- Email: [email protected]
The evolution of deformation microstructure in electron beam melted Ta-2.5W alloy during cold rolling
G Model
ARTICLE IN PRESS
FUSION-9282; No. of Pages 5
Fusion Engineering and Design xxx (2017) xxx–xxx
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
The evolution of deformation microstructure in electron beam melted Ta-2.5W alloy during cold rolling S. Wang a,b,d , L. Niu a , C. Chen b,d,∗ , Y.L. Jia c , M.P. Wang c , Z. Li c , Z.H. Zhong b,d , P. Lu a,e , P. Li b,d , Y.C. Wu b,d a
Institute of Industry and Equipment Technology, Hefei University of Technology, Hefei, Anhui,230009, China School of Materials Science and Engineering, Hefei University of Technology, Hefei, Anhui, 230009, China c School of Materials Science and Engineering, Central South University, Changsha, Hunan, 410083, China d National-Local Joint Engineering Research Center of Nonferrous Metals and Processing Technology, Hefei University of Technology, Hefei, Anhui, 230009, China e State Key Laboratory for Strength and Vibration of Mechanical Structures, Xian Jiaotong University, Xian, Shanxi, 710049, China b
h i g h l i g h t s • • • •
This article systematically sheds light on the characteristics of microbands in Ta-2.5W during cold rolling. The morphology of microbands is quite different at different reduction levels. The evolution of dislocation structure in EPBs is investigated by TEM in detail. The development mechanism of microbands is proposed.
a r t i c l e
i n f o
Article history: Received 22 September 2016 Received in revised form 17 March 2017 Accepted 20 March 2017 Available online xxx Keywords: Ta-W alloy Cold-rolling Deformation microstructure Microbands Extended planar boundaries
a b s t r a c t The evolution of deformation microstructure in electron beam melted Ta-2.5W alloy was investigated. The morphology of microbands appear different in different reduction levels. The microbands are relatively straight at low to medium strain and can cross the grain boundary. They become wavy when the reduction reaches 70%. The microbands are parallel with rolling direction when the reduction is up to 90%. The average inclination angles between microbands and rolling direction is about 35◦ at 20% reduction and get smaller and smaller with increasing strain. EPBs are developed when the reduction is 20%. Most of grains contain one or two groups of EPBs occupying almost the whole grain interiors when the reduction is larger than 30%. The EPBs are fragmented, diffused and curved in the early stages of development. When the reduction reaches 40%, the EPBs become more well-developed and homogeneous. The dislocations within EPBs tend to rearrange themselves with increasing strain in a sequence, from tangled dislocations, followed by parallel dislocations and finally into dislocation nets. The evolution of microbands is related with both the cross-slip of dislocations and the splitting of high dense dislocation walls. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Ta-W alloy is a kind of Ta-based solid-solution strengthened refractory alloy, which is widely used in high-tech applications, such as power, aerospace and nuclear engineering [1]. Ta and Ta-W alloys are candidate protective first wall materials for fusion reactors and target material for spallation neutron sources which are
∗ Corresponding author at: School of Materials Science and Engineering, Hefei University of Technology, Hefei, Anhui, 230009, China. E-mail addresses: [email protected], [email protected] (C. Chen).
also subject to a high flux of high-energy neutrons [2,3]. It has been found that the interaction between two helium atoms in tantalum shows repulsive or weak repulsive other than attractive in tungsten, which suggests that helium atoms are easy to move other than to be a cluster in tantalum [4]. It has also been found that tungsten grains with different orientations exhibited different irradiation resistance under deuterium (D), helium (He) and heavy ions bombardment [5]. More blisters tend to be formed in grains with the <111> orientation. It can be inferred that the irradiation resistance is closely related with their texture. Therefore, to get the beneficial texture, it is important to correlate the microstructures with
http://dx.doi.org/10.1016/j.fusengdes.2017.03.101 0920-3796/© 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: S. Wang, et al., The evolution of deformation microstructure in electron beam melted Ta-2.5W alloy during cold rolling, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.03.101
G Model FUSION-9282; No. of Pages 5
ARTICLE IN PRESS
2
S. Wang et al. / Fusion Engineering and Design xxx (2017) xxx–xxx
Fig. 1. EBSD ND IPF orientation images of Ta-2.5W alloy. Step size: 0.2 m. (a)5%, (b)10%, (c)20%, the orientated stereographic projection for the location A. (d)30%, the orientated stereographic projection for the location B. (e)40%, (f)60%, (g)70%, (h)90%. The dashed lines show the traces of EPBs.
