Deformation behavior and microstructure evolution during hot working of Ti60 alloy with lamellar starting microstructure

Deformation behavior and microstructure evolution during hot working of Ti60 alloy with lamellar starting microstructure

Accepted Manuscript Deformation behavior and microstructure evolution during hot working of Ti60 alloy with lamellar starting microstructure Wei Chen,...

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Accepted Manuscript Deformation behavior and microstructure evolution during hot working of Ti60 alloy with lamellar starting microstructure Wei Chen, Weidong Zeng, Jianwei Xu, Dadi Zhou, Simin Wang, Shengtong He PII:

S0925-8388(19)31183-1

DOI:

https://doi.org/10.1016/j.jallcom.2019.03.345

Reference:

JALCOM 50109

To appear in:

Journal of Alloys and Compounds

Received Date: 3 February 2019 Revised Date:

24 March 2019

Accepted Date: 25 March 2019

Please cite this article as: W. Chen, W. Zeng, J. Xu, D. Zhou, S. Wang, S. He, Deformation behavior and microstructure evolution during hot working of Ti60 alloy with lamellar starting microstructure, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.03.345. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Deformation behavior and microstructure evolution during hot working of Ti60 alloy with lamellar starting microstructure Wei Chen*, Weidong Zeng, Jianwei Xu, Dadi Zhou, Simin Wang, Shengtong He

China

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State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an 710072, P.R.

Defense Technoloies Innovation Center of precision forging and ring rolling, School of Materials Science and

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Engineering, Northwestern Polytechnical University, Xi'an, 710072, China

E-mail: [email protected]

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*Corresponding Author: Wei Chen

Postal address: State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, No. 127

Youyi Xilu, Xi’an 710072, P.R. China

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Abstract

The hot deformation behavior and microstructure evolution of Ti60 alloy with lamellar starting microstructure were investigated through hot compression test in the temperature range 970-1030

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°C with 20 °C intervals, strain rate range 0.01-10 s-1 and height reductions of 40%, 60% and 80%.

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On the basis of the flow stress obtained as a function of the strain rate and deformation temperature, the constitutive equation was established. It was found that the hyperbolic sine law equation is more applicable than the exponential law equation since the relative standard deviation of lnε -ln[sinh(ασ)] plot is lower than lnε -σ plot, and the hot deformation activation energy is calculated to be 839 kJ/mol. The results also showed that the deformation mechanism mainly involves the globularization of the α lath. As a thermal diffusion process, the strain rate plays a remarkable effect on the globularization. The fraction of globularized α lath increased with the

ACCEPTED MANUSCRIPT increase of the strain rate. Meanwhile, the globularization is sensitive to the orientation. At lower strain, only local favorable orientation α lath occurred globularization. With the increase of the strain, more α lath became kinking and globularization. The optimum deformation parameter for

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the globularization is in the temperature rang 990-1010 °C, strain rate 0.01 s-1 and height reduction of 60%.

Key words:

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Hot deformation; Constitutive equation; Microstructure evolution; Dynamic globularization

1. Introduction

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Titanium alloys have been widely used in aerospace applications, chemical industries and automotive due to their high specific strength, corrosion resistance, excellent fracture toughness [1, 2]. In order to extend the temperature range of the conventional titanium alloy, some near-α titanium alloys, such as Ti-1100, IMI834, and Ti60 alloy were designed for a service temperature

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of 600 °C [3, 4]. As a potential candidate of materials for the aerofoil blade and disc, Ti60 alloy has attracted strong interest in China, recently. Compared with IMI834 alloy, more Si element is

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added to the Ti60 alloy to improve the creep performance. A small amount of Ta and C element are added to improve its heat-resistant and widen the α+β phase field. It is well known that the hot

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deformation parameter and initial microstructure affect the flow behavior and microstructure evolution, and then the final microstructure determines the mechanical properties. Therefore, the understanding of the relationship between thermomechanical processing and microstructure is particularly critical for sustaining further improvement in the control of the microstructure and property [5, 6]. For the most of titanium alloys, thermomechanical processing is associated with two major aspects: (1) ingot breaks down above the β transus, which converts a cast ingot into a worked

