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EFFECT OF MICROSTRUCTURE ON HIGH CYCLE FATIGUE BEHAVIOR OF Al-Li BINARY ALLOY
Z. Di, S. Saji, S. Hori
To cite this version:
Z. Di, S. Saji, S. Hori. EFFECT OF MICROSTRUCTURE ON HIGH CYCLE FATIGUE BEHAV- IOR OF Al-Li BINARY ALLOY. Journal de Physique Colloques, 1987, 48 (C3), pp.C3-753-C3-759.
�10.1051/jphyscol:1987388�. �jpa-00226620�
EFFECT OF MICROSTRUCTURE ON HIGH CYCLE FATIGUE BEHAVIOR OF A1-Li BINARY ALLOY
Z. DI* , S. S A J I and S. HORI
Department of Materials Science and Engineering, Faculty of Engineering, Osaka University, Yamadoka 2-1, suita 565, Osaka, Japan
" ~ r a d u a t e School, Osaka University, Yamadoka 2-1, suita 565, Osaka, Japan
Abstract
Effects of microstructures obtained by the three kinds of thermomechanical treatments on high cycle fatigue behaviors in Al- 1.8wt%Li alloy were investigated by means of optical and transmission electron microscopy. The three kinds of treatments were as follows:
1 ) solution treatment, 2) solution treatment-aging and 3) solution
treatment-cold rolling-aging. The results obtained are summarized as follows: (1) Fatigue strength increases with age-hardening due to 6 : ~ l ~ ~ i precipitates. Fatigue cracks nucleate preferentially at the sharp or coarse slip bands and propagate mainly along them in the solution-treated specimens and the aged specimens. (2) Deformation bands perpendicular to t h e rolling direction are f o r m e d heterogeniously within the elongated grains of the cold-rolled specimens, and sub-structure shows many microbands and cells. Fatigue strength in the thermomechically-treated specimens is higher than those in the aged specimens. (3) Fatigue strength in the specimens with the deformation bands parallel to the axis of fatigue stress is higher than that in the specimens with deformation bands perpendicular to the axis
.
The deformation bands perpendicular to the axis of fatigue stress can be preferential nucleation Sites and propagation paths of fatigue cracks. Microstructures consisting of disloction cell structure and alleviate the localized planar slip of dislocations and retard the initiation of fatigue cracks.Introduction
It is well known that lithium additions to aluminum alloys provide the greatest reduction in density of any alloying element and offer the additional advantage of increasing the elastic modulus.
These improvements have been attractive for structural applications in especially aerospace industry where various specific strengths shoud be as high as possible. It is necessary to investigate fatigue behaviors in aluminum alloys containing lithium in views of use for a structural material. Age hardening due to the S l ~ l ~ ~ i particles brings about a re arkable decrease in ductility and toughness of A1-Li binary The low ductility and toughness is expected to be improved by thermomechanical treatments, appropriate complex itation, grain refinement, elimination of impurities,and so o n
prfSfP5). Micorstructure is an important factor to improve various mechanical properties. Fatigue behaviors of metal materials are also influenced by microstructures w h i c h control the slip m o d e o f dislocations, initiation and propagation of fatigue crack. There are some reports(6-8) o n low cycle fatigue behaviors in A1-Li system alloys but no on high cycle fatigue behaviors. In the present work,
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987388
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effects of the various microstructures obtained by the three kinds of thermal and thermomechanical treatments on the fatigue-strength and- life, initiation and propagation of cracks during high cycle fatigue test in an Al-1.8wt%Li alloy containing $Lparticles were investigated.
Experimental
Pure aluminum (99.99%) was first melted in an alumina crucible under LiCl flux, and then metallic lithium (99.6%) was added. The molten alloy was cast into iron-moulds. A stream of argon gas was passed during the entire operation. The chemical composition of the alloy used is shown in Table 1
.
The as-cast ingots were homogenized at 570°C for 24h and scalped on the surface. The ingot was hot-rolled at 450-500°C and cold-rolled to final thickness of 2.3mm, and fatigue specimens shown in Fig.1 were cut from the sheets. Specimens were solution-treated at 500°C for Ih, followed by quenching into water with ice, and then aged in oil bath at 1 SO0 C for various periods. For the therrnomechanically treated specimens, the alloy was cold-rolled to final thickness of 2.3mm by thickness reduction of 67% after solution- treatmentat570°c for 40 min.The fatigue specimens were cut from the sheets i n the direction parallel and perpendicular to the rolling direction, respectively, a n d aged i n oil bath a t 150°c for various periods. Bending fatigue tests were carried out under a constant load at a load ratio of R=-I and a frequency of 30 Hz. Micro Vickers hardness was examined using 50gr load.Table 1 Chemical compsltion of the specimen used
wt%
-- - .-
Li
--TTl;-TT-
1.82 0 0 0 3 9 0.0091 0.0007 0.0031 bal - - - ---- -
Fig.1 Dimension of the specimen Microstructures were examined using optical and transmission electron microscopy ( TEM ) , at a n operating voltage of 200Kv. Thin foil for TEM were obtained by single-jet electropolishing method using an electrolyte of 10% perchloric acid-ethanol solution, at -20°C and a potential difference of 15 volts. All specimens surface for tests were groundedby hand with 600 grit emery paperfand then electropolished with 10% perchloric acid-ethanol mixture at -5-O°C and at a optential difference of 20 volts.
