The flow behavior and microstructure evolution during (α + β) deformation of β wrought TA15 titanium alloy

The flow behavior and microstructure evolution during (α + β) deformation of β wrought TA15 titanium alloy

    The flow behavior and microstructure evolution during (α + β) deformation of β wrought TA15 titanium alloy A.M. Zhao, H. Yang, X.G. F...

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    The flow behavior and microstructure evolution during (α + β) deformation of β wrought TA15 titanium alloy A.M. Zhao, H. Yang, X.G. Fan, P.F. Gao, R. Zuo, M. Meng PII: DOI: Reference:

S0264-1275(16)30892-9 doi: 10.1016/j.matdes.2016.07.001 JMADE 2005

To appear in: Received date: Revised date: Accepted date:

23 April 2016 22 June 2016 1 July 2016

Please cite this article as: A.M. Zhao, H. Yang, X.G. Fan, P.F. Gao, R. Zuo, M. Meng, The flow behavior and microstructure evolution during (α + β) deformation of β wrought TA15 titanium alloy, (2016), doi: 10.1016/j.matdes.2016.07.001

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ACCEPTED MANUSCRIPT The flow behavior and microstructure evolution during (α+β) deformation of β wrought TA15 titanium alloy

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A.M. Zhao, H. Yang, X.G. Fan*, P.F. Gao, R. Zuo, M. Meng

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State Key Laboratory of Solidification Processing, School of Materials Science and Engineering,

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Northwestern Polytechnical University, Xi’an, 710072, P.R. China. *Corresponding author. Tel./fax: +86-29-88460212-809; E-mail: [email protected]

Abstract: The hot compressions in (α+β) region were conducted on β wrought TA15

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titanium alloy, so as to investigate the effect of β working on the flow behavior and dynamic globularization of lamellar microstructure. The results indicated that initial β

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grain size had no significant influence on flow stress. The dynamic globularization rate of α laths in smaller β grains was higher due to the difference in lamellar length. β predeformation would decrease the flow stress of (α+β) deformation. Most of α laths

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in the microstructure after β predeformation were short with a geometric orientation

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vertical to β grain boundary and rotated together during compression, which would

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promote the formation of α/α substructures, thus highly enhanced the dynamic globularization rate of α laths. While β heat treated microstructure had long α lamellae with a random orientation, which tended to become kinked first and then break up

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into short ones, resulting in the delay of dynamic globularization. In addition, prior β deformation changed the initial α phase texture and dynamic globularization rate would be affected by strain path. When the compressive direction in (α+β) region was vertical to that of β predeformation, the globularized fraction of α lamellae was apparently high. Key words: TA15 titanium alloy; β working; (α+β) deformation; flow behavior; dynamic globularization 1. Introduction TA15

(Ti-6Al-2Zr-1Mo-1V)

is

a

near-α

titanium

alloy

with

moderate

room-temperature and high-temperature strength, superior thermal stability, good welding performance, low growth rate of fatigue crack and strong corrosion resistance. It is widely used to manufacture structural components in aerospace industry [1, 2].

ACCEPTED MANUSCRIPT For example, the large scale integral complex components of TA15 titanium alloy have been successfully applied to the airplane bulkhead [3]. The conventional metallurgy processing of two-phase titanium alloy billet usually

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consists of a series of steps, each of which has an influence on the following step due to microstructure heredity. The primary hot working is to obtain an equiaxed/bimodal

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structure from a transformed microstructure, i.e., the colony or Widmannstatten structure, of which the phase constitution and morphology can be regulated by deformation and heat treatment in β region. Astarita et al. [4] conducted beta forging

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on a Ti-6Al-4V component for aeronautic applications, and found that the morphology and dimension of α lamellae would be influenced by β deformation and

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its cool rate.

