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Microstructural evolution of equal channel angular drawn purity titanium at room temperature
Journal of Alloys and Compounds 811 (2019) 152002
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Microstructural evolution of equal channel angular drawn purity titanium at room temperature Hong Zhao, Yuping Ren**, Bo Yang, Gaowu Qin* Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), School of Materials Science and Engineering, Northeastern University, Shenyang, 110819, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 June 2019 Received in revised form 29 July 2019 Accepted 24 August 2019 Available online 26 August 2019
Thin commercial-purity titanium (CPeTi) wire was successfully acquired by equal channel angular drawing (ECAD) at room temperature with route Bc using a 90 die at a relatively high drawing speed of 10 mm s1. The as-drawn CPeTi wires were of good quality free of cracks and segmentation on their surface. The grain size of CPeTi was reduced from ~32 mm for the as-annealed wire to ~700 nm for 12passes equal channel angular drawn wire. The grain experienced transition from microband to thin lath and to equiaxed subgrains with the increment in drawing passes. Face-centered cubic (FCC) phase was triggered obviously to accommodate the large shear strain induced by ECAD at the drawing rate of 20 mms1. The thickness of the FCC phase increased with an increase in drawing passes, and no equiaxed subgrains were formed in CPeTi. Accordingly, the drawing speed significantly affects the deformation mode and microstructural evolution of CPeTi during ECAD. A lower drawing speed provides a longer time for the structure recovery, thus resulting in the occurrence of dynamic recovery when ECAD was performed at room temperature. Additionally, f1012g tension twinning and f1122g compression twinning occurred simultaneously to accommodate ECAD shear deformation. The success in processing CPeTi rods at room temperature through multiple passes of ECAD provides a new perspective to efficiently fabricate ultrafine grained small-sized materials continuously. © 2019 Elsevier B.V. All rights reserved.
Keywords: ECAD Microbands FCC phase Small size Drawing speed
1. Introduction As one of the most typical severe plastic deformation (SPD) techniques [1], equal channel angular pressing (ECAP) is an efficient process to produce titanium with ultrafine grains. Consequently, improved mechanical properties of CPeTi are comparable to those of Ti-6Al-4V alloy and suitable for use as biomedical devices, such as bone pins and dental implants [2e5]. The exploitation of CPeTi is in coincidence with for the concept of compositional plainification and sustainability improvement [6]. In general, the processing of CPeTi by ECAP is conducted at elevated temperatures (473e873 K) [7e12] with a relatively slow ram speed of 0.25e8 mm s1 [7,12e15]. The grain size of CPeTi can be refined to 100e700 nm after four to eight passes of ECAP. However, CPeTi processed by ECAP at room temperature was inclined to split when it was pressed at a speed higher than 10 mm s1 despite using specially
* Corresponding author. ** Corresponding author. E-mail address: [email protected] (G. Qin). https://doi.org/10.1016/j.jallcom.2019.152002 0925-8388/© 2019 Elsevier B.V. All rights reserved.
designed die and composite lubricant [16e20]. Apart from restrictions in temperature and ram speed, the dimensions of billets for ECAP were confined to be no less than 10 mm in diameter and no more than 150 mm in length. Whereas with regard to biomaterials, the diameter of most bone pins and dental implants is below 2 mm and even is as small as 0.5 mm [21,22]. Therefore, medical pure titanium rod with ultrafine grains is difficult to be obtained directly by ECAP. Chakkingal et al. [23,24] found that the coarse grain size of 2000 mm could be reduced significantly to 1 mm by conducting six passes of ECAD on cast aluminum bars at room temperature. In comparison with ECAP, ECAD is a more innovative technique to draw samples through intersecting channels [23,24] and is more feasible to continuously process longer wires with smaller diameter. Therefore, ECAD will become a new processing method to produce ultrafine-grained metal wires. We aim to apply ECAD technology to process CPeTi rods and investigate the effect of high drawing speed on microstructural evolutions. This research provides a new approach to efficiently fabricate small-sized CPeTi with ultrafine grains continuously at room temperature.