Results and Discussion 1. Effects of sLparticles
The S-N curves on high
cycle fatigue are shown in 11
0 Pure-Al(999Yf.)
Fig.2. This figure shows X) - that fatigue strength of Al-
1.8%Li alloy is considerably higher t h a n t h a t o f p u r e aluminum, even for solution- treated specimens. Fatigue strength of alloy increases with age-hardening due to
4 - 3 -
Fig.2 S-N curves for as-quenched 2 - -0-
and variously aged specimens 1
of AL-1.8%Li alloy and pure lo' lor 10' td
alminm. N C F I ~
ture, initiation and propagation of crack. The results obtained by continuous observation are shown in Fig.3 for the solution- treated specimen which was fatfgue-tested at a stress just above the fatigue limit. Fine slip bands are homogeniously observed within grains in the early stage of fatigue test as shown in Fig.3(a) and (b). Some of the slip bands become coarse and sharp with increasing cycle numbers, as indicated by an arrow in Fig.3(c), and fatigue cracks nucleate at the sharp slip bands and then propagate along them, as seen in Fig.3(d). A final fatigue crack propagates along the above mentioned sharp slip bands and some grain boundaries.
The arrows in Fig.3(c) and (d) indicate the same position o n the specimen surface. Fig.4 and 5 show initiation and propagation of fatigue cracks in the under-aged and the peak-aged specimens, respectively. The allows indicate the same position on the surface of each specimen. Sharp slip bands which bring about the initiation and propagation of a fatigue crack, occur more inhomogeniously with increasing age-hardening and radius of 8 ~ p a r t i c l e s , as seen in comparison with Fig.3, 4 and 5. Microstructures of the solution- treated, under-aged and peak-aged specimens fatigue-tested variously, were observed by means of TEM. Fig.6 is a micrograph of the solution- treated specimen fatigue-tested under the indicated conditions, showing high density of dislocations arranged along the <110>
direction which is a characteristic micorstructure in fatigue deformation of this alloy. These dislocations may be caused by the motion of paired dislocations on (1 1 1 ) plans in the < I 10, direction w h i c h i s u e to the dispersion of 8/-particles f o r m e d during quenching(9P from the solution-treatment and may be responsible for sharp slip bands on surfaces which lead to nucleation and propagation of fatigue cracks. Fig.7(a) and (b) are micrographs of the under-aged specimen fatigue-tested under two stress levels, showing different
Fig.3 Slip bands and crack nucleation in as-quenched specimen of Al-1 .8%Li alloy during fatigue, G-= 5.6~g/mm~ ( ~ C Z =I .3), OM.
Fig.4 Crack nucleation at the inhomogeneous slip band during fatigue in specimen aged at 150'~ for 24h of A1-1.8%Li alloy,
@ = 9 . 2 ~ g / m m ~ (a/G2=0.83 ) ,OM.
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Fig.5 Crack nucleation and propagation during fatigue in specimen aged at 1 5 0 ' ~ for 500h, corresponding t o the peak hardness of Al-1 .8%Li alloy, G-= 9 . 2 ~ ~ / r n r n ' ( @/+,, = 0 . 5 8 ) ,OM.
higher stress level, cells consisting of higher density of dislocations are formed and most of dislocations within the cells are single dislocations as seen in Fig.7(b), shearing of
8'- articles
by a lot of planar dislocation slips under the higher stress,may bring about localized destruction of the ordered structure in 6-particles.Fig.8(a) and (b) are micrographs of Jcparticles in the peak-aged specimen fatigue-tested under the indicated stress level (a). The
x/-
particles seem to be cut into slices by dislocations during fatigue test.
2 Effects of thermomechical treatments
Surfaces of the specimen cold-rolled (67%) after solution- treatment were examined by means of optical microscopy. Grains are elongated along therolling direction and deformation bands (arrow) perpendicular to the rolling direction are observed inhomogeniously within grains, as shown in Fig.9 (a). Fig.9 (b) shows a micrograph (TEM) corresponding to one of the deformation bands. There are fine microbands w i t h 0.2-0.5um i n w i d t h and dislocation cells.
Misorientation, between two adjoining microbands is about 2-6 deg.
