Flow behavior and deformation mechanism in the isothermalcompression of the TC8 titanium alloy

K. Wang, M.Q. Li n

School of Materials Science and Engineering, Northwestern Polytechnical University, Xi'an 710072, China

a r t i c l e i n f o

Article history:Received 16 January 2014Received in revised form31 January 2014Accepted 3 February 2014Available online 12 February 2014

Keywords:Flow behaviorKinetic analysisDynamic recoveryDynamic recrystallization

a b s t r a c t

The TC8 titanium alloy was isothermally compressed at the deformation temperatures ranging from820 1C to 980 1C, strain rates of 10 s�1, 30 s�1 and 50 s�1, and a height reduction of 60%. An opticalmicroscope (OM) and a transmission electron microscope (TEM) were used to examine the micro-structure. The flow stress decreases with the increasing of deformation temperature and decreasing ofstrain rate. The strain rate sensitivity exponent m increases gradually to a maximum value as thedeformation temperature increases from 820 1C to 940 1C, and then decreases at the deformationtemperature of 980 1C. The strain hardening exponent n decreases with the decreasing of deformationtemperature, and gets a maximum value at the strain rate of 30 s�1 and a given deformationtemperature. According to the microstructure examination, the variation of flow stress, m and n valuesare found to depend on the phase transformation, grain morphology, dislocation content, dynamicrecovery (DRV) and dynamic recrystallization (DRX) of primary α and β phases. The apparent activationenergy for deformation is 429.766780.394–383.478778.734 kJ mol�1, and indicates that the disloca-tion climbing is not the main deformation mechanism. This deduction agrees well with the micro-structure examination which shows that the DRX of primary α phase and β phase play an important rolein the isothermal compression of TC8 titanium alloy.

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

Hot deformation of titanium alloys in the two phase region isassociated with two major requirements: (1) the production ofusable shapes through primary working (ingot breakdown);(2) the optimization of mechanical properties through microstruc-ture control. However, titanium alloy is not easy to be deformed athigh temperature because the resistance of deformation is sensi-tive to temperature, which results in the narrow temperaturerange of hot deformation. It is therefore important to gain a goodinsight into the flow behavior and deformation mechanism duringhot deformation so as to optimize the processing parameters andcontrol the microstructure of titanium alloys.

Many investigators studied the flow behavior of different kindsof titanium alloy, and found that the flow stress is very sensitive tothe processing parameters [1–6]. It is well known that the strainrate sensitivity exponent m is an important parameter in deter-mining the tensile ductility of superplastic material and related tothe deformation mechanism [7,8], and the strain hardeningexponent n is an important parameter in controlling the amount

of uniform plastic strain which the material can undergo beforestrain localization, necking and failure [9]. In order to study theflow ability and the dependence of flow stress on the processingparameters, many investigations focused on the variation of m andn values in the hot deformation of titanium alloy. Some investi-gators pointed out that the m and n values were sensitive to theprocessing parameters [10,11]. Luo et al. [12,13] studied thevariation of m and n values in the isothermal compression ofTi60 alloy and Ti–6Al–4V alloy, and found that the dependence ofm and n values on the processing parameters was related to theβ phase content and primary α grain size. Huang et al. [14] studiedthe flow behavior in the hot deformation of Ti–24Al–14Nb–3V–0.5Mo alloy, and suggested that the grain boundary slipped as them values exceed 0.3. Meanwhile, the apparent activation energyfor deformation is an important parameter for the kinetic analysisin the high temperature deformation of titanium alloy. Wanjaraet al. [15] and Jia et al. [16] pointed out that the dislocationclimbing was the main deformation mechanism if the apparentactivation energy for deformation is close to that for self-diffusion.However, it is necssary to further analyze the relationship betweenflow behavior and microstructure evolution, such as dynamicrecovery (DRV) and dynamic recrystallization (DRX).

The TC8 titanium alloy (corresponding to Russia titanium alloyBT8) is a kind of α/β titanium alloy which has been widely used in

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Materials Science & Engineering A

http://dx.doi.org/10.1016/j.msea.2014.02.0020921-5093 & 2014 Elsevier B.V. All rights reserved.

n Corresponding author. Tel.: þ86 29 88460328; fax: þ86 29 88492642.E-mail address: honeymli@nwpu.edu.cn (M.Q. Li).

Materials Science & Engineering A 600 (2014) 122–128

the aeronautical industries due to its highly attractive properties,such as high strength, low density, high toughness and goodhigh-temperature properties. In the recent investigations, theauthors [17] studied the precipitation mechanism of the secondaryα phase in the TC8 titanium alloy compressed in the αþβ phasefield. However, there are few reports focusing on the deformationbehavior in the hot deformation of TC8 titanium alloy.

