Observations of cracking behavior and non-unique cracking thresholds during LME of aluminum

Scripta METALLURGICA

Vol. 20, pp. 1433-1438, 1986 Printed in the U.S.A.

Pergamon Journals Ltd. All rights reserved

OBSERVATIONS OF CRACKING BEHAVIOR AND NON-UNIQUE CRACKING THRESHOLDS DURING LME OF ALUMINUM D. A. Wheeler and R. G. Hoagland Dept. o f M e t a l l u r g i c a l Engineering The Ohio S t a t e U n i v e r s i t y Columbus, Ohio 43210

(Received ,July 14, 1986) (Revised August II, 1986) Introduction Although t h e r e i s good agreement between r e s e a r c h e r s t h a t some adsorbing l i q u i d metals tend to decrease the s t r e n g t h of atomic i n t e r a c t i o n s in o t h e r s o l i d m e t a l s , t h e r e remains some ambig u i t y over how t h i s phenomena r e l a t e s to crack growth. While many t h e o r i e s have been o f f e r e d (1i 0 ) , no s i n g l e mechanism has emerged to t o t a l l y d e f i n e the c r i t i c a l c o n d i t i o n s and i n t e r a c t i o n s of LME. In f a c t , i t has been suggested (11) t h a t , j u s t l i k e s t r e s s - c o r r o s i o n c r a c k i n g and hydrogen e m b r i t t l e m e n t , several mechanisms are p o s s i b l e , depending on the c o n d i t i o n s of the system in q u e s t i o n . Several a r t i c l e s r e v i e w i n g many proposed mechanisms, t h e i r advantages and t h e i r d i s a d vantages can be found (11-13). The p r e s e n t work focusses on the c h a r a c t e r i z a t i o n o f crack i n i t i a t i o n and p r o p a g a t i o n events of aluminum in a l i q u i d mercury environment. In p a r t i c u l a r , d e t e r m i n a t i o n o f crack v e l o c i t i e s as a f u n c t i o n of a p p l i e d s t r e s s i n t e n s i t y was performed in an a t t e m p t t o l o c a t e a t h r e s hold s t r e s s i n t e n s i t y f o r crack p r o p a g a t i o n and the K-dependence o f the crack p r o p a g a t i o n v e l o c i t i e s . Previous i n v e s t i g a t o r s have performed s i m i l a r experiments on the AI-Hg system ( 1 4 , 1 5 ) , and have shown unique crack v e l o c i t y vs. s t r e s s i n t e n s i t y b e h a v i o r . The p r e s e n t r e s u l t s , howe v e r , show a d e f i n i t e v a r i a t i o n i n s t r e s s i n t e n s i t y t h r e s h o l d s f o r LME crack p r o p a q a t i o n . Experimental Procedure Double c a n t i l e v e r beam (DCB) specimens were prepared o u t of 7075-T651 aluminum a l l o y p l a t e . These specimens were of the dimensions shown i n Figure i , and were machined w i t h the l e n g t h o r i e n t e d t r a n s v e r s e t o the r o l l i n g d i r e c t i o n . Wetting the crack t i p w i t h l i q u i d mercury proved to be d i f f i c u l t because mercury w i l l n o t wet the o x i d e f i l m . In o r d e r to p e n e t r a t e the o x i d e , a small drop of HF acid ( d i l u t e d in w a t e r 1:1) was a p p l i e d to the crack t i p . Through t h i s a c i d , a p p r o x i m a t e l y I ml o f l i q u i d mercury was a p p l i e d . Once the mercury c o l l a p s e d onto the crack t i n , w e t t i n g occurred and the specimen was i m m e d i a t e l y t e s t e d . Using c o n s t a n t d i s p l a c e m e n t c o n t r o l of 1.27 mm/min, the specimens were loaded u n t i l a s i g n i f i c a n t d e f l e c t i o n in the l o a d - d i s p l a c e m e n t curve was d e t e c t e d , s i g n a l l i n g crack i n i t i a t i o n . The crosshead was then stopped, a l l o w i n q the crack t o propagate in e s s e n t i a l l y c o n s t a n t d i s p l a c e m e n t c o n d i t i o n s . As the crack extends, the s t r e s s i n t e n s i t y decreases u n t i l a t h r e s h o l d value i s reached, wherein the crack a r r e s t s . The specimens were then loaded again and a l l o w e d to propagate in c o n s t a n t d i s p l a c e m e n t c o n d i t i o n s . This was repeated u n t i l the specimens were c o m p l e t e l y separated. D u r i n g these e x p e r i m e n t s , l o a d , d i s p l a c e m e n t and time was recorded and used to c a l c u l a t e the crack l e n g t h from the compliance equation given by (16): d/P - aE- ~b J h43 [1 + 1 . 9 2 ( h / a ) + 1 . 2 2 ( h / a ) 2 + O.39(h/a) 3]

f o r c > 2h

where c i s the uncracked l i g a m e n t ( F i g . 1) o f the DCB specimen. The measured compliances were f i r s t c o r r e c t e d f o r load t r a i n compliance. Stress i n t e n s i t i e s were then determined from (16): K =