orientations and study the evolution of microstructures during processing. It has been found that deformation texture with different deformation microstructures was formed in Ta-W alloy during cold rolling [6,7]. The deformation microstructure in micro-scale can be divided into cells or microbands, which are comprised of contin-
uous parallel extended planar boundaries (EPBs). The microbands play an important role in the properties of Ta-W alloy. Therefore, this article focuses on investigating the evolution of microbands during cold-rolled process.
Please cite this article in press as: S. Wang, et al., The evolution of deformation microstructure in electron beam melted Ta-2.5W alloy during cold rolling, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.03.101
G Model FUSION-9282; No. of Pages 5
ARTICLE IN PRESS S. Wang et al. / Fusion Engineering and Design xxx (2017) xxx–xxx
3
Fig. 2. Typical TEM images of Ta-2.5W alloy cold rolled by different reductions. (a)5%, (b)10%, (c)20%, (d)20%. The dashed lines show the traces of EPBs, which are aligned close to {110}. The arrows show dislocation loops.
2. Experiments The Ta-2.5W ingot with a diameter of 16 mm was prepared by an electron beam melting method. The casting cylinders with an average grain size of 4 mm were hot forged at 1473K–1073 K, the deformation ratio is about 40%. Then, the forged plates were annealed at 1573 K for 1 h, and a fully recrystallization plates with 10 mm in thickness can be obtained. The grain size was 100–200 m in the annealed plates. Finally, the annealed plates were cold rolled to different reduction at room temperature. The reduction per pass is 10–20%. Electron backscatter diffraction (EBSD) images were taken by FEI-Sirion 200 field emission scanning electron microscope (SEM) equipped with EDAX OIMTM Data Analysis software. EBSD investigations were done on ND (Normal direction) – RD (Rolling direction) planes of the plates. The TEM observation was conducted with a JEOL 2010 transmission electron microscope. The operated accelerating voltage is 200 KV. The specimens were jet polished in a mixing solution of HF, H2 SO4 and CH3 OH with ratio of 1:5:94 at 273 K. The specimens characterized by TEM were also cut from ND-RD sections. 3. Results and discussion Fig. 1 shows the typical EBSD microstructure of the Ta-2.5W alloy cold-rolled by different reduction levels. It can be clearly seen that the color in the grains is homogenous when the reduction is 5% (Fig. 1a). The color gradient can be found in grains when cold-rolled by 10% (Fig. 1b). Microbands appear when the reduction reaches 20%, as shown in Fig. 1c. It can be seen that the microbands in a grain are straight and mutually parallel. The dashed lines show the traces of microbands. It can be concluded from the analysis of the traces of these microbands through the inserted orientated stereographic
projection and EBSD data that the habit planes of these microbands are mainly aligned with the slip planes of {110}. When the strain is larger than 20%, the density of microbands increases dramatically with increasing strain. Meanwhile, the contrast of microbands becomes more and more distinct, which means the misorientation across EPBs gets larger and larger. When the reduction increases to 30%, the grains contain one or two groups of microbands occupying most region of the grain interiors, as shown in Fig. 1d. Most of the traces of microbands are still aligned close to {110}. It can also be clearly seen that the deformation microstructure in different orientations is quite different. That is to say, some orientations contain microbands and the others do not. When the strain increases to 60%, the microbands can cross the high angle boundaries, as shown in Fig. 1f, which can also be called as shear bands. The microbands become wavy when the reduction reaches 70% (Fig. 1g). They are usually inclined at about 10◦ with RD. Finally, it can be found that microbands are parallel with RD when the reduction is up to 90% (Fig. 1h). It can also be found from the ND orientation maps that the normal direction of most grains rotates to <111> or <100> with increasing strain, which means the deformation textures are developed. Fig. 2 shows the typical dislocation microstructure in Ta-2.5W alloy at low strain. After cold-rolled by 5%, plenty of long straight dislocations are formed without dislocation tangles, as shown in Fig. 2a. As shown in Fig. 2b, dislocation tangles are the typical dislocation microstructure in the specimens cold-rolled by 10%. However, there are no well-developed dislocation boundaries. Two typical kinds of dislocation boundaries, including cells boundaries and EPBs, are developed when the reduction reaches 20%, as shown in Fig. 2c and d, respectively. The orientation effects on the plastic deformation behaviors in body centered cubic metals have been studied [6,7]. The formation of cells or EPBs mainly depends on
Please cite this article in press as: S. Wang, et al., The evolution of deformation microstructure in electron beam melted Ta-2.5W alloy during cold rolling, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.03.101
G Model FUSION-9282; No. of Pages 5
ARTICLE IN PRESS
4
S. Wang et al. / Fusion Engineering and Design xxx (2017) xxx–xxx
Fig. 3. Typical EPBs structure of Ta-2.5W cold rolled by different reductions. (a)30%, (b)30%, (c)40%. The dashed lines show the traces of microbands. Two groups of EPBs can be seen in Fig. 3b and c. (d) The sketch shows the EPBs in Fig. 3c.
