Long crack growth mechanisms in Ti-6Al-4V alloy

TGoswamiMechanical Engineering Department, T J Smull College of Engineering, Ohio Northern University,

Ada, OH 45810, USA

Received 3 December 2004, accepted 8 July 2005

A transition mechanism of long crack growth from structure sensitive (Regime I) to insensitive region (Regime II) wasinvestigated at (i) room temperature under high humidity conditions, and (ii) elevated temperatures for Ti-6AI-4V alloy andother materials. Constant amplitude tests were conducted using several stress ratios (R = 0.05, 0.1, 0.4 and 0.7) for the highhumidity tests and 0.1 for room and elevated temperature tests. Conventionally forged Ti-6AI-4V alloy disks, processed tothe solution treated and over-aged condition, were studied for both programs. An increase in stress ratio and temperaturelowered the transitional stress intensity factor range where Regime I transitioned to Regime II. Higher stress ratios (R = 0.7)accelerated the fatigue crack growth rates many times the crack growth rates obtained at lower stress ratios (R = 0.05).However, higher temperatures (345°C) influenced the crack growth rates only marginally. The mechanisms controllingelevated temperature fatigue crack growth in Ti-6AI-4V were by the secondary crack formation, striations and some cavityfeatures on the fracture surface. The damage was localized on the ex platelets, where a type dislocations found with the

- -Burgers vectors (aI2) <11 20> g = 1010 near (10 I 0).

IPC Code: C22C14/00

Indian Journal of Engineering & Materials SciencesVol. 12, October 2005, pp. 367-375

Crack growth behaviour of Ti-6Al-4V shows abilinear behaviour in that two slopes representRegimes I and II respectively. Figure 1 schematicallyshows the stress intensity factor ranges as a functionof the crack growth rates for the two Regimes. Themechanisms of FCP in Ti-6Al-4V are structuresensitive at the low 11K values and depend upon thea-f3 packet size'"!'. The reversed plastic zone at thetip of a fatigue crack has been related with a-f3packet size"!', in that the reversed plastic zone sizesmaller than the a-f3 packet size causes Regime Igrowth and vice-versa for Regime II. The transition

The growth of long fatigue cracks in Ti-6Al-4V is afunction of testing parameters and materialparameters such as microstructure. The microstructureof Ti-6Al-4V, which is a a-f3 alloy, is achieved byheat treatment summarized elsewhere'. A number ofstudies are found in the literaturei'" describing the.fatigue crack growth behaviour of Ti-6Al-4V alloyand establishing microstructure-property correlations.With increased attention given to high cyclefatiguel2

,17-20 being one of the potential failure modesin military aircraft engine disks and blades made oftitanium alloys, room and elevated temperaturefatigue crack propagation (FCP) behaviour iscurrently being re-evaluated. The evaluation ofinteraction effects of mixed fatigue (interactionsbetween low and high cycle fatigue), foreign objectdamage and fretting aspects are discussedelsewhere'<'". This paper provides additional fatiguecrack growth rate data at elevated temperatures for Ti-6Al-4V. Since a major percent of component life isspent in nucleation and growth of a crack to adetectable size, crack growth studies are conductedboth in Regime I (low ranges of da/dN and 11K) andRegime II or Paris region. Thus, the study oftransition of Regime I to II and modeling thisbehaviour will be useful in structural integrityassessment of aircraft structural components.

da/dN

Regime II

Regime I

Stress intensity factor range

Fig. I-Schematic representation of Regime I and Regime IIcrack growth behaviour in Ti-6AI-4V alloy

368 INDIAN J. ENG. MATER. SCI., OCTOBER 2005

between structure sensinve Regime I behaviour tostructure insensitive Regime II behaviour, occurs atintermediate range of stress intensity factor range. Inthe literature'"!', Mode I M was reported to be from10 to 30 MPa-,Jm where transition occurs. This paperexamines the parameters that influence the transitioncharacteristics of long crack growth for a number ofstructural materials used in aircraft structure anddevelops an empirical model to predict the transitionpoint.

Fatigue crack growth data from two test programsare presented revisiting the mechanisms of FCP underdifferent stress ratios (R) at high humidity andelevated temperature environments.

