Materials Science & Engineering A 582 (2013) 108–116

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The microstructure evolution in the isothermal compressionof Ti-17 alloy

Z. Mu a,b, H. Li a, M.Q. Li a,n

a School of Materials Science and Engineering, Northwestern Polytechnical University, Xi'an 710072, PR Chinab Beijing Aeronautical Materials Institute, Aviation Industry Corporation of China (AVIC), Beijing 100095, PR China

a r t i c l e i n f o

Article history:Received 2 November 2012Received in revised form31 May 2013Accepted 13 June 2013Available online 21 June 2013

Keywords:Ti-17 alloyIsothermal compressionDeformation behaviorMicrostructure

93/$ - see front matter & 2013 Elsevier B.V. Ax.doi.org/10.1016/j.msea.2013.06.043

esponding author. Tel.: +86 29 88460328.ail address: honeymli@nwpu.edu.cn (M.Q. Li).

a b s t r a c t

The effects of processing parameters on flow curves and microstructure evolution in the isothermalcompression of Ti-17 alloy are investigated. The experiments are conducted in a deformation tem-perature range of 800–920 1C, strain rate range of 0.01–10.0 s−1 and height reduction range of 30–70%.The flow stress increases with a decrease in deformation temperature or an increase in strain rate. Basedon the optical microstructure observations, both of deformation temperature and strain rate have a greateffect on the volume fraction of α grains. The effect of strain rate on primary α grains morphology andsoftening mechanism is of first-order importance compared with the effects of deformation temperatureand strain. Transmission electron microscope (TEM) analysis is carried out at a deformation temperatureof 840 1C, strain rate of 0.1 s−1 and 10.0 s−1 to reveal the two mechanisms for the morphology evolution ofprimary α grains. When the strain rate is relatively low, the primary α grains are globularized after thepenetration of β phase along α/α subboundaries. While the strain rate is relatively high, the primary αgrains are elongated, and the dislocation density in grain interior is relatively high.

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1. Introduction

Titanium alloys are widely used in the aerospace industry dueto their low density, high strength and excellent high-temperatureperformances. However, it is more difficult to process titaniumalloys comparing with aluminum alloys and steel, and the proces-sing window for titanium alloys is quite narrow [1–3]. Therefore,a large amount of efforts have been devoted to investigate theeffects of processing parameters on deformation behavior andmicrostructure evolution [4–8]. As a typical “beta-rich” α+β tita-nium alloy, Ti-17 alloy exhibits outstanding characteristics, such asexcellent corrosion resistance, superior fracture toughness andhigh hardenability. It has been widely used to fabricate jet engineand compressor components [9,10]. Some efforts have beendevoted to investigate the mechanical properties of Ti-17 alloy.Ning et al. [11] and Li et al. [12] investigated the hot deformationbehavior of Ti-17 alloy by using processing maps and the apparentactivation energy for deformation has been calculated. Zhao et al.[13] investigated the deformation behavior of Ti-17 powdercompact, and found that the flow behavior was sensitive to strainrate. However, the effects of processing parameters on the

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microstructure evolution are not analyzed in the above mentionedliteratures.

With the developments of modern industry, the application oftitanium alloys becomes much wider and the demands for highstrength, high fracture toughness and low crack growth rate becomemore critical. Therefore, alternative routes have been explored.The modification in chemical composition is one way to improvethe comprehensive performance [14,15]. Meanwhile, the control ofmicrostructure by optimizing the processing parameters is also aneffective route. It is well known that microstructure has a great effecton the mechanical properties of titanium alloys. The fully lamellarmicrostructure enables the alloy to obtain good creep resistance andhigh fracture toughness, while fine grain size and equiaxed micro-structure usually exhibits excellent fatigue properties. Therefore, theinvestigation on microstructure evolution is very important for themechanical property improvement of titanium alloys. Weiss et al.[16] investigated the microstructure evolution of Ti–6Al–4V and themodification of α grains morphology during isothermal compression.Jia et al. [17] investigated the effect of processing parameters onmicrostructure evolution of Ti60 alloy and found that the lamellarprimary α grains were distorted, broken up and globularized in theα+β two phase region. Wang et al. [10] analyzed the dynamicglobularization kinetics of Ti-17 alloy and proposed a predictionmodel for describing the non-linear relationship between thedynamic globularization fraction and the processing parameters.