the orientations of the grains. EPBs can almostly appear in all orientations except the orientation of {001} <110> [6]. The dislocation configuration appears in a sequence with increasing strain: from long straight dislocations, followed by dislocation meshes and dislocation tangles, finally to cell boundaries and EPBs. A lot of dislocation loops appear in all these images, as shown by the arrows, which means that dislocations cross slip frequently during the deformation process. It can be inferred that the cross-slip plays an important role in the formation of microbands [6]. The typical microstructure of EPBs in different reduction levels is shown in Fig. 3. It can be seen that there are one group of EPBs in Fig. 3a and two groups of EPBs in Fig. 3b and c. Comparing with the EPBs in Fig. 2d, the EPBs in Fig. 3 are more mature and longer. Fig. 3a shows the evidence of the early stages of formation of a set of EPBs. There are still many individual dislocations between neighbor EPBs. The EPBs incline at a similar angle to RD at this condition, which is about 35◦ . Meanwhile, the boundaries are aligned with {110} slip planes. The discontinuities in the EPBs can also be
Fig. 4. Typical dislocation microstructure in EPBs of Ta-2.5W at different stages. (a)Dislocation tangles at 20% reduction, (b) Parallel dislocation sets at 30% reduction, (c) Dislocation nets at 30% reduction, (d) Dislocation nets at 60% reduction.
seen in this condition. Furthermore, the EPBs are still not strictly planar, as some boundary segments are wavy, and their projected width varies locally. Some sharp and curved high dense dislocation walls can be seen. Meanwhile, some long straight wide dislocation boundaries can also be seen. The morphology of these EPBs can be divided into types: (i) thin, intensive and straight boundaries, (ii) wide and diffused boundaries. Especially, both two types of EPBs can appear in the same boundaries, as indicated by the arrows. It can be inferred that the EPBs are in the early stage of development. Fig. 3b shows a representative TEM micrograph showing a prominent group of EPBs 1 intersected by a complementary set of less well-defined EPBs 2. EPBs 1 are long and curved. The space of EPBs 1 is more homogeneous and smaller than that of group 2. The space of group 2 is very inhomogeneous. The larger space between two EPBs of group 2 is 3–4 m. The relatively smaller space between two EPBs of group 2 is about 0.5 m.