The transformation from the lamellae to the equiaxed microstructure, known as the

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globularization process, is significant for the final performance of titanium alloy

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components. Thus much attention has been paid to the globularization process and many investigations have been established recently [5-10]. It is widely accepted that

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the globularization process includes three apparent stages: formation of intraphase α/α boundaries in α lamellae, separation of α lamellae into low-aspect ratio α particles, and the coarsening of isolated α grains by means of the termination migration and

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Ostwald ripening [1]. The formation of intraphase α/α boundaries might originate from recovered substructures, dynamic recrystallization, localized shear bands, or twinning deformation [11, 12]. Nowadays, many investigations also have been carried out to clarify the effect of initial microstructure on primary hot working, in which the grain size and morphology intensively depended on prior β working. For instance, Semiatin and Prasad [13, 14] found that prior β grain size would noticeably affect the microstructural refinement during (α+β) deformation and the mechanical properties of titanium alloys. Bieler et al. [15] established the effect of α lamellar thickness on the plastic flow of Ti-6Al-4V with a transformed microstructure and found that initial lamellar thickness was mainly affected by prior β heat treatment. Semiatin et al. [16] investigated the inhomogeneous deformation in a colony-α microstructure during primary hot working, which mainly focused on how the globularization efficiency of

ACCEPTED MANUSCRIPT α lamellae was affected by the initial colony orientation. Mallikarjun [17] proposed a new schedule involving processing in β and (α+β) phases to obtain good superplastic properties, and found that the prior β working had a marked effect on superplastic,

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due to the influence of β deformation on microstructural refinement. From above-mentioned analysis, it can be found that the deformation behavior and dynamic

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globularization of α lamellae would be affected by the state of initial microstructure, e.g. grain size and lamellar orientation. However, the existing studies only concerned the effect of β forging on precipitation of α lamellae or the influence of initial

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microstructure on (α+β) deformation, respectively. There are close relationships between β and (α+β) forging in the traditional metallurgy processing, but the direct

been systematically established.

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influences of β forging and heat treatment on subsequent (α+β) deformation have not

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The present aim of this study is to examine the effects of prior β deformation and heat

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treatment on (α+β) deformation of TA15 titanium alloy, in which the special attention is paid to the influences of initial grain size, β predeformation and strain path. To

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achieve this goal, different prior β workings were conducted on TA15 titanium alloy with primary equiaxed microstructure. The X-Ray Diffraction (XRD) technique was applied to investigate the initial texture of β wrought microstructure. The orientation

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distribution of globularized microstructure was also checked by electron back-scattered diffraction (EBSD). The present work could provide an insight into the effect of prior β working on the flow behavior and dynamic globularization of TA15 titanium alloy and give a guidance for conventional metallurgy processing of two-phase titanium alloys. 2. Material and procedures 2.1. Material The material used in the present work was TA15 titanium alloy, with chemical composition (wt.%) of 6.06 Al, 2.08 Mo, 1.32 V, 1.86 Zr, 0.30 Fe, and balanced Ti. The β-transus temperature of TA15 was about 985°C measured by the metallographic method. The as-received microstructure of TA15 consisted of approximately 60% primary equiaxed α phase within transformed β matrix and the size of primary α phase

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was ~10μm, as shown in Fig.1.

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Fig.1. The microstructure of as-received TA15 titanium alloy.

2.2. Experimental procedures

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The specimens with the size of 50mm×50mm×50mm manufactured from the as-received material were heated to 1020°C in the resistance furnace SXL 1200, held for 15min, 30min and 60min, respectively, and then cooled in furnace (FC). The

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corresponding microstructures were denoted as sample A, sample B and sample C, as

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shown in Fig.2 (a)-(c). The prior β grain sizes after heat treatment were respectively 296μm, 549μm and 753μm obtained by quantitative measurement. Another sample O

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heated at 1020°C, held for 15min and air cooled was used, of which the lamellar orientations were random, as shown by scanning electron microscope (SEM) micrograph in Fig.2 (d). In addition, the prior β deformation was conducted on a

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cylinder specimen of Φ30mm×45mm from the as-received material with the universal testing machine CMT5205 at 1020°C and a strain rate of 1s-1, to the height reduction of 50%. The sample was held at 1020°C for 15min before deformation and cooled in air (AC) after compression testing. The optical micrograph (OM) and orientation image micrograph (OIM) of microstructure with β predeformation are illustrated in Fig.3 (a) and (b). The initial β grains were elongated along the material flow direction. The α lamellae precipitating from β matrix during air cooling had a preferred orientation, which were nearly vertical to the elongated β grain boundary. Some α lamellae even traversed the integral grain. The samples for the subsequent (α+β) deformation were manufactured from the β-deformed specimen by two methods of the following schematic illustration (Fig.3(c)), and the two samples obtained were

ACCEPTED MANUSCRIPT denoted as sample P, sample V, respectively. The specimens after prior β working were manufactured to cylinder samples of 10 mm in diameter and 15 mm in height for

(b)

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subsequent compression in (α+β) region.