Z. Mu et al. / Materials Science & Engineering A 582 (2013) 108–116 109

The present research is motivated to investigate the micro-structure evolution of Ti-17 alloy in the isothermal compression.The effects of processing parameters, e.g. deformation tempera-ture, strain rate and strain, on flow curves and microstructure areexamined in both α+β two phase region and β single phase region.Meanwhile, the variation of primary α phase morphology ofisothermally compressed Ti-17 alloy with strain rate is analyzedin depth.

2. Material and experiment procedures

2.1. Material

The Ti-17 alloy (Ti–5.0Al–2.3Sn–2.0Zr–4.0Mo–4.0Cr, wt%) usedin this research was supplied in the form of forged bar with adiameter of 30.0 mm. The β transformation temperature wasmeasured to be approximately 895 1C by metallographic observa-tions. The initial microstructure of Ti-17 alloy parallel to compres-sion axis is illustrated in Fig. 1. As shown in Fig. 1, there areapproximately 15% equiaxed and elongated primary α grainswithin the β matrix, and the primary α grains locate at the β grainboundaries homogeneously. Since the received material has everbeen hot forged, it can be seen in Fig. 1 that elongated primary αgrains are along the vertical direction.

2.2. Experimental procedures

Cylindrical specimens of 8.0 mm in diameter and 12.0 mm inheight were electro-discharged machined from the forged bar.

Fig. 1. Initial microstructure of as-received Ti-17 alloy.

Fig. 2. Flow curves of the Ti-17 alloy isothermally compressed

High temperature compression tests were conducted on a Gleeble-1500 thermal simulator. The deformation temperatures are 800 1C,840 1C, 880 1C, 900 1C, 920 1C, strain rates are 0.01 s−1, 0.1 s−1,1.0 s−1, 10.0 s−1 and height reductions are 30–70% with a 10%interval. Thermocouples were welded at the middle height ofthe specimens to measure the actual temperature of specimens.Specimens were firstly heated to elevated temperatures with aheating rate of 20 1C/s, homogenized for 3 min and then com-pressed at a constant strain rate. The load–displacement data inisothermal compression of Ti-17 alloy was collected by a dataacquisition system and then was converted to the flow stress–strain curves. After isothermal compression, the specimens wereair cooled to room temperature.

To observe the microstructure evolution, the deformed speci-mens were sectioned parallel to the compression axis and the cutsurfaces were prepared for metallographic observation usingstandard procedures. The polished samples were etched with asolution of 10% HNO3+15% HF+75% H2O and microstructures at thecentral area of each specimen were observed on an OLYMPUSGX71 optical microscope. The microstructure variables, such asvolume fraction and grain size, were measured using quantitativemetallographic image analysis software (Image-pro plus 6.0).In order to minimize the statistical error of calculated α phasevolume fraction and grain size, at least three micrographs weremeasured for the specimens at the same deformation parameters.The ratio between α phase area and measured area is taken as theα phase volume fraction. TEM samples mechanically thinned to50–70 μm were prepared. The observation of samples was carriedout on a Tecnai F30 G2 FEG transmission electron microscope withan applied voltage of 300 kV after jet polishing by a double-jetpolisher.

3. Experimental results and analysis

3.1. Flow stress

Fig. 2 shows the typical flow curves of Ti-17 alloy isothermallycompressed at different strain rates, respectively. It can be seen inFig. 2(a) that the shape of flow curves of isothermally compressedTi-17 alloy is apparently different at various strain rates in the α+β twophase region. The flow curves exhibit typical steady-state featurewhen the strain rate is lower than or equal to 0.1 s−1. The flow stressdecreases slightly after a peak value, then it remains nearly constant asthe strain increases. This steady-state feature of flow curves indicatesthat the effect of dynamic softening is sufficiently fast to cancel outthat of work hardening in the isothermal compression. But the flowstress exhibits continuous flow softening feature after the peak valuewhen the strain rate is higher than 1.0 s−1, it might result from thedeformation heat in isothermal compression. Most of the deformation

at deformation temperatures of: (a) 840 1C and (b) 920 1C.

Fig. 3. (a) Flow curves of the Ti-17 alloy isothermally compressed at a strain rate of 0.1 s−1and (b) peak flow stress of the Ti-17 alloy isothermally compressed at differentdeformation temperatures.

Fig. 4. Variation of α phase volume fraction with deformation temperature of Ti-17alloy at a strain rate range of 0.01–10.0 s−1.