Please cite this article in press as: S. Wang, et al., The evolution of deformation microstructure in electron beam melted Ta-2.5W alloy during cold rolling, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.03.101
G Model FUSION-9282; No. of Pages 5
ARTICLE IN PRESS S. Wang et al. / Fusion Engineering and Design xxx (2017) xxx–xxx
Fig. 3c shows typical microbands microstructure in 40% coldrolled Ta-2.5W. It can also be seen that two group of EPBs are well-developed. Compared with the microbands in Fig. 3b, these microbands are more planar, mature and uniform. The average space between neighbor EPBs is about 0.5 m. The inclination angle between the two groups of EPBs is about 60◦ . Both of them are parallel with slip planes of {110}, inclining at ±30 ± 5◦ to the RD. The sketch in Fig. 3d shows the EPBs in Fig. 3c. Both of these two sets of EPBs are not always strictly planar, as some boundary segments are slightly wavy. Meanwhile, the splitting of high dense dislocation walls to form microbands can be seen. There are mainly two kinds of formation mechanism of microbands, which are double crossslip mechanism and boundary-splitting mechanism [7]. It has been proved that the misorientation of microbands formed by double cross-slip is usually below than 1◦ . The splitting of high dense dislocation walls can form wavy microbands. It can be concluded that the double cross-slip mechanism play an important role at the early stage of development. The boundary splitting mechanism becomes dominate at the late stage. Fig. 4 shows the evolution of dislocation structure in EPBs during cold rolling at different reduction levels. When Ta-2.5W is coldrolled by 20%, the dislocation boundaries are diffused, which are composed of dislocation tangles. Many redundant dislocations can be seen between EPBs. The dislocation boundaries become more and more intensive with increasing strain. When the reduction increases to 30%, the dislocation density between the EPBs is very low, as shown in Fig. 4b and c. The dislocations within EPBs become more regular. Although some dislocation tangles can be still seen in EPBs, one set of parallel dislocations are the main feature of the EPBs. Furthermore, regular cross dislocation nets formed by two main sets of dislocations can be seen in EPBs. These two sets of dislocations can reaction to form a type of sessile [100] dislocations [6]. These reaction dislocations can make the dislocation boundaries stable during deformation. Dislocation nets can still be seen in Fig. 4d. Compared with the dislocation nets in Fig. 4c, the dislocation density is much higher in this situation. That is to say, the space of parallel dislocations gets smaller and the misorientation of dislocation boundaries becomes larger. It means that the dislocation density in the EPBs become larger and larger with increasing strain.
5
(2) The density of EPBs increases with increasing strain. Most of grains contain one or two groups of EPBs occupying almost the whole grain interiors when the reduction is larger than 30%. (3) The EPBs are curved at the early stage. The density of dislocations in the EPBs increases with increasing strain. The dislocations within EPBs tend to rearrange themselves with increasing strain in a sequence, from tangled dislocations, followed by parallel dislocations and finally into dislocation nets. Acknowledgments This work is supported by Special Foundation for State Major Basic Research Program of China with Grant No. 2014GB121000, Anhui Natural Science Foundation with Grant Nos. 1608085QE95 and 1708085QE94, the fundamental research funds for the central universities with Grant No. JZ2016HGTA0689. References [1] M. Kuznietz, Z. Livne, C. Cotler, et al., Diffusion of liquid uranium into foils of tantalum metal and tantalum-10 wt% tungsten alloy up to 1350 ◦ C, J. Nucl. Mater. 152 (1988) 235–245. [2] K. Yasunaga, H. Watanabe, N. Yoshida, et al., Correlation between defect structures and hardness in tantalum irradiated by heavy ions, J. Nucl. Mater. 283–287 (2000) 179–182. [3] T.S. Byun, S.A. Maloy, Dose dependence of mechanical properties in tantalum and tantalum alloys after low temperature irradiation, J. Nucl. Mater. 377 (2008) 72–79. [4] W. Yin, X. Jia, Q. Yu, et al., First-principles study of the interaction between helium and the defects in tantalum, J. Nucl. Mater. 480 (2016) 202–206. [5] M. Miyamoto, D. Nishijima, Y. Ueda, et al., Observations of suppressed retention and blistering for tungsten exposed to deuterium–helium mixture plasmas, Nucl. Fusion 49 (2009) 65035. [6] S. Wang, C. Chen, Y. Jia, et al., Evolution of texture and deformation microstructure in Ta-2.5W alloy during cold rolling, J. Mater. Res. 30 (2015) 2792–2803. [7] S. Wang, L. Niu, C. Chen, et al., The orientation spreading in ␥-fiber of electron beam melted Ta-2.5W alloy during cold rolling, J Alloy Compd 699 (2017) 57–67.
4. Conclusions (1) The morphology of microbands in different reduction is different. The microbands are relatively straight at low strain and can cross the grain boundary. They become wavy when the reduction reaches 70%. They are parallel with RD when the reduction is up to 90%.