(d)

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Fig.2. The TA15 microstructure after prior β heat treatments of (a)1020°C, 15min, FC, sample A; (b)1020°C, 30min, FC, sample B; (c)1020°C, 60min, FC, sample C; (d) the SEM micrograph of sample

(a)

AC

O heated at 1020°C for 15min, AC.

(b)

(c)

Fig.3. (a) The OM of prior β deformation at 1020°C and 1s-1, to height reduction of 50% and air cooled; (b) the corresponding OIM; (c) Schematic illustration of the method to obtain samples P and V, and the compression axis (CA) of prior β deformation was shown by arrow.

The samples with prior β working were then isothermally compressed at 900°C and the nominal strain rates of 0.01s-1, 0.1s-1 and 1s-1 to the height reductions of 30%, 50% and 70% respectively by a Gleeble-3500 simulator. Each specimen was held for 5min

ACCEPTED MANUSCRIPT at 900°C to make sure that the temperature distributed homogeneously before compression. The graphite layers were used in order to reduce the friction between the anvils and the sample. After compression, the samples were quenched in water to

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keep the high-temperature microstructures. The compressed specimens were sectioned along the compression axis, mechanically grinded and polished, then etched with a

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solution of 13%HNO3, 7%HF and 80%H2O for the microstructure observations, which were taken from the central portion of each specimen with a Leica DMI3000 optical microscope. The quantitative analysis was obtained using the software

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Image-pro plus 6.0. Globularization was taken to be an α-phase morphology with aspect ratio less than 2.5. For EBSD observations, the compressed samples were also

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electropolished in a solution of 5% perchloric acid, 65% methyl alcohol and 30% butanol at 28°C with a voltage of 25V and the polishing time was 40s. The

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electropolished samples were then detected on a TESCAN MIRA3 XMU scanning

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electron microscope equipped with a NordlysMax EBSD detector. The analysis of crystal orientation evolution after deformation was carried out with the HKL-Channel

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5 software. The macrotexture of the samples after prior β workings was determined on PANAlytical Xpert Pro X-ray automatic detector. The X-ray beam was derived from Cu-Kα radiation in a sealed tube operated at 40kV and 40mA.

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3. Results

3.1. Flow behavior Fig.4 (a) shows the stress-strain curves of sample A and sample C deformed at 900°C and strain rates of 0.01s-1, 0.1s-1, 1s-1, respectively. Each stress-strain curve exhibited a peak stress at low strains, a wide region of flow softening, and a near-steady flow stage at large strains, irrespective of strain rate and initial microstructure, which was considered to be an obvious feature of two-phase titanium alloys with a lamellar α initial microstructure deformed in (α+β) region [15,18-21]. Meanwhile, the flow stress of each deformed sample apparently enhanced with strain rate increasing. At a given strain rate, the stress-strain curves of sample A and sample C were nearly overlapped and the biggest difference of flow stress was only about 9.4%, indicating that there was a small influence of initial β grain size on the flow stress at the same

ACCEPTED MANUSCRIPT processing parameters. It is well accepted that the flow stress of initial lamellar microstructure deformed in (α+β) field is closely related to the dislocation glide of α lamellae [22]. That is to say, flow behavior is mainly affected by the α lamellar

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thickness and orientation distributions, because the orientations of α laths determine the activation of slip systems and the lamellar thickness will affect slip transfer.

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However, the orientation and thickness are intensively dependent on the cooling rate of prior β working but are less affected by initial β grain size [23]. Thus, there is no significant influence of initial β grain size on the flow stress during (α+β)

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deformation.

The stress-strain curves of samples P, V and O deformed at 900°C and strain rates of

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0.01s-1, 0.1s-1 and 1s-1 are also illustrated in Fig.4 (b). These stress-strain curves exhibited the similar flow softening behavior as the above-mentioned. Also, the flow

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stress was closely related to the strain rate at the same compressive temperature.

