Effect of Aging on Fatigue Crack Growth Behavior of a High Temperature Titanium Alloy

Effect of Aging on Fatigue-Crack Growth Behavior

of a High-Temperature Titanium Alloy

Jianrong Liu

, Shouxin Li

, Dong Li

and Rui Yang

Titanium Alloy Laboratory, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, P.R. China

2_{Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences,}

72 Wenhua Road, Shenyang 110016, P.R. China

The effects of aging on the room-temperature fatigue-crack growth (FCG) behavior of a newly developed high-temperature near- titanium alloy (Ti-5.6Al-4.8Sn-2Zr-1.0Mo-0.32Si-0.8Nd) with both bimodal and lamellar microstructures were investigated. The results indicate that aging had a relatively small effect on the FCG behaviors of the concerned alloy. The effect of aging is more pronounced on the lamellar than on the bimodal microstructure. This effect reduces with aging time especially for the lamellar microstructure. TEM and SEM studies were performed to gain insight into the deformation behavior and crack propagation mechanisms of the alloy. Dependence of the FCG rates on aging treatment is rationalized by considering changes in crack path tortuosity, roughness-induced crack closure and deformation uniformity. Slip reversibility is also considered. The work suggests that the amount and size of2particles based on Ti3Al is mainly responsible

for this dependence in the highKrange.

(Received November 10, 2003; Accepted January 27, 2004)

Keywords: aging effect, fatigue crack growth behavior, high-temperature titanium alloy

1. Introduction

Microstructural stability of high temperature near-

titanium alloys during service is of significant practical concern from the view point of component life and safety. For near-titanium alloys, depending on alloy composition, aging may result in the precipitation of incoherent silicide and coherent 2 phase based on Ti3Al, as well as the

breaking-up of interplatelet films.1–14) _These

microstruct-rual changes will, in turn, lead to a decrease in ductility and a certain extent of gain in strength.2–5)_{The aging treatment may}

also affect the creep and low cycle fatigue (LCF) properties in a complicated way.15,16) _{Since such microstructural}

changes occur over both short and long time periods, the effects of aging must be evaluated in order to make better estimations of the component life or to guide the alloy designer in improving the alloy composition.1)

Effects of microstructural parameters that can be varied by primary heat treatment on fatigue crack growth (FCG) behavior of titanium alloys have been widely studied,17–24) while only limited studies concerning the effect of secondary heat treatment, viz., aging conditions on the FCG behavior have been reported.3,16,25,26) _{Previous investigations on Al}

alloys27–31)_{showed that underaged specimens (with shearable}

and coherent precipitates) exhibit decreased FCG rate as compared to that of the over-aged (with unshearable and incoherent precipitates), due to the enhanced slip planarity by cutting of the coherent precipitates and hence the increased slip reversibility at the crack tip. In most near titanium alloys, aging may produce both 2 phase based on Ti3Al

(shearable and coherent) and incoherent silicide particles, which may impose a complicated effect on the FCG behaviors. Moreover, the coherent Ti3Al particles

precipi-tated in Ti-Al alloy are almost free of stress field, while a high stress field exists around the coherent particles precipitated in the Al-alloy.31)This difference in stress field may probably

result in different effects of aging on FCG behaviors between the two alloy systems. Furthermore, one should note that the FCG rate depends strongly on the environmental conditions and load ratio, R (¼Kmin=Kmax; Kmin and Kmax are the

minimum and maximum values, respectively, of the stress intensity factor during a fatigue stress cycle). The effect of aging on the FCG behavior may be changeable if either or both factors of the environmental conditions and load ratio,

R, are changed. Study of Widmansta¨tten Ti-11003)_indicates

that FCG rate was increased by aging at room temperature, while a slight decrease was produced by aging at high temperature (593_{C). Yet the effects of aging on FCG}

behavior at differentRratios are not clear so far. The purpose of this work was to investigate the effects of aging on the room-temperature fatigue crack growth behavior of Ti-5.6Al-4.8Sn-2.0Zr-1.0Mo-0.32Si-0.8Nd (mass%), a near- alloy for use up to 600_{C, heat treated to obtain two typical}

microstructures (bimodal and lamellar), and to discuss the underlying mechanisms that account for these effects at differentRratios.

