Hot deformation behavior and microstructure evolution of TA15 titanium alloy with nonuniform microstructure

Hot deformation behavior and microstructure evolution of TA15 titanium alloy with nonuniform microstructure

Materials Science & Engineering A 689 (2017) 243–251 Contents lists available at ScienceDirect Materials Science & Engineering A journal homepage: w...

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Materials Science & Engineering A 689 (2017) 243–251

Contents lists available at ScienceDirect

Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea

Hot deformation behavior and microstructure evolution of TA15 titanium alloy with nonuniform microstructure

MARK



Pengfei Gao, Mei Zhan , Xiaoguang Fan, Zhenni Lei, Yang Cai State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, P.O. Box 542, Xi’an 710072, PR China

A R T I C L E I N F O

A BS T RAC T

Keywords: TA15 titanium alloy Nonuniform microstructure Hot deformation behavior Microstructure evolution

The flow behavior and microstructure evolution of a near α titanium alloy with nonuniform microstructure during hot deformation were studied by isothermal compression test and electron backscatter diffraction technique. It is found that the nonuniform microstructure prior to deformation consists of equiaxed α, lamellar α in the colony form and β phase, and the α colony keeps the Burgers orientation relationship with β phase. The flow stress of nonuniform microstructure exhibits significant flow softening after reaching the peak stress at a low strain, which is similar to the lamellar microstructure. Nevertheless, the existence of equiaxed α in nonuniform microstructure makes its flow stress and softening rate be lower than the lamellar microstructure. During deformation, the lamellar α undertakes most of the deformation and turns to be rotated, bended and globularized. Moreover, these phenomena exhibit significant heterogeneity due to the orientation dependence of the deformation of lamellar α. The continuous dynamic recrystallization and bending of lamellar α lead to the “fragmentation” during globularization of lamellar α. The bending of lamellar α is speculated as a form of plastic buckling, because the bending of lamellar α almost proceed in the manner of “rigid rotation” and presents opposite bending directions for the adjacent colonies.

1. Introduction Titanium alloys have been gaining extensive applications in the aviation and aerospace fields due to their excellent mechanical and physical properties, such as high specific strength, good thermal stability, excellent corrosion resistance and superior creep resistance [1–4]. Since the titanium alloy components often serve as the key loadbearing structures under severe working conditions, not only precision macroscopical shape but also well microstructure and mechanical properties are required for the forming of those components. The thermomechanical processing (TMP) involving multistage hot deformation is the most applicable forming method to manufacture the titanium alloy components as it can tailor the microstructure while shaping [5,6]. In the TMP of titanium alloy components, the final properties are significantly dependent on the deformation behavior and microstructure evolution during hot deformation, which are very sensitive to the initial microstructure and processing conditions (such as the deformation temperature, deformation amount, etc.) [7,8]. The nonuniform microstructure of titanium alloy means a microstructure mixed with the equiaxed α, lamellar α and β phase, which has been defined by



Semiatin et al. [9] in the Ti6242Si alloy. The authors [10] also found the nonuniform microstructure in the isothermal local loading forming (ILLF) of TA15 titanium alloy. To obtain the tri-modal microstructure with well mechanical properties in ILLF, the authors proposed the processing scheme of near-β forging followed with conventional forging. In this processing scheme, the nonuniform microstructure was generated prior to conventional forging due to the high-low temperature history. As a result, the deformation behavior and evolution mechanisms of nonuniform microstructure during conventional forging play crucial roles in the final tri-modal microstructure and mechanical properties. Therefore, understanding the deformation behavior and microstructure evolution of nonuniform microstructure is a topic of fundamental and technological importance to reveal the deformation mechanisms in depth and control the microstructure evolution during the hot working of titanium alloy. By now, considerable researches have been carried out on the flow behavior and microstructure evolution at various conditions during the hot deformation of titanium alloys. According to the initial microstructure, these works can be broadly classified into two groups aiming at the lamellar microstructure and equiaxed microstructure. Semiatin et al. [11,12] found that the significant flow softening and dynamic

Corresponding author. E-mail address: [email protected] (M. Zhan).

http://dx.doi.org/10.1016/j.msea.2017.02.054 Received 18 October 2016; Received in revised form 18 January 2017; Accepted 16 February 2017 Available online 17 February 2017 0921-5093/ © 2017 Elsevier B.V. All rights reserved.

