Fatigue crack growth transitions in Ti-6Al-4V alloy

Scripta METALLURGICA

Vol. 23, pp. 1685-1690, 1989 Printed in the U.S.A.

Pergamon Press plc All rights reserved

FATIGUE CRACK GROWTH TRANSITIONS IN Ti-6AI-4V ALLOY

K.S.Ravichandran* and E.S.Dwarakadasa Department of Metallurgy Indian Institute of Science Bangalore-560 012. India.

(Received March 16, 1989) (Revised July 17, 1989) I INTRODUCTION The fatigue crack growth (FCG) behavior of titanium alloys has been investigated[I-3] extensively . FCG curves for these alloys exhibit[I] two stages consisting of a hypertransition regime (microstructure-insensitive) and a hypotransition regime (microstructure-sensitive) with the transition occurring at a AK level (AKcpzs=cs) at which the cyclic plastic zone size (CPZS) at the crack tip becomes equal to the Widmanstatten colony size. This aspect has been studied in detail in several titanium alloys[I,3] and in other alloys[4] as well. Widmanstatten colonies exhibit[5] large crack closure levels during FCG so that CPZS can be expected to be uniquely related to aKeff (effective or closure-free stress intensity range) rather than AK. AKeff can be expressed as: AKeff = Kmax- Kmin if Kcl < Kmin

AKeff = Kmax- Kcl

if Kcl >Kmin

...

(1)

where Kmax, Kmin and Kcl are respectively the maximum, minimum and closure stress intensities. Previous studies[I,3,4] computed CPZS using AK and this might be misleading. Further, apart from Widmanstatten colonies, individual o~ laths can limit slip at ~/[~ interfaces if the obstructing [~ phase is thick. Hence, crack growth transitions can be induced when the dimensions of the cyclic plastic zone and (x laths are similar. In the course of reexamination of transitions in FCG in Ti-6Al-4V alloy, the occurrence of transitions induced by cyclic as well as monotonic plastic zone sizes (MPZS) were observed and are reported here. II EXPERIMENTAL PROCEDURE To produce a Widmanstatten colony structure, a Ti-6AI-4V alloy was beta annealed at 1040°C for 30 min. and cooled at rate of 2°C/rain. Fatigue crack propagation testing was performed on 10mm thick compact tension specimens by a manual load shedding procedure. The objective was to generate data on near threshold fatigue crack growth behavior and crack closure for this microstructure. Fractography and crack path profile examination were performed in a scanning electron microscope. IH RESULTS The microstructure consisted of coarse Widmanstatten colonies of average size 132 I.tm, and ct laths of average thickness 11.5 I.l.m . The laths were separated by a [~ phase of thickness of about 1-2 ~ m . The tensile properties were: 755 MPa (YS), 853 MPa (UTS) and 8% Elongation. The magnitudes of total (AKth), effective ( AKeff,th) and closure (I(cl,th) stress intensities were 5, 2.8 and 2.4 MPa..qi=i, respectively.

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1685 0036-9748/89 $3.00 + .00 Copyright (c) 1989 Pergamon Press plc

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FATIGUE CRACK GROWTH IN T i - 6 A 1 - 4 V

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Figs. 1 (a & b) illustrate fatigue crack growth rates as functions of AK and AKeff for two tests, respectively, with the same microstructure. Scatter in d a / d N is common in microstructures of this type due to extensive crack deflection and branching. The trends in crack growth are indicated by solid lines. Two transitions in FCG behavior are evident. The first transition occurs in the form of a plateau in crack growth rate. At the second transition there is a change in slope. In test 1, transition 1 occurred at a slightly lower crack growth rate but at the same AK range (12-15 MPa¢/ii ) compared to test 2. Similarly, the transition 2 occurred at the same AK level but at significantly different crack growth rates in both tests. By means of the relations:

=

![AKeffl 2 31I [_ 2gy ..]

