Materials Science and Engineering A266 (1999) 303–309

Fatigue crack growth in an in-situ titanium matrix composite

S. Dubey a,*, Y. Li a, K. Reece a, W.O. Soboyejo a, R.J. Lederich b

a Department of Materials Science and Engineering, The Ohio State Uni6ersity, Columbus, OH 43201, USAb The Boeing Company, P.O. Box 516, St. Louis, MO 63166, USA

Received 6 October 1997; received in revised form 8 June 1998

Abstract

The results of a recent investigation of fatigue crack growth in the in-situ titanium matrix composite, Ti–6Al–4V–0.5B, arepresented. An effort is made to understand the crack-tip plasticity in a TiB whisker reinforced titanium matrix composite usingcrack-tip transmission electron microscopy (TEM) techniques. The results of crack/microstructure interaction studies arediscussed, and an effort is made to quantify the effects of fatigue crack bridging. © 1999 Published by Elsevier Science S.A. Allrights reserved.

Keywords: Fatigue crack growth; Titanium matrix composite; Interaction

1. Introduction

Since its development at Battelle, Columbus, OH,about 50 years ago, Ti–6Al–4V has been widely usedin the aerospace and offshore industries due to itsunique combination of damage tolerance, corrosionresistance and low density [1]. However, it is unlikelythat further significant improvements in the balance ofproperties of Ti–6Al–4V can be engineered via intrin-sic modification, i.e. by changing the internal structureof Ti–6Al–4V by heat treatment or processing varia-tions. Extrinsic modification (composite) approachesare therefore needed for the engineering of furtherimprovements in the mechanical properties of Ti–6Al–4V.

This study is an extension of previous studies inwhich ingot metallurgy (I/M) and rapid solidificationpowder metallurgy (RSP/PM) processing techniqueshave been successfully used to produce in-situ titaniummatrix composites reinforced with TiB whiskers [2–8].The main driving force for the development of thesecomposites is the increase in modulus (5–20%) that canbe achieved without significant loss of ductility [2]. It isimportant to note here that even a marginal increase inmodulus (�5%) is significant with respect to the designof high stiffness aerospace and automotive components.

The in-situ composites produced by ingot metallurgytechniques are also relatively inexpensive and havehigher elevated-temperature strengths [3] and low densi-ties (�4.8 g cm−3). They are, therefore, attractivecandidates for high performance structural aerospaceapplications in the intermediate temperature regimebetween 400 and 650°C.

The fatigue and fracture behavior of an UM Ti–6Al–4V–0.5B in-situ composite reinforced with TiBwhiskers are examined in this study. Toughening isshown to occur as a result of bridging by stiff elasticTiB reinforcements. The toughening due to this bridg-ing is quantified using a micromechanics model. Thecrack-tip deformation mechanisms are elucidated viacracktip TEM studies. Crack-tip plasticity in the com-posite is compared with that of the unreinforced matrixalloy.

2. Material

The ingot metallurgy alloy composite Ti–6Al–4V–0.5B was produced by induction skull melting andconventional casting by Duriron, Dayton, OH. Theresulting ingot had a diameter of 70 mm, and the actualcomposition is shown in Table 1. Boron addition wasdone in the elemental form during casting. The castingot was then extruded to form a 20 mm diameter rod.* Corresponding author.

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S. Dubey et al. / Materials Science and Engineering A266 (1999) 303–309304

Table 1Chemical composition of I/M in-situ composite

OV BAl H (ppm)NCAlloy

0.48 300.13 0.02 0.003Ti–6Al–4V–0.5B 6.5 4.2

Single edge notched (SEN) specimens, with a size of50×13×6.35 mm, were fabricated from this rod byelectro-discharge machining (EDM). These specimenswere then annealed at 704°C for 1 h followed by aircooling.

The microstructure of the Ti–6Al–4V–0.5B com-posite used in this study is shown in Fig. 1(a). Thecomposite has a Widmanstatten a+b structure with a

laths in b phases. The a laths have a mean length of�12 mm and a mean width of �1 mm. The volumefraction of the b phase is about 36%. A rather homoge-neous distribution of TiB whiskers aligned along theextrusion direction was observed in the composite. TheTiB whiskers have a mean aspect ratio of 8:1, withlengths ranging from 7.0 to 23.0 mm and widths be-tween 1.0 and 1.3 mm. Fig. 1(b) shows typical TEMmicrograph of the undeformed composite. The Wid-manstatten a+b structure and the TiB reinforcementsare clearly evident in Fig. 1(b).