In practice, the minimum value of strain rate in the forgingof titanium alloy is 10 s�1, so this work selected the strain rate as10–50 s�1 for the isothermal compression of TC8 titanium alloy inthe two phase region. An optical microscope (OM) and a transmis-sion electron microscope (TEM) were used to observe the micro-structure. The flow behavior, including flow stress–strain curve,strain rate sensitivity exponent, strain hardening exponent and

kinetic analysis, was investigated with the help of the microstruc-ture examination of TC8 titanium alloy.

2. Experimental procedures

The TC8 titanium alloy used in this study was received in theform of a rod with a diameter of 25 mm and with the chemicalcomposition (wt%) of 6.5 Al, 3.3 Mo, 0.3 Si, 0.06 Fe, 0.01 C, 0.002 H,0.075 O, 0.005 N, and a balance of Ti. The microstructure of the as-received TC8 titanium alloy is shown in Fig. 1. The β transustemperature is measured to be 1000 1C [17]. Cylindrical specimensthat were 8.0 mm in diameter and 12.0 mm in height weremachined from the TC8 titanium alloy rod. The alloy was iso-thermally compressed on a Gleeble 3500 thermal simulator at thedeformation temperatures ranging from 820 1C to 980 1C with aninterval of 40 1C, the strain rates of 10 s�1, 30 s�1and 50 s�1, and aheight reduction of 60%. Before compression, the specimens wereheated to the deformation temperature and held for 3 min toestablish a uniform temperature. After compression, the speci-mens were cooled in wind to room temperature.

To observe the microstructure evolution, the compressed speci-mens were sectioned along the compression axis and prepared formicrostructure examination by standard metallographic techni-ques. For the OM examinations, the sectioned specimen wasprepared following standard grinding/polishing procedures andetched in a solution of 5% HF, 15% HNO3, and 80% H2O. For the TEMexamination, the sectioned specimen was ground to 60–80 μmfollowed by twin-jet electropolishing. An OLYMPUS GX71 OM anda Tecnai F30 G2 TEM were used to examine the microstructure.Fig. 1. Microstructure of the as-received TC8 titanium alloy.

0

100

200

300

400

500

600

980

940

900

860

820

Strain

Flow

stre

ss/M

Pa

0

100

200

300

400

500

600

980

940900

860

820

Strain

Flow

stre

ss/M

Pa

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.80

100

200

300

400

500

600

980

940

900

860

820

Strain

Flow

stre

ss/M

Pa

Fig. 2. Flow stress–strain curves in the isothermal compression of TC8 titanium alloy at the strain rates of: (a) 10 s�1, (b) 30 s�1, and (c) 50 s�1.

K. Wang, M.Q. Li / Materials Science & Engineering A 600 (2014) 122–128 123

3. Results and discussion

3.1. Flow stress–strain curves

Fig. 2 shows a series of flow stress-–strain curves in theisothermal compression of TC8 titanium alloy at different strainrates. It can be seen from Fig. 2 that the flow stress decreases withthe increasing of deformation temperature. This phenomenon canbe interpreted by the microstructure observation. Fig. 3 shows theeffect of deformation temperature on the content and grainmorphology of primary α phase and β phase of TC8 titanium alloyisothermally compressed at a strain rate of 10 s�1. As shown inFig. 3, the β phase content increases with the increasing ofdeformation temperature. The body centered cubic (bcc) β phasehas more operative slip systems and lower strength than hexago-nal close-packed (hcp) α phase, so the increasing of β phasecontent induces the decreasing of flow stress. Meanwhile, thedecreasing of flow stress is related to the variation of dislocation inprimary α phase with the increasing of deformation temperature.Fig. 4 shows the TEM observation of TC8 titanium alloy isother-mally compressed at a strain rate of 30 s�1 and different deforma-tion temperatures. As shown in Fig. 4, the dislocation content inthe primary α phase decreases with the increasing of deformation

temperature. This phenomenon can be due to that the increasingof deformation temperature enhances the thermal activation ofatoms and makes the dislocation moves more easily, and thenresults in that the deformation of TC8 titanium alloy need fewerdislocations. So the decreasing of dislocation content in theprimary α phase plays an important role in the decreasing of flowstress with the increasing of deformation temperature.