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However, an increasing scatter of flow stress was observed for samples P, V and O after prior β deformation. In particular, the flow stress of sample O for the strain rates

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of 0.01s-1 and 1s-1 was apparently higher than that of sample P and sample V. At the strain rate of 0.1s-1, though the flow stress of sample O was not the highest among that of three samples, its overall stress was high and the peak stress was the highest. This

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result suggests that prior β deformation would decrease the flow stress of (α+β) deformation. In fact, this feature is associated with the effect of β predeformation on initial microstructure. The α laths in samples P and V had a preferred orientation while the orientation in sample O was random. Hence, the deformation resistances of sample P and sample V were low. In addition, the residual dislocations produced by prior β deformation were also beneficial to (α+β) deformation, thus decreased the deformation resistance.

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900C

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C

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Fig.4. The stress-strain curves of (a) samples A , C; and (b) samples P, V, O deformed at 900°C and strain rates of 0.01s-1, 0.1s-1, 1s-1.

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3.2. Microstructure observation

Microstructures of sample A, sample B and sample C after deformation at 900°C and

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a strain rate of 0.1s-1, to the height reduction of 50% are shown in Fig.5, to check the effect of β grain size on dynamic globularization of α laths in (α+β) region. The

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globularized fractions of sample A, sample B and sample C were 35%, 32%, 25%

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obtained by quantitative measurement, suggesting that the microstructure with a small initial β grain would have a high dynamic globularization rate of α lamellae. It could

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be easily found that α layer in β grain boundary was apparently visible in the microstructure of sample A, since its grain size was relatively small and the proportion of β grain boundary was large. The α laths inside β grains were straight and

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have substantially broken up into short platelets, most of which were aligned to the material flow direction. On the other hand, the bent α layers were observed in sample B and sample C, and the corresponding dynamic fragmentation of α lamellae was little. As the α laths were longer in sample B and C with relatively larger initial β grains, the long lamellae tended to become kinked and then break up into short ones at the beginning of compressive deformation. In fact, the rotation of short α laths was easy and the interfacial shear of α/β phase during lamellar rotation would lead to the loss of coherency between α and β phase [5], which would promote the formation of α/α substructures and dynamic globularization of α lamellae in sample A. Though it has been accepted that the globularized regions were always found in the area in which the laths have formed kink [9], the area of kinked platelets in sample B and C was apparently less compared with the unbent area as the lamellar kinking was

ACCEPTED MANUSCRIPT limited. Therefore, the globularized fractions of samples B and C were low. Fig.6 illustrates the microstructures of sample O, sample P and sample V after deformation at 900°C and a strain rate of 1s-1, to the height reduction of 50%. It could

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be observed that the dynamic globularization rates of α lamellae in sample P and sample V were higher than that in sample O (the globularized fractions of samples O,

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P, V were 18%, 27% and 35%, respectively), indicating that β predeformation would highly promote the dynamic globularization of α laths in (α+β) region. The orientation distributions of the broken lamellae were disordered in sample O while the short laths

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in samples P and V were nearly aligned to the material flow direction. It could be deduced that the initial orientation of α laths in sample O was more disordered than

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that in samples P and V. The initial microstructure with β predeformation is illustrated in Fig.3. As mentioned above, the α laths precipitating from the deformed β grains

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were comparatively short and formed a preferred orientation, most of which were

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nearly perpendicular to the straight β grain boundary. The initial β grains after β deformation have been elongated and the specific surface area was increased, leading

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to the increase of α lamellar nucleation site and the decrease of lamellar length. Thus, the short α laths with this preferred orientation in samples P and V tended to rotate together at the beginning of compression. On the contrary, the α platelets in initial

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microstructure of sample O were longer and the orientation distributions were comparatively random. The long α laths with different orientations would become kinked firstly. Hence, the lamellar rotation would enhance dynamic globularization rate in the viewpoint of α/α substructure and interfacial coherency, while the globularization of long lamellae was delayed due to the limited kinking at first. In addition, there were dislocations left in the initial microstructure after β predeformation, which would also destroy the interfacial coherency of α/β phase during precipitation of α laths in air cooling. Therefore, prior β deformation would promote the dynamic globularization of α laths.

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Fig.5. Microstructures of (a) sample A ; (b) sample B; (c) sample C after deformation at 900°C and a strain rate of 0.1s-1, to height reduction of 50%.

(b)

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(a)

(c)

Fig.6. Microstructures of (a) sample O; (b) sample P; (c) sample V after deformation at 900°C and a strain rate of 1s-1, to height reduction of 50%.