ACCEPTED MANUSCRIPT billet and eliminates casting inhomogeneities. Depending on the cooling rate from the β phase field, the transformed microstructure with different morphologies can be obtained; (2) the ideal microstructure is obtained through hot deformation in α+β phase field. This stage plays an

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important role in both the microstructure evolution and properties for the final product [7, 8, 9, 10], and it has been received the increasing attention over the years. Jianwei Xu [11] studied the globularization behavior and tensile of Ti-17 alloy and pointed out that the fraction of the

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globularization increased with the increase of the prestrain and the tensile strength exhibited the

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increasing tendency with the fraction of the globularization. J.K.Fan et al [12] investigated the hot deformation of Ti-7333. The results showed that the deformation mechanism was dominated by dynamic recovery, globularization and dynamic recrystallization, while the globularization trend of α phase was remarkable at higher temperature and lower strain rate. Fengcang Ma [13] studied

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the microstructure evolution of a near-α Ti-1100 alloy with TiC (1 vol.%) for four types of hot deformation and revealed that the final microstructure depended on the thermomechanical processing parameter and the cooling rate after hot deformation. P. Wanjara [14] analyzed the

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microstructure evolution of IMI834 in the β phase field with bimodal staring microstructure and

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found that the dynamically recrystallized grain size was determined to follow a Zener-Hollomon relationship. Yong Niu [15] illustrated the hot deformation characteristics of a near-α Ti600 with equiaxed α grains by constructing its processing map and obtained that the superplastic deformation occured under the condition of strain rate lower than 0.01 s-1 and temperature about 920 °C. However, there are few studies focusing on the deformation behavior and the microstructure evolution of Ti60 alloy with lamellar starting microstructure. In the present work, the isothermal compression tests of Ti60 alloy with lamellar starting

ACCEPTED MANUSCRIPT microstructure were carried out to explore the hot deformation behavior, and analyze the effect of deformation parameters on the microstructure evolution. In order to describe the relationship between strain rate, deformation temperature, and flow stress, the hyperbolic sine constitutive

2. Materials and experimental procedure

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equation was established. In addition, it can provide an optimum parameter for the globularization.

The nominal composition (wt %) of Ti60 alloy used in the present work as follows: 5.6Al,

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1.0Mo, 2.0Zr, 4.8Sn, 0.05C, 0.35Si, 0.85Nb, and balance Ti. The β transus temperature of the

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alloy was approximately 1054 °C by metallographic method. As received bar stocks were first forged in β phase field and then quenched in water to obtain lamellar starting microstructure. As shown in Fig. 1, it consists of the lamellar α colonies with length and width of 900~1000 um and 400~500 um. The thickness of the α lath is about 4 um.

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In order to analyze the effect of different deformation parameters on the flow behavior and microstructure evolution, isothermal compression tests were carried out in the temperature range 970-1030 °C with 20 °C intervals, strain rate range 0.01-10 s-1 and the height reduction of 40-80%

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on a Gleeble-3500 simulator. The specimens were machined into cylinder with 8 mm in diameter

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and 12 mm in height. Thermocouples were welded in the middle surface of the specimen to measure the actual temperature. The top and bottom surfaces of the specimen were lubricated by tantalum chip. The specimens were resistance heated to the corresponding test temperatures with the rate of 5 °C/s, soaked for 5 min to ensure homogenized temperature, and then deformed up to the corresponding height reduction at a constant strain rate. After isothermal compression, the specimens were quenched immediately in water to retain the hot deformed microstructure. The true stress-strain curves were recorded automatically.

ACCEPTED MANUSCRIPT The α lath precipitated from the single-phase field is unstable. Its morphology would change during heating treatment before deformation. In order to exclude this interfering factor, a number of specimens with 8 mm in diameter and 12 mm in height were cut at the same location as the

range 970-1030 °C for 5 min followed by water quench.