3 E h b FI + O . 6 4 ( h / a ] 2 a 2 [ I + 1 . 9 2 ( h / a ) + 1 . 2 2 ( h / a ) 2 + O.39(h/a) 31

1433 0036-9748/86 $3.00 + .00 Copyright (c) 1986 Pergamon Journals Ltd.

f o r c > 2h

1434

CRACKING

OF A1

Vol.

20, No. i0

When c < 2h, more general equations must be used in order to account f o r free end e f f e c t s in f i n i t e length DCB specimens (16). Crack v e l o c i t i e s f o r a given p o i n t " i " were approximated:

da/dt ( i ) = [ a ( i + l ) - a ( i - 1 ) ] / [ t ( i + l ) - t ( i - 1 ) ] After fracture, specimens were examined in an SEM to characterize t h e i r surface appearance. Results Previous investigations concerning crack propagation of metals in liquid environments appear to show a unique threshold stress intensity for crack propagation. An example of this is seen in Figure 2, taken from Speidel (14). Results derived from the present work revealed a drastic variation in the cracking threshold. Figure 3 shows this data obtained by the crack propagation experiments described e a r l i e r . The cracking threshold was observed to decrease for each propagation-arrest event in a single specimen. This reduction in threshold was seen for each specimen tested. The maximum velocity seen for crack propagation was 2-3 cm/sec for the transverse specimens. These values occurred f a i r l y consistently throughout the specimens tested. Only one such set of data points is shown in Figure 3 for c l a r i t y . These experimental observations suggest that as the crack velocity approaches some c r i t i c a l value, 2-3 cm/sec in this case, the crack velocity becomes independent of stress intensity, and probably controlled by l i q u i d transport to the advancing crack t i p . Specimens fractured in a i r and in the mercury environment were observed in the scanning electron microscope (SEM). Figure 4 shows a range of magnifications of the i n - a i r fracture surface, revealing a ductile-dimple fracture appearance. Special emphasis is placed on the high magnification (lO,O00x) image, as i t reveals ductile-dimple regions on a very fine scale. Figure 5 shows similar magnifications used to examine the mercury-embrittled samples. These reveal a f l a t , stepped fracture appearance. The high magnification (lO,O00x) image shows a f l a t "cleavage-like" appearance, with l i t t l e evidence of any fine scale ductile-dlmple regions. In addition, Figure 6 shows the mercury-embrittled samples using backscattered electron imaging to highlight the atomic number contrast between mercury and aluminum. The bright particles, v e r i fied to be mercury by x-ray microanalysis, are seen to l i e on top of the fracture surface, as evidenced by the Shadows in Figure 6(b). However, no other inhomogeneous distributions of mercury were seen on the fracture surface. Discussion Variable cracking thresholds were seen for sequential crack propagation and arrest in a single DCB specimen (Fig. 3). This non-uniqueness tends to suggest a rather i n t r i c a t e crack-tip embrittlement process. The fact that each subsequent cracking event in a single specimen produced a lower cracking threshold tends to imply a kinetic or time dependent crack-tip process. Competition between mercury and oxygen is suggested as a possible explanation. Indeed, the formation of an oxide f i l m on the freshly exposed fracture surface could deplete the r e l a t i v e surface area to which the mercury has access. I f the oxide becomes complete, crack growth should cease. Thus, the apparent threshold would be controlled by the rate of oxide film formation r e l ative to the rate of free surface production, and there may e x i s t a range of crack velocities within which oxidation rates are s u f f i c i e n t to quench the crack growth process. This explanation is tentative, and needs to be investigated further. Fractographic evidence presented by Lynch (5-?,17) shows regions of very fine scale ductile-dlmple fracture on LME fracture surfaces, and is represented as evidence of enhanced shear triggering the nucleation and growth of voids ahead of the advancing crack t i p . However, present observations (Fig. 5) show l i t t l e evidence of extensive plastic flow on the surface of LME fractures compared with those fractured without mercury (Fig. 4). Closer examinations of these emb r l t t l e d fracture surfaces indicated that the f l a t fracture regions tended to have well defined chevron markings, examples of which are in Figures 5(b) and 6(d). These patterns may be crack jump sites, and along with the lack of evidence of ductile-dimple appearance, tend to support a cleavage-type fracture process. Conclusions Present work on 7075-T651 aluminum in l i q u i d mercury has shown the crack propagation threshold to be non-unlque. This indicates that other crack-tip interaction processes are occurring and possibly controlling. Competition between mercury embrittlement and surface oxidation at the crack t i p has been suggested as a possible explanation for this phenomena. Maximumcrack propagation v e l o c i t i e s were on the order of 2-3 cm/sec.