2. Experimental Procedure

The material used in this study was supplied in the form of 20 mm diameter bar which was (þ) forged. The alloy composition was listed in Table 1. The as-received material was solution treated above and below thetransus for 2 h to obtain the fully lamellar and bimodal microstructures, respectively. Optical micrographs of the microstructures produced in the two solution treatment conditions are shown

Table 1 Composition of the alloy used in this study (mass%). Element Al Sn Zr Mo Nd Si O Fe Ti Content 5.6 4.5 3.0 1.0 0.8 0.34 0.071 0.03 Bal. Special Issue on Recent Research and Developments in Titanium and Its Alloys

in Fig. 1. The bimodal microstructure here consists of equiaxed primary phase (10pct in volume fraction and

15mmin size, hereafter referred to asp) with a

colony-type lamellar matrix (the average colony size is about 35mm) of alternatingplates and thinfilms within priorgrains, Fig. 1(a). The lamellar microstructure consists of packets of alignedplates with thinfilms separating them, Fig. 1(b). Thegrain size in Fig. 1(b) is about 450mmand the average

colony size is about 100mm. Here one should note that the present alloy system contains about 1 pct of rare earth Nd, which forms rare earth phase based on Nd3Sn (black particles

in Fig. 1). The two microstructures were then subjected to aging at 700_{C for different lengths of time. Aging did not}

produce any noticeable changes in the microstructural features as observed by optical microscopy. However, an obvious difference can be found under TEM, as will be discussed later. Heat treatment schemes adopted in the present experiment and the tensile test results as well as fracture toughness KIC are given in Table 2. For both

microstructures, slight gains in strength occur upon aging, while the ductility and fracture toughness are reduced. A little difference in mechanical properties between the two micro-structures is that the lamellar microstructure obtained by 700_{C/2 h aging exhibited higher strength than its unaged}

and 700C/12 h aged counterparts, while the bimodal micro-structure obtained by 700C/12 h aging exhibited the highest strength.

The three-point bending specimens used in crack growth studies are 8 mm16 mm64 mm in dimension, contain-ing a through-thickness notch with a depth of about 1 mm

prepared by spark-cutting. All fatigue crack growth tests were performed in laboratory air at room temperature using a 20 kN Schenck fatigue testing machine under constant load control, at a frequency of 45 Hz. Preparation of pre-crack was conducted using a conventional load-shedding procedure and the crack length,a, was measured optically with an accuracy of 0:01mm. Values of da=dN and K were calculated from a seven-point incremental polynomial method. All specimens were mechanically polished in order to facilitate crack detection and later scanning electron microscope (SEM) observation of crack-microstructure interaction. Thin-foil specimens for observation of the microstructure and the deformed zone ahead of the fatigue crack in a transmission electron microscope (TEM) were prepared using the conventional twin-jet method and the foils were observed in an EM-420 TEM at an accelerating voltage of 100 kV.

3. Results

3.1 Microstructural changes induced by aging

TEM micrographs show the changes in bimodal micro-structure caused by different aging treatments (Fig. 2). In the unaged condition, no precipitate was observed and the inter-plateletfilms are intact. The 2 h- and 12 h-aged conditions show silicide precipitation along the platelet boundaries, Figs. 2(a)–(b), with some coarsening in the 12 h-aged condition. After aging of 700_{C/2 h tiny}

2 particles can be

seen within the primaryphase (Fig. 2c) but no superlattice reflections can be detected within theplatelets, suggesting

20µm

(a)

10µm

(b)

Fig. 1 Optical micrographs of the solution-treated microstructures: (a) bimodal; (b) lamellar.