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isothermal compression test specimens. The heat treatment was conducted in the temperature

Fig. 2 shows the simulation strain distribution of the deformed specimen at the height

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reduction of 60%. It was found that the strain appears the larger gradient distribution at the edge of

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the deformed specimen. However, at the central region of the deformed sample, the stain is more uniform. Meanwhile, it possesses the largest strain and the microstructure evolution is more obvious than other regions. If taken the metallographic photograph at the edge, the different positions in the photograph can possess different strain values and it leads to the selection bias. In

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order to eliminate the selection bias, the central regions of the cross section were selected for the metallographic examination using standard procedure. The optical micrographs of the specimens were obtained through an Olympus/PMG3 microscope, and then the micrographs were examined

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by quantitative image analysis software Image-pro plus 6.0.

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3. Results and discussion 3.1. Flow behavior

Fig. 3 shows the true stress-strain compression curves obtained at different deformation

temperatures (970 °C, 990 °C, 1010 °C, and1030 °C) and strain rates (0.01 s-1, 0.1 s-1, 1 s-1and 10 s-1) with height reduction of 60%. As shown in the Fig. 3, the initial strain hardening followed by the flow softening behavior can be observed at all deformation temperatures and strain rates. At the lower strain rates (0.01 s-1, 0.1 s-1), the flow stress remains nearly constant with the increase of

ACCEPTED MANUSCRIPT the strain, which represents the steady state flow behavior. However, at the higher strain rates (1.0 s-1, 10 s-1), the flow stress reaches a peak value at a small strain, and continuously decreases with the further straining. Meanwhile, the flow stress decreases with the increase of the deformation

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temperature and the decrease of the strain rate. 3.2. Flow softening behavior

Due to the low thermal conductivity and high strength, titanium alloys often occur the

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temperature rise during hot deformation, and it is probably one of reasons for the flow softening

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[16]. To verify this phenomenon, the temperature rise of Ti60 alloy during hot deformation can be calculated by the following equation [17, 18]: ∆T =

0.95η ρc

ε

0 σ ε

(1)

Where ∆Τ is the temperature rise, ρ is the material density (g/cm3), c is the specific heat

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(J/gK-1),

σ ε is the amount of work done by the deformation being equal to the area under the stress-strain curves, η is the efficiency and can be calculated by :

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1  η = (3 + log ε& ) / 3 0 

ε& ≥ 1.0 s −1 −1

0.001 s <ε& <1.0 s

(2) −1

ε& ≤ 0.001 s −1

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According to Eqs. (1) and (2), the temperature rise of Ti60 alloy is illustrated in Table 1. Table 1 shows that the temperature rise increases with the increase of the strain rate and decrease of the temperature. It is because that compared to the lower strain rates, there is not enough time for deformation heating to conduct at higher strain rates at a given strain. Meanwhile, the thermal conductivity of titanium alloy decreases with the decrease of the temperature[19]. Therefore, the heat is difficult to conduct at lower temperature than the high temperature. It is seen that the temperature rise plays a role in the flow stress at higher strain rate and lower temperature.

ACCEPTED MANUSCRIPT In addition, the temperature rise can be also verified by the microstructure observation. As shown in Fig. 4, the band of the flow localization occurred at the strain rate of 10 s-1, which is caused by the temperature rise. It is worth noting that this flow localization may induce

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the flow instability and it should be avoided during hot deformation. Whether there are other sources resulted in the flow softening, it would be discussed in the subsequent section. 3.3. Constitutive modeling

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The constitutive equation is used to predict the relationship between strain rate, deformation

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temperature, and flow stress for Ti60 alloy. The effect of the deformation temperature and strain rate on the deformation behavior is expressed by Zener-Holloman parameter (Z) in an exponent-type equation [20, 21]:

Z = ε exp(Q/RT)

(3)

Where ε& is strain rate (s-1), Q is the activation energy for the hot deformation (kJ/mol), R is

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the universal gas constant (8.314 J/mol·K), and Τ is the absolute deformation temperature (K). Meanwhile, depending on the level of the stress, the constitutive equation can be described in

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three different forms of Z parameter as follows [22]: Z1 = A1 σn1

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Z = A2 exp (βσ) Z = A[sinh(ασ)n