Experimental ProcedureRoom temperature tests

Forged bars 16 were used in this program fromwhich the specimens were machined in the R-Lorientation. The bar was processed to solution treatedand aged condition (STOA). The processingparameters for STOA were kept same for bothprograms (Table 1). Tests were conducted to studymore fully the transition mechanisms from Regime Ito Regime II at different stress ratios. Since theenvironment influences the FCP behaviour and a widerange of transitional stress intensity (10-30 MPa-,Jm)

Table \- Parameters of solution treated and over-agedcondition

Solution treat

Water quench

Overage

Room temperature

700°C, for 2 h, air cool

Fig. 2-Microstructure of Ti-6AI-4V in STOA condition

reported in the literature"!', tests were conductedbelow Mode I stress intensity factor range (11K,) of 20MPa-,Jm to document the onset of the transition. Bothpre-cracking and FCP tests were conducted in highhumidity (over 85% relative humidity) using afrequency of 10 Hz. All the samples were preparedper ASTM E-647-95 by inducing an EDM notch atthe center of the specimen. Crack growthmeasurements were made using resistance gaugesmounted on each face of specimens. A total of twelvespecimens were tested,S each with R = 0.05 and 0.7,and 2 with R = 0.4.

Elevated temperature testsForged disks" were supplied by AlliedSignal in the

solution treated and over-aged (STOA) condition.Details of STOA parameters are outlined in Table 1.Ti-6A1-4V is typically solution treated between 955and 970DC and water quenched. When the material wassubsequently held at or near the annealing temperature,which was above normal aging temperature (700DC),an over-aged condition resulted. The applied heattreatment resulted in the development of duplexmicrostructure of primary a, and platelets of ~ in ~(Fig. 2), containing acicular alpha with an aspect ratioof approximately 10:1. However, a very differentmicrostructure was observed when a specimen wasmade from a different area (flange area) (Fig 3). Thismicrostructure (Fig. 3) contains slightly distorted,coarse, plate like alpha grains with randomly equiaxedalpha grains (light) in beta phase (seen dark), with anaspect ratio of 10:1. Specimens were machined such away that the loading was in the radial direction,whereas the crack growth occurred in longitudinaldirection (R-L), representative of actual stressdistribution in an engine disk from the blade

Fig. 3- Microstructure of Ti-6AI-4V in STOA condition

altlV.

CI

tcP

PdJDVI

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GOSW AMI: LONG CRACK GROWTH MECHANISMS IN Ti-6AI-4V ALLOY 369

attachment root or from bolt-holes. The dimensions ofthe compact tension (CT) specimens used for the testswere in conformance with ASTM E647.88. A directcurrent potential difference (DCPD) system was usedto record the crack growth of CT specimens andprocedure reported elsewhere'v'".

A total of sixteen CT specimens were tested in thisprogram, eight from each disk and each test wasduplicated. Specimens numbering 1 and 2 representDisk I and 3-4, Disk II, respectively. It is likely thatvariations in the data may arise from locations fromwhere specimens were machined, product form (diskand bar), forging ratio (different % volume reductionratio) used for bar and disks, and methods used tomonitor crack growth. Since the plane strain fatiguecrack growth rate data were generated for both highhumidity testing and elevated temperature testing, thedata were compared to derive trends in behaviour andrelating the transitional stress intensity factor rangefor each condition. Monotonic properties arepresented in Table 2 for different temperatures. Thematerial became slightly ductile (measured by %elongation) followed by lower yield strength, andmodulus with temperature increase.