Table 2 Heat treatment schemes and the mechanical properties of the alloy.

Solution treatment Aging treatment YS UTS El. RE KIC

(MPa) (MPa) (%) (%) (MPa m1=2₎

1010_{C/2 h/AC*} Unaged 944 1029 15.6 32.8 72.7

(Bimodal) 700

_{C/2 h/AC 985 1057 12.2 21.3 45.1}

700_{C/12 h/AC 994 1066 12.0 20.0 44.8}

1030_{C/2 h/AC} Unaged 933 1020 12.4 23.2 71.6

(Lamellar) 700

_{C/2 h/AC 981 1052 8.4 12.5 54.0}

700_{C/12 h/AC 970 1042 6.5 8.0 47.4}

that no detectable 2 particles exist and only short range

order may exist in these platelets under this aging condition. In the 12 h-aged condition, however, 2 can be

seen in both the primary phase and platelets. Fig. 2(d) shows copious amount of2precipitation from a transformed

platelet.

The effect of aging on lamellar microstructure is similar to that on the bimodal microstructure, Figs. 3(a)–(f). No precipitation exists in the unaged condition and the inter-platelet layers are intact, Fig. 3(a); both silicide and 2

phase are found in the 2 h- and 12 h-aged conditions, with some growth of both types of precipitates in the 12 h-aged condition, Figs. 3(b, c, e, f). Another distinguishing feature in the 12 h-aged condition is the apparent breaking-up of inter-platelet layers, which makes the platelet interface dis-continuous (Fig. 3d) in contrast to that in the unaged condition (Fig. 3a). These microstructural changes are summarized in Table 3.

3.2 Effect of aging on the FCG behavior

Theda=dNKcurves of the differently aged bimodal and lamellar microstructures at a load ratio of R¼0:1 are presented in Fig. 4(a) and Fig. 4(b), respectively. For both microstructures the unaged specimen exhibits lower FCG rate than the 12 h-aged over the wholeK ranges tested. A

transition exists in the FCG curves of the 2 h-aged specimen: for bimodal microstructure, the FCG rate of the 2 h-aged specimen is nearly equal to that of the 12 h-aged in the near-threshold regime (K<6:6MPa m1=2_{); with increasingK}

range theda=dN vsKcurve departs from that of the 12 h-aged specimen until the K range reaches 8.9 MPa m1=2; beyond this point, there is no difference between the crack growth rates of the unaged and 2 h-aged microstructures. For lamellar microstructure, the difference between FCG rates of the 2 h- and 12 h-aged specimens is very small in the near-threshold regime (<6:9MPa m1=2_{); at K>6:9MPa m}1=2_,

the da=dN vs K curve of the 2 h-aged specimen departs from that of the 12 h-aged specimen and finally settles roughly between that for the unaged and 12 h-aged con-ditions. Comparison of Fig. 4(a) and Fig. 4(b) indicates that the lamellar microstructure suffers from a more pronounced aging effect than the bimodal microstructure does. As can be seen, another apparent difference between the FCG curves of the differently aged bimodal and lamellar microstructures is that the FCG curve of the 2 h-aged specimen with lamellar microstructure lies closer to that of the 12 h-aged than its bimodal counterpart over the wholeKrange. This may be due to the homogeneous precipitation of the2phase in both

the 2 h- and 12 h-aged specimens with lamellar micro-structure and will be discussed later.