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energy will convert into deformation heat when Ti-17 alloy isisothermally compressed, and the deformation heat will make theactual deformation temperature of specimens rise, which may induceinstability flow, such as flow localization and adiabatic shear band.Consequently, the flow stress decreases gradually and the flow curvesexhibit continuous flow softening feature. This can be validated bythe microstructure observation in Section 3.2.2. When the isothermalcompression was conducted in the β single phase region (Fig. 2(b)), allof the flow curves exhibit steady-state feature at strains exceeding 0.6.

Fig. 3 shows the typical flow curves of Ti-17 alloy isothermallycompressed in the deformation temperature range of 800–920 1Cand strain rate 0.1 s−1. As illustrated in Fig. 3(a), the deformationtemperature also has a great effect on the shape of flow curves.Continuous flow softening feature of flow curves is observed at adeformation temperature of 800 1C, while steady-state feature isobserved at a deformation temperature of 920 1C. Meanwhile, itcan be seen in Fig. 3(b) that the peak flow stress decreases rapidlywith an increase in deformation temperature. This is probablyattributed to the decreased α phase volume fraction and acceler-ated dislocation motion at high deformation temperatures. Thiseffect of deformation temperature on the flow stress will befurther explained in Section 3.2.1 with microstructure evidence.

3.2. Microstructure evolution

3.2.1. Effect of deformation temperatureFig. 4 shows the variation of α phase volume fraction with

deformation temperature at a strain rate range of 0.01–10.0 s−1.It can be seen that the effect of deformation temperature on the α

phase volume fraction is related to strain rate. The α phase volumefraction decreases rapidly as the deformation temperatureincreases at strain rates of 0.01 s−1 and 0.1 s−1, while it decreasesslightly or even fluctuated at a relatively high strain rates, such as1.0 s−1 and 10.0 s−1. The decreasing of α phase volume fractionmainly results from the enhanced phase transformation with anincrease in deformation temperature. However, a certain time isrequired for α-β phase transformation. A lower strain rateindicates a long deformation time, in which the phase transforma-tion can be adequately implemented. But the short deformationtime at a higher strain rate is insufficient for phase transformation.So the decreasing of α phase volume fraction is less obvious whenthe strain rate is relatively high. The rapid decreasing of α phasevolume fraction with an increase in deformation temperature at arelatively low strain rate can be validated in Fig. 5(a)–(c). Conse-quently, it is reasonable to see in Fig. 3 that the peak flow stressdecreases with an increase in deformation temperature. The αphase with close-packed hexagonal structure has less slip systemsfor deformation compared with the body-centered β phase [18],which makes it perform much harder than the β phase. Thus thepeak flow stress would be much lower at a higher deformationtemperature for there are only a few primary α grains contained.

Meanwhile, it can be seen in Fig. 5(a)–(c) that the grain size ofprimary α phase reduces and the morphology of primary α phasevaries as the deformation temperature increases owing to theeffect of motivated phase transformation process. Compared withthe initial microstructure of Ti-17 alloy in Fig. 1, most of theelongated primary α grains are kinked and the grain patterns areirregular at the deformation temperature of 800 1C. While theprimary α grains are fully equiaxed and the grain patterns becomeregular when the Ti-17 alloy are isothermally compressed atdeformation temperatures of 840 1C and 880 1C.

As the deformation temperature increases to 900 1C, above βphase transformation temperature, primary α grains disappear inFig. 5(d). The grain size of β phase at 900 1C is much larger thanthat at 880 1C in Fig. 5(c). This is because that the primary α phaseat β grain boundaries plays a role of pining to limit the growth of βgrains at the deformation temperature of 880 1C. However, thegrowth of β grains is no longer limited due to the implement ofphase transformation when Ti-17 alloy is isothermally compressedin the β single phase region, thus the grain size of β phaseincreases rapidly with an increase in deformation temperature.As illustrated in Fig. 5(d) and (e), the grain size of β phase increasesfrom 2174 μm to 3176 μm with an increase in the deformationtemperature range from 900 1C to 920 1C, much larger than1073 μm at 880 1C.

3.2.2. Effect of strain rateThe variation of α phase volume fraction with strain rates at

deformation temperatures between 800 1C and 880 1C is shown in

Fig. 5. Microstructure of the Ti-17 alloy isothermally compressed at a strain rate of 0.01 s−1, height reduction of 60% and different deformation temperatures: (a) 800 1C,(b) 840 1C, (c) 880 1C, (d) 900 1C and (e) 920 1C.