(ασ<0.8) (ασ>1.2) (for all ασ)

(4) (5) (6)

Where A1, A2, A, n1, n and β are the material constants, and α = β/n1, σ is the flow stress

(MPa). Taking natural logarithm of the both sides of Eqs. (4)-(6) yields: lnZ1=lnε +Q1/(RT)=lnA1+n1lnσ

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lnZ2=lnε  +Q2/(RT)=lnA2+βσ

(8)

lnZ=lnε + Q/RT = lnA + nln[sinh (ασ)

(9)

According to Eqs. (7)-(9), it reveals that there is a linear relationship between lnε -lnσ, lnε -σ

ACCEPTED MANUSCRIPT and lnε -ln[sinh(ασ). Therefore, n1, β and n can be expressed as the slopes of corresponding curves at a fixed temperature. The values of the peak flow stress and strain rate are substituted into Eqs. (7)-(9), and then linear fitting gives the relationship between the strain rate and flow stress, as

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shown in the Fig. 5. The average values of n1, β and n are calculated to be 4.67, 0.04 and 3.43. The stress multiplier α could be estimated as α = β/n1 =0.091. According to the values of correlation coefficients (R), the R2 of lnε -σ plot and lnε -ln[sinh(ασ)] plot are higher

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than lnε -lnσ plot. It means that the hyperbolic sine law equation and exponential law equation can

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describe the trend of peak stress more accurately than the power law constitutive equation. Meanwhile, in order to reduce the adverse effect of the deformation temperature on the material constant β and n, the RSD relative standard deviation is introduced to quantitatively depiction the dispersion of β and n.

standard deviation × 100% Average value

(10)

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RSD=

As shown in Fig. 5, the RSD of lnε -ln[sinh(ασ) plot is lower than lnε -σ plot. It means that the n value calculated by the hyperbolic sine law equation has smaller dispersion than β value

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calculated by exponential law equation at different deformation temperatures.

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As previously described, it was found that the hyperbolic sine law equation is more suitable to describe the flow behavior of Ti60 alloy. According to the Eq. (9), the Q can be defined as: ε

Q = R ln [sinh (ασ) ×

ln [sinh(ασ) (1/T)

= RnS

(11)

As shown in Eq. (11), the value of the deformation activation energy Q can be obtained from

the average slopes of the lnε -ln [sinh(ασ)and ln[sinh(ασ) -T -1 plot at various strain rates, as illustrated in Fig. 5(c) and Fig. 6. The average value of S is 29868.02. Hence, the Q is calculated to be 839 kJ/mol.

ACCEPTED MANUSCRIPT By use of the least square method, the lnZ is plotted versus corresponding ln [sinh(ασ), as shown in Fig. 7. According to the Eq. (9), the lnA is the intercept with the value 77. Therefore, the value of A can be calculated as 2.7×1033 and the correlation coefficient R2 for the linear regression

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is 0.99. It indicates that the hyperbolic sine law equation exhibits a good linear correlation between Z value and flow stress. Meanwhile, it proves once more that the hyperbolic sine law equation is effective for Ti60 alloy.

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The constitutive equation for Ti60 alloy with lamellar starting microstructure in α+β phase region is:

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ε = 2.7×1033 [sinh(0.091σ)3.43 exp [−8.39 × 105 /(RT)

(12)

3.4. Effect of deformation parameter on the microstructure evolution 3.4.1. Effect of heat treatment before deformation

The as-received Ti60 alloy bar was forged in α+β temperature field. It is in metastable state

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and would occur the complex microstructure evolution during heat treatment. In order to accurately investigate the effect of deformation parameter on the microstructure evolution, the

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microstructure evolution during heat treatment should be considered. The microstructure after heat treatment is shown in Fig. 8.