Please cite this article in press as: S. Wang, et al., The evolution of deformation microstructure in electron beam melted Ta-2.5W alloy during cold rolling, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.03.101
ARTICLE IN PRESS
FUSION-9282; No. of Pages 5
Fusion Engineering and Design xxx (2017) xxx–xxx
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
The evolution of deformation microstructure in electron beam melted Ta-2.5W alloy during cold rolling S. Wang a,b,d , L. Niu a , C. Chen b,d,∗ , Y.L. Jia c , M.P. Wang c , Z. Li c , Z.H. Zhong b,d , P. Lu a,e , P. Li b,d , Y.C. Wu b,d a
Institute of Industry and Equipment Technology, Hefei University of Technology, Hefei, Anhui,230009, China School of Materials Science and Engineering, Hefei University of Technology, Hefei, Anhui, 230009, China c School of Materials Science and Engineering, Central South University, Changsha, Hunan, 410083, China d National-Local Joint Engineering Research Center of Nonferrous Metals and Processing Technology, Hefei University of Technology, Hefei, Anhui, 230009, China e State Key Laboratory for Strength and Vibration of Mechanical Structures, Xian Jiaotong University, Xian, Shanxi, 710049, China b
h i g h l i g h t s • • • •
This article systematically sheds light on the characteristics of microbands in Ta-2.5W during cold rolling. The morphology of microbands is quite different at different reduction levels. The evolution of dislocation structure in EPBs is investigated by TEM in detail. The development mechanism of microbands is proposed.
a r t i c l e
i n f o
Article history: Received 22 September 2016 Received in revised form 17 March 2017 Accepted 20 March 2017 Available online xxx Keywords: Ta-W alloy Cold-rolling Deformation microstructure Microbands Extended planar boundaries
a b s t r a c t The evolution of deformation microstructure in electron beam melted Ta-2.5W alloy was investigated. The morphology of microbands appear different in different reduction levels. The microbands are relatively straight at low to medium strain and can cross the grain boundary. They become wavy when the reduction reaches 70%. The microbands are parallel with rolling direction when the reduction is up to 90%. The average inclination angles between microbands and rolling direction is about 35◦ at 20% reduction and get smaller and smaller with increasing strain. EPBs are developed when the reduction is 20%. Most of grains contain one or two groups of EPBs occupying almost the whole grain interiors when the reduction is larger than 30%. The EPBs are fragmented, diffused and curved in the early stages of development. When the reduction reaches 40%, the EPBs become more well-developed and homogeneous. The dislocations within EPBs tend to rearrange themselves with increasing strain in a sequence, from tangled dislocations, followed by parallel dislocations and finally into dislocation nets. The evolution of microbands is related with both the cross-slip of dislocations and the splitting of high dense dislocation walls. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Ta-W alloy is a kind of Ta-based solid-solution strengthened refractory alloy, which is widely used in high-tech applications, such as power, aerospace and nuclear engineering [1]. Ta and Ta-W alloys are candidate protective first wall materials for fusion reactors and target material for spallation neutron sources which are
∗ Corresponding author at: School of Materials Science and Engineering, Hefei University of Technology, Hefei, Anhui, 230009, China. E-mail addresses: [email protected], [email protected] (C. Chen).
also subject to a high flux of high-energy neutrons [2,3]. It has been found that the interaction between two helium atoms in tantalum shows repulsive or weak repulsive other than attractive in tungsten, which suggests that helium atoms are easy to move other than to be a cluster in tantalum [4]. It has also been found that tungsten grains with different orientations exhibited different irradiation resistance under deuterium (D), helium (He) and heavy ions bombardment [5]. More blisters tend to be formed in grains with the <111> orientation. It can be inferred that the irradiation resistance is closely related with their texture. Therefore, to get the beneficial texture, it is important to correlate the microstructures with
http://dx.doi.org/10.1016/j.fusengdes.2017.03.101 0920-3796/© 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: S. Wang, et al., The evolution of deformation microstructure in electron beam melted Ta-2.5W alloy during cold rolling, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.03.101
G Model FUSION-9282; No. of Pages 5
ARTICLE IN PRESS
2
S. Wang et al. / Fusion Engineering and Design xxx (2017) xxx–xxx
Fig. 1. EBSD ND IPF orientation images of Ta-2.5W alloy. Step size: 0.2 m. (a)5%, (b)10%, (c)20%, the orientated stereographic projection for the location A. (d)30%, the orientated stereographic projection for the location B. (e)40%, (f)60%, (g)70%, (h)90%. The dashed lines show the traces of EPBs.