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globularization of lamellar α are two main features in the hot deformation of Ti-6Al-4V alloy with lamellar microstructure. Then, Semiatin and Bieler conducted series of investigations on the flow softening mechanism and its dependence on the processing parameters [11], initial lamellar α thickness [13], colony size [11], morphology of lamellar α [14] and texture [15,16]. The evolution mechanisms of lamellar α, especially the dynamic globularization, have also been studied [17–21]. Moreover, Babu and Lindgren [22], Park et al. [23] and Gao et al. [24] have developed unified prediction models of flow softening and dynamic globularization kinetic, which were successfully applied in the hot deformation of Ti-6Al-4V and TA15 alloys with lamellar microstructure. As for the hot deformation of equiaxed microstructure, two common flow behaviors, i.e., softening and steady state, have been analyzed in detail and related to the mechanisms of dynamic recrystallization (DRX) and dynamic recovery (DRV). In addition, the microstructure evolution mechanisms have been revealed from various aspects, such as the apparent deformation activation energy, strain rate sensitivity, processing map and microstructure observation [25–30]. On the other hand, the effects of processing parameters on microstructure parameters (volume fraction and grain size of equiaxed α, etc.,) have been widely studied [31–34]. Moreover, Fan et al. [35,36] proposed an internal-state-variable based selfconsistent constitutive model to realize the unified prediction of flow behavior and microstructure evolution in the hot working of two-phase titanium alloys with equiaxed microstructure. However, little work has been conducted on the hot deformation behavior and microstructure evolution of titanium alloy with nonuniform microstructure, thus, which is an urgent subject to be investigated. The aim of present paper is to address the flow behavior and microstructure evolution of TA15 alloy with nonuniform microstructure during hot deformation, and compare with the lamellar and equiaxed microstructures. The results will enrich the knowledge about the deformation and microstructure evolution mechanisms and provide technical guidance for the microstructure control in the hot deformation of titanium alloys.

Fig. 1. Microstructure of the as-received TA15 alloy.

At last, the deformed specimens were sectioned parallel to the compression axis and the cutting surface was processed and prepared for metallographic examination. For electron backscatter diffraction (EBSD) examination, the samples were electropolished with a solution of 5% perchloric acid, 65% methyl alcohol and 30% butanol at 28 °C with a voltage of 25 V and the polishing time was 40 s. The EBSD observations were conducted on a Hitachi S-3400N scanning electron microscope equipped with EBSD detector. The grain crystal orientation and other microstructure characteristics were analyzed through the HKL-Channel 5 software. Besides, the microstructure was also observed by optical microscope and scanning electron microscope using standard techniques. Two regions of the deformed specimens were examined in detail, as shown in Fig. 2. Region A is at the center of the specimen where the local effective strain is near 1.10 (obtained by finite element simulation), and Region B is half way from the center to the outer diameter where the local effective strain is near 0.85.

3. Results and discussion

2. Material and methods

3.1. Microstructure prior to deformation

The material employed in this work was a near-alpha TA15 titanium alloy provided by Western Superconducting Technologies Co., Ltd. Its chemical composition is shown in Table 1, and the measured β-transus temperature is 985 °C. The microstructure of the as-received TA15 alloy is a typical equiaxed microstructure, as shown in Fig. 1. It consists of about 50% equiaxed α within the transformed β matrix. Three cylindrical specimens of 10 mm in diameter and 15 mm in height were machined from the as-received wrought plate, of which the axis was parallel to the normal direction of the plate. Before the hot compression test, the pre-treatment (970 ℃/20 min/AC) was carried out on one specimen to obtain the nonuniform microstructure prior to deformation. For comparison, another specimen was pre-treated (1020 ℃/5 min/AC) to obtain the initial lamellar microstructure. While, the last one specimen without pre-treatment would get the equiaxed microstructure prior to hot deformation. In the isothermal compression test, three specimens were heated to 930 ℃, held for 15 min, compressed to 50% reduction at the constant strain rate of 0.01 s−1, and then air cooled. The microstructures prior to deformation of three specimens were obtained by water quenching after heating and holding, which will be discussed in Section 3.1.