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It is clearly observed from the Fig. 8 that the content of the primary α phase decreases with

the increase of the temperature. Compared with initial microstructure, the α lath are split and globularized in varied degrees. The quantitative statistics are shown in Fig. 9 through metallographic method. As seen from Fig. 9(a), the primary α phase is sensitive to the temperature change. The content of the primary α phase is as high as 82% at 970 °C, but it has dropped to 22% when the temperature rises up to 1030 °C. It indicates that the primary α phase has a large transformation

ACCEPTED MANUSCRIPT ration with the increase of the temperature. In addition, the time consumed in the following hot deformation is shorter than the heat-treatment and the volume fraction of primary α phase has little change during hot deformation. Therefore, the content of the primary α phase after

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heat-treatment can be regarded as the total content that involved in the subsequent deformation and dynamic globularization. As shown in Fig. 9(b), the static globularized fraction increases with the increase of the temperature. The globularization process is controlled by the diffusion of solute

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and the diffusivity of solutes is influenced by the temperature. Thus, the static globularization is

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accelerated with the increase of the temperature [23]. To better analyze the effect of deformation parameter on the microstructure evolution, the fraction of the static globularization should be subtracted in the subsequent dynamic globularized statistics. 3.4.2 Effect of strain rate

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According to the latest research, the globularization process consists of three stages: the dislocation generation in the α lath, the separation of α lath, and formation of the globule. As shown in Fig. 10. First, the dislocation of both signs is generated along the shear plane. The

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recovery causes annihilation of the opposite sign dislocation and leaves behind group of the

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dislocation with the same sign. This stage provides driving force for the globularization [24]. Second, the accumulation of the dislocation causes unbalance at the intersections of intraphase. To achieve the balanced state, the β phase penetrates into α lath at intraphase boundaries and the diffusion occurs in the interface between them. It is so-called the thermal erosion ditch [25, 26]. Last, driven by the interfacial energy, the migration of the interface forms the globule. The mechanism has been verified by the transmission electron micrograph [27].

ACCEPTED MANUSCRIPT Fig. 11 shows the globularization development of the lamellar microstructure at 990 °C and different strain rates under height reduction of 60%. These micrographs clearly reveal that the fraction of the globularized α lath increases with the decrease of strain rate. The specific

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globularization values are 63.8%, 49.6%, 33.2% and 29.1%. It indicates that the strain rate has an important effect on the globularization. At lower strain rates, there is sufficient time for the dislocation to cross slip and annihilate, whereas insufficient time for the dislocation to react at

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higher strain rates so that it cannot form enough subboundaries in α lath. Corresponding to the

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microstructure evolution, the flow stress increases with the increase of strain rate. Meanwhile, a large quantity of dislocation causes a high initial work hardening and peak stress (Fig. 3) at higher strain rates. Similarly, the diffusion of the solute cannot proceed completely at higher strain rate, which makes the α lath difficult to be separated by the penetration of β and forms the globule.

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3.4.3 Effect of the deformation temperature

The micrographs of the specimen deformed at various deformation temperatures are shown in Fig. 12. It can be obviously seen that the higher the deformation temperature is, the higher the

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fraction of globularized α lath is. At the temperature of 970 °C, the α lath is in absence of

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globularization and only severe bending occurs. With the increase of the temperature, the α/β interface becomes fuzzy and the α lath appears globularization. The specific globularization values are 7%, 25%, 42.5% and 52.2%. Meanwhile, the content of the α lath decreases with the increase of the temperature and the microstructure is akin to a β transformed one at the temperature of 1030 °C. At lower temperatures, the deformation degree of the each α lath is small due to the high content. It leads to generate less intraphase dislocation in α lath so that it is difficult to be separated and globularized. However, it is contrary to the previous situation at

ACCEPTED MANUSCRIPT higher temperatures. In addition, both the split of the boundary and the migration of the interfaces are controlled by the diffusion and the diffusivity is determined by the deformation temperature, so the fraction of the globularization increases with the increase of the temperature.

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It should be noted that the globularization process can be considered to be a type of DRX [28]. Accordingly, the flow stress decreases with the increase of the deformation temperature (Fig. 3).