(a)

500nm (b)

500nm

100nm (c)

200nm (d)

Fig. 2 TEM micrographs of the bimodal microstructure subjected to different aging treatments: (a) and (b) dark field images of silicide precipitated at=interface in the 2 h- and 12 h-aged conditions, respectively; (c) dark field of2precipitated in the primaryphase in

A significantReffect was found in both the bimodal and lamellar microstructures as expected (Figs. 4(c) and (d)). For a givenK, a great increase of FCG rate was found atR¼

0:6as compared to that atR¼0:1 for both microstructures irrespective of their aging conditions. For bimodal micro-structure, the effect ofRwas decreasing with increasingK,

viz.a decreased ‘m’ value upon raising theRratio (here,mis the exponent in the Paris power law relationship); while for lamellar microstructure, this phenomenon is less apparent. Further investigation found that the same phenomenon does

exist in the lamellar microstructure if more data are collected in the high K regime. This may imply that a significant crack closure effect exists in the threshold regime for both bimodal and lamellar microstructures at a stress ratio of 0.1 and the closure effect decreases with increasingK.

The effect of aging on FCG rate at R¼0:6 follows the same trend as that at R¼0:1, with the unaged specimen exhibiting the lowest FCG rate and the 12 h-aged specimen exhibiting the highest for both bimodal and lamellar micro-structures. It can be seen from Fig. 4(c) that theda=dNvsK

(c)

200nm (b)

100nm (a)

500nm

(d)

200nm

(e)

100nm (f)

200nm

Fig. 3 TEM micrographs of the lamellar microstructure subjected to different aging treatments: (a) intact inter-plateletlayers in the unaged condition; (b) and (e) dark field images of2precipitated in theplatelets under 2 h- and 12 h-aged conditions, respectively;

curves of differently aged specimens with bimodal micro-structure at R¼0:6 become closer to one another in comparison with those at R¼0:1 and this implies a weakened effect of aging on the FCG behavior of bimodal microstructure at increased load ratio. This phenomenon also exists in the lamellar microstructure (note the scale ofK)

but is less apparent than its bimodal counterparts.

3.3 Fatigue crack path

Figures 5(a)–(c) are SEM micrographs showing the crack paths of the differently aged bimodal microstructure at

R¼0:1. The parameter , as illustrated in Fig. 5, was used Table 3 Summary of the microstructure changes induced by the different aging treatments.

Solution Aging Aging-induced microstructural changes

Unaged No precipitation phase

700_{C/2 h/AC} Coherent ordered2particles homogeneously inp

1010_{C/2 h/AC} and non-coherent silicide atplatelet boundaries

(Bimodal) More and larger coherent ordered2particles in both

700_{C/12 h/AC}

pandplatelets; non-coherent silicide atplatelet

boundaries

Unaged No precipitation phase

700_{C/2 h/AC} Coherent ordered2particles throughout the

1030_{C/2 h/AC platelets and non-coherent silicides at=interface}

(Lamellar) _{More and larger coherent ordered2particles and} 700_{C/12 h/AC non-coherent silicide atplatelet boundaries; apparent}

breaking-up of inter-plateletfilms.

10 1E-10

1E-9

da/dN

/m

cycle

-1

∆K /MPa. m1/2

Lamellar unaged Lamellar 2h aged Lamellar 12h aged R=0.1

(b)

10 1E-10

1E-9 1E-8

Bimodal microtructure

da/dN

/m

cycle

-1

∆K /MPa.m1/2 Unaged

2h-aged 12h-aged Unaged 2h-aged 12h-aged

R=0.6 R=0.1

(c)

10 1E-10

1E-9

Lamellar microstructure

da/dN

/m

cycle

-1

∆K /MPa.m1/2

Unaged 2h-aged 12h-aged Unaged 2h-aged 12h-aged

R=0.6 R=0.1

(d)

10 1E-10

1E-9 1E-8

da/dN

/m

cycle

-1

∆K /MPa. m1/2

Bimodal unaged Bimodal 2h-aged Bimodal 12h-aged R=0.1

(a)

.

. .

.