3. Experimental procedures

Fatigue crack growth tests were conducted on singleedge notched (SEN) bend specimens under three-pointbend loading in air at room temperature. The polishedspecimens were precracked under far-field compressionfatigue loading at a stress ratio, R=Kmin/Kmax of 0.1,and a stress intensity range, DK of �23 MPa m.After pre-cracking, fatigue crack growth tests werecarried out at a stress ratio, R=Kmin/Kmax of 0.1, anda cyclic frequency of 20 Hz. Fatigue crack growth wasmonitored using a high resolution Questar™1 (2.5 mm)telescope connected to a video monitoring unit. Thefatigue test was stopped prior to specimen fracture forcareful optical and scanning electron microscopy(SEM) studies of the interactions of the cracks with theunderlying microstructures. The SEN specimens werethen fractured under monotonic loading for fracto-graphic analyses by SEM. Crack-tip transmission elec-tron microscopy (TEM) foils (3 mm in diameter and200 mm in thickness) were sliced off (in the directionparallel to the crack face) from the fatigued regions ofthe fracture surfaces. The foils were then ground withsilicon carbide paper, dimpled and ion-milled, for ex-amination under a transmission electron microscope(TEM).

4. Results and discussion

4.1. Fatigue crack growth beha6ior

The fatigue crack growth rates obtained for theTi–6Al–4V–0.5B composite are presented in Fig. 2.The fatigue crack growth rates of the matrix alloy(Ti–6Al–4V extrusion), both in the as-extruded andheat treated conditions, are also presented for compari-son. The fatigue crack growth rates for both the matrixalloy and the composite are very similar. The additionof boron to Ti–6Al–4V therefore does not appear tohave a strong effect on the fatigue crack growth rates.

SEM studies of the polished sides of the SEN speci-mens revealed useful evidence of the interaction of thecrack with the underlying microstructure. Elastic bridg-ing by TiB reinforcements was observed to occur in theTi–6Al–4V–0.5B composite (Fig. 3). In Fig. 3, certainregions have been magnified in micrographs 1–6 toprovide clear evidence of the crack/microstructure in-teractions. In region 1, it is clear that some of the TiBwhiskers were fractured during crack opening. Debond-ing was also observed along the TiB whisker/matrixinterface. In region 6, elastic bridging by TiB whiskersis observed. Again, debonding is also observed alongthe bridging TiB whisker/matrix interface. The bridgelength (distance from the crack-tip to the farthest un-broken TiB whisker behind the crack-tip) is �22.6 mm.

The fractured surfaces of the Ti–6Al–4V–0.5B com-posite reveal a ductile dimpled fracture mode in theoverload failure region (Fig. 4(a)), and a ductile trans-granular fracture mode in the fatigue regime (Fig. 4(b)).However, fatigue striations in Ti–6Al–4V–0.5B (Fig.4(b)) are not as distinguishable as those in the matrixTi–6Al–4V alloy (Fig. 4(c)).

TEM micrographs of the fatigued region of Ti–6Al–4V–0.5B reveal that most of the dislocations wereconcentrated in the a grains (Fig. 5(a)). The TiBwhiskers do not appear to undergo plastic deformation,as suggested by the complete lack of dislocations in thewhiskers (Fig. 5(a)). Fig. 5(b) shows important evidenceof slip transmission through the b phase. A dislocationloop in one a lath transmits slip across the b phase intoanother a lath. The difference in the contrast of theloop sections (in the different laths) is probably due toslight misorientation of the laths with respect to eachother. Dislocation networks were also observed in the a

phases (Fig. 5(c)), and the dislocations appear to belong and wavy.

1 Questar is a trademark of the Questar Corporation of New Hope,PA.

S. Dubey et al. / Materials Science and Engineering A266 (1999) 303–309 305

Fig. 1. (a) SEM micrograph of TI–6Al–4V–0.5B (704°C/1 h/AC) showing a+b Widmanstatten structure and second phase TiB whiskers alignedin the extrusion direction. (b) TEM micrograph of the undeformed Ti–6Al–4V–0.5B showing a grains in a b matrix as well as TiB whiskers.