Vol.

20, No.

i0

CRACKING

OF A1

1435

Fractographic observations of the embrittled specimens revealed l i t t l e evidence of d u c t i l e dimple fracture appearance, even on a fine scale. Chevron-like markings along regions of f l a t fracture tend to imply a cleavage-type fracture process. Acknowledgment We wish to acknowledge the helpful discussions with Prof. J. P. Hirth, and the support of this work under NSF Contract DMR 8418159. References 1. 2. 3. 4.

N. S. Stoloff, T. L. Johnson, Acta. Met., vol. 11, (1963), pg. 251. A.R.C. Westwood, M. H. Kamdar, Phil. Mag., vol. 8, (1963), pg. 787. A.R.C. Westwood, C. M. Preece and M. H. Kamdar, ASM Trans., vol. 60, (1967), pg. 723. S. P. Lynch, Embrittlement by Liquid and Solid Metals, pg. 105, AIME-TMS Conf. Proc., M. H. Kamdar, ed., 1983. 5. S. P. Lynch, Environmental Degradation of Engineering Materials in Aggressive Environments, pg. 229, NASA-NACEConf. Proc., M. R. Louthan, R. P. McNitt, R. D. Sisson, eds., 1981. 6. S. P. Lynch, Acta. Met., vol. 32, (1984), pg. 79. 7. S. P. Lynch, J. Mat. Science, vol. 20, (1985), pg. 3329. 8. W. M. Robertson, Trans. AIME-TMS, vol. 236, (1966), pg. 1478. 9. M. A. Krishtal, Sov. Phys. Dokl., vol. 15, (1970), pg. 614. 10. P. Gordon, J. H. An, Met. Trans. A, vol. 13, (1982), pg. 457. 11. N. S. Stoloff, Atomistics of Fracture, pg. 921, NATOConf. Proc., R. M. Latanision, J. R. Pickens, eds., 1982. 12. M. H. Kamdar, Prog. Mat. Sci., vol. 15, (1973), Dg. 289. 13. N. S. Stoloff, Embrittlement by Liquid and Solid Metals, pg. 3, AIME-TMS Conf. Proc., M. H. Kamdar, ed., 1983. 14. M. O. Speidel, The Theory of Stress Corrosion Cracking in Alloys, pg. 289, NATOConf. Proc., J\ D. Scully, ed., 1971. 15. J. A. Kapp, Embrittlement by Liquid and Solid Metals, pg. 117, AIME-TMS Conf. Proc., M. H. Kamdar, ed., 1983. 16. M. F. Kanninen, Int. J. of Fract., vol. 9, (1973), pg. 83. 17. S. P. Lynch, Scripta Met., vol. 13, (1979), pg. 1051.

Double Cantilever Bemm Specimen 7075-T851

I I

~b--

L

°

ili !

1.

[--

a

L = 165. i mm

l

] h ~ I £ . 05 mm

--]

c=L-o u = 38. 1 mm

.

b ~ 6. 35 mm bn

Fig. 1 Double cantilever beam specimen.

-

1.27 mm

F-

bn

CRACKING OF A1

1436

0.2

"'

o-o

O.I

Do~

._~

&Ill

: n ,,

,,

.

- n

a

o /

O.05

.ti

Vol.

o ~

IZ(~

I

0.O(~

I

i

4

2

I

6

e

I

I

1O

I

12

14

IB

K Q4Pa-t]") Fig. 2 Results from Speidel (14).

~05

k

\

O.O1

.~

I1.001

O.NII~

-

O. 00015

,

?

8

,

9

!

lO

.L

11

I

12

l

13

I

14

K 04Pa-'t~) Fig. 3 Present work on 7075-T651 in l i q u i d mercury.

,

15

20, No. 10

Vol.

20, No. I0

CRACKING

OF A1

1437

a)

r

c)

d)

, Fig. 4 7075-T651

fractured

Fig.

in a i r

5

7075-T651 f r a c t u r e d

in mercury

1438

CRACKING

OF A1

c)

Vol.

d)

Fig. 6 7075-T651 fractured in mercury a) secondary electron image b) backscattered electron image of a) c) secondary electron image d) backscattered electron image of c)

20, No.

I0