Fig. 4 FCG rate curves in differently aged conditions of (a, c) bimodal and (b, d) lamellar microstructures atR¼0:1(a, b) and atR¼0:1

here to assess the fracture surface roughness, similar to that in Ref. 32. It can be seen from Figs. 5(a) and (b) that the 2 h-aged specimen shows a slightly larger value and hence a slightly rougher crack path than the 12 h-aged one. The unaged specimen exhibited the largest value, but the deflection in this condition (Fig. 5(c)) is somewhat macro-scopic and is less periodic than in the aged condition.

4. Discussion

In the current experiment, the FCG rate of a near-

titanium alloy increases with increasing aging time, consis-tent with results of similar alloys reported in the litera-ture.3,4,26–31) It should be noted here that with prolonged aging, both the amount and size of2particles are increased.

Yet no by-passing mechanism was involved in the present experiment since the largest coherent2particle (as seen in

Fig. 2(d) and Fig. 3(e)) is still not large enough to activate this mechanism. In order to interpret the present experimental results, changes in (a) crack path tortuosity and the associated roughness-induced crack closure, (b) slip reversibility and (c) deformation uniformity need to be considered.

(a) Crack path tortuosity and roughness-induced crack closure in different aging conditions as well as the resultant effects on FCG behaviors

The experimental results indicated that the FCG curves of the differently aged specimens atR¼0:6became closer to each other than atR¼0:1. The effect ofRon FCG behavior was related to the change of the external resistance such as crack deflection and roughness-induced crack closure under most circumstances in titanium alloys.19,21,22,25) Since at a higherR-ratio of 0.6, the effects from crack closure and crack deflection have been largely removed,22) _{it can be deduced}

from the experimental results that the unaged specimens exhibited the largest decrease in crack growth resistance, namely, atR¼0:1, the unaged specimens suffered from the most pronounced effect of crack deflection and closure. Figures 5(a)–(c) shows that the unaged specimen exhibited the most pronounced deviation of crack path from its nominal growth plane while the 12 h-aged specimen exhibited the least. This may be due to the fact that the crack in the unaged condition deflects predominantly at colony boundaries and priorgrain boundaries because thefilms andplatelets are not thick enough to deflect fatigue cracks,24)_{Fig. 6(a),}

while in the aged conditions the fatigue crack also deflects at

platelet boundaries due to the retained phase or large silicide particles along the platelet boundaries, Fig. 6(b). The frequent deflection of crack in the 12 h-aged condition leads to a smoother fracture surface than that in the 2 h-aged and unaged conditions, especially in the near-threshold regime. It is well accepted that the nominal stress intensity range to propagate a more deflected crack is always greater than that for an undeflected one at the same rate as a linear crack even in the absence of roughness-induced crack closure.33,34)_{The pronounced deviation of fatigue crack from}

its nominal growth plane may reduce the driving force for crack propagation. Another consideration related to the crack path deflection at low stress ratios is the roughness-induced crack closure. The magnitude of roughness-induced crack closure is dictated by both the crack path tortuosity and slip reversibility.28–31,33) The slip reversibility does not play a dominant role in the present study, as will be discussed later. The unaged specimens in the current study exhibited the most pronounced macroscopic crack path deflection and were also thought to suffer from a higher extent of roughness-induced crack closure as compared to the 2 h- and 12 h-aged

speci-Bimodal 12h-aged, R=0.1 (a)

200µm

δ1

FCG direction

(b) _{Bimodal 2h-aged, R=0.1}

200µm

δ2

FCG direction

(c)

200µm

Bimodal unaged, R=0.1

δ3

FCG direction

mens, which, together with effects from crack deflection, partially explains the higher near-threshold fatigue crack growth resistance measured with the unaged specimens at

R¼0:1.