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2. Material and methods The die for ECAD comprises two circular channels with diameters of 2 mm intersecting at an angle of 90 , as shown in Fig. 1(a), which is designed to yield an effective strain of 0.6 by a single drawing [25]. An annealed coarse-grained (30 mm, as shown in Fig. 1(b)) CPeTi (grade 1) wire with diameter of 2 mm and length over 1 m was drawn for 1e12 passes at room temperature through route Bc, with a clockwise rotation of 90 between adjacent passes. According to the manufacturer's specifications, the composition (in wt.%) was 0.11% O, 0.015% H, 0.001% N, 0.01% C, and 0.03% Fe, with the remainder is Ti. To provide a quantitative evaluation, the grain size was measured using the linear intercept method with a count of at least 200 grains for CPeTi sample. ECAD was performed at two constant drawing speeds of 10 and 20 mm s1 and graphite paste was carefully applied prior to and during drawing. Microstructural images were observed after ECAD using both a JSM-7001F field emission scanning electron microscope (SEM) and a JEM-2100F transmission electron microscopy (TEM) on polished samples cut parallel to the drawing direction. The samples used for SEM were etched with a solution of 16% hydrofluoric acid, 27% nitric acid and 57% water. Specimens for TEM analysis were prepared by mechanically polishing down to 40 mm thickness followed by double-jet polishing with an electrolyte of 5% perchloric, 35% butanol, and 60% methanol at 30 C with a constant voltage of 25 V. Selected area electron diffraction (SAED) patterns obtained at 200 kV were used to analyze the structure. 3. Results and discussion Fig. 2(a) shows a bright-field TEM image of starting material CPeTi. Few dislocations and twins are shown within the grains. Fig. 2(b) shows a series of wires drawn through 2, 5, 8, and 12 passes at 10 and 20 mm s1. It is apparent that all wires are in excellent quality with smooth and crack-free surfaces, while the billets were segmented at the same speeds for ECAP even at 598 K [26]. Our present experimental results demonstrate that thin CPeTi wires with excellent quality are produced successfully by ECAD at room temperature. Fig. 3 shows SEM images of as-drawn rods at low magnification. As shown in Fig. 3, compared with the image for the as-annealed wire (Fig. 1(b)), more twins and elongated grains distribute uniformly, as seen in these images for the as-drawn wires. This implies that the imposed strain during ECAD is partly accommodated by twinning and dislocation slip. To observe microstructures in more detail, TEM micrographs and SAED patterns of the CPeTi samples drawn at 10 mm s1 are
presented in Fig. 4, revealing the development of microbands after 2, 5, 8, and 12 passes of ECAD. Fig. 4(a) shows that many elongated parallel sub-microbands of width 700 nm are generated after 2 passes of ECAD. High-density dislocations appear within the microbands, which are similar to the slip bands formed in ECAPdeformed Ti [17,18]. The formation of the microbands was reported to result from slip on dominant slip systems and subsequently propagated by multiple-slip or cross-slip events owing to stress concentration [27]. After 5 passes, as shown in Fig. 4(b), the microbands continue to evolve. Some dislocation walls and irregular cell blocks are formed at some locations in microbands containing high-density dislocations, as indicated by the red arrows and white circle, respectively. This is the initiation of the grain refinement process. Fig. 4(c) shows that the dislocations within the microband tend to accumulate at certain locations (indicated by large arrows) that further divide the microband into several thin-lath structures with a width of approximately 500 nm. The microstructures are refined by further increasing the strain after 8 passes of ECAD. As shown, most of the lath boundaries are straight and remain perpendicular to the microband boundaries, as opposed to the results of longitudinal split that generally occur prior to an intense transverse breakdown in cold-rolled titanium [28]. Furthermore, high-density dislocations accumulate to break down the thin-lath into smaller blocks, as indicated by small white arrows. Fig. 4(d) shows more clearly defined equiaxed subgrains with grain size of approximately 700 nm when the wire is drawn up to 12 passes, and the dislocation density in the subgrains reduces prominently owing to the dynamic recovery. Fig. 4 clearly indicates that the dislocation slip dominates the ECAD process at 10 mm s1 and facilitates grain refinement, similar to that of CPeTi processed by ECAP [17,18] or Al processed by ECAD [23,24]. TEM micrographs and SAED patterns of the CPeTi samples drawn at 20 mm s1 are shown in Fig. 5. Fig. 5(a) and (b) show many elongated microbands with the width of approximately 0.5e2 mm after 2 and 5 passes of ECAD, and high-density dislocations appear in the band. It is noteworthy that elongated microbands are wider and their boundaries are more cambered compared with the results in Fig. 4(a) and (b). This phenomenon may be attributed to the rapid strain accumulation at the higher drawing rate. Moreover, some acicular lamellas in the microbands can be observed in Fig. 5(a) and (b), as indicated by yellow arrows and are defined as deformation microtwins in ECAP [29]. After 8 passes, as shown in Fig. 5(c), the acicular lamellas continue to increase. By increasing the number of passes to 12 (Fig. 5(d)), the interfaces of the band structure cannot be clearly identified owing
Fig. 1. (a) Schematic diagram of ECAD process, (b) optical micrograph of the as-annealed CPeTi.
H. Zhao et al. / Journal of Alloys and Compounds 811 (2019) 152002
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Fig. 2. (a) TEM images of the received CPeTi, (b) appearance of the wires after ECAD at room temperature, where the left-hand side was drawn at 10 mm s1 and the right-hand side was drawn at 20 mm s1.
Fig. 3. SEM images of the microstructures after ECAD at room temperature through (a1) 2 passes, (b1) 5 passes, (c1) 8 passes and (d1) 12 passes at 10 mm s1, and (a2) 2 passes, (b2) 5 passes, (c2) 8 passes and (d2) 12 passes at 20 mm s1.