Fig. 6. Variation of α phase volume fraction with strain rate of Ti-17 alloy atdeformation temperatures between 800 1C and 880 1C.

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Fig. 6. It can be seen that the α phase volume fraction increasesrapidly with an increase in strain rate at the deformation tem-perature of 880 1C, while it increases slightly at the deformationtemperature of 840 1C. When the deformation temperature is

800 1C, the variation of α phase volume fraction with strain rateis no longer regular and the value just fluctuates between 16.63%and 23.52%. It can be concluded that the influence of strain rate onα phase volume fraction is obvious at a higher deformationtemperature but limited at a lower deformation temperature.The reason is that the rate of phase transformation is relativelyslow at a lower deformation temperature. The increasing of αphase volume fraction with an increase in strain rate can bevalidated in Fig. 7. As shown in Fig. 7, the α phase volume fractionis only 13.25% at the strain rate of 0.01 s−1, while the values reachto 15.59%, 18.92% and 19.56% when the strain rates are 0.1 s−1,1.0 s−1 and 10.0 s−1, respectively. Wanjara et al. [19] pointed outthat the phase transformation would not reach a steady state athigh strain rates. When Ti-17 alloy is isothermally compressed at arelatively low strain rate, the extent of phase transformation willbe higher due to the longer deformation time. Consequently, the αphase volume fraction of Ti-17 alloy at the strain rate of 0.01 s−1 isless than that at the strain rates of 0.1 s−1 and 10.0 s−1.

The effect of strain rate on the morphology of primary α grainsis evident as well. As shown in Fig. 7(a) and (b), the primary αgrains are almost equiaxed and the elongated primary α grains areseldom present at the strain rates of 0.01 s−1 and 0.1 s−1. It suggeststhe dynamic globularization for primary α grains at the strain rateslower than or equal to 0.1 s−1. However, some elongated primary αgrains are still remained when the strain rate increases to 1.0 s−1

Fig. 7. Microstructure of the Ti-17 alloy isothermally compressed at a deformation temperature of 840 1C, height reduction of 60% and different strain rates: (a) 0.01 s−1,(b) 0.1 s−1, (c) 1.0 s−1 and (d) 10.0 s−1.

Fig. 8. Flow localization of Ti-17 alloy isothermally compressed at a deformationtemperature of 840 1C, strain rate of 10.0 s−1 and height reduction of 60%.

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and 10.0 s−1 (Fig. 7(c) and (d)). Meanwhile, it is noteworthy thatthe elongated α grains have changed to the directions perpendi-cular to the compression axis, which is quite different from theinitial primary α grains in Fig. 1. The microstructure at a lowermagnification (Fig. 8) reveals that this phenomenon results fromthe occurrence of flow localization at the central area of speci-mens. These observations on flow localization are reasonable toexplain the rapid flow softening feature of flow curves at the strainrates higher than 1.0 s−1 in Fig. 2(a). It is well known that the flowlocalization is a kind of unstable flow which is closely connectedwith deformation heat. Owing to the low thermal conductivity oftitanium alloys, deformation heat cannot be conducted in a shorttime during the isothermal compression, thus the actual tempera-ture is much higher at the central area of specimens. Thetemperature rise would promote the phase transformation andincrease the dislocation mobility, thus the deformation resistancereduces and further deformation is preferred in these areas[20,21]. Meanwhile, the microstructure change, such as globular-ization or kinking of primary α grains, would also lower thedeformation resistance during isothermal compression [11,22].Consequently, it leads to the large deformation at the central ofspecimens, and the deformation resistance decreases greatly.Finally, the flow localization occurs and the flow stress decreases.

Fig. 9 reveals the effect of strain rate on the microstructureevolution of Ti-17 alloy isothermally compressed at a deformationtemperature of 920 1C. As it shows, the microstructure of Ti-17alloy isothermally compressed at a strain rate of 0.01 s−1 consistsof equiaxed β grains. However, the β grains are elongated perpen-dicular to the compression axis and serrated β grain boundariesare observed when the strain rate increases to 1.0 s−1. Smallrecrystallized β grains are observed at the serrated β grainboundaries, which give a necklace-appearance of small β grainsaround the elongated β phase. Apparently, the β grains at highstrain rates show a typical dynamic recrystallization characteristicand the β grains exhibit a trend to be refined. These observationssuggest that the main softening mechanisms are dynamic recrys-tallization at high strain rates and dynamic recovery at low strainrates respectively, and an increase in the strain rate would result in

a decrease in grain size of β phase in β single phase region. Similartrend was also observed in the isothermal compression of IMI834alloy [19]. These differences of softening mechanisms at differentstrain rates can be attributed to the effect of strain rate ondislocations. When the strain rate is relatively low, the rate ofdislocation annihilation is sufficient to offset dislocation multi-plication and the density of accumulated dislocations is not highenough to drive dynamic recrystallization. While at a relativelyhigh strain rate, the amount of dislocations increases rapidly in ashort time, thus the dislocation density increases to a critical valuequickly which leads to the occurrence of dynamic recrystallization.