(b) The role of slip reversibility

At a higherR-ratio of 0.6, the effects from crack closure should be largely removed.22)_{Yet difference among the FCG}

curves of the differently aged specimens still exists (Figs. 4c and d), suggesting that the difference among the FCG curves at R¼0:1 is not totally due to crack path deflection and roughness-induced crack closure. The slip reversibility argu-ment must be considered here since it played an important role in influencing the FCG behaviors of age-hardened Al-alloys.27–31)

It is well established21,27)_{that slip reversibility might play a}

role predominantly in the near threshold FCG regime where the plastic zone size is small relative to the grain or colony size. If the slip reversibility is responsible for this dependence of FCG behavior on aging conditions, then in the Paris-law regime where the slip reversibility is less active, this dependence should minimize or disappear. However, large difference in FCG curves was still found between the unaged and 12 h-aged bimodal microstructures and among the unaged and aged lamellar microstructures at higher K

regime. This indicates that the slip reversibility is not the main cause of the difference in FCG behaviors among the different aged specimens.

(c) Deformation uniformity in differently aged specimens

Since the dependence of FCG behaviors on aging conditions found in the present investigation cannot be fully rationalized by the factors discussed above, changes in deformation uniformity induced by aging should be consid-ered here. From Table 2, it can be seen that both yield and ultimate tensile stresses of the present alloy were increased after aging. This phenomenon can be largely attributed to the formation of the2phase, which may lead to the increase of

the critical resolved shear stress (CRSS).3,11) When the coherent2particles are cut by the moving dislocations in the

aged condition, their strengthening effect is reduced. The active slip plane is weakened and subsequent slip will occur on this slip plane preferentially,9,11)which makes the width of the individual slip band decreased and the spacing between

the adjacent slip bands increased, viz. the intensity of slip bands decreased in the aged specimens (Fig. 7(a)). The plastic strain ahead of the crack tip was concentrated to these localized slips, leading to an increased strain localization in the loading half cycle. The extent of strain localization increases with both the amount and size of coherent and shearable phase.29)_{This means that the 12 h-aged specimens}

shall suffer from the highest extent of strain localization in the present experiment. At the same time, the continuous interplatelet phase was broken up in the 12 h-aged condition and the platelet boundaries were replaced by incoherent silicide particles and retainedphase surrounded bymatrix with high density of mismatch dislocations, Fig. 3(d). The slip at the crack tip may be impeded by the incoherent silicide particles or by interaction with the mismatch dislocations and hence results in a lower slip reversibility in the unloading half cycle. The high strain localization combined with low slip reversibility associated with the 12 h-aged specimens results in a high strain accumulation.

On the other hand, the slip bands in the unaged condition exist in a large number and are rather uniformly distributed, Fig. 7(b). The plastic strain ahead of the crack tip was divided among them and hence leads to a less localized plastic strain. At the same time, the slip at the crack tip would not be impeded by the=interfaces or byphase as proposed by Ravichandran,24)and hence a somewhat higher slip reversi-bility at the crack tip was expected in this condition. The low strain localization combined with high slip reversibility associated with the unaged specimens resulted in a lower strain accumulation for the same number of cycles as compared to the aged specimens. This may imply that for a givenK, the unaged specimens would exhibit lower FCG rates than the aged ones even in the absence of roughness-induced crack closure at lowerK values.

The transitions on the FCG curves of both the 2 h-aged bimodal and lamellar microstructures indicate a decreased sensitivity of FCG behavior to short-time aging with increasing K. Yet at higher K ranges, the 12 h-aged specimen exhibited significantly higher FCG rate than its unaged counterparts; similar phenomena were also reported before.3,26)_{It is interesting to note that the 2 h-aged specimen}

10µm

(b)

FCG direction

10µm

(a)

FCG direction

with bimodal microstructure contains much less2phase as

compared to its 12 h-aged counterpart since 2 phase was

only found in the primaryphase in the 2 h-aged condition and the p only accounts for 10pct of the total volume

fraction. As a result, a considerable difference in FCG rate between them was found, Fig. 4(a). The 2 h- and 12 h-aged lamellar specimens had comparable amount of2phase and

the two specimens showed comparable FCG rates, Fig. 4(b). It seems reasonable to conclude that the amount and size of