ACCEPTED MANUSCRIPT Figs.7 (a)-(c) show the α phase evolution of samples O, P and V by inverse pole figure (IPF) compressed at 900°C and a strain rate of 0.01s-1 to the height reduction of 50%, in which the deformation direction was vertical and the orientation perspective

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was parallel to the compression axis (CA). In general, the α phase exhibited a more random orientation distribution after compressive deformation, comparing with the

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OIM of initial microstructure in Fig.3 (b). The majority of α lamellae showed the crystal orientations of [11-20] and [10-10] parallel to CA, appearing in blue or green, suggesting that most non-globularized α platelets have rotated to the direction

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perpendicular to CA after 50% height reduction. The globularized new grains tended to have a random crystal orientation, appearing in red or yellow, which was different

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from that of surrounding α lamellae. It could be also found that the orientation distribution and the number of globularized grains in samples P and V were more than

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that in sample O. This result could also confirm that the dynamic globularization rate

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of the initial microstructure with β predeformation was high. Figs.8 (a)-(c) illustrate the corresponding distribution of grain boundary misorientation of α phase in sample

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O, sample P and sample V. The fractions of low-angle boundaries (LABs) in Figs.8 (a)-(c) were 19.6%, 25.5% and 21.3%, respectively, which indicated that the LABs in sample O were the least for three samples and the fragmentation of α lamellae in

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sample O was little. This also meant the deformation of α laths with a preferred orientation in samples P and V would promote the formation of α/α substructures, thus led to the increase of subsequent globularized fraction. Moreover, the high-angle boundaries (HABs) in sample O were mainly around 60° and 90°, which was the typical orientation distribution of α laths precipitating from β phase after β heat treatment [24]. Hence, this feature meant that the initial orientation relationship in sample O was still kept after 50% height reduction and few α lamellae have been broken up, i.e., the low dynamic globularization rate in sample O. In Fig.7, the HABs were denoted by black lines and the white lines represent LABs. It could be observed that many subgrains have formed in the non-globularized α laths. These substructures usually derived from the dislocation glide and then turned into high-angle grain boundaries by the migration and rotation of subgrains, which was the characteristic of

ACCEPTED MANUSCRIPT continuous dynamic recrystallization (CDRX). That is to say, the globularization mechanism of the present material and processing parameters is CDRX, for which the intraphase boundary in α lamellae is firstly formed to generate new subgrains without

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the nucleation of grain. Then the α/α substructures will migrate and rotate into HABs

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near initial grain boundary of α lamellae, which is illustrated by white ellipses in Fig.7.

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This process is driven by the release of the stored dislocation energy.

Fig.7. Inverse pole figure (//CA) for α phase of TA15 titanium alloy isothermally compressed at 900°C and a strain rate of 0.01s-1 to the height reduction of 50% respectively for (a) sample O, (b) sample P, (c) sample V.

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Sample O LABs:19.6%

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Sample V LABs:21.3%

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Fig.8. Distribution of grain boundary misorientation of α phase in TA15 titanium alloy compressed at 900°C and 0.01s-1 to the height reduction of 50% for (a) sample O, (b) sample P, (c) sample V.

4. Discussion

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4.1. Mechanism of flow softening The stress-strain curves of each sample exhibited the same characteristic of flow softening, and corresponding flow softening rates (dσ/dε) are illustrated in Fig.9,

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which were deformed at 900°C and strain rates of 0.01s-1, 0.1s-1 and 1s-1. In Fig.9 (a), the flow softening rates of samples A and C were similar with each other at the same deformation temperature and strain rate, as shown by black arrows. Also, the flow softening rates in Fig.9 (b) were more similar for each sample. The trends of dσ/dε curves were consistent at the same strain rate. The above results reflect that flow softening rate is closely related to strain rate at the same deformation temperature, but less dependent on initial sample. In addition, the quantitative flow softening values of stress-strain curves for different initial samples are shown in Table 1. This flow softening values were obtained by estimating the value of Δσ/σp at the true strain of 0.5 and the σp represented the peak stress. It could be found that the flow softening values decreased with strain rate increasing. Moreover, these values at the same strain

ACCEPTED MANUSCRIPT rate were similar respectively for samples A, C and samples O, P, V. This meant that flow softening was mainly affected by strain rate for the identical deformation temperature, but less influenced by initial microstructure and strain path. As

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mentioned above, the initial microstructures of each sample were regulated by β deformation and heat treatment, and the corresponding dynamic globularization rates

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were different at the same deformation parameters. Therefore, the flow softening of stress-strain curves is closely related to processing parameters, i.e. strain rate and deformation temperature, but it is independent of the dynamic globularization and

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prior β working.