It can be also seen from Fig. 2 that the flow stress increasessharply to a peak value with the increasing of strain at thebeginning of deformation, and then decreases with the furtherincreasing of strain. The hot deformation of titanium alloy is oftena competition process between the work-hardening and dynamicsoftening. The propagation and piling of the dislocations result inthe work-hardening effect. The work hardening effect is muchstronger than the dynamic softening effect at the beginning ofdeformation. After the flow stress reaches a peak value, thedynamic softening effect becomes superior to the work hardeningeffect, which results in that the flow stress decreases with theincreasing of strain in the isothermal compression of TC8 titaniumalloy. Meanwhile, Fig. 2 shows that the flow stress decreases withthe decreasing of strain rate. In order to further study the flowability and dependence of flow stress on the processing para-meters, it is necessary to calculate the strain rate sensitivity

Fig. 3. Typical microstructure at the strain rate of 10 s�1, height reduction of 60% and different deformation temperatures of: (a) 820 1C, (b) 860 1C, (c) 900 1C, (d) 940 1C,and (e) 980 1C.

K. Wang, M.Q. Li / Materials Science & Engineering A 600 (2014) 122–128124

exponent and strain hardening exponent in the isothermal com-pression of TC8 titanium alloy, which are discussed as follows.

3.2. Strain rate sensitivity exponent

Based on the flow stress data in isothermal compression of TC8titanium alloy, the strain rate sensitivity exponent m at a givenstrain and deformation temperature could be calculated in the

following form [18]:

m¼ ∂ ln s∂ ln _ε

����ε;T

ð1Þ

where m is the strain rate sensitivity exponent, s is the flow stress(MPa), _ε is the strain rate (s�1), ε is the strain and T is the absolutedeformation temperature (K).

Fig. 5 shows the variation of m values with deformationtemperature in the isothermal compression of TC8 titanium alloy.As shown in Fig. 5, the m values increase gradually to a maximum

Fig. 4. TEM micrographs of isothermally compressed TC8 tatanium alloy at a strain rate of 30 s�1, height reduction of 60% and different deformation temperatures of:(a) 820 1C, (b) 860 1C, (c) 900 1C, (d) 940 1C, and (e) 980 1C.

K. Wang, M.Q. Li / Materials Science & Engineering A 600 (2014) 122–128 125

value as the deformation temperature increase from 820 1C to940 1C, and then it decreases at the deformation temperature of980 1C. The effect of deformation temperature on the m values canbe explained based on the microstructure examination. As shownin Fig. 3, the β phase content increases with the increasing ofdeformation temperature. Meanwhile, it can be seen from Fig. 3that the primary α grain shows an elongated morphology asthe deformation temperature is not more than 900 1C, and theprimary α and β grains become equiaxed and small-sized as thedeformation temperature increases to 940 1C. The m value repre-sents the capacity of material to resist necking and affects theoverall deformation and stability in hot deformation [19]. It isbelieved that the appropriate increasing of β phase content canenhance the deformation stability, and the finer grain can result ina more uniform deformation. Therefore, the increasing of β phasecontent and refining of primary α and β grains strengthen thestability of high temperature deformation, and then result in theincreasing of m values of TC8 titanium alloy. However, as shown inFig. 3, the primary α phase content becomes very small at thedeformation temperature of 980 1C, so the hindering effect ofβ grain weakens, and then causes the coarsening of β grain.The coarsening of β grain restrains the uniform deformation, sothe m values decrease as the temperature increases from 940 1C to980 1C.

Stüwe and Les [20] suggested that the strain rate sensitivityexponent m of material is supposed to be determined by thermalactivation of dislocation slipping. It is well known that the clusterof dislocations could result in the stress concentration, and thenreduce the deformation stability and m value of high temperaturedeformation. As mentioned in Fig. 4, the dislocation contentdecreases apparently with the increasing of deformation tempera-ture. The decreasing of dislocation content in primary α phaserelieves the stress concentration, so plays an important role inthe increasing of m values with the increasing of deformationtemperature.

3.3. Strain hardening exponent

The strain hardening exponent n results from a competitionbetween hardening mechanisms and softening mechanisms, anddepends mainly on the strain and time respectively [20]. Inpresent work, the strain hardening exponent is calculated in thefollowing form [18]:

n¼ ∂ ln s∂ ln ε

����_ε;T

ð2Þ

where s is the flow stress (MPa), ε is the strain, _ε is the strain rate(s�1), and T is the absolute deformation temperature (K). Based onthe flow stress data in the isothermal compression of TC8 titaniumalloy, the n values are calculated at a given strain rate anddeformation temperature.