Additionally, in a separate test effort, cyclic strainaccumulation tests were conducted under total straincontrol and was recorded for several hourglassspecimens in Fig. 4 showing behaviour under

Table 2--Mechanical properties of Ti-6AI-4V alloy disks

Temperature°C

Yieldstrength

MPa

%elongation

Youngmodulus

GPa

Room175230290

813800772760

12.4

17.118.720

120110109100

o.o14r----------------,

0.012-<>- Room Temp..•.. lt5°C.••. 230°C

-I>- 290°C0.010

0.008.!:~ 0.006Vi

0.002

°0L--~4~00~-~8~00~-~1~20~0~~1~60~0~~20~00Numbe, of cycles

Fig. 4-Strain accumulation in Ti-6Al-4V alloy at roomtemperature, 175,230 and 290°C.

repeated cyclic plastic deformation for roomtemperature, 175,230, and 290°C. At the same cyclesto failure (400 cycles) the strain accumulated at roomtemperature was 0.004, whereas at 290°C, 0.0055(Fig. 4), which is over 35% higher, meaning higherstrain accumulation and reduced cyclic life. As aresult, the low cycle fatigue behaviour of Ti-6AI-4Vat high temperature is inferior from its high cyclefatigue resistance. Similar trends are shown for othercombination of test parameters.

For the fatigue crack growth tests, the tests werecontinued with the crack length to specimen widthratio, (a/W) of 0.24 to 0.7 within the range specifiedin ASTM E-647-88. Fatigue crack growth rates werecalculated from incremental measurements of crackgrowth.

The incremental crack growth rates were correlatedwith !Y..K values derived from the mean of the crackgrowth interval, (a i+l+a i)l2.

ResultsRoom temperature tests

A number of stress ratios were studied rangingfrom 0.05 to 0.7. Pre-cracking was performed withsinusoidal waveform at 10 Hz in a high humidityenvironment. Each specimen was pre-cracked 6 mmon the middle tension (MT) specimens 101 mm wideand 6.25 mm thick on the average. The crack typicallypropagated to a length of 45-46 mm, where !Y..Kl was13-16 MPa"m, and cycles applied ranged from 2 x105 to nearly 7 x 106• The number of cycles to pre-crack the specimen varied from a few hundredthousand cycles to several million cycles.

.OO~7,----------~------~~

.OO~8"R'=OOS

R='O<4

XR::().iI

.AoR=O.7

·R=O.7°R::Q.05AR=O.7liR:>O.7aR=O.7

.:OR::O.056R=O.05

(l>R~.05

.OOE-09

.OOE-IO .'----------------------l10 100

Fig. 5-Crack propagation behaviour of Ti-6AI-4V in highhumidity environment.

370 INDIAN·]. ENG. MATER. SCI., OC!'9BER 2005

Environment for all the tests was high humidity forboth pre-cracking and fatigue crack growth phase.Tests were performed under constant load.

Fatigue crack growth behaviour of Ti-6AI-4V alloyis presented in Fig. 5. The data presented in Fig. 5shows the daldN and 11K! for various stress ratios. Allthe data were below 11K of 20 MPav'm to show thetwo Regimes in crack growth and capture the onset oftransition.

Elevated temperature testsPre-cracking was performed with sinusoidal

waveform at 20 Hz using a stress ratio (R) of O.l.Upon the specified pre-crack length was achieved, thespecimens were tested for fatigue crack growth phaseusing the same test parameters except the maximumload reduced. A 11K gradient of 4 MPa-v'm was usedfor load increments as the crack progressed. Thefatigue crack growth behaviour is presented in Fig. 6for elevated temperature tests at a constant stress ratioof (0.1), however, temperature varied from 175 to345°C.

DiscussionRoom temperature tests

The data presented in Fig. 5 shows the structuresensitivity effect for the lower stress ratio testsconducted at R=0.05 and 0.4. For these tests,microstructure has been found to dictate themechanics of crack growth at the lower ranges of 11K!.As a result, slope of the daldN and I1KJ changes earlyinto the cracking. This effect is also interpreted in theliterature in terms of texture effect9

,1O. As the stress.ratio increases to R=0.7, the crack growth rates

1.00£..04,------------------.

IOTI'$lI 115 • Test 1 11$flTrsll 11.5 iIoYc:st4 I1S

Test I 2.30 YCSll noTesll 230 TCSOI" 230

toTes:t1 290 oTesll 2SIOo Tcst 3 290 .• TCSl4 290

Tcsal l-4S T(:SIl. 3")TCSll ).IS To:st~ H~

i.oos-os

~~ i.oos-os!~ 1.00&07-o

1.00&08

l.ooE-09 '- ~_~_~ ~ ....l.