... (2a)

Monotonic plastic zone size =

~1 [Kmaxl 2 31-1!_ ay ..I

... (2b)

Cydic plastic zone size

the magnitudes of Kmax, mc, AKeff,cc (respectively the Kmax and AKeff levels at which the MPZS and CPZS become equal to colony size) and Kmax,ml, AKeff,cl (respectively the Kma x and AKeff levels at which the MPZS and CPZS become equal to alpha lath size) were computed and are presented in Table I. The crack growth rates together with AKeff at transition 1 and Kma x at transition 2 are also included. Transitions 1 and 2 coincide with AKeff,cl and Kmax, ml respectively. The crack closure levels also exhibited (fig.2) a transition and the maximum closure occurred at AKeff,cl suggesting that the cyclic plastic zone influences the magnitude of crack closure. The SEM crack path profile at AKeff,cl is presented in fig.3a. Crack branching can be seen to occur at AKeff=14.4 MPa~t-~ which is approximately equal to AKeff,cl. At a slightly lower AK level, arrest of slip band cracks at thick [~ phases can be seen (fig.3b). Fracture surfaces exhibited a microstructure-sensitive, lath related fracture morphology (fig.4a) at low AK, a mixed mode fracture at the transition 1 (fig.4b) and a fiat transgranular cleavage like fracture (fig.4c) at high AK, commensurate with above observations. IV DISCUSSION Transitions in FCG were seen to occur when both the monotonic [7] and cyclic plastic zones[3,4] become equal to microstructure size, here, the alpha lath size. When CPZS become equal to lath size, enhanced slip reversibility (unimpeded by (z/13 interfaces) causes less fatigue damage. In spite of slip compatibility[8] between IZ and 13, the interface can act as a barrier to slip through differences in moduli, strain partitioning and intrinsic strength levles of (Z and 13. Slip reversibility also increases the transportation of environmental species such a s H2 dissociated from the moist air at the crack tip, thus causing H2 saturation in ~. As 13 has higher solubility for H2 than (%, it can act as a reservoir to cause hydride induced cracking along the slip bands in (Z. This can lead to multiple crack branching (fig.3). Hence, a combined environmental and CPZS effect could cause a plateau (transition I ) in FCG curve. In the literature, a consensus exists[I,3,4] with regard to the equality of CPZS with microstructure size during the transition in FCG. However, it is argued[9] that below a certain critical crack growth rate, the crack tip environmental species such as H2 may encounter favourable conditions to diffuse to the crack tip, thus bringing in an environmentally assisted cracking mechanism. This could occur when the rate of creation of fresh fracture surfaces matches the rate of adsorption of environmental species (H 2 or 02) on these surfaces. Moist laboratory air is a potential contributer in this respect and a two step process could prevail at the crack tip. The first step would involve diffusion and dissociation of H20 molecules releasing H2 and 02. The second step involves adsorption of 02 on freshly created surfaces and the diffusion or dislocation transport of H2 to regions ahead of the crack tip. Adsorption of 02 prevents slip reversibility and H2 would cause embrittlement: both in combination accelerating crack growth. In planar slip materials such as Ni-base alloys and titanium alloys, this mechanism is

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FATIGUE CRACK GROWTH IN T i - 6 A I - 4 V ALLOY