Fig. 3 shows the microstructure prior to deformation of three specimens with different morphology, of which the quantitatively measured microstructure parameters are given in Table 2. The nonuniform microstructure (Fig. 3(a)) consists of 9.2% equiaxed α, 36.0% lamellar α and balance β phase. The grain size of equiaxed α is 12.1 µm, and the thickness of lamellar α is 1.1 µm. Fig. 4 shows the inverse pole figure and local crystallographic relationship between the adjacent lamellar α and β phases (in the yellow circle), respectively. It can be found that the (0001) plane and one of < 11–20 > directions of lamellar α phase are parallel to one of the {110} planes and one of the < 111 > directions of β phase, respectively, for the colony A, B and C, which suggests that the lamellar α and β phases in a local colony keep the Burgers orientation relationship (OR) in the nonuniform microstructure. Fig. 3(b) shows the lamellar microstructure, a typical transformed microstructure, with the β grain size of 164.5 µm, the colony size of 68.2 µm and the lamellar α thickness of 1.1 µm. From Fig. 5, it can be seen that the lamellar α and β phases in a local colony also keep the Burgers OR in the lamellar microstructure. The equiaxed

Table 1 The chemical composition of TA15 alloy. Element

Al

Zr

Mo

V

Fe

O

N

C

Ti

Content (wt%)

6.69

2.26

1.77

2.25

0.14

0.12

0.001

0.005

Balance Fig. 2. Schematic of the microstructure observation locations.

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Fig. 3. The microstructure prior to deformation of three specimens with different morphologies: (a) nonuniform microstructure; (b) lamellar microstructure; (c) equiaxed microstructure.

nonuniform microstructure is calculated as 0.39. The flow stress of lamellar microstructure also presents obvious softening behavior after the peak stress at the strain of 0.045 (Fig. 6), whose softening index is 0.41. While, the stress curve of lamellar microstructure is above that of the nonuniform microstructure. The sample of equiaxed microstructure almost exhibits the steady flow stress in the whole strain range, only a very small stress drop occurs after the peak stress at the strain of 0.028, as shown in Fig. 6. The work hardening rate and softening rate after peak stress of three kinds of microstructure were calculated and shown in Fig. 7. It can be seen that the dσ/dε of nonuniform microstructure is very close to the lamellar microstructure in the whole strain range (Fig. 7(a)). Nevertheless, a small difference exists in the maximum of softening rate for these two kinds of microstructure, as shown in Fig. 7(b). The lamellar microstructure presents obviously greater softening rate than the nonuniform microstructure at the beginning of softening stage. On the other hand, the equiaxed microstructure presents different variation trend of softening rate compared to the nonuniform microstructure and lamellar microstructure, as shown in Fig. 7(b). Its softening rate quickly decreases to near 0 at the strain about 0.1 and then fluctuates around 0. Moreover, the maximum softening rate of equiaxed microstructure is smaller than those of the nonuniform and lamellar microstructures. Previous studies [11,13,15] have pointed out that the flow softening of titanium alloy with lamellar microstructure is essentially related to the deformation mechanisms of lamellar α. The slip transmission across α/β interfaces and the resulting loss of Hall-Petch strengthening account for the flow softening to a first order. Besides, the evolution of substructure and bending/kinking of lamellar α play a second-order

Table 2 Possible misorientation between two α variants inherited form the same β grain [17,39]. Misorientation angle (deg.)

Axis

Probability at random condition (%)

0 10.5 60 60.8 63.3 90

– [0 0 0 1] [1 1 −2 0] [−10 −7 17 3] [−10 5 5 3] [7 −17 10 0]

– 9.1 18.2 36.4 18.2 18.2

microstructure, composed of equiaxed α and β phases, is shown in Fig. 3(c). Its content and grain size of equiaxed α are 38.1% and 15.3 µm, respectively. 3.2. Flow behavior The flow stress curves of different initial microstructures tested at 930 ℃ and 0.01 s−1 are shown in Fig. 6. The flow stress of sample with nonuniform microstructure quickly reaches the peak stress at a low strain (0.046) and then decreases with increasing strain exhibiting significant flow softening. When the strain increases to the order of 0.69, the flow stress tends to exhibit a steady state. The softening index defined by Semiatin et al. [16] is applied to quantify the flow softening degree, as follows:

γ = (σp − σss )/ σp

(1)

where σp is the peak stress and σss is the stead state stress. Here, the stress at the strain of 0.69 is used as σss. The softening index of