Fig. 6 shows the variation of n values with deformationtemperature and strain rate in the isothermal compression ofTC8 titanium alloy. It can be seen from Fig. 6 that the n values areabout zero at the deformation temperature of 940 1C and 980 1C,and decrease with the decreasing of deformation temperature, andreach to a negative value at the deformation temperature below940 1C. This phenomenon indicates that the softening effect playsan important role and becomes more apparent with the decreas-ing of deformation temperature in the isothermal compression ofTC8 titanium alloy. The variation of n values with the deformationtemperature can be interpreted by microstructure examination. Asshown in Fig. 4, the dislocation content increases with thedecreasing of deformation temperature. Meanwhile, it can be seenfrom Fig. 4 that the DRX and DRV occur in the primary α phase andinduce the formation of subgrain and DRX grain at the lower

temperature, while only the DRV occurs in the primary α phaseand induces the formation of subboundaries at the higher tem-perature. Subsequently, the formation of DRX grain and subgrainreduces the dislocation content more apparently, so the softeningeffect expresses more apparently and the n values become smallerat the lower deformation temperature.

It is worth noted that a large of equiaxed β grain appears in theβ phase at the deformation temperatures of 940 1C and 980 1C, andindicates the occurrence of DRX in β phase. However, the n valuesat the deformation temperatures of 940 1C and 980 1C is near thezero, and is much higher than that at the deformation tempera-tures below 940 1C where no DRX occurs in the β phase. Thisphenomenon suggests that the softening effect resulted from DRXof β phase is weaker than that resulted from DRX and DRV ofα phase, so the DRX of β phase does not result in the apparentdecreasing of the n values in the isothermal compression of TC8titanium alloy at the deformation temperatures of 940 1C and980 1C.

It can be also seen from Fig. 6 that the n values reach to amaximum value at the strain rate of 30 s�1 at a given deformationtemperature. This is possibly because that the lower strain rate(10 s�1) can provide longer deformation period and the higherstrain rate (50 s�1) can accumulate higher dislocation content. Thelonger deformation period and higher dislocation content canpromote dislocation annihilation by the DRV or DRX, so thedislocation annihilation occurs more slowly at the strain rate of30 s�1 than that at 10 s�1 and 50 s�1. As seen from Fig. 7, thedislocation content at the strain rate of 50 s�1 is much higher thanthat at the strain rate of 30 s�1, and some subgrain and DRX grainsappear at the strain rate of 50 s�1 in the isothermal compressionof TC8 titanium alloy. So the high dislocation content makes the

800 840 880 920 960 10000.00

0.04

0.08

0.12

0.16

0.20

0.24

0.28

Stra

in ra

te s

ensi

tivity

exp

onen

t

Deformation temperature/

Strain: 0.4 0.5 0.6 0.7

Fig. 5. Strain rate sensitivity exponent m in the isothermal compression of TC8titanium alloy.

10 20 30 40 50-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

Strain rate/s-1

Stra

in h

arde

ning

exp

onen

t

940 980 1030

820 860 900

Deformation temperature

Fig. 6. Strain hardening exponents n in the isothermal compression of TC8titanium alloy.

K. Wang, M.Q. Li / Materials Science & Engineering A 600 (2014) 122–128126

flow stress increase, and induces the formation of subgrain andDRX grain, which reduces the dislocation content and flow stressmore apparently. So the softening effect at 10 s�1 and 50 s�1 isstronger than that at 30 s�1, and the strain hardening exponent nvalue reaches to a maximum value at the strain rate of 30 s�1 anda given deformation temperature.

3.4. Kinetic analysis

The hot deformation behavior of titanium alloy in two phaseregion is complicated because it includes the concurrent deforma-tion process of α and β phases as well as the phase transformationfrom α to β phase. Calculation of some kinetics parameters is usefulto understand the mechanisms of hot deformation. The apparentactive energy for deformation describes the activation barrier thattransition of atom requires to overcome and represents the work-ability of alloys [21]. The temperature and strain rate dependenceof flow stress in hot deformation is generally expressed in terms ofa kinetic rate equation [22] given by

_ε¼ Asn1 exp � QRT

� �ð3Þ

where _ε is the strain rate (s�1), Q is the apparent activation energyfor deformation (J mol�1), s is the peak flow stress (MPa), n is thestress exponent related to the strain rate sensitivity exponent m,

n1 ¼ 1=m, T is the absolute deformation temperature (K), and R isthe gas constant (8.3145 J mol�1 K�1).