orientations and study the evolution of microstructures during processing. It has been found that deformation texture with different deformation microstructures was formed in Ta-W alloy during cold rolling [6,7]. The deformation microstructure in micro-scale can be divided into cells or microbands, which are comprised of contin-
uous parallel extended planar boundaries (EPBs). The microbands play an important role in the properties of Ta-W alloy. Therefore, this article focuses on investigating the evolution of microbands during cold-rolled process.
Please cite this article in press as: S. Wang, et al., The evolution of deformation microstructure in electron beam melted Ta-2.5W alloy during cold rolling, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.03.101
G Model FUSION-9282; No. of Pages 5
ARTICLE IN PRESS S. Wang et al. / Fusion Engineering and Design xxx (2017) xxx–xxx
3
Fig. 2. Typical TEM images of Ta-2.5W alloy cold rolled by different reductions. (a)5%, (b)10%, (c)20%, (d)20%. The dashed lines show the traces of EPBs, which are aligned close to {110}. The arrows show dislocation loops.
2. Experiments The Ta-2.5W ingot with a diameter of 16 mm was prepared by an electron beam melting method. The casting cylinders with an average grain size of 4 mm were hot forged at 1473K–1073 K, the deformation ratio is about 40%. Then, the forged plates were annealed at 1573 K for 1 h, and a fully recrystallization plates with 10 mm in thickness can be obtained. The grain size was 100–200 m in the annealed plates. Finally, the annealed plates were cold rolled to different reduction at room temperature. The reduction per pass is 10–20%. Electron backscatter diffraction (EBSD) images were taken by FEI-Sirion 200 field emission scanning electron microscope (SEM) equipped with EDAX OIMTM Data Analysis software. EBSD investigations were done on ND (Normal direction) – RD (Rolling direction) planes of the plates. The TEM observation was conducted with a JEOL 2010 transmission electron microscope. The operated accelerating voltage is 200 KV. The specimens were jet polished in a mixing solution of HF, H2 SO4 and CH3 OH with ratio of 1:5:94 at 273 K. The specimens characterized by TEM were also cut from ND-RD sections. 3. Results and discussion Fig. 1 shows the typical EBSD microstructure of the Ta-2.5W alloy cold-rolled by different reduction levels. It can be clearly seen that the color in the grains is homogenous when the reduction is 5% (Fig. 1a). The color gradient can be found in grains when cold-rolled by 10% (Fig. 1b). Microbands appear when the reduction reaches 20%, as shown in Fig. 1c. It can be seen that the microbands in a grain are straight and mutually parallel. The dashed lines show the traces of microbands. It can be concluded from the analysis of the traces of these microbands through the inserted orientated stereographic
projection and EBSD data that the habit planes of these microbands are mainly aligned with the slip planes of {110}. When the strain is larger than 20%, the density of microbands increases dramatically with increasing strain. Meanwhile, the contrast of microbands becomes more and more distinct, which means the misorientation across EPBs gets larger and larger. When the reduction increases to 30%, the grains contain one or two groups of microbands occupying most region of the grain interiors, as shown in Fig. 1d. Most of the traces of microbands are still aligned close to {110}. It can also be clearly seen that the deformation microstructure in different orientations is quite different. That is to say, some orientations contain microbands and the others do not. When the strain increases to 60%, the microbands can cross the high angle boundaries, as shown in Fig. 1f, which can also be called as shear bands. The microbands become wavy when the reduction reaches 70% (Fig. 1g). They are usually inclined at about 10◦ with RD. Finally, it can be found that microbands are parallel with RD when the reduction is up to 90% (Fig. 1h). It can also be found from the ND orientation maps that the normal direction of most grains rotates to <111> or <100> with increasing strain, which means the deformation textures are developed. Fig. 2 shows the typical dislocation microstructure in Ta-2.5W alloy at low strain. After cold-rolled by 5%, plenty of long straight dislocations are formed without dislocation tangles, as shown in Fig. 2a. As shown in Fig. 2b, dislocation tangles are the typical dislocation microstructure in the specimens cold-rolled by 10%. However, there are no well-developed dislocation boundaries. Two typical kinds of dislocation boundaries, including cells boundaries and EPBs, are developed when the reduction reaches 20%, as shown in Fig. 2c and d, respectively. The orientation effects on the plastic deformation behaviors in body centered cubic metals have been studied [6,7]. The formation of cells or EPBs mainly depends on
Please cite this article in press as: S. Wang, et al., The evolution of deformation microstructure in electron beam melted Ta-2.5W alloy during cold rolling, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.03.101
G Model FUSION-9282; No. of Pages 5
ARTICLE IN PRESS
4
S. Wang et al. / Fusion Engineering and Design xxx (2017) xxx–xxx
Fig. 3. Typical EPBs structure of Ta-2.5W cold rolled by different reductions. (a)30%, (b)30%, (c)40%. The dashed lines show the traces of microbands. Two groups of EPBs can be seen in Fig. 3b and c. (d) The sketch shows the EPBs in Fig. 3c.