Fig. 4. The inverse pole figure (CA, compression axis) (a) and orientation relationship between the adjacent lamellar α and β phases in the yellow circle (b) of the initial nonuniform microstructure. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

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Fig. 5. The inverse pole figure (CA, compression axis) (a) and orientation relationship between the adjacent lamellar α and β phases in the red ellipse (b) of the initial lamellar microstructure. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

lamellar microstructure may be caused by the extra 9.2% equiaxed α in the nonuniform microstructure. The existence of equiaxed α decreases the contents of lamellar α and α/β interfaces, and increases the deformation coordination, which would then reduce the flow stress and softening rate. 3.3. Microstructure evolution 3.3.1. General observation Fig. 8 shows the deformed microstructure of sample with initial nonuniform microstructure at two regions with different strains. Comparing to the initial microstructure (Fig. 3(a)), the morphology and spatial trace orientation of lamellar α produce great changes. The orientation of lamellar α trace presents a trend of concentrated distribution to the perpendicular direction of compression axis, and the concentration degree increases with strain (Fig. 8(b)). The variation from homogeneous distribution (initial microstructure) to concentrated distribution with strain suggests that the lamellar α would gradually rotate themselves perpendicular to the compression direction during deformation. Besides, some lamellar α are bent or globularized generating many small equiaxed α, as indicated by the red circle in Fig. 8. By comparing Fig. 8(a) and (b), it also can be found that the globularization fraction of lamellar α increases with strain. On the other hand, it should be noted that the spatial trace orientation, bending and globularization of lamellar α exhibit significant heterogeneous characteristic. Although the trace orientation of lamellar α keeps consistent broadly in one colony, the trace orientation changes of

Fig. 6. The true stress – true strain curves of TA15 alloys with different initial microstructures hot compressed at 930 ℃ and 0.01 s−1.

role in the flow softening. In this work, the nonuniform microstructure presents 36.0% lamellar α and keeping the Burgers OR with β phases in the local colony (Fig. 4), which is the same as that of lamellar microstructure (Fig. 5). As a result, the deformation mechanisms of lamellar α in the nonuniform microstructure may be close to that of lamellar microstructure (this will be discussed in Section 3.3), thus presenting the similar flow softening behavior. However, the lower flow stress and softening rate of nonuniform microstructure than the

Fig. 7. The variations of work hardening rate (a) and softening rate (b) with strain.

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Fig. 8. The deformed microstructure of sample with initial nonuniform microstructure (the compression direction is vertical): (a) Region B with strain of 0.85; (b) Region A with strain of 1.10. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

proportion of deformation and whose morphology changes little, as shown in Fig. 9(b). For the initial nonuniform microstructure, it can be found that the morphology of equiaxed α also changes little while the lamellar α changes greatly during deformation by comparing Fig. 8 with Fig. 3(a). This indicates that the combination of lamellar α and β phase still deforms more easily than the equiaxed α, and undertakes a large proportion of deformation. The lamellar α in nonuniform microstructure plays a dominant role in the deformation and presents very similar evolution laws to the lamellar microstructure, which explains why these two microstructures present the similar flow softening behavior as described in Section 3.2.

various colonies are different from each other. Moreover, the globularization fractions of lamellar α present great heterogeneity for one colony and different colonies. At larger strains, many lamellar α have been globularized into small equiaxed grains but some of them remain almost undeformed (Fig. 8(b)). The deformed microstructure of sample with initial lamellar microstructure and equiaxed microstructure are given in Fig. 9(a) and (b), respectively. It can be found that the rotation, bending and globularization of lamellar α produced in the nonuniform microstructure are close to those in the lamellar microstructure. It has been reported that in the hot deformation of lamellar microstructure, the evolution of lamellar α mainly depends on its initial orientations (trace orientation and crystallographic orientation) to the compression axis and the crystallographic orientation relationship between the lamellar α and β phase [18,37]. As described in Section 3.1, the lamellar α in the nonuniform microstructure and lamellar microstructure both exist in colony with random orientation and keep Burgers OR with β phase. Thus, the evolution of lamellar α in the nonuniform microstructure is close to the lamellar microstructure. However, the deformation heterogeneity of lamellar α in the nonuniform microstructure is smaller than that in the lamellar microstructure. This may be related to the colony size of nonuniform microstructure is much smaller than the lamellar microstructure due to the existence of equiaxed α, which is beneficial to improve the deformation homogeneity. On the other hand, in the hot deformation of equiaxed microstructure, the equiaxed α particles act as hard inclusions in the soft β matrix and β phase is easier to deform than equiaxed α [35,38]. As a result, the equiaxed α undertakes a small