Actually, the increasing dislocation density and dislocation intersection firstly resulted

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in work hardening in stress-strain curves in Fig.4 [25]. The stage of rapid flow softening was mainly associated with the platelet kinking, as shown in Figs.5 and 6,

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since dynamic globularization would occur at the strains much larger than those at

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which the flow softening effectively took place [26]. Moreover, the deformation of the colony microstructure could make slip transfer across α/β interfaces easier and some

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lamellar α phase would rotate to the orientations with low Taylor factors [5], which would also lead to a load drop. At large strains, the lamellar fragementation has occurred and the long α lamellae left in the microstructure were almost parallel to the

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metal flow direction. The formation of substructure in lamellar α phase was responsible for the flow softening of this stage and the division of α laths was associated with boundary splitting by dislocation migration. When the work hardening and recovery reached a dynamic equilibrium, the near-steady flow stress would be obtained [25]. Consequently, the phenomenon of flow softening is mainly associated with the change of phase morphology and the formation of substructure. For example, the platelet kinking observed at the beginning of compressive deformation is one of the reasons for flow softening. In addition, the evolution of α/α substructures would decrease the dislocation density, resulting in the decrease of flow stress. Thus, the effect of prior β working on flow softening behavior was negligible.

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Sample A Sample C Sample A Sample C Sample A Sample C

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The flow softening rate d/dPa

The flow softening rate d/dPa

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Fig.9. The flow softening rates (dσ/dε) of (a) samples A, C, (b) samples O, P, V deformed at 900°C and

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strain rates of 0.01s-1, 0.1s-1 and 1s-1.

Table 1 The quantitative flow softening values of samples A, C and samples O, P, V

Temperature/°C

900

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Sample C

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900

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Strain rate/ s-1

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Initial microstructure

0.01 0.1 1 0.01 0.1 1 0.01 0.1 1 0.01 0.1 1 0.01 0.1 1

Flow softening Δσ/σp 0.47 0.41 0.28 0.48 0.44 0.32 0.52 0.48 0.31 0.55 0.41 0.35 0.48 0.45 0.33

4.2. Effect of strain path The dynamic globularization of α lamellae was found to be significantly influenced by the relationship of strain paths between β predeformation and (α+β) compressive deformation. Fig.10 shows the microstructures after deformation at 900°C and a strain rate of 0.01s-1, to height reductions of 30%, 50%, 70% respectively for sample P and sample V. In Figs.10 (a)-(b), the lamellar fragmentation and dynamic globularization were different for the two kinds of loading directions. The majority of α laths still kept straight and the α platelets in the same colony were also parallel to each other in

ACCEPTED MANUSCRIPT sample P. The α laths and β matrix in some area still kept the initial orientation relationship. In fact, the dynamic fragmentation of α lamellae was only observed in the colonies with a specific crystal orientation, in which the basal and prism slip

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systems of α laths could operate easily. The other laths with hard orientations just rotated a little to the metal flow direction with strain increasing and the distribution of

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the globularized grains was greatly heterogeneous. On the other hand, the α laths in sample V have almost broken up into short platelets, suggesting that the slip systems of α lamellae in sample V have been activated effectively. Meanwhile, the

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non-globularized laths left in sample V have kinked, and if further compressive deformation was followed to be conducted on it, the kinked laths would completely

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break up into globularized grains quickly. In Fig.10 (c), the α platelets perpendicular to the compressive axis were elongated along the metal flow direction. In addition, the

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other orientations of α laths would also be observed in sample P. However, there were

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few α laths with the geometric orientation perpendicular to the compressed axis left in sample V (Fig.10 (d)), and most of α lamellae have globularized into the equiaxed

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particles. Also, the distribution of globularized grains in sample V was more homogeneous than that in sample P. When the height reduction increased to 70% (Figs.10 (e) and (f)), most of α laths in sample V have globularized but some long α