the orientations of the grains. EPBs can almostly appear in all orientations except the orientation of {001} <110> [6]. The dislocation configuration appears in a sequence with increasing strain: from long straight dislocations, followed by dislocation meshes and dislocation tangles, finally to cell boundaries and EPBs. A lot of dislocation loops appear in all these images, as shown by the arrows, which means that dislocations cross slip frequently during the deformation process. It can be inferred that the cross-slip plays an important role in the formation of microbands [6]. The typical microstructure of EPBs in different reduction levels is shown in Fig. 3. It can be seen that there are one group of EPBs in Fig. 3a and two groups of EPBs in Fig. 3b and c. Comparing with the EPBs in Fig. 2d, the EPBs in Fig. 3 are more mature and longer. Fig. 3a shows the evidence of the early stages of formation of a set of EPBs. There are still many individual dislocations between neighbor EPBs. The EPBs incline at a similar angle to RD at this condition, which is about 35◦ . Meanwhile, the boundaries are aligned with {110} slip planes. The discontinuities in the EPBs can also be
Fig. 4. Typical dislocation microstructure in EPBs of Ta-2.5W at different stages. (a)Dislocation tangles at 20% reduction, (b) Parallel dislocation sets at 30% reduction, (c) Dislocation nets at 30% reduction, (d) Dislocation nets at 60% reduction.
seen in this condition. Furthermore, the EPBs are still not strictly planar, as some boundary segments are wavy, and their projected width varies locally. Some sharp and curved high dense dislocation walls can be seen. Meanwhile, some long straight wide dislocation boundaries can also be seen. The morphology of these EPBs can be divided into types: (i) thin, intensive and straight boundaries, (ii) wide and diffused boundaries. Especially, both two types of EPBs can appear in the same boundaries, as indicated by the arrows. It can be inferred that the EPBs are in the early stage of development. Fig. 3b shows a representative TEM micrograph showing a prominent group of EPBs 1 intersected by a complementary set of less well-defined EPBs 2. EPBs 1 are long and curved. The space of EPBs 1 is more homogeneous and smaller than that of group 2. The space of group 2 is very inhomogeneous. The larger space between two EPBs of group 2 is 3–4 m. The relatively smaller space between two EPBs of group 2 is about 0.5 m.