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hypothesised[10] for the occurrence of crystallographic fracture at low AK. A strong pile-up is then crucial and the condition that CPZS be equal to microstructure size favours this condition. In other words, a combined requirement of favourable transport conditions for environment and the CPZS equal to the lath size appear to be necessary to induce crystallographic cracking. This could be the reason why the transition 1 in both tests occurred at the same ZhK level and slightly different crack velocities, still satisfying the required transport rates of environment to the crack tip. Since the local orientation of microstructure at AKefLc1 will vary specimen to specimen, this can be expected. The transition 2 occurs at Kmax, ml (cf. Table I) due to the constriction of monotonic plastic zone within the lath. This transition transfers the continuum deformation at the crack tip to single crystal behavior. Similar transitions due to MPZS have been noted[Ill during FCG in Cobalt-base directionally aligned eutectics consisting of lamellar rnicrostructure. The observation of a maximum Kcl at AKeff,cl is also consistent with these explanations. The onset of crystallographic fracture by multiple crack branching causes an increase in the roughness of the crack wake, thereby increasing closure levels. At low AK, the absolute value of closure is a function of the crack tip sliding displacement along the slip plane and the disregistry of asperities behind the crack tip[12]. Hence as AK decreases, the slip band length of CPZS would decrease proportionately reducing closure level. The discrepancies observed in two tests on closure are not clear at present as closure is also sensitive to several crack wake parameters. V CONCLUSIONS There are two transitions during fatigue crack growth in a titanium alloy, caused by cyclic and monotonic plastic zones when their respective sizes become equal to the alpha lath size. The transition due to cyclic plastic zone size appears to be a combined action of environment and the stage where cyclic plastic zone size becomes equal to lath size. Correspondingly, crack closure levels also exhibited a transition. The crack path exhibited multiple crack branching at this transition. The cyclic plastic zone size is uniquely related with AKeff as against AK in situations of large crack closure. Similarly, a transition also occurred when the monotonic plastic zone size was of a size equal to the alpha lath size and this is attributed to the change from continuum to single crystal behavior at the crack tip. Both the transitions were induced by alpha laths because of the presence of thick interplatelet beta layer in the microstructure. ACKNOWLED GEMENTS The authors thank Dr. D. Banerjee, DMRL for his assistance in heat treatment and valuable discussions. REFERENCES 1. G.R.Yoder, L.A.Cooley and T.W.Crooker, Proc. 2nd Int. Conf. Fatigue and Fatigue Thresholds, C.J.Beevers (ed.), EMAS Publication, 1(1984)351. 2. A.Yuen, S.W.Hopkins, G.R.Leverant and C.A.Rau, Metall. Trans., 5(1974)1833 3. G.R.Yoder, L.A.Cooley and T.W.Crooker, Metall. Trans., 15A(1984)183. 4. G.R.Yoder, L.A.Cooley and T.W.Crooker, NRL Memorandum: 4576, Nav. Res. Lab., Washington D.C., USA. 5. M.A.Hicks, R.H.Jeal and C.J.Beevers, Fat. Engg. Mat. Struct., 6(1983)51 6. K,S.Ravichandran, PhD Thesis, Dept. of Metallurgy, Indian Institute of Science, Bangalore-560 012, India, 1988. 7. R.O.Ritchie and S.Suresh, MetaU. Trans., 13A(1982)937. 8. Sreeramamurthy Ankem and H.Margolin, Metall. Trans., 11A(1980)963. 9. G.F.Pittinato, Metall. Trans., 3(1972)235 10. D.J.Duquette and M.GelI, Metall. Trans., 2(1971)1325. 11. C.M. Austin, N.S.Stoloff and D.J.Duquette, Metall. Trans., 8A(1977)1621. 12. K.Minakawa and A.J.McEvily, Scr. Metall., 16(1981)618.

FATIGUE CRACK GROWTH IN T i - 6 A I - 4 V ALLOY

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TABLE I: CRACK GROWTH C O N D I T I O N S D U R I N G TRANSITIONS

Parameter

Transition I Test 1

Transition 2

Test 2

Tes~ 1

9.0 X 10 -7

da/dN (mm/cyde)

4.5 X 10 -5

8.0 X 10 -5

zXKeff

15.0

13.4

( MPaq-m )

Test 2

4.0 X 10 -6

8.0

Kmax ( MPa'/-m ~

8.0

AKeff, cc = 53.3 MPaq-m

AKeff,cl = 15.8 MPaq-m

(Calculated from eqn. 2a)

Kmax,mc = 26.6 MPaV-m

Kmax,ml = 7.9 MPaq-m

(Calculated from eqn. 2b)

10-3

10-~,

t'

z • io ~ ,,,m/, ~,.1~

J u -5 >,10 u

~,

-g

¥

E

,,d6 *p~'

. 2 ~ ~ ~I~

Tr~

z 106 TEST

:1

7"?:

~6~

16 e 2

3

10

(a)

20

30

!00

2

3

10

(b)

Fig. 1 : Fatigue crack growth curves (a) for test I and Co) for test 2.

20

30

100

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FATIGUE

CRACK

GROWTH

IN T i - 6 A I - 4 V

ALLOY

1689

8

I a Test * Test ~

1 2

6

&Keff,CL

= 15.8 MPa;m-

0_

~

4

- -

DO

C)

~,

,y,

2

I

0

10

I

l

20 30 40 AKeff (MPa ~r'm)

1

50

Fig. 2 : Crack closure stress i n t e n s i t y variation as a f u n c t i o n of

~eff

Fig.3 : SEM crack p a t h m o r p h o l o g y ; (a) at a b o u t ~Keff, cl a n d Co) at a lower

AK level

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FATIGUE CRACK GROWTH IN Ti-6AI-4V ALLOY

(a)

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(b)

(c) Fig. 4 : Fractography of fatigue fracture surface ; (a) at AK < AKeff,cl , (b) a p p r o x i m a t e l y at AKeff,cl and (c)at AK > ~ e f f , cl