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3.4.4 Effect of strain

According to the analysis of the previous section, the Ti60 alloy has a higher fraction of

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globularization at lower strain rate. To analyze the effect of the strain on the microstructure evolution, a series of experiments were carried out at the temperature of 970 °C and strain rate of 0.01s-1. As shown in Fig. 13, it is found that the fraction of globularized α lath increases with the

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increase of strain. At the height reduction of 40%, the α/β interface begins to appear fuzzy, which is caused by the β penetration. It should be emphasized that a part of α lath begin to break and globularize as shown inside the circle of the Fig. 13(a) compared to other section and the fraction

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of globularization is 9.6 %. With the increase of the strain, the α lath becomes kinking (Fig. 13(b))

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and the fuzziness of the α/β interface is more obvious. Meanwhile, the α lath with higher feret ration get more globularized than the previous height reduction. At the height reductions of 60% and 80%, the fraction of globularized α lath is 40.9% and 68.7%. As shown in Fig. 10, with the increase of the strain, a large excess of dislocation were

generated in the α lath and the strain energy increased, which provides the premise for globularization. Since the globularization requires deformation of the α lath and the deformation is sensitive to orientation [29], only certain favorable orientation α lath occur globularization

ACCEPTED MANUSCRIPT firstly at the lower strain, as shown in the circle region of Fig. 13(a). With the increase of the strain, a part of α lath with low Schmidt factor occur kinking since it is difficult to occur basal or prismatic slip [30]. However, it is convenient for β phase to penetrate α lath at the kinking area

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and form the globular α grain. That is why there is no kinking in the Fig. 13(c) compared with Fig. 13(b), and the fraction of globularized α lath at height reduction of 80% is higher than the height reduction of 60%.

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Using quantification metallographic method, the fraction of globularized α lath at different

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deformation parameters are counted in this paper, as shown in Fig. 14. It is found that the trend of the globularization is consistent with the above discussion. The strain required to achieve the same fraction of the globularization is less at high temperature while is more at high strain rate,. Meanwhile, at the strain rate of 0.01 s-1, the fraction of globularized α lath significantly increases

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than other lower strain rates. It indicates that the strain rate has a greatly effect on the globularization. In addition, above the temperature of 1010 °C, the content of α phase begins to plunge which may cause adverse effect on the mechanical properties. Therefore, the maximum

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temperature for the globularization is restricted to 1010 °C. Meanwhile, at the temperature of 970

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°C, the maximum globularization value is only 70% at the strain of 1.6 and strain rate of 0.01 s-1. Therefore, the optimum deformation temperature for the globularization is range of 990 °C-1010 °C.

4. Conclusions

In the present study, the isothermal compression tests of Ti60 alloy with lamellar starting microstructure were carried out in the temperature range 970-1030 °C and strain rate range 0.01-10 s-1. The hyperbolic sine constitutive equation was established. The effect of the

ACCEPTED MANUSCRIPT deformation parameters on the microstructure evolution was discussed, respectively. The main outcomes of this work could be summarized as follows: (1) At the strain rate of 0.01-0.1 s-1, the flow curves are characterized by the steady state flow

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behavior. When the strain rate is higher than 0.1 s-1, the Ti60 alloy exhibits peak flow stresses followed by continuous flow softening.

(2) The hyperbolic sine law equation is more applicable than the power law equation and the

Meanwhile,

the RSD

(relative

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higher than lnε -lnσ plot.

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exponential law equation. It is because that the R2 of lnε -σ and lnε -ln[sinh(ασ)] plot are standard

deviation) of

lnε -ln[sinh(ασ) plot is lower than ln!-" plot. The hyperbolic sine law equation for Ti60 alloy

with

lamellar

starting

microstructure

in

α+β

phase

region

is:

ε = 2.7×1033 [sinh(0.091σ)3.43 exp [−8.39 × 105 /(RT).

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(3) The flow softening is mainly attributed to the globularization of the α lath at the strain rates of 0.01 s-1and1 s-1, while attributed to the temperature rise at the strain rate of 10 s-1. Meanwhile, at the high height reduction of 80%, the band of the flow localization occurs and it should be

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avoided during hot deformation.