Please cite this article in press as: S. Wang, et al., The evolution of deformation microstructure in electron beam melted Ta-2.5W alloy during cold rolling, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.03.101
G Model FUSION-9282; No. of Pages 5
ARTICLE IN PRESS S. Wang et al. / Fusion Engineering and Design xxx (2017) xxx–xxx
Fig. 3c shows typical microbands microstructure in 40% coldrolled Ta-2.5W. It can also be seen that two group of EPBs are well-developed. Compared with the microbands in Fig. 3b, these microbands are more planar, mature and uniform. The average space between neighbor EPBs is about 0.5 m. The inclination angle between the two groups of EPBs is about 60◦ . Both of them are parallel with slip planes of {110}, inclining at ±30 ± 5◦ to the RD. The sketch in Fig. 3d shows the EPBs in Fig. 3c. Both of these two sets of EPBs are not always strictly planar, as some boundary segments are slightly wavy. Meanwhile, the splitting of high dense dislocation walls to form microbands can be seen. There are mainly two kinds of formation mechanism of microbands, which are double crossslip mechanism and boundary-splitting mechanism [7]. It has been proved that the misorientation of microbands formed by double cross-slip is usually below than 1◦ . The splitting of high dense dislocation walls can form wavy microbands. It can be concluded that the double cross-slip mechanism play an important role at the early stage of development. The boundary splitting mechanism becomes dominate at the late stage. Fig. 4 shows the evolution of dislocation structure in EPBs during cold rolling at different reduction levels. When Ta-2.5W is coldrolled by 20%, the dislocation boundaries are diffused, which are composed of dislocation tangles. Many redundant dislocations can be seen between EPBs. The dislocation boundaries become more and more intensive with increasing strain. When the reduction increases to 30%, the dislocation density between the EPBs is very low, as shown in Fig. 4b and c. The dislocations within EPBs become more regular. Although some dislocation tangles can be still seen in EPBs, one set of parallel dislocations are the main feature of the EPBs. Furthermore, regular cross dislocation nets formed by two main sets of dislocations can be seen in EPBs. These two sets of dislocations can reaction to form a type of sessile [100] dislocations [6]. These reaction dislocations can make the dislocation boundaries stable during deformation. Dislocation nets can still be seen in Fig. 4d. Compared with the dislocation nets in Fig. 4c, the dislocation density is much higher in this situation. That is to say, the space of parallel dislocations gets smaller and the misorientation of dislocation boundaries becomes larger. It means that the dislocation density in the EPBs become larger and larger with increasing strain.
5
(2) The density of EPBs increases with increasing strain. Most of grains contain one or two groups of EPBs occupying almost the whole grain interiors when the reduction is larger than 30%. (3) The EPBs are curved at the early stage. The density of dislocations in the EPBs increases with increasing strain. The dislocations within EPBs tend to rearrange themselves with increasing strain in a sequence, from tangled dislocations, followed by parallel dislocations and finally into dislocation nets. Acknowledgments This work is supported by Special Foundation for State Major Basic Research Program of China with Grant No. 2014GB121000, Anhui Natural Science Foundation with Grant Nos. 1608085QE95 and 1708085QE94, the fundamental research funds for the central universities with Grant No. JZ2016HGTA0689. References [1] M. Kuznietz, Z. Livne, C. Cotler, et al., Diffusion of liquid uranium into foils of tantalum metal and tantalum-10 wt% tungsten alloy up to 1350 ◦ C, J. Nucl. Mater. 152 (1988) 235–245. [2] K. Yasunaga, H. Watanabe, N. Yoshida, et al., Correlation between defect structures and hardness in tantalum irradiated by heavy ions, J. Nucl. Mater. 283–287 (2000) 179–182. [3] T.S. Byun, S.A. Maloy, Dose dependence of mechanical properties in tantalum and tantalum alloys after low temperature irradiation, J. Nucl. Mater. 377 (2008) 72–79. [4] W. Yin, X. Jia, Q. Yu, et al., First-principles study of the interaction between helium and the defects in tantalum, J. Nucl. Mater. 480 (2016) 202–206. [5] M. Miyamoto, D. Nishijima, Y. Ueda, et al., Observations of suppressed retention and blistering for tungsten exposed to deuterium–helium mixture plasmas, Nucl. Fusion 49 (2009) 65035. [6] S. Wang, C. Chen, Y. Jia, et al., Evolution of texture and deformation microstructure in Ta-2.5W alloy during cold rolling, J. Mater. Res. 30 (2015) 2792–2803. [7] S. Wang, L. Niu, C. Chen, et al., The orientation spreading in ␥-fiber of electron beam melted Ta-2.5W alloy during cold rolling, J Alloy Compd 699 (2017) 57–67.
4. Conclusions (1) The morphology of microbands in different reduction is different. The microbands are relatively straight at low strain and can cross the grain boundary. They become wavy when the reduction reaches 70%. They are parallel with RD when the reduction is up to 90%.
Please cite this article in press as: S. Wang, et al., The evolution of deformation microstructure in electron beam melted Ta-2.5W alloy during cold rolling, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.03.101