1 10MCMPaim

Fig. 6-Elevated temperature fatigue crack propagation behaviourin Ti-6AI-4V alloy

become much larger (a 3 to 5 times) at comparable11K! found in this study. However, an order ofmagnitude variation at comparable I1KJ has also beenreported in the literature'. Several factors contribute tothis accelerated crack growth, viz., the packet size ofa-13 structure, and reversed plastic zone effect"!'which depends on packet size. For the higher stressratio tests, as R increases, structure sensitivity(causing multiple slopes in daldN versus 11K! below10 MPa-v'm) tends to diminish as observed in Fig. 6.For all the tests conducted in high humidity, thetransition of structure sensitive to structure insensitivemode (Regime I to Regime II) was found to be lowerthan the tests conducted in laboratory airenvironment"!'. The transitional 11K was found to befrom 3 to 10 MPa -v'm,also found in some studies inliterature and higher within 30 MPa -v'msummarizedelsewherei". Elevated temperature tests show distinctregions where Regime I and II exist and transitioneffects. High humidity in pre-cracking phase has beenfound to initiate pits from where cracking wasobserved in high strength aluminum alloys and atypical crack growth behaviour is presented in Fig. 7for 7075- T7651 I. A pronounced texture and/ormicrostructure effect is exhibited for this material atlower stress ratios. However, as the R became higher(0.7) the transitional I1Kl reduced considerably. Figure7 also shows the pronounced crack acceleration at the

1.00E·04

f--.j.-I

•• 1

II IIiIN ~ II .111

II Iii

1 I III

ifjl II!

/~ 1'1

1.00E-05

1.00E-06'i'U>.

iz-e 1.00E-07iiI

"

1.00E-08

R=0.05OR=0.05l>R=0.05oR=0.7iii R=O.7

• R=0.7OR=0.05

1.00E-091 10 100

100

Fig. 7-Fatigue crack propagation behaviour of aluminum alloy7075-T76511.

fI

GOSWAMI: LONG CRACK GROWTH MECHANISMS IN Ti-6AI-4V ALLOY 371

Regime I with low tl.K,. Corrosion, as found inaluminum alloys, may be one of the contributingfactors for accelerated crack growth in Ti-6AI-4V thatlowers the transitional tl.KT for Regime I phase athigher R ratios (e.g., R = 0.7).

Elevated temperature testsCrack growth rates vary from 10.8 to 10-5 (m/cycle)

(Fig. 6) within the range of Mode I stress intensityfactor 10 to 53 MPa-Ym. Straight line fits for the datawere determined within the range of crack growthrates from 10-8to 10-6m/cycle, representative of linearor power law regime. The parameters of the Parisequation, daldN = C (tl.Kr)m (slope of linear line, m,and material parameter, C), for each disk arepresented elsewhere I5, for all temperatures for boththe disks (these parameters are material dependent). Atwo-parameter analysis of variance method was usedto correlate the disk-to-disk variation in crack growthrates and appeared to be very small 15.Also FCP ratesobtained in this study for elevated temperature testswere compared with other studies3,4,6,8 in earlierwork". A marginal variation was observed fordifferent studies3,4,6,8,15.It is evident from Fig. 6 thatcrack growth rates within the mid-range (>10-7

rnIcycle), no substantial influence of temperature hasbeen observed. Crack propagation rate at 345°C wasslightly higher than at room temperature test. Thismay be a result of lower strength high ductilityexhibited by Ti-6AI-4V at high temperatures. Similarresults were reported for Ti-6AI-4V and other Ti-8AI-

13 h . . fl Mo-I V at elevated temperatures - . T e transiuon 0

Regime I cracking to II occurred for all the casesabove 10 MPa-Ym(refs 1-3, 7-10).

Each region, Regime I and II, has distinct features,shown schematically in Fig. 1. For the materialsstudied in this paper, the structural sensitivity in thecrack growth rates occurs for stress intensity factorbelow 10 MPa-Ymbeyond this point the crack growthprocess is likely to be independent of microstructure.The slope of the crack growth rate curve in Regime Iis higher than the Regime II, drawn schematically inFig. 1. This behaviour has been interpreted in terms ofcyclic plastic zone becoming equal to the a-13 packetsize9,1O. This point also describes the boundarieswithin which structural sensitivity occurs. In theliterature, several expressions have been proposedempirically':" describing cyclic plastic zone in

aluminum and titanium alloys.