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(4) The globularization process is sensitive to orientation. Only local favorable orientation α lath become globularization at lower strain. Meanwhile, as a thermal diffusion process, the strain rate has a remarkable effect on the globularization. The fraction of the globularizaion increases with the increase of deformation temperature and strain, and decreases with the increase of strain rate. The optimum deformation parameters for the globularization is at 990-1010 °C/0.01 s-1 and height reduction of 60%.

Acknowledgements

ACCEPTED MANUSCRIPT This study was financially supported by the program of National Key Research and Development Plan of China(NO.2016YFB0301203). The authors would like to gratefully acknowledge the Analytical & Testing Center of Northwestern Polytechnical University for

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conveniences of experimental tests.

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ACCEPTED MANUSCRIPT [18]R.L. Goetz, S.L. Semiatin, The adiabatic correction factor for deformation heating during the uniaxial compression test, J. Mater. Eng. Perform. 10 (2001) 710-717. [19]Q. Luo, L. Wang, Z. R. Zhou, S. W. Liu, Relationship between temperature and thermal

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physical properties of TA16 and TA17 Titanium alloys, Materials for Mechanical Engineering. 33 (2009) 30-33.

[20]H.J. Mcqueen, N. D. Ryan, Constitutive analysis in hot working, Mater. Sci. Eng. A 322 (2002)

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43-63.

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[21]D. Samantaray, S. Mandal, A.K. Bhaduri, Constitutive analysis to predict high-temperature flow stress in modified 9Cr-1Mo (P91) steel, Mater. Des. 31(2010) 981-984. [22]H. Mirzadeh, J.M. Cabrera, J.M. Prado, A. Najafizadeh, Hot deformation behavior of a medium carbon microalloyed steel, Mater. Sci. Eng. A 528 (2011) 3876-3882.

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[23]X.G. Fan, H. Yang, S.L Yan, P.F Gao, J.H. Zhou, Mechanism and kinetics of static globularization in TA15 titanium alloy with transformed structure, J. Alloys. Compd. 533 (2012) 1-8.

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[24]T. Seshacharyulu, S.C. Medeiros, W.G. Frazier, Y.V.R.K. Prasad, Microstructural mechanisms

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during hot working of commercial grade Ti–6Al–4V with lamellar starting structure, Mater. Sci. Eng. A 325 (2002) 112-125.

[25]Weiss. I, E.H. Froes, D. Eylon, G.E. Welsch, Modification of Alpha Morphology in Ti-6Al-4V by Thermomechanical Processing, Metall. Mater. Trans A 17 (1986) 1935-1947.

[26]X.G. Fan, H. Yang, S.L. Yan, P.F. Gao, J.H. Zhou, Mechanism and kinetics of static globularization in TA15 titanium alloy with transformed structure, J. Alloys. Compd. 533 (2012) 1-8.

ACCEPTED MANUSCRIPT [27]J.K. Fan, H.C. Kou, M.J. Lai, B. Tang, H. Chang, J.S. Li, Characterization of hot deformation behavior of a new near beta titanium alloy: Ti-7333, Mater. Des. 499 (2013) 945-952. [28]N.Stefansson, S. L. Semiatin, Mechanisms of globularization of Ti-6Al-4V during static heat

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treatment, Metall. Mater. Trans A 34 (2003) 691-698. [29]T.R. Bieler, S.L. Semiatin, The origins of heterogeneous deformation during primary hot working of Ti-6Al-4V, Int. J. Plast 18 (2002) 1165-1189.

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[30]R. Shibayan, S. Satyam, The influence of temperature and strain rate on the deformation

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response and microstructural evolution during hot compression of a titanium alloy Ti-6Al-4V-0.1B, J. Alloys. Compd. 548 (2013) 110-125.

Table caption

Figures captions

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Table 1. The temperature rise of Ti60 alloy deformed at different temperatures and strain rates

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Fig. 1. The initial microstructure of Ti60 alloy at different amplifications.

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Fig. 2. The simulation strain distribution of the deformed specimen at the height reduction of 60%. Fig. 3. The true stress-strain curves of Ti60 alloy at the strain rate of 0.01-10 s-1and various temperatures: (a) 970 °C, (b) 990 °C, (c) 1010 °C, (d) 1030 °C.