Taking logarithm of both sides of Eq. (3) and then partialdifferentiating, the apparent activation energy Q at a given strainrate can be obtained from Eq. (3) as follows:

Q ¼ n1R∂ ln s∂ð1=TÞ

����_ε

¼ Rm

∂ ln s∂ð1=tÞ

����_ε

� Rm

Δ ln sΔð1=TÞ

����_ε

ð4Þ

the m value can be calculated according to Eq. (1).Fig. 8 shows the relationship between the flow stress and

deformation conditions in the isothermal compression of TC8titanium alloy at a strain of 0.4. It can be seen from Figs. 8(a)and 5 that the m value at the deformation temperature of 820 1C islargely different from that at other temperatures. Because of thesharp decreasing of m value at the deformation temperature of820 1C, it can be concluded that the deformation mechanism at adeformation temperature of 820 1C is different with that at thetemperatures ranging from 860 1C to 980 1C. For the accuracy ofthe kinetic analysis in the isothermal compression of TC8 titaniumalloy, the present work just deals with the kinetic analysis by Eq. (4)at the temperatures ranging from 860 1C to 980 1C. The averageΔ ln s=Δð1=TÞ value at the temperatures ranging from 860 1C to980 1C can be obtained by linear regression of data in Fig. 8(b).Therefore the apparent activation energy Q for deformation ofisothermally compressed TC8 titanium alloy at a strain of 0.4 can becalculated to be 429.77780.4 kJ mol�1 according to Eq. (4).

4.5

5.0

5.5

6.0 820 860 900 940 980

ln(F

low

stre

ss/M

Pa)

ln(strain rate/s-1)

2.0 2.5 3.0 3.5 4.0 4.5 0.80 0.82 0.84 0.86 0.884.6

4.8

5.0

5.2

5.4

5.6

5.8

6.0

ln(F

low

stre

ss/M

Pa)

1000/T/K

10 s-1

30 s-1

50 s-1

Fig. 8. Relationship between the flow stress and deformation conditions in the isothermal compression of TC8 titanium alloy at the strain of 0.4.

Fig. 7. TEM micrograph in the isothermal compression of TC8 titanium alloy at the deformation temperature of 900 1C and strain rates of: (a) 30 s�1 and (b) 50 s�1.

K. Wang, M.Q. Li / Materials Science & Engineering A 600 (2014) 122–128 127

In the same way, the apparent activation energy for deforma-tion of isothermal compressed TC8 titanium alloy at every straincan be calculated as shown in Fig. 9. It can be seen from Fig. 9 thatthe apparent activation energy for deformation decreases from429.77780.4 kJ mol�1 to 383.48778.73 kJ mol�1 as the strainincreases from 0.4 to 0.7, which indicates that the softening effectplays an important role at the strains ranging from 0.4 to 0.7. TheseQ values are much higher than that for self-diffusion of α-Ti(169 kJ mol�1) and β-Ti (153 kJ mol�1) [23], so it seems to ruleout the rate controlling mechanism by dislocation climbing in theisothermal compression of TC8 titanium alloy. The results ofmicrostructure examination (Figs. 3 and 4) indicates that DRX ofprimary α phase and DRV of β phase play an important role at thedeformation temperature below 900 1C, while DRV of primaryα phase and DRX of β phase play an important role at thedeformation temperature above 900 1C. Therefore, the deductionfrom the apparent activation energy for deformation agrees wellwith the microstructure examination.

4. Conclusions

The effect of processing parameters on the flow stress, strainrate sensitivity exponent, strain hardening exponent and kineticanalysis was studied in the isothermal compression of TC8titanium alloy. The deformation mechanism is discussed wellbased on the microstructure examination. The following conclu-sions are drawn from the present investigation:

(1) The flow stress decreases with the increasing of deformationtemperature due to the increasing of β phase content anddecreasing of dislocation content in the primary α phase.

(2) The strain rate sensitivity exponent m increases gradually to amaximum value as the deformation temperature increasesfrom 820 1C to 940 1C due to the increasing of β phase content,decreasing of dislocation content in primary α phase, andrefining of primary α and β grains. Them value decreases at thedeformation temperature of 980 1C due to the coarsening ofβ grain.

(3) The strain hardening exponent n decreases with the decreas-ing of deformation temperature and gets a maximum value ata strain rate of 30 s�1 and a given deformation temperature.The DRX and DRV of primary α phase play a more importantrole in the softening effect than the DRX of β phase.

(4) The apparent activation energy for deformation decreasesfrom 429.766780.394 kJ mol�1 to 383.478778.734 kJ mol�1

as the strain increases from 0.4 to 0.7, which suggests that thedislocation climbing is not like the main deformation mechan-ism. This deduction agrees well with the microstructureexamination.

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0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75250

300

350

400

450

500

550

Strain

App

aren

t act

ivat

ion

ener

gy/k

J m

ol-1

Fig. 9. Variation of the apparent activation energy for deformation in theisothermal compression of TC8 titanium alloy.

K. Wang, M.Q. Li / Materials Science & Engineering A 600 (2014) 122–128128