Reversed plastic zone size = 0.132 [tl.K,IFty]2 ... (1)

where, tl.K, is the range of Mode I stress intensityfactor in MPa -Ym,and Fty the tensile yield strength inMPa of the material.

For titanium alloys transition point was determinedto be

Transition point (tl.KT) = 5.50 r; (d)O.5 .. , (2)

*Part of an internal study conducted in 1997-98 for Durability andDamage Tolerance Group.

where, d is the grain diameter using the ASTMspecifications.

A number of aluminum and titanium alloys wereanalyzed by the author, in a separate effort, that areused in aircraft structures (work performed at CessnaAircraft Co., Wichita, KS)* and the correlations weremade with the following equations.

Reversed plastic zone = 0.5 [tl.K,IFtyf ... (3)

Whereas,

Transition point was at (tl.KT) = 2.8 Fty (d)0.5 ... (4)

These expressions were verified with the materialsused in this study (Figs 5-7) and other materials andresults show very good agreement for both high-humidity and elevated temperature tests. All the dataanalyzed with Eq. (2) were tested in high humidityenvironment only. A two-parameter analysis ofvariance (ANOV A) was performed on the crackgrowth rate data and parameters of crack growth rate.>.

Fig. 8-Fractographic features of Regime I crack propagation,arrow showing the crack path

372 INDIAN J. ENG. MATER. SCI., OCTOBER 2005

equation (C and m) were analyzed'<. The crackgrowth rate equation parameters (Paris equation) donot show disk to disk variation and both C and mvalues appeared to be very close.

Regime I fatigue is normally observed on highcycle-low stress fractures and is frequently absent inlow cycle-high stress fatigue (Fig. 4) where fracturetopography contained mainly striations (Fig. 9). Thecrack follows crystallographic planes but changesdirection at discontinuities such as grain boundariesand other particles for Regime 1. A typical Regime Ifracture is shown in Fig. 8, showing the crack paththrough the crystallographic planes. The fracturetopography contained such features as facets oftenresembling cleavage where striations do not occur.Figure 8 shows the Regime I behaviour observed at

Fig. 9- Typical Fractographic features of Regime II crackpropagation (striations).

Fig. lO-Cavity formation in Ti-6AI-4V at 345°C (arrowsshowing typical cavities)

345°C, implying that these features were common,more pronounced for lower temperature tests.

Figure 9 shows trans granular fracture, which ismore influenced by the magnitude of the alternatingstress than by mean stress or microstructure. RegimeII fracture surface topography contained such featuresas striations in Fig. 9 ranging from 175 to 345°C. Avariation in stress, temperature, microstructure,frequency and environment can change the orientationof the plane of fracture and alter the direction ofstriation alignment. These features are observed forall the tests as the alignment of striation planes weredifferent for each case. These alignments (Fig. 9) aredue to the alignment of a-~ planes in themicrostructure and have not been investigated in thisstudy.

Fatigue crack growth process is a complexinteractive phenomenon among material, test,

(a)

(b)

(c)

Fig. ll-Secondary cracking at 345°C

(d

Fimdil(

Fig. 12-lntergranular fracture and dimples on the fracturesurface indicating overloading failure at 345°C.

on,

is109

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re

GOSW AMI: LONG CRACK GROWTH MECHANISMS IN Ti-6A1-4V ALLOY 373

(a) (e)

(b)

(f)

(c)

(g)

(h)(d)

Fig. 13-TEM micrographs of the elevated temperature fatigue crack growth mechanisms in Ti-6AI-4V alloy: (a) and (b) typicalmicrostructure showing platelets of q in ~; (c) Persistent slip band features at 290°C; (d) Persistent slip band features at 345°C; (e) a type

- - - -dislocations with Burger's vector (a/2)< 11 20>g = 10 10 near (10 10); (f) a type dislocations with Burger's vector (a/2)< 112 O>g =10I0 near (10 10); (g) dislocation network at 345°C; (h) dislocation network at 175°C