3.3.2. Deformation mechanism of lamellar α The deformed nonuniform microstructure at different strains are shown as EBSD maps (Euler angle) in Fig. 10. It can be found that the globularization fraction of lamellar α varies with its color (orientation), which verifies that the heterogeneous evolution of lamellar α is determined by its orientation. To further reveal the deformation mechanism of lamellar α, the evolution of grain boundary misorientation during deformation was measured, as given in Fig. 11. In this work, the boundaries with misorientation between 2° and 15° were defined as low-angle grain boundaries (LABs), and those of misorientation over 15° as high-angle grain boundaries (HABs). The microstructure prior to deformation (Fig. 11(a)) possesses quantities of HABs (83.8%) and presents higher probabilities at the misorientation of 60° and 90°. This is the consequence of lamellar α transforming from β phase and satisfying the Burgers OR. It

Fig. 9. The deformed microstructure of samples with initial lamellar microstructure (a) and equiaxed microstructure (b) in Region A (the compression direction is vertical).

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Fig. 10. The Euler orientation map of the deformed sample with initial nonuniform microstructure (the compression direction is vertical): (a) Region B with strain of 0.85; (b) Region A with strain of 1.10.

is known that 12 equivalent α variants with the Burgers OR can be formed in the parent β grain. There are 144 relative orientations between adjacent α variants from the same β grain, whose possible misorientation between each other and the corresponding probability at random condition are listed in Table 2 [17,39]. After deforming to the strain of 0.85 (Fig. 11(b)), the misorientation peaks related to the Burgers OR are weakened greatly, and the fraction of LABs dramatically increases from 16.2% (prior to deformation) to 56.3%. Increasing the strain further, the boundary misorientation increases gradually with some LABs transforming to

HABs. When the strain increases from 0.85 (Fig. 11(b)) to 1.10 (Fig. 11(c)), the fraction of HABs increases from 43.7% to 49.3% and the globularization fraction of lamellar α increases concurrently. The above grain boundary evolution coincides with the continuous dynamic recrystallization(CDRX), i.e., the process of “tangled cell structure→ subgrain bounded by LABs→HABs”, during deformation [40,41]. This suggests that CDRX is an underlying mechanism leading to the “fragmentation” during globularization of lamellar α in the deformation of nonuniform microstructure.

Fig. 11. Distribution of grain boundary misorientation in α phase for the sample with initial nonuniform microstructure: (a) prior to deformation; (b) Region B with strain of 0.85; (c) Region A with strain of 1.10.

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Fig. 12. The SEM image (a) and corresponding inverse pole figure (b) of a region containing different morphology of lamellar α in the sample with initial nonuniform microstructure. (In the inverse pole figure, β phase is indicated by gray areas, the black lines correspond to HABs, and the silver lines represent LABs).

smooth bending, but the bending direction, angle (the angle included by two bended sides) and curvature are different from each other. The bending angle of colony B is the smallest, which may be the early stage of bending process. Many LABs but few HABs can be found in this colony, as shown in Fig. 12(b). Colonies C and D were bent to a greater extent with the bending angles approaching to 90°. At these situations, the lamellar α experienced significant crystallographic rotation. Some HABs were generated and the crystallographic misorientation between two bended parts reaches about 60° and 50° for C and D, respectively. These phenomena suggest that the bending of lamellar α is accompanied by the generation of LABs and HABs resulted from the dislocation movement. The LABs and HABs accommodate the bending of lamellar α during straining. Moreover, the HABs associated with bending can also result in the “fragmentation” of initial lamellar, and then promote the globularization of lamellar α. Besides the CDRX, this is another major mechanism responsible for the globularization of lamellar α in the deformation of nonuniform microstructure. The kinking of lamellar α is also an important phenomenon in the deformation of lamellar microstructure, as observed by Semiatin et al. [11,14] in the hot working of Ti-6Al-4V with lamellar microstructure. It was thought that the kinking may be a form of plastic buckling analogous to that occurs during the compression of beams. However, there is still a lack of strong evidence to support their conclusion. In this work, it was found that the lamellar α and β phases in colony C still nearly keep the Burgers OR with only a very small deviation after great bending (Fig. 14). This means that the bending almost proceeded in the manner of “rigid rotation”. In addition, colonies C and D are very close in location but bended in the opposite directions, which is also applicable for the colonies A and B. According to the above two points, it can be concluded that the bending of lamellar α in the nonuniform microstructure may be a form of plastic buckling rather than the shear band or deformation band. For the deformation of lamellar microstructure, it has been proposed that the kinking of lamellar α preferentially takes place at the colony with hard orientations where the c-axis is nearly parallel to the compression axis [18]. In this work, the c-axis of colony B is parallel to the compression axis, exhibiting the hard orientation, as shown in Fig. 12(b). However, its bending degree is very small. This may be because the local principal stress is different from the compression axis. It suggests that the

