Fracture toughness of AlLiX alloys at ambient and cryogenic temperatures

Scripta METALLURGICA

Vol. 22, pp. 1553-1556, 1988 Printed in the U.S.A.

Pergamon Press plc All rights reserved

FRACTURE TOUGHNESS OF AI-Li-X ALLOYS AT AMBIENT AND CRYOGENIC TEMPERATURES K.V. Jata* and E.A. Starke, Jr.** ~ i v e r s l t y of Dayton Research Instltute, Dayton, Ohio 45469 ---Department of Materials Science, University of Virginia, Charlottesville, Virginia 22901 °

°

°

{Received June 20, 1988) {Revised July 5, 1988~ Introduction Recent research on the mechanical behavior of some AI-Li-X alloys indicates that the c r y o g e n i c fracture toughness in the L-T o r i e n t a t i o n could be substantially higher than at room temperature (1-5). Similar to the 2219 A1 alloy (6) the yield and ultimate tensile strengths, fracture strain and strain hardening exponents at cryogenic temperatures have also been observed to be higher than at room temperature. Since there is a great interest in employing the AI-Li-X alloys in the cryogenic temperature range in some advanced aerospace vehicles, recent research has been directed towards obtaining an understanding of the mechanisms that govern the improvement of fracture toughness. Several different mechanisms have already been proposed to explain the higher fracture toughness at cryogenic temperatures. Briefly, the higher fracture toughness has been attributed to: (i) low melting point grain boundary and remain liquid at room temperature

phases which (1,4);

solidify

at low temperatures

(2) a larger number of crack delaminations perpendicular to the short transverse direction or perpendicular to the fracture surface in an L-T oriented specimen (2,5) and in plane crack deflections (5) at cryogenic temperatures; and (3) higher strain hardening

capacity at low temperatures

(3).

Since AI-Li-X alloys are s t r e n g t h e n e d l a r g e l y by c o h e r e n t ordered precipitates such as AI3Li and AI2CuLi , planar slip is a dominant mechanism that not only governs deformation mode but also the resultant fracture (7) through transgranular shear fracture (slip band decohesion) and crack deflections (8). In a previous publication, the authors suggested that higher fracture toughness in underaged alloys is due to slip dispersal, and lower fracture toughness in near-peakage alloys is due to extensive slip localization. Depending on the aging treatment and alloy chemistry the fracture at peakage could be either transgranular or intergranular. In either case, slip bands formed by precipitate shearing largely contribute to the fracture process (7,9). Since higher fracture toughness through slip dispersal or less slip localization is expected in these alloys, it is the objective of the present study to examine the effects of slip localization and dispersal on the fracture toughness of some AI-Li-X alloys at ambient and liquid nitrogen temperatures. Experimental

Procedures

An experimental 2090 type aluminum plate, rolled to 12.7 mm thickness with actual chemical composition of 2.53Li-l.77Cu-0.5Mg-0.13Zr (Fe and Si 0.3%) was obtained from Reynolds Aluminum Company. A second plate, 8090 type, (2.28Li-

1S53 0036-9748/88 $3.00

+ .00

FRACTURE TOUGHNESS OF A I - L i - X ALLOYS

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0.86Cu-.9Mg-.13Zr-0.13Fe-.06Si) rolled to 25 mm t h i c k n e s s was o b t a i n e d from Alcan I n t e r n a t i o n a l Ltd., U.K. The 2090 p l a t e was s o l u t i o n heat t r e a t e d at 823 K for one hour, s t r e t c h e d to 2% and then aged at 463 K for 4 and 8 hours. One set was aged for 8.5 hours w i t h o u t stretch. The A l c a n p l a t e was r e c e i v e d in the T-351 c o n d i t i o n and was aged at 4 6 3 4 K f~r 4 and 8 h o u r s T e n s i o n tests were p e r f o r m e d at a s t r a i n rate of i*i0s- , and 10.5 m m t h i c k c o m p a c t tension specimens w e r e t e s t e d w i t h a c r a c k o p e n i n g d i s p l a c e m e n t g a u g e b o t h ~t ambient and liquid n i t r o g e n temperatures. A l t h o u g h the t h i c k n e s s c r i t e r i o n was met in some specimens, some stable c r a c k g r o w t h was also o b s e r v e d in t h o s e specimens, and thus the f r a c t u r e t o u g h n e s s is d e s i g n a t e d as KQ. T h i n foils from fractured tensile samples w e r e e x a m i n e d u n d e r and <112> zone axis and a (iii) 2-beam c o n d i t i o n to o b t a i n slip b a n d w i d t h and slip b a n d s p a c i n g from at least twenty sets of micrographs. F r a c t u r e surfaces w e r e a n a l y z e d w i t h a s c a n n i n g electron microscope. R e s u l t s and D i s c u s s i o n M e c h a n i c a l p r o p e r t y data of 2090 and 8090 alloys at 300 and 77 K (Table I) clearly show an i n c r e a s e in the fracture t o u g h n e s s and the a c c o m p a n i e d increase in the strength, s t r a i n h a r d e n i n g e x p o n e n t n and f r a c t u r e strain.