374 INDIAN J. ENG. MATER. SCI., OCTOBER 2005

environment and other parameters. Therefore,accurate prediction of FCGR is a challenge. Highertemperature and aggressive environment may reducegreatly the crack growth resistance of a material.Cyclic cleavage, secondary cracks and voids formedas the temperature increased (Fig. 8). Secondarycracks often reduce the stress severity at the tip of themain crack leading to increased crack growthresistance as the test temperature increased. However,there are competitive mechanisms among secondarycracks, secondary cracks interacting with the primarycrack, and other interactions are very difficult tointerpret and quantify. Some voids were observed at290 and 345°C as shown in the fractograph Fig. 10,which may provide crack linking path for theadvancing crack.

Secondary cracks were observed in the Regime IIof the fatigue crack growth region (see Fig. 11). Inthat area fine fracture features were seen compared tothe early Regimes of crack growth process wherefracture surface features were very rough showingcleavage, crack path changes and crack growth rateinteracting with microstructure.

The room temperature crack growth process wasdictated by the nature of the a-f3 grain structure, anddistribution of alpha platelets in beta grain, in whichcyclic cleavage was more deleterious and increasedFCP rates. For higher temperature tests (at 345°C)some cavities formed indicative of creep effect. Atsome point towards the end of Regime II, in RegimeIII, dimples were observed and fracture was byintergranular mode as shown in Fig. 12. Thesefeatures were similar to that reported earlierl-3,1O.

Mechanisms of fatigue crack growthThe fatigue crack growth mechanism of Ti-6AI-4V

has been evaluated with transmission electronmicroscope (TEM) of specimens tested at R = 0.1, 10Hz frequency and tested at 175,230,290 and 345°C21.A parallel, thin layer of material was removedcontaining the fracture surface to prepare the samplesfor TEM studies. Disk samples were removed atvarious locations along the fracture path simulatingslow, steady and rapid crack growth. Special care wastaken to locate the TEM foil within the crack tipplastic zone to capture the mechanisms. Figure 13 (a-b) shows a typical alpha-beta microstructure underTEM. The features of persistent slip bands werevisible in Fig. 13 (c-d) at two temperatures shown.The dislocations were mainly concentrated in a-

grains with only a small fraction present in f3-grains.Within the a-grains, <a> type dislocations formedwit~ Burgers vectors (aI2) <11 2: 0> g = 1010 near

(1010) (Fig. Be). The same location was seen in Fig.13f with a tilt g = 0002 near (1010). Some twinningtoge~her with other features appeared to be <c><10 10> type_dislocations in planner arrays with g =0002 near (1 2 10) was found.

The observation of thin foils also revealed anincrease in the number and density of bothdislocations and twins as the cycles and crack lengthincreased. However, the observed arrangement ofdislocations was generally random with lines ofdislocations emanating from the grain boundaries.The presence of deformation twins suggests that theintersecting twins are potential sites for micro-crackformation. As the temperature increased, 290-345°C,increased activity in the secondary crack formationperpendicular andlor at some angle to the load linewas documented in the fracture surface (Fig 9). For aTi-48AI-2Cr-2Nb intermetallic similar observationswere made22 and found that presence and interactionof the deformation twins is strongly dependent on themutually competitive influences of (i) orientation, (ii)grain size, and (iii) texture of the specimen. At thelower cyclic strain amplitude the resultant lowerresponse stress, a greater number of dislocations wasobserved in regions of single phase, v-grains andcoarse twin-related v-lamellae with no a2 present, withonly a marginal increase in the y lamellae.

The marginal effects of temperature on the fatiguecrack growth rates may have been due to thefollowing: (i) greater activity of dislocations Fig. 13(g-h) and (ii) an increased activation of the dislocationsources.

A major amount of strain energy, from the crackgrowth test, may have been utilized in deformationprocess to initiate twins. However, dislocationmobility may have supplemented this process latter onthe crack growth process showing the marginaldifferences in crack growth rates (Fig. 6).