Fig. 4. The band of flow localization occurred at 10 s-1 and height reduction of 80%: (a) 970 °C, (b) 990 °C. Fig. 5. Plots of lnε -lnσ, lnε -σ and lnε -ln[sinh(ασ)]. Fig. 6. Plot of ln[sinh(ασ) -T -1 .

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Fig. 9. Volume fraction of the primary α phase and Fraction of globularized α lath during heat treatment.

Fig. 10. Schematic illustrations of the globularization mechanism of α lath: a dislocations wall,

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(b) Intraphase subboundary, (c) the separation of α lath, (d) formation of the globule.

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Fig. 11. The micrographs of Ti60 specimens deformed at the temperature of 990 °C and height reduction of 60%: (a) 0.01 s-1, (b) 0.1 s-1, (c) 1 s-1, (d) 10 s-1.

Fig. 12. Microstructures of Ti60 alloy at strain rate of 0.01 s-1 and height reduction of 40%: (a) 970 °C, (b) 990 °C, (c) 1010 °C, (d) 1030 °C.

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Fig. 13. Microstructures of Ti60 alloy compressed at various height reduction at the temperature of 970 ℃and strain rate of 0.01 s-1: (a) 40%, (b) 60%, (c) 80%. Fig. 14. The fraction of globularized α lath at different strain rates and temperatures: (a) 970 °C,

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(b) 990 °C, (c) 1010 °C, (d) 1030 °C.

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1

Strain rate(s - ) 0.1

1.0

10.0

970

7.02

25.7

54

61.9

990

3.53

17.2

33.3

45.2

1010

3.13

17.95

25.6

41.76

1030

2.64

12

22.2

38.42

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0.01

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Fig. 1. The initial microstructure of Ti60 alloy at different amplifications.

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Fig. 10. Schematic illustrations of the globularization mechanism of α lath:

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(a)dislocations wall; (b) Intraphase subboundary; (c) the separation of α lath; (d) formation of the globule.

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Fig. 11. The micrographs of Ti60 specimens deformed at the temperature of 990 °C and height reduction of 60%:

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(a) 0.01 s-1 (b) 0.1 s-1 (c) 1 s-1 (d) 10 s-1.

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Fig. 12. Microstructures of Ti60 alloy at strain rate of 0.01 s-1 and height reduction of 40%:

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(a) 970 °C, (b) 990 °C, (c) 1010 °C, (d) 1030 °C.

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Fig. 13. Microstructures of Ti60 alloy compressed at various height reduction at the temperature of 970 ℃

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and strain rate of 0.01 s-1: (a) 40%, (b) 60%, (c) 80%.

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Fig. 14. The fraction of globularized α lath at different strain rates and temperatures:

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(a) 970 °C, (b) 990 °C, (c) 1010 °C, (d) 1030 °C.

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Fig. 2. The simulation strain distribution of the deformed specimen at the height reduction of 60%.

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Fig. 3. The true stress-strain curves of Ti60 alloy at the strain rate of 0.01-10 s-1and various temperatures:

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(a) 970 °C, (b)990 °C, (c)1010 °C, (d)1030 °C.

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Fig. 4. The band of flow localization occurred at 10 s-1 and height reduction of 80%: (a) 970 °C, (b) 990 °C.

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Fig. 5. Plots of lnε -lnσ, lnε -σ and lnε -ln[sinh(ασ)].

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Fig. 6. Plot of lnሾsinhሺασሻሿ -ܶ -1 .

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Fig. 7. Relationship of ln-ln [sinhασ].

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Fig. 8. Microstructure after heat-treatment for 5 minutes at various temperatures:

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(a) 970 °C, (b) 990 °C, (c) 1010 °C, (d) 1030 °C.

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Fig. 9. Volume fraction of the primary α phase and Fraction of globularized α lath during heat treatment.

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Highlights 1) The hyperbolic sine law equation is more applicable to describe the flow behavior. 2) The primary α phase is sensitive to the temperature change during heat treatment.

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3) The strain rate plays a remarkable effect on the globularization.

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4) Only local favorable orientation α lath become globularization at lower strain.