M i c r o g r a p h s ( T E M ) o f t h e s p e c i m e n a g e d a t 1 5 0 ° ~ f o r 67hafter cold rolling are shown in Fig.lO(a)-(c). Dislocation density within cells and microbands becomes lower, moreover, distribution of SLparticles is relatively homogeneous. This result supports that precipitation of 6/- particles are nearly unaffected by dislocations and that S'particles are easy to precipitate, because of the small mi in .erfacial energy between the matrix and Jcparticle
7%
t and the low.
rolling direction
Fig.9 Micrographs of the specimen cold-rolled(67%). (a) OM, (b) TEM corresponding to photo(a),(BFI).
Fig.10 Transmission electron micrographs of the specimen cold- rolled(67%) and aged(150°~-67h). (a), (b) BFI, (c) DFI corresponding to photo (b).
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S-N curves for various cold- 1 0 0 IRD
rolled and aged specimens are shown 1 3 - A A A l l RD
together with these of solution- , A'
treated specimen and the specimen % aged at 1 50°C-500h in Fig.1 I. l 1
Fatigue strength of the cold-rolled 5 and aged specimens is higher than
$
9 -those of the solution-treated specimen, the solution-treated and 7 -
cold-rolled (67%) specimen and the
peak-aged specimen. The higher I
fatigue strength of thermomechanical ld lo6 ld ld treated specimens is due to the N Cycle
dislocation cell structure with&;
particles. Fatigue strength of the Fig.11 S-N curves for the various specimens with deformation bands cold-rolledandaged specimens parallel to the axis of fatigue The axis of fatigue stress was stress is higher than that of the parallel or perpendicular to specimens with deformation bands the rolling direction (RD).
perpendicular to the axis of fatigue stress, that is, anisotropy of
fatigue strength in the rolling direction is recognized. Fig.12 (a)- (c) are micrographs showing crack nucleation and propagation during fatigue test under the indicated condition in a cold-rolled (67%) and aged (150°C-67hr) specimen. In the early stage of fatigue test, indistinct fine slip bands appear within grains, and the fatigue crack nucleates preferentially at the deformation bands as shown by an arrow in Fig.12 (a) with increasing cycle numbers. This crack propagates along the deformation band, as seen in Fig.l2(b) and (c) and lead to fatigue fracture. Fatigue cracks are easy to nucleate and propagate at deformation bands perpendicular to the axis of fatigue stress because of the favorable condition of stress concentration. On the other hand, in the case of deformation bands parallel to the axis, nucleation of crack occurs preferentially at the slip bands formed during fatigue test and propagates along some of the slip bands and grain boundaries as seen in Fig.13.
rollinn direction. fatigue stress
Fig.12 Optical microstructures showing c ~ a c k nucleation and propagation in a cold-rolled (67%), aged (150'~-67h) and then fatigued specimen. Applied stress:
W = 9.7Kgfmm
.
(a) N=4.37x10:, ( b ) ~ = 4 . 6 3 x 1 0 ~ , (c) N=4.79x10.
6 =I 1. 3k9/mm2,
(a) ~ = 8 . 7 x 1 0 ~ , (b) N = ~ . ~ x I o ~ , (c) ~=9.36x10~.
Conclusion
1. Fatigue strength of the solution-treated specimens of A1-1.8%Li alloy is rather higher than that of pure aluminium and it increases with increasing age-hardening due to
S L A ~ ~ L ~
particles.2. Fatigue cracks nucleate preferentially at the sharp or coarse slip bands and propagate mainly along them in the solution-treated specimens and the aged specimens.
3. Fatigue strength of the thermomechanically-treated specimen is higher than that of the aged specimen. The higher fatigue strength is due to the dislocation cell structure combined with &particles. The cell structure alleviates the localization of dislocation on a slip plain during fatigue and retards the initiation of fatigue crack.
4. Fatigue strength in the specimens with the deformation bands parallel to the axis of fatigue stress is higher than that in the specimens with deformation bands perpendicular to the axis.
5. Fatigue crcak nucleats, and propagates preferentially along deformation bands when the deformation bands are perpendicular to the axis of fatigue stress.
References
B-Noble, S.J.Harris and K-Dinsdale: J.Met.Sci.,16(1982),425 M.Furukawa, Y-Miura, M.Nemoto: Bulletin of the Japan Inst.
Met., 23(1984),172
A.K.Vasudevan, E.A.Ludwiczak, S.F.Baumann, P.H.Howel1,
R.D.Doherty and M.M.Kersker: Mat.Sci. and Tec., 2(1986),1205 E.A.Starke, Jr.and F.S.Lin: Met.Trans.,l3A(1982),2259
P.J.Gregson and H.M.Flower: Acta Met.,33(1985),527
T.H.Sander, Jr. and E.A.Starke, Jr.: Acta Met.,30(1982),927 A.K.Vasudevan and S.Suresh: Met.Trans.,l6A(1985),475
D-Webster: Met-Trans. ,10A(1979),1913
Z.Di, S.Saji and S.Hori: J.Japan Inst Light Metals, 3 6 ( 1 9 8 6 ) , 4 3 6 B.Noble and G.E.Thompson: Met,Sci,J.,5(1971 ),I14