ConclusionsThe following conclusions may be drawn from this

study: (i) The transition mechanisms in Ti-6AIA Vwere a function of environment in which tests wereperformed. (ii) High humidity and elevatedtemperature environments reduced the stress intensity

Fig,

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=

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GOSWAMI: LONG CRACK GROWTH MECHANISMS IN Ti-6AI-4V ALLOY 375

range where Regime I transitioned to Regime II. Thiswas also influenced by stress ratio. (iii) At higherstress ratio, R = 0.7, the transition phenomenondiminished in high humidity environment. Testsconducted at elevated temperatures showed bilinearslopes in the crack growth rate showing distinctRegime I and II,respectively. (iv) Cyclic plastic zonesize calculated empirically was found to predicttransitional stress intensity factor satisfactorily for anumber of aluminum and titanium alloys. (v)Complex interactions of environment, microstructure,and test parameters were not as significant at lowertemperatures and stress ratios. However, moresecondary cracks and cavities formed at 20 Hz at orabove 290°C and had a mixed effect on fatigue crackgrowth rates as secondary cracks may have reducedthe stress intensity at the crack tip of a dominatingcrack and voids may have accelerated the growth ofmain dominating crack, needs to be investigatedfurther. (vi) The dislocations were mainlyconcentrated in a-grains with only a small fractionpresent in ~-grains. Within the a-grains, <a> typedislocations formed· with Burgers vectors (aI2)

- - -<1120> g = 1010 near (1010) (Fig. 13 (e-f)). Sometwinning together with other features appeared to be<c> type dislocations in planner arrays with g = 0002near (1210) was found.

4 Salivar G C & Haake F K, Eng Fract Mech, 37 93)(1990)505-17.

5 Vesier L S & Antolovitch S D, Eng Fract Mech, 37 (4)(1990) 753-75.

6 Raizenne M D, AGARD SMP SC-33, Engine disk testprogramme fatigue crack growth rate data and modelingcases for Ti-6Al-4V, IMI 685 and Ti-17, National ResearchCouncil of Canada, LTR-ST-1785, Nov. 1990.

7 Zeghloul A & Petit J, Fat Fract Eng Mater Struct, 8 (4)(1985) 341-48.

8 Ogawa T, Hayashi Y, Tokaji K & Hirose M, Fatigue crackgrowth of Ti-6Al-4V at elevated temperatures, JapaneseSociety of Materials Science.

9 Yoder G R, Cooley L A & Crooker T W, Metall Trans, llA(1979) 805-816.

10 Eylon D & Hall J A, Metall Trans, 8A (1977) 981-90.11 Wanhill R J H, Galatolo R & Looije C E W, Int J Fatigue, 11

(6) (1989) 407-16.12 Cowles B A, Int J Fract, 18 (1) (1996) 1-16.13 Ramchandran N, Arakere N & Goswami T, High Temp

Mater Process, 19 (5) (2000).357-370.14 Ritchie R 0, Crack growth monitoring: some

considerationson the electrical potential method, TechnicalReport, Dept. Metall. Mater. Sci., University of Cambridge,1972.

15 Arakere N, Krohn J, Goswami T & Ramchandran N, HighTemp Mater Process, 21 (4) (2003). 229-236.

16 Goswami T, Mater & Design, 24 (2003) 423-433.17 Campbell J P & Ritchie R 0, Eng Fract Mech, 67 (2002)

209-227.18 Campbell J P & Ritchie R 0, Eng Fract Mech, 67 (2002)

229-249.19 Boyce B L & Ritchie R 0, Eng Fract Mech, 68 (2001) 129-

147.20 Peters J 0 & Ritchie R 0, Eng Fract Mech, 67 (2002 193-

20721 Goswami T, Gelles D & Jones R H, Mechanisms of elevated

temperature fatigue crack growth of Ti-6Al-4V, presented at132 Annual TMS meeting in San Diego, CA Mar. 2-6, 2003.

22 Srivatsan T S, Stangwood M & Soboyejo W 0, in Fatigueand Fracture of Ordered Intermetallic Materials, edited bySobyejo W 0, Srivatsan T S & Ritchie R 0, TMS 1994, p.113-132.

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metallurgical factors on the fatigue crack growth rates ina-f3 titanium alloys, AFML-TR-78-68, May 1978.

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