2particles may account for this apparent aging effect in the

high K regime. The coherent 2 particles are sheared by

moving dislocations irrespective ofKrange. A significant difference in the amount and size of 2 particles may also

lead to an appreciably different degree of strain localization. Yet the slip reversibility is less active in the highKregime. The different degree of strain localization between the unaged and 12 h-aged specimens resulting from the large difference in the amount and size of2particles may be the

most probable causes for their apparent difference in FCG resistance at high K ranges. Since silicide precipitated along the platelet boundaries also facilitates slip local-ization,2)the large silicide particles in the 12 h-aged speci-mens may also help to accelerate the FCG rate.

It should be mentioned that the crack propagation along the interplatelet boundaries may also help increase fatigue growth rate as proposed by Hornbogen and Gahr.27)Yet this mechanism is not dominant here since it fails to rationalize the experimental finding that there is a large difference between the FCG curves of the unaged and 2 h-aged lamellar microstructures at high K ranges (Fig. 4b), whereas this difference was not found between their bimodal counterparts, Fig. 4(a).

The addition of rare earth element Nd to the present alloy was intended to improve the high-temperature strength, creep resistance and ductility after thermal exposure. The influence of rare earth phases on low cycle fatigue behavior and tensile and creep properties has been reported by other authors.35,36) We have not, however, found any correlation between the rare earth containing phase and FCG behavior of the present alloy so far. Comparison tests done on alloys with and without Nd addition showed no difference of FCG curves.37)

Because the size of the Nd-rich particles (110mm) in the present alloy is much smaller than the thickness (8000mm) of the FCG specimens, the Nd-rich particles may not influence the FCG on a macroscopic level. One may imagine that, when the thickness of the FCG specimen becomes compa-rable to the Nd-rich particle size, the soft Nd-rich particles may modify the stress/strain concentration ahead of the crack and may impose significant influence on FCG behavior of the alloy.

5. Conclusions

The effect of aging on the bimodal and lamellar micro-structures of a newly developed Nd-containing near-

titanium alloy has been investigated. Room-temperature FCG behaviors in the different aging conditions were compared and the underlying mechanism was discussed.

(1) The FCG rate for a givenKincreases with prolonged aging time. The unaged specimens exhibited the higher FCG resistance than that of the 2 h-aged and 12 h-aged for both the bimodal and lamellar microstructures at Rratio of both 0.1 and 0.6. For the 2 h-aged specimen, a transition exists in its FCG curve: in the threshold regime, the crack velocity of the 2 h-aged specimen is nearly equal to that of the 12 h-aged specimen; with increasingK range, the FCG curve of the 2 h-aged specimen departs from that of the 12 h-aged specimen and finally becomes nearly identical with that of the unaged specimen for bimodal microstructure, lying roughly between the FCG curves of the unaged and 12 h-aged specimens for lamellar microstructure.

(2) The effect of aging on the lamellar microstructure is larger than that on the bimodal microstructure; difference between the FCG curves of the 2 h-aged and 12 h-aged lamellar microstructures is less pronounced than that between their bimodal counterparts.

(3) The effect of aging on FCG behavior is slightly different at differentRratios: a larger aging effect was found atR¼0:1than atR¼0:6. This difference is attributed to the different extent of crack path tortuosity and the associated roughness-induced crack closure accompanying fatigue crack growth of differently aged specimens atR¼0:1.

(a)

200nm (b)

200nm

(4) The overall dependence of the FCG behavior on aging conditions is rationalized by considering the difference in crack path tortuosity, roughness-induced crack closure and deformation uniformity among different aging conditions. Slip reversibility is also considered. The amount and size of

2phase based on Ti3Al is thought to be responsible for this

dependence at higherKvalues.

Acknowledgments

The authors wish to acknowledge the financial support of this research from the National Key Project of Basic Research in China (TG19990650).

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