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platelets still existed in sample P. The initial lamellar microstructure of sample V has almost transformed into the equiaxed microstructure. The globularized fraction of sample V was obviously higher than that of sample P (92% and 81% respectively). Comparing the distribution of grain boundary misorientation in Figs.8 (b) and (c), the fraction of HABs in sample V was higher than that in sample P. Also, the globularized new grains with random crystal orientation were more in sample V. This also suggests the dynamic globularization would be promoted by the strain path of sample V. From the OM of initial microstructure after prior β deformation in Fig.3 (a), it could be found that the geometric orientations of most α laths were nearly parallel to the compressive axis of (α+β) deformation in sample P and vertical to that in sample V. Now, it seems to be accepted that dynamic globularization of α lamellae is easier if its geometric orientation is parallel to compressive direction, in which the kinking of α

ACCEPTED MANUSCRIPT lamellae and subsequent fragmentation would easily take place. Namely, the geometric orientation of α lamellae in sample P is thought to be more beneficial to dynamic globularization than that in sample V. However, the globularized fraction of

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sample V was higher than that of sample P. This phenomenon indicates that the

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geometric orientation of α lamellae parallel to compressive axis could not certainly

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promote dynamic globularization. It is highly possible that the crystal orientation is more significant for the lamellar globularization, due to its relationship with the

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activation of slip systems. (b)

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(a)

(d)

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(c)

(e)

(f)

Fig.10. Microstructures after deformation at 900°C and a strain rate of 0.01s -1, to the height reductions of (a)30%, (c)50%, (e)70% for sample P and (b) 30%, (d)50%, (f)70% for sample V.

ACCEPTED MANUSCRIPT The initial microstructures of samples P and V were identical but the corresponding dynamic globularization rates were different, which was mainly associated with the relationship between crystal orientations of α laths and corresponding compressive

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direction. The α phase textures represented by {0002} of initial microstructures are

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illustrated in Fig.11. Compared with the texture of the microstructure with β heat

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treatment, the microstructure after prior β deformation had an apparent preferred orientation. To be specific, the angle between CA (of β predeformation) and [0002] crystal orientation of close-packed hexagonal was similar for most α laths, from 28°

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to 40°, and the most intensive distribution was around 34° from CA. On the contrary, the orientation distribution was a little disordered in Fig.11 (a) and the most intensive

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[0002] crystal orientation was nearly parallel to CA. Hence, the special initial texture was another reason for the comparatively low dynamic globularization rate of sample

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O. As reported previously, the lowest globularization rate of a two-phase titanium

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alloy was observed in the hard orientations with the c-axis ([0002] crystal orientation) tilted less than 15° from the compressed direction [16]. In these hard crystal

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orientations, basal and prism slip systems could not be effectively activated, and relatively little slip and lamellar fragmentation would take place. When the c-axis of close-packed hexagonal was tilted 55° from the compressed axis, the highest

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efficiency of dynamic globularization would be observed, due to the simultaneous operation of basal and prism slip systems in α laths [16]. In Fig.9 (b), it could be quantitatively calculated that the [0002] crystal orientation of α laths was averagely tilted 56° from the compressive direction of sample V and approximately 34° from that of sample P during (α+β) deformation, suggesting that the operating efficiency of basal and prism slip systems in sample V was higher than that in sample P. In fact, the prism slip can slice α lamellae, making ribbons of lamellar microstructure from the initial planar geometry. The ribbons will be then chopped into many equiaxed particles by the basal slip, which finally promotes the fragmentation of α laths. Therefore, it was specific α lamellar texture produced by prior β deformation that led to the higher dynamic globularization rate in sample V. In addition, it could be found that the formation of LABs in sample V was mainly along the length of α laths while

ACCEPTED MANUSCRIPT the LABs in sample P was transverse, as indicated by black arrows in Figs.7 (b) and (c). The lengthwise and transverse LABs were closely related to the operation of prism and basal slip in α laths respectively, suggesting that the operation of slip

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systems in sample P and V was different. This further confirmed that initial texture

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would affect the fragmentation and dynamic globularization of α laths with different

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strain paths. (a)

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(b)

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Fig.11. α phase texture represented by {0002} of the initial microstructure after (a) β heat treatment at

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1020°C, 15min, AC; (b) β deformation at 1020°C and a strain rate of 1s -1, to the height reduction of 50%.