4.2. Crack bridging

4.2.1. Monotonic loadingExamination of the polished sides of the SEN speci-

mens showed evidence of crack bridging by the elasticTiB whiskers (Fig. 3). The bridge length (distance fromthe crack tip to the farthest unbroken whisker bridgingthe crack) was measured to be �22.6 mm. An attemptwas made to estimate the crack-tip shielding due to theelastic bridging mechanism [9–12], as shown in Fig. 6.Due to the lack of whisker/matrix interfacial propertydata, a simplified elastic crack bridging model wasdeveloped. It is assumed that the TiB whiskers simplybridge the cracks and effectively pin the cracks. Thistherefore increases the resistance to crack extension.The overall fracture energy is given by:

Jc=Vm Jm+DJ (1)

where Jc is the overall fracture energy, Jm is the matrixfracture energy, Vm is the volume fraction of the ma-trix, DJ is the energy change due to the bridgingprocess. The overall toughness of the composite is,therefore, given by:

Kc= (EcVm Jm+EcDJ)1/2 (2)

where Ec is the overall Young’s modulus of thecomposite.

The energy change associated with the bridging pro-cess is a function of the bridging stress/traction, Tu, andthe crack opening displacement, u, and is defined as[13,14]:

DJ=& umax

0

Tudu (3)

where umax is the maximum displacement at the end ofthe bridging zone, as shown in Fig. 6(a).

The maximum crack opening displacement at the endof the bridging zone, umax, is equal to the tensiledisplacement in the bridging TiB whisker at the pointof failure:

umax=owfldb (4)

where owf is the strain to failure of the TiB whisker, andldb is the length of the debonded matrix–whisker inter-face in Fig. 7. The strain to failure of the whisker canbe defined as:

owf=swf

Ew

(5)

where swf is the ultimate tensile stress of the TiBwhisker, and Ew is the Young’s modulus of the TiBwhisker. The interfacial debond length depends on theratio of the fracture energy of the whisker, gw, to theinterfacial fracture energy,gi. The debond length isgiven by [15]:

ldb=rgw

6gi

(6)

where gw/gi is the ratio of the fracture energy of thebridging whisker to that of the whisker–matrix inter-face, and r is the radius of TiB whisker.

It is assumed that the bridging stress increases lin-early from zero at the crack tip to a maximum at theend of the bridging zone and immediately decreases to

Fig. 2. Fatigue crack growth rate for Ti–6Al–4V–0.5B comparedwith the growth rates in Ti–6Al–4V extrusions.

S. Dubey et al. / Materials Science and Engineering A266 (1999) 303–309306

Fig. 3. Montage showing crack/microstructure interaction in Ti–6Al–4V–0.5B alloy. Unbroken TiB whiskers throughout the plastic zone.Bridging TiB whiskers seen throughout the length of the crack.

zero, as shown schematically in Fig. 6(b). The energychange associated with the bridging process (Eq. (3))then reduces to [14]:

DJ=Tmaxumax

2(7)

where Tmax is the maximum closure stress imposed bythe reinforcing whisker, Tmax is the product of thefracture strength of the whiskers, swf, and the areafraction of whiskers intercepting with the crack plane,Aw, and is given by:

Tmax=swfAw$swfVw (8)

where Aw is approximated by the volume fraction, Vw,for whiskers which have large aspect ratios. The energychange due to bridging process can therefore be writtenas:

DJ=swf

2 Vwrgw

12Ewgi

(9)

Substituting Eq. (9) into Eq. (2), the overall tough-ness of the composite is:

Kc=�

EcVm Jm+Ec�swf

2 Vwrgw

12Ewgi

�n(10)

The toughening ratio, Gb, is therefore given by:

Gb=Kc/Km=�

Vm

Ec

Em

+Ec

Km2

�swf2 Vwrgw

12Ewgi

�n1/2

(11)

where Vm is the volume fraction of matrix (=1−Vw),Em is the Young’s modulus of the matrix, Ew is theYoung’s modulus of TiB whiskers, Ec is the overallYoung’s modulus of the composite, Km is the toughnessof the matrix. swf Is the ultimate tensile stress of TiBwhiskers, Vw is the volume fraction of TiB whiskers, ris the average radius of TiB whiskers, gw is the fractureenergy of TiB whiskers and gi is the fracture energy ofthe whisker–matrix interface.