TABLE 1 M e c h a n i c a l P r o p e r t y Data at A m b i e n t and L i q u i d N i t r o g e n T e m p e r a t u r e s Alloy YS YS + (MPa) A g i n g 300K 77K 2090 4 h 8 h T-6 8 h 8090 4 h 8 h

UTS UTS (MPa) 300K 77K

K~ %el. (MP a ~m I/2 ) 300K 77K 300K

n

77K

300K

77K

503 507

525 564

548 542

619 630

23 20

37 33

14 12

17 15

.06 .04

.i0 .07

407

478

493

627

24

38

13

23

.09

.15

387 406

392 411

456 464

542 566

35 32

56 52

9.4 9.4

16 16

.08 .07

.09 .i0

As shown in F i g u r e i, the f r a c t u r e t o u g h n e s s samples at liquid n i t r o g e n t e m p e r a t u r e e x h i b i t d e e p e r and larger numbers of d e l a m i n a t i o n s normal to the crack p l a n e at all aging c o n d i t i o n s and in b o t h alloys. F r a c t o g r a p h y of the samples reveals the f r a c t u r e m e c h a n i s m to be slip b a n d d e c o h e s i o n at b o t h ambient and liquid n i t r o g e n temperatures. However, as shown in the m i c r o g r a p h s Figure 2 (a) and (b), the t r a n s g r a n u l a r shear or slip b a n d facets are o b s e r v e d to be c o n s i s t e n t l y w i d e r at 77 K. T r a n s m i s s i o n e l e c t r o n m i c r o s c o p y of t h i n foils o b t a i n e d from f r a c t u r e d t e n s i l e samples showed w i d e r slip b a n d s and s m a l l e r slip b a n d s p a c i n g at 77 K, T a b l e 2. These results are consistent with the o b s e r v a t i o n s m a d e b e f o r e on A I - L i - X a l l o y s , w h e r e h i g h e r s t r a i n h a r d e n i n g exponents and w i d e r slip bands (SBW) w i t h s m a l l e r s p a c i n g (SBS) r e s u l t e d in h i g h e r f r a c t u r e t o u g h n e s s (7). The p r e s e n t results s u g g e s t that the f r a c t u r e t o u g h n e s s i m p r o v e m e n t at liquid n i t r o g e n t e m p e r a t u r e c o u l d be e x p l a i n e d by the d e f o r m a t i o n p r o c e s s e s that occur in the A I ~ L i - X alloys. F r o m T a b l e s 1 and 2, it is c l e a r t h a t the increase in strain h a r d e n i n g c a p a c i t y and total e l o n g a t i o n at 77 K is r e l a t e d to w i d e r slip b a n d s and s m a l l e r slip b a n d spacings. At 300 K w h e r e slip l o c a l i z a t i o n is more prevalent, i.e., n a r r o w e r slip bands and larger spacings, a d e c r e a s e in the strain h a r d e n i n g e x p o n e n t and f r a c t u r e strain occurs. In n o t c h e d specimens, such as a c o m p a c t t e n s i o n specimen, the c r a c k i n i t i a t i o n p r o c e s s is c o n t r o l l e d by the

Vol.

22, No.

9

,

,~,

,

<

FRACTURE TOUGHNESS OF AI-Li-X ALLOYS

..

..~,'i,,:,,

:

t ',~,.--~.::':

IL~,"

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i~,~.r~,[~'

~t:.

..".,-,--.~ :,t ~ ; . "

~':.~,,~

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:

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- ~ ',..-..~.~, - " I , "

' ,,~ "- ~.

FIG. i.

~:

i

'~-

~

'-!1.

:),-,~'. * t ~

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:'

'~-.>'""':~ ~" ;'.,%:..,~

"'

..:.~

':tl,l',~¢b~'~[.