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4.3. Globularization kinetics

The quantitative measurements of lamellar α globularized fraction as a function of true strain respectively for samples O, P and V are summarized in Fig.12. The

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globularized fraction f g is assumed by Avrami equation:

f g  1  e x p[k (   c ) n ]

(1)

where  c in Eq.(1) refers to the critical true strain for the initiation of lamellar globularization. The non-linear curve fitting using Eq.(1) showed that the dynamic globularization rate increased with strain in a typical sigmoid model and agreed well with the experimental results. The dynamic globularization of α laths is a representative thermal activation process and its occurrence requires the lowest activation energy. In the present work, the critical strains predicted by Eq.(1) were 0.358, 0.301 and 0.297 respectively for samples O, P and V, which were slightly lower than that in TC11 titanium alloy with a colony alpha microstructure (0.42~0.65) [27]. This was associated with the difference in lamellar thickness of the initial

ACCEPTED MANUSCRIPT microstructure. The thickness of α platelet in current work was thinner and dynamic globularization of thin α laths would be easier. The critical strain of sample O was comparatively high, indicating that its dynamic globularization of α lamellae was

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difficult. Furthermore, the critical strain (0.29~0.36) for the initiation of dynamic globularization was noticeably higher than the peak strain of stress-strain curves (less

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than 0.1). This suggested that the mechanism of globularization for present material and deformation conditions was different from dynamic recrystallization (DRX), for which the critical strain was less than the strain of peak stress [27]. DRX is highly

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dependent on dislocation density for the nucleation of new grains, which would just reach the maximum value at the peak stress. While the strain for initiation of

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globularization was higher than that of peak stress and the dislocation glide has been activated right now, resulting in the decrease of dislocation density. Thus, the present

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globularization mechanism should be continuous dynamic recrystallization (CDRX),

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for which the new grains are formed by dislocation movement without nucleation. This shows a good agreement with the conclusion drawn from EBSD results. In

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addition, the kinking of α laths has taken place before the initiation of dynamic globularization, leading to the reorientation of lamellae and the increase of critical strain for dynamic globularization.

Globularization fraction

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1.0 0.9 0.8 0.7 0.6 0.5 0.4

Sample O Sample P Sample V

0.3 0.2 0.1 0.0

0.3

0.6

0.9

1.2 1.5 True strain

1.8

2.1

2.4

Fig.12. The globularization fractions of sample O, sample P and sample V to the different height reductions at 900°C and a strain rate of 0.01s-1.

5. Conclusions Experimental study has been carried out to check the effect of prior β workings on

ACCEPTED MANUSCRIPT (α+β) compressive deformation of TA15 titanium alloy with an initial lamellar microstructure. The flow behaviors and dynamic globularization of α lamellae were discussed in depth. The main conclusions drawn above are as follows:

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(1) The initial β grain size has no significant influence on flow stress, but prior β

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deformation will decrease flow stress of (α+β) deformation. Flow softening is

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independent of microstructure phenomena related to dynamic globularization. (2) The long lamellae in large grains tend to become kinked and then break up into short platelets, leading to the delay of dynamic globularization. The β

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predeformed microstructure has a high globularized fraction, because the formation of α/α substructures is promoted by the rotation of short α laths.

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(3) The dynamic globularization in β predeformed microstructure is affected by strain path. When compressive directions of (α+β) deformation and β predeformation are

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vertical, the globularization rate can be highly enhanced.

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(4) The dynamic globularization rate is not necessarily associated with the geometric orientation of α lamellae. The crystal orientation is more important for dynamic

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globularization in terms of its relationship with the activation of slip systems.

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Acknowledgement

The authors would like to gratefully acknowledge the support of National Natural Science Foundation of China (No. 51575449), Research Fund of the State Key Laboratory of Solidification Processing (NWPU), China (No. 104-QP-2014).

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ACCEPTED MANUSCRIPT [14] Y.V.R.K. Prasad, T. Seshacharyulu, S.C. Medeiros, et al., Effect of prior β-grain size on the hot deformation behavior of Ti-6Al-4V: Coarse vs coarser, J. Mater. Eng. Perform. 9 (2000) 153-160.

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Alloys Comp. 480 (2009) 922-927.

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Graphical abstract

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Research Highlights

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1. Effect of prior β working on the flow behavior of TA15 titanium alloy during (α+β)

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deformation is revealed.

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2. β predeformation will promote the dynamic globularization of α laths. 3. The globularization rate in the microstructure with β predeformation is affected by

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strain path.