For the Ti–6Al–4V–0.5B in-situ titanium matrixcomposite, the volume fraction of TiB whiskers wasestimated as follows: an addition of 0.5 wt.% B corre-sponds to 2.6 at.% B which yields 3.1 vol.% TiBreinforcements (i.e. Vw=3.1%)

The overall composite modulus, Ec can be estimatedfrom the constant strain rule-of-mixtures. This gives:

Ec=VmEm+VwEw= (1−Vw)Em+VwEw (12)

The fracture energy of the bridging TiB whisker, gw,is given by:

S. Dubey et al. / Materials Science and Engineering A266 (1999) 303–309 307

gw=swfowf

2=

swf2

2Ew

(13)

The value of fracture energy of the whisker–matrixinterface, gi, is not available. However, it can be as-sumed that the maximum fracture energy of whisker–matrix interface is equal to the fracture energy of thematrix. Hence, the interfacial fracture energy is givenby:

gi=gm (14)

where gm is the fracture energy of the Ti–6Al–4Vmatrix. Fig. 8 shows a typical stress–strain curve forthe Ti–6Al–4V matrix alloy. The ultimate tensilestrength, sy, is �881 MPa and the plastic elongation is�15.5%. After yielding, the Ti–6Al–4V does not ex-hibit significant hardening. The stress–strain behaviormay therefore be idealized as elastic-perfectly plasticbehavior. Hence, the fracture energy of Ti–6Al–4Vmatrix is given by:

gm=syo1

2+sy (of−o1)=sy

�of−

12

o1�

=sy�

of−sy

2Em

�(15)

Taking Vw=3.1%, Ec=123 GPa, Em=112 GPa,Ew=480 GPa, Km=39.9 MPa m, swf=3500 MPa,r=1 mm, sy=881 MPa, the toughening ratio, Gb, isestimated to be �1.05. Kc is, therefore, estimated from

Eq. (11) to be 42 MPa m. This is comparable tothemeasured composite fracture toughness of 44 MPam.

4.2.2. Cyclic loadingThe above toughening ratio can be used to calculate

the crack-tip stress intensity, Ktip, under monotonicloading. As shown by McMeeking and Evans [9], thecrack-tip stress intensity factor range under cyclic load-ing can be estimated by the crack tip stress intensity atthe mean stress level when the stress ratio, R, is zero.Since the stress ratio used in our study is close to zero(R=0.1), the following relationship due to McMeekingand Evans [9] was used to estimate the effective stressintensity factor range, DKtip:

DKtip=2Ktip�Ds

2�

(16)

The DKtip calculated with Eq. (16) was comparable tothe applied stress intensity factor range, DKapp. Thesimilarities between the fatigue crack growth rates inthe monolithic Ti–6Al–4V alloy and the Ti–6Al–4V–0.5B in-situ composite may therefore be explained bythe very small extent of shielding due to bridging. Thesmall effect of crack bridging may also be offset by theeffects of deformation restraint that can be promotedby the second phase reinforcements. These may result

Fig. 4. Typical fatigue fracture modes. (a) Overload failure region in Ti–6Al–4V–0.5B showing distinct ductile dimple fracture mode. (b) fatiguedregion in Ti–6Al–4V–0.5B (striations are not as clearly visible as they are in Ti–6Al–4V–0.5B matrix in (c). (c) fatigue striations clearly visiblein Ti–6Al–4V–0.5B matrix alloy.

S. Dubey et al. / Materials Science and Engineering A266 (1999) 303–309308

Fig. 5. (a) Crack-tip TEM micrograph of Ti–6Al–4V–0.5B after being subjected to cyclic loading (most of the dislocations appear to be in thea region). (b) Slip transmission between two a laths across b phase (the arrows mark the parts of the dislocation loop in the different laths—thedifference in contrast to the parts of the loop is probably due to misorientation between the a laths. (c) Dislocation network in the a regions(dislocations are long and wavy—slip bands were not observed).

in a slight increase in the extent of damage, and hencethe overall composite fatigue crack growth rates are notsignificantly different from those in the matrix alloy.

5. Conclusions

1. Elastic bridging of the crack by the TiB whiskerswas observed to occur under cyclic loading. How-ever, the toughening due to crack bridging wasrelatively small, lb�1.02–1.05, but the shieldingcontribution due to this bridging was probably off-set by the deformation restraint due to the presenceof the stiff second phase TiB whiskers. Hence, crackbridging does not appear to have a strong effect onthe fatigue crack growth resistance of the Ti–6Al–4V–0.5B composite.