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Fracture surfaces of compact tensions tested at (a) 300 K and (b) 77 K.

r;

FIG. 2.

1555

.

:

Fractography of compact tension specimens tested at (a) 300 K and (b) 77 K.

deformation process ahead of the crack tip. For a critical strain controlled fracture process, the ability to concentrate strain in the slip bands is a critical factor. When slip is dispersed, such as at 77 K, larger numbers of wider slip bands accommodate strain whereas at 300 K when slip is localized, fewer and narrower slip bands (Table 2) accommodate strain. Thus at 77 K, the critical strain in one of the potential slip bands where cracks initiate is reached at a later stage when compared to that at 300 K. Following a previous model for fracture toughness of AI-Li alloys (7) in terms of slip band width (SBW) and slip band spacing (SBS), fracture toughness at 300 and 77 K is calculated for the 2090 alloy using

KIc

= [8 S i n s E O y s D .~SBW) ~.

c ] 112

(e)F

where E is the Youngs modulus, O vs is the yield stress, D is the plastic ~one width, SBW is the slip band widt~ and SBS is the slip band spacing and (e)F is the critical fracture strain.

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FRACTURE TOUGHNESS OF AI-Li-X ALLOYS

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Although delaminations exist in samples tested at 77 K the material between the delaminations still acts as a notched fracture specimen except that it is now in the plane stress condition due to reduced thickness. The deformation p r o c e s s e s t h a t c o n t r o l t h e f r a c t u r e t o u g h n e s s in the A I - L i - X alloys, as described in detail in cf. (7) are still applicable. The previous model (7) has been used here to calculate the fracture toughness values and are shown in Table 2. TABLE 2 Calculated and Experimental Fracture Toughness Values Alloy Aging +

2090 4 h 8 h T6 8 h

SBS 300K 77K

1.8 2.1 1.25

SBW 300K 77K

KQ(meas.) 300K-MPam1/277K

KQ(Calc.) 300K M P a m l ~ K

1.07 1.22

.26 .20

.45 .36

23 20

37 33

19.6 12

47 34

.85

.29

.58

25

38

32

56

Conclusions The fracture toughness of 2090 and 8090 alloys increases at liquid nitrogen temperature. This is accompanied by an increase in yield and ultimate tensile stress, strain hardening exponent, and fracture strain. A larger number of delaminations occur at liquid nitrogen temperature. These delaminations cause the s p e c i m e n to b e h a v e as if it w e r e u n d e r p l a n e stress. H o w e v e r , the deformation processes, i.e., less slip l o c a l i z a t i o n at l i q u i d n i t r o g e n temperature when compared to intense slip localization at room temperature, still control the fracture process. Less slip localization at 77 K results in wider and closely spaced slip bands, whereas at 300 K slip bands are narrower and widely spaced. The fracture mechanism at both temperatures is slip band decohesion with larger facets at 77 K and smaller facets at 300 K. Acknowledgement This work was sponsored by the U.S. Office of Naval Research under Contract No. N00014-85-5-0526, Dr. Donald Polk, Contract Monitor. References i. 2. 3. 4. 5. 6. 7. 8. 9.

D. Webster: AI-Li Alloys III, C. Baker, P.J. Gregson, S.J. Harris and C.J. Peel, eds., Institute of Metals, London, U.K., 1986, pp.602-609. R.C. Dorward: Scripta Metall., 1986, 20, pp. 1379-83. J. Glazer, S.L. Verzasconi, R.R. Sawtell and J.W. Morris: Met. Trans. A, 1987, vol. 18A, pp. 1695-1701. D. Webster: Met. Trans. A, 1987, vol. 18A, pp. 2181-93. K.T. Venkateswara Rao, H.F. Hayashigatani, W. Yu and R.O. Ritchie: Scripta Metall., 1988, vol. 22, pp. 93-98. F.G. Nelson and G.J. Kauffman: in Fracture Toughness Testing at Cryogenic Temperatures, ASTM STP 496, American Society for Testing of Materials, Philadelphia, PA, 1971, pp. 27-39. K.V. Jata and E.A. Starke: Met. Trans. A, 1986, vol. 17A, pp. 1011-26. S. Suresh, A.K. Vasudevan, M. Tosten and P.R. Howell: Acta. Metall., 1987, pp. 25-46. T.H. Sanders and E.A. Starke: Acta Metall., 1982, vol. 36, p. 927-937.