Fig. 12 shows the scanning electron microscope (SEM) image and corresponding inverse pole figure of a region containing different morphology of deformed lamellar α. Since the sample was titled 70° in the EBSD test, there exists a slight difference in size between these two images. From the SEM image, it is seen that some lamellar α still keep flat colony form (in the center), while others were bent to some extent (indicated by A, B, C, D). For the flat α colony, some LABs produced internally, as shown in Fig. 12(b). In addition, the cumulative misorientations along and across the flat α colony (L1 and L2 in Fig. 12(b)) are obtained by the "Accumulated Misorientation Profile" function in HKL-Channel 5 software, as shown in Fig. 13. The zero point for x axis of Fig. 13 is just the reference first point of line. For both of L1 and L2, the left end points are the first points, i.e., the cumulative misorientations are from left to right along the lines. The misorientation data in Fig. 13 indicates that the orientations along and across the flat α colony both change greatly, and the maximum misorientations in two directions both exceed 20°. This is the reason for the heterogeneous evolution of lamellar α in the same colony. As for the bending of lamellar α, it can be seen from the SEM image (Fig. 12(a)) that four colonies (A, B, C, D) all produce continuous and

Fig. 13. Point-to-origin misorientation along L1 and L2 in Fig. 12(b).

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Fig. 14. Orientation relationship between the bended lamellar α and β phases in colony C of Fig. 12: (a) the upper part (white circle); (b) the lower part (blue circle). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

References

orientation of colony relative to the local stress may be the major influencing factor for the occurrence of bending.

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4. Conclusions In this paper, the flow behavior and microstructure evolution of titanium alloy with nonuniform microstructure during hot compression are investigated and compared to the lamellar and equiaxed microstructures. The conclusions can be summarized in three parts as follows: (1) Microstructure prior to deformation. Prior to deformation, the nonuniform microstructure consists of equiaxed α, lamellar α in the colony form and β phase. The α colony keeps the Burgers orientation relationship with β phase, which is the same as the lamellar microstructure. However, the size of α colony in the nonuniform microstructure is much smaller than the lamellar microstructure. (2) Flow behavior. As the lamellar microstructure, the flow stress of nonuniform microstructure exhibits significant flow softening after reaching the peak stress at a low strain. However, the existence of equiaxed α in nonuniform microstructure makes its flow stress and softening rate lower than the lamellar microstructure. (3) Microstructure evolution. The lamellar α in nonuniform microstructure undertakes most of the deformation and presents the evolution process of lamellar trace rotation, bending and globularization. In addition, these evolution phenomena exhibit significant heterogeneity. The continuous dynamic recrystallization and bending of lamellar α are two mechanisms leading to the “fragmentation” during the globularization of lamellar α. The bending of lamellar α is regarded as a form of plastic buckling rather than the shear band or deformation band. Acknowledgements The authors would like to gratefully acknowledge the support of National Natural Science Foundation of China (Nos. 51605388 and 51575449), 111 Project (B08040), the Research Fund of the State Key Laboratory of Solidification Processing (NWPU), China (Grant no. 131QP-2015), the Fundamental Research Funds for the Central Universities, and the Open Research Fund of State Key Laboratory of Materials Processing and Die & Mould Technology, Huazhong University of Science and Technology. 250

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