2. Crack-tip TEM studies indicate that most of thedeformation is confined in the a phase during crack-tip deformation under cyclic loading. The disloca-

tions observed were long and wavy. There was alsoevidence of slip transmission between the a lathsacross the b phase. No evidence of plasticity wasobserved in the TiB whiskers.

3. SEM studies of the fracture surfaces indicate aductile transgranular fracture mode in the fatigueregime and a ductile dimpled fracture mode in theoverload region. Fatigue striations are not as clearlyvisible in the composite as those on the fracturesurfaces of the matrix Ti–6Al–4V alloy.

Acknowledgements

The research was funded by The Boeing Corporationand The Division of Materials Research of The Na-tional Science Foundation. The authors are grateful tothe NSF Program Monitor, Dr Bruce MacDonald, forhis encouragement and support.

S. Dubey et al. / Materials Science and Engineering A266 (1999) 303–309 309

Fig. 6. A schematic illustration of crack bridging by TiB whiskers (a)and the bridging stress distribution in the wake of the crack-tip (b).

Fig. 7. A schematic illustration of debonding along the matrix/TiBwhisker interface. At the end of the bridging zone, the maximumcrack opening is equal to the displacement in the TiB whiskercorresponding to its fracture stress.

Fig. 8. A typical stress–strain curve for the Ti–6Al–4V matrix alloy..

in-situ titanium matrix composites, in: J.J. Lewandowsk, W.Himt (Eds.), Proceedings of the Symposium on Intrinsic andExtrinsic Fracture Mechanisms in Discontinuously ReinforcedComposites, The Metallurgical Society, Warrendale, PA, 1995,pp. 167–181.

[4] W.O. Soboyejo, R.J. Lederich, S.M.L. Sastry, Fatigue and frac-ture of in-situ composites, in: S.K. Das, C.P. Ballard, F. Marikar(Eds.), Proceedings of High Performance Composites for the1990’s, The Metallurgical Society, Warrendale, PA, 1991, pp.127–141.

[5] R.J. Lederich, W.O. Soboyejo, T.S. Srivatsan, Preparing dam-age-tolerant titanium–matrix composites, J. Metals 46 (11)(1994) 68–71.

[6] H.J. Rack, P. Ratnaparkhi, Damage tolerance in discontinuouslyreinforced metal–matrix composites, J. Metals 40 (11) (1988)55–57.

[7] T. Saito, T. Furuta, T. Yamaguchi, Development of low costtitanium alloy matrix composite, recent advances in titaniummetal matrix composites, Proc. of the 1994 MRS Fall Meeting,Rosemont, IL, Oct. 2–6 1994, pp. 33.

[8] J.K. Shang, R.O. Ritchie, Monotonic and cyclic crack growth ina TiC-particulate-reinforced Ti–6Al–4V metal–matrix com-posite, Scripta Metall. 24 (1990) 1691–1694.

[9] J.W. Hutchinson, H.M. Jensen, Mech. Mater. 9 (1990) 139–163.[10] R.M. McMeeking, A.G. Evans, Mech. Mater. 9 (1990) 217–227.[11] M.F. Ashby, F.J. Blunt, M. Bannister, Acta Metall. 37 (1989)

(1847) 1857.[12] K.S. Chan, Acta Metall. 41 (1993) 761–768.[13] P.F. Becher, Crack bridging processes in toughened ceramics, in:

S.P. Shah (Ed.), Toughening Mechanisms in Quasi-Brittle Mate-rials, Kluwer, Netherland, 1933, p. 1991.

[14] J.R. Rice, in: H. Liebowitz (Ed.), Mathematical Analysis in theMechanics of Fracture, in Fracture, vol. II, Academic Press,New York, 1986.

[15] B. Budiansky, J.W. Hutchinson, A.G. Evans, Matrix fracture infiber-reinforced ceramics, J. Mech. Phys. Solids 34 (2) (1986)167–189.

References

[1] F.H. Froes, D. Eylon, H.B. Bomberger, Titanium Technology:Present Status and Future Trends, The Titanium DevelopmentAssociation, Boulder, CO, 1985.

[2] W.O. Soboyejo, R.J. Lederich, S.M.L. Sastry, Acta Metall.Mater. 42 (1994) 2579–2591.

[3] W.O. Soboyejo, R.J. Lederich, T.S. Srivatsan, K. Reece, Me-chanical properties of damage tolerant TiB whisker-reinforced