3.2.3. Effect of strainThe effect of strain on microstructure of Ti-17 alloy isothermally

compressed at a deformation temperature of 800 1C and strain rate of

Fig. 10. Microstructure of Ti-17 alloy isothermally compressed at a deformation temperature of 800 1C, strain rate of 0.1 s−1 and different height reductions: (a) 30%, (b) 50%and (c) 60%.

Fig. 11. Microstructure of Ti-17 alloy isothermally compressed at a deformation temperature of 920 1C, strain rate of 0.1 s−1 and different height reductions: (a) 40% and(b) 70%.

Fig. 9. Microstructure of Ti-17 alloy isothermally compressed at a deformation temperature of 920 1C, height reduction of 60% and different strain rates: (a) 0.01 s−1 and(b) 1.0 s−1.

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0.1 s−1 is shown in Fig. 10. Elongated primary α grains still exist at aheight reduction of 30% and most of them distribute along thedirections parallel to compression axis. As the height reductionincreases to 50%, most of primary α grains are equiaxed and only afew elongated primary α grains are remained. While at a heightreduction of 60%, the elongated α grains disappear and all of the

primary α grains are equiaxed. It can be concluded that the morphol-ogy of primary α grains becomes equiaxed with an increase in strainwhen the Ti-17 alloy is isothermally compressed in the α+β two phaseregion. The reason is that sufficient deformation is necessary for theglobularization of primary α grains. Large deformation providesenough time for the completion of globularization and more

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deformation energy for globularization can be accumulated at a higherdeformation degree.

Fig. 11 shows the effect of strain on microstructure of Ti-17 alloyisothermally compressed at a deformation temperature of 920 1C.It can be seen that the amount of recrystallized β grains increaseswith an increase in strain. As shown in Fig. 11(a), the primary βgrains are elongated transverse to the compression axis and only asmall amount of recrystallized grains are observed along the elon-gated primary β grain boundaries at a height reduction of 40%.While the height reduction increases to 70%, a large amount ofrecrystallized β grains are observed, and the size of recrystaillized βgrains becomes larger. Therefore, it can be concluded that the deg-ree of dynamic recrystallization increases with an increase in strainwhen the compression is conducted in the β single phase region.

3.3. Discussion

As illustrated in Section 3.2.2, when the deformation tempera-ture is 840 1C, the elongated primary α grains are fully equiaxed ata relatively low strain rate while they change to the directionsperpendicular to compression axis at a relatively high strain rate.It reveals that there are two different mechanisms for themorphology evolution of primary α grains at low and high strainrates. Therefore, TEM observations have been conducted on thespecimens at a deformation temperature of 840 1C and differentstrain rates in order to further investigate the mechanisms formorphology evolution of primary α grains.

It can be seen in Fig. 7 that during the deformation of Ti-17alloy at a deformation temperature of 840 1C, the globularizationof primary α grains is obvious at strain rates of 0.01 s−1 and 0.1 s−1.

Fig. 12. (a) Bright-field TEM image of primary α grain at a deformation temperature of 8diffraction patterns of primary α grain and β matrix in (a).

Stefansson et al. [23] suggested that dislocation substructureswould be formed to drive boundary splitting at the initial stageof globularizaiton. Weiss et al. [16] showed that hot workingdevelops both low and high angle subboundary across the αlamellae with misorientation angles ranging from a few degreesup to 301, which led to the separation of primary α grains. In thispaper, substructure with tangled dislocations in primary α grainshas been observed at a deformation temperature of 840 1C, strainrate of 0.1 s−1 and height reduction of 30%, as the arrows depictin Fig. 12(a). It can be seen that the dislocations rearrange like awall within the primary α grain. Misorientation across the dis-location wall has been estimated by calculating the data recordedon the TEM machine and the value is approximately 101. Mean-while, the brightness of right side is much darker than that of leftside in the primary α grain, which results from the occurrence ofcrystal diffraction at the right side. In order to distinguish thedifferent phases in Fig. 12(a), corresponding diffraction patterns ofprimary α grain and β matrix are shown in Fig. 12(b) and (c).

