p o n s i b l e for the c l u s t e r i n g seems to be the same as p r o p o s e d by Makin (ll) which allows v a c a n c i e s to condense to p r i s m a t i c loops with Burgers vector a/2


To give an i n t e r p r e t a t i o n of the m e a s u r e d increase of flow stress ~F with i n c r e a s i n g f r e q u e n c y 6"F will be d e v i d e d a c c o r d i n g to (12) mal part ~ * and an athermal part ~ . *

after fatigue into a a ther-

The athermal part d e s c r i b e s the c o n t r i b u t i o n of the stress fields of d i s l o c a t i o n s to the flow stress and the thermal part m a i n l y the c o n t r i b u t i o n of the clusters to the flow stress. As the d i s l o c a t i o n a r r a n g e m e n t and the d i s l o c a t i o n d e n s i t y remain nearly u n a f f e c t e d by the frequency, the thermal part has to be r e s p o n s i b l e for the influence of the frequency on the cyclic h a r d e n i n g b e h a v i o u r of AI. This means that the c o n t r i b u t i o n of clusters to the flow stress has to be considered. As known from i n v e s t i g a t i o n s on neutron i r r a d i a t e d Cu (13) the c o n t r i b u t i o n of clusters to the flow stress is not only d e p e n d e n t on their density but also on the mean cluster size. At d e n s i t i e s of io I~ - 1015 cm -3 a saturation could be found in the c o n t r i b u t i o n of clusters with a d i a m e t e r ~Z.Snm to the flow stress. The shape of the curves shown in FIG. 1 lead to the c o n c l u s i o n to apply these results to AI. From this point of view the influence of the frequency on the flow stress of A1 after cyclic d e f o r m a t i o n might be e x p l a i n e d q u a l i t a t i v e l y as shown in FIG. 3 Summary I n v e s t i g a t i o n s of the cyclic h a r d e n i n g (flow stress after deformation) c o r r e s p o n ding to a cyclic sinusoidal tension - c o m p r e s s i o n d e f o r m a t i o n at frequencies of 2, 200 and 2.104 Hz showed an increase of the h a r d e n i n g with i n c r e a s i n g frequency. T E M - i n v e s t i g a t i o n s led to the c o n c l u s i o n that this f r e q u e n c y effect could be exp l a i n e d by the increase of P D - c l u s t e r s with i n c r e a s i n g frequency.

Vol.

12,

No.

11

CYCLIC HARDENING OF ALUMINIUM

1049

Acknowled@ement These i n v e s t i g a t i o n s were conducted under the s p o n s o r s h i p of the Fonds zur F ~ r d e r u n g der w i s s e n s c h a f t l i c h e n Forschung, Vienna, Austria. The authors are greatly indebted to Prof. R. Stickler for helpful discussions. References 1 2 3 4 5 6 7 8 9

iO. ii. 12. 13.

W. Hoffelner, K. Kromp, B. Weiss, B. Wielke, Scripta Met. 9, 1157 (1975) W. Hoffelner, Thesis U n i v e r s i t y of V i e n n a (1977) C.E. Feltner, C. Laird, Trans AIME, 242, 1253 (1968) P. Lukas, M. Klesnil, Met. Sci. Eng. ii, 345 (1973) P. Lensing, P. Mayr, E. Macherauch, Z . M e t a l l k u n d e 69, H 6, 394 (1978) J. Awatani, K. Katagiri, Bull. ISMR 12, 53, 940 (1969) D. Hull, I n t r o d u c t i o n to Dislocations, 135, Pergamon Press (1965) P.C.J. Gallagher, Met. Trans. i, 2429 (1970) M. Badon, P. Guyot, 4th I n t e r n a t i o n a l C o n f e r e n c e on the Strength of Metals and Alloys, Conf. Proc. Vol. 2, p. 804, L a b o r a t o i r e de Physique du S o l i d e - N a n c y C~dex, France (1976) G. Schoeck, A l u m i n i u m and A l u m i n i u m l e g i e r u n g e n p. 210, Ed.D.G. Altenpohl, Springer, Berlin (1965) M.J. Makin, B. Hudson, Phil. Mag. 8, 447 (1963) J.C. Grosskreutz, W. Frank, Phys. Stat. Sol., ii, 567 (1972) M.J. Makin, A.D. Whapam, F.J. Minter, Phil. Mag. 6, 465 (1961) Figures

....~....---/

,,, i

. O f

..i . f

,/ .zo

°'4o

4o"

The i n f l u e n c e t i o n at RT.

~

'oz

FIG. 1 o f f r e q u e n c y on t h e ~ O , l - y i e l d

4oI

404

4~ s

strength

o f A1 a f t e r

cyclic

deforma-

1050

CYCLIC H A R D E N I N G OF A L U M I N I U M

a) 200 Hz

~t = 2'8"10-4

C) 200 Hz

~ t = 7 . lO -4

FIG.

Vol.

b)

12, No. Ii

2.104 Hz

d) 2.104 Hz

2

The influence of f r e q u e n c y on the defect a r r a n g e m e n t of A1 after cyclic deformation at RT (N = i.iO ~)

FIG.

3

The influence of f r e q u e n c y on the flow stress of A1 after cyclic d e f o r m a t i o n at RT (schematic)