The dislocation density would increase as the strain increases,thus the misorientation across dislocation substructures develops. Itcan be seen in Fig. 13(a) that clear subboundary has formed at adeformation temperature of 840 1C, strain rate of 0.1 s−1 and heightreduction of 60%. Meanwhile, the β phase has formed a cusp alongα/α subboundary and the primary α grain is being separated bypenetration of β phase. Thus the formation of low and high angleboundaries across primary α grains is considered to be the first stepto the break-up of lamellae α. The similar phenomenon wasobserved in the investigations on Ti600 alloy [24]. If the α lamellaewidth is less than two times the penetration distance of the β phasecusps, the separation of primary α grains occurs readily [16]. After

40 1C, strain rate of 0.1 s−1 and height reduction of 30%; (b) and (c) Corresponding

Fig. 13. Bright-field TEM images of primary α grains and corresponding diffraction patterns at a deformation temperature of 840 1C, strain rate of 0.1 s−1 and height reductionof 60%.

Fig. 14. Bright-field TEM image of primary α grain and corresponding diffraction pattern at deformation temperature of 840 1C, strain rate of 10.0 s−1 and height reductionof 60%.

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these processes, the separated primary α grains are globularizedsubsequently by diffusion process in order to minimize the surfaceenergy. Some equiaxed primary α grains with low dislocationdensity are observed after the compression at a deformationtemperature of 840 1C, strain rate of 0.1 s−1 and height reductionof 60%, as shown in Fig. 13(c). It implies that the globularization ofprimary α grains completes during the isothermal compression.

However, the TEM images are quite different when Ti-17 alloy isisothermally compressed at a deformation temperature of 840 1C,strain rate of 10.0 s−1 and height reduction of 60%. It can be seen inFig. 14(a) that the primary α grain is elongated and the grain interior

contains a large amount of dislocations (shown by arrows). It impliesthat the rate of softening by dynamic recovery is insufficient tobalance strain hardening. The dislocation density of primary α grainsin isothermal compression depends on two competing processes:dislocation multiplication and annihilation. The dislocation multiplica-tion is easily activated due to the quickly accumulated deformationenergy when Ti-17 alloy is isothermally compressed at a relativelyhigh strain rate. But the rate of dislocation annihilation is limited dueto the short deformation time. Therefore, the increase in dislocationdensity by strain hardening is much faster than the decrease indislocation density by dynamic recovery. Then the dislocation density

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in primary α grains is remained at a high level when the deformationis finished. This characteristic of microstructure with high dislocationdensity reveals that the main softening mechanism of Ti-17 alloy athigh strain rates contains dynamic recovery. Meanwhile, the highdislocation density in the primary α grain is reasonable to explain thehigher flow stress at a deformation temperature of 840 1C and strainrate of 10.0 s−1 in Fig. 2.

4. Conclusions

In this work, the deformation behavior and microstructureevolution in isothermal compression of Ti-17 alloy have beensystematically investigated. According to the analysis, followingconclusions can be drawn from this work.

(1)

Steady-state feature of flow curves is observed in both α+β twophase and β single phase region except continuous flowsoftening feature is observed at the strain rate higher than1.0 s−1 in the α+β two phase region.

(2)

The volume fraction and grain size of primary α grainsdecrease with an increase in deformation temperature. Thegrain size of β phase increases rapidly with an increase indeformation temperature in the β single phase region.

(3)

The primary α grains are fully equiaxed at low strain rates(_ε≤0.1 s−1) and flow localization is observed at a high strainrate (10.0 s−1). In the β single phase region, the main softeningmechanisms of Ti-17 alloy are dynamic recrystallization athigh strain rates and dynamic recovery at low strain rates.

(4)

The primary α grains become equiaxed as the strain increasesin the α+β two phase region. Dynamic recrystallization degreeof β grains increases with an increase in strain when thecompression is conducted in β single phase region.

(5)

The primary α grains are equiaxed by the penetration of βphase along α/α subboundaries when the strain rate is rela-tively low, while they are elongated when the strain rate isrelatively high.

Acknowledgment

The authors would like to thank the financial support from theNational Natural Science Foundation of China with Grant no.51275416

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