Fig. 4. TEM micrographs and corresponding SAED patterns of CPeTi samples after different ECAD passes at constant drawing speed of 10 mm s1: (a) 2 passes, (b) 5 passes, (c) 8 passes, and (d) 12 passes.
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Fig. 5. TEM micrographs and corresponding SAED patterns of CPeTi samples after different ECAD passes at constant drawing speed of 20 mm s1: (a) 2 passes, (b) 5 passes, (c) 8 passes, (d) 12 passes, and (e) an HRTEM image of the FCC lamella and HCP matrix, viewed along the < 1213 > HCP// < 112 > FCC.
to the interference of increased acicular lamellas. Furthermore, dense acicular lamellas containing high-density dislocations intersect with each other. Based on Gatan Digital Micrograph software, the maximum thickness of lamellas after 2 passes of ECAD is no more than 30 nm, and it increases to 70 nm after 5 passes, then it increases further to 100 nm and 180 nm after 8 and 12 passes, respectively. While the length of the lamellas after 2, 5, 8 and 12 passes is 500 nm, 2000 nm, 3000 nm and even more than 4000 nm, respectively. It is evident the lamellas grow thicker and longer gradually with the increase of the drawing passes. For this condition, the acicular lamellas are morphologically similar to the phase in cold-rolled CPeTi that was demonstrated to be FCCstructured Ti [30,31]. To better understand the acicular structures, we performed an HRTEM and SAED analysis of the sample after 5 passes of ECAD at 20 mm s1. As shown in Fig. 6(a), the crystal structure of the acicular lamellar is indeed FCC accompanied by the proof of the fast Fourier transformation (FFT), differing from that of the matrix.
Meanwhile, the orientation relation between hexagonal closepacked (HCP) matrix and FCC phase is [0001]HCP//[001]FCC, f0110g HCP//{110}FCC and f2110g HCP// f110g FCC, which is consistent with the phase relationship in literatures [30,31]. Previous studies in ECAP failed to examine crystal structures under highresolution transmission conditions. Therefore, the FCC structure had not been detected exactly and effectively [29,32]. It is clear that several steps exist along the interface between the FCC phase and HCP matrix (Fig. 6(a)), lying on f2110g HCP, as indicated by green lines. The SAED pattern of the interface in Fig. 6(b) indicates that in addition to the fundamental spots corresponding to the HCP matrix (red solid circle) and FCC phase (yellow hollow circle), independent spots between the fundamental spots appear along f2110g HCP (green box), corresponding to the steps in Fig. 6(a). Moreover, it is noteworthy that the steps typically correspond to stacking faults (Fig. 6(c)), indicating that the steps are ordered and coherent to a high degree. Meanwhile, the FCCeTi can grow longer and thicker (Fig. 5(c) and (d)) through the ledge slipping along the interface,
Fig. 6. (a) HRTEM image of the FCC lamella after 5 passes of ECAD at 20 mm s1, viewed along the <0001>HCP//<001>FCC, (b) SAED pattern of an area containing both HCP matrix and FCC lamella, and (c) FCC lamella and enlarged area in green ellipse showing the stacking faults in interphase. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
H. Zhao et al. / Journal of Alloys and Compounds 811 (2019) 152002
thus strongly implying the shearing characteristic of the FCC phase growth [30,33,34]. It can be concluded that shear-induced HCP to FCC transformation can produce numerous defects and dislocation gliding, which can accommodate the large plastic strain when the drawing speed increases. At low temperature, the FCC phase is reported as stable as the HCP phase [33,35]. Thus, the boundaries of the FCC phase can effectively segment the microstructure and facilitate grain refinement. Additionally, Fig. 7 shows more details about lenticular f1012g tension twinning and f1122g compression twinning developed after 5 passes of ECAD at 20 mm s1, as seen in Fig. 7 (a) and (c). Their corresponding SAED patterns were illustrated in Fig. 7(b) and (d). It is noteworthy that both twins are observed under other ECAD passes drawn at 10 and 20 mm s1, which is consistent with Fig. 3, indicating that twinning accommodates deformation during the
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entire processing owing to the relatively high strain rate and low temperature. This result is in agreement with those of previous investigations, where the f1012g < 1011 > tension twins and the f1122g < 1123 > compression twins are the most active twins during other conventional deformation at ambient temperature [36,37]. However, it is noticeable that deformation twinning is typically identified as either f1011g < 1012 > type or f1012g < 1011 > type in most ECAPedeformed CPeTi at room temperature [12,18], instead of being observed simultaneously. Apart from in the matrix, the FCC phase develops in the deformation twins to accommodate the plastic strain, as shown in Fig. 7(e). 4. Conclusions In this study, ECAD process was applied to commercial CPeTi wires to evaluate the feasibility of continuous processing at room
Fig. 7. TEM image of CPeTi sample after 5 passes of ECAD at constant drawing speed of 20 mm s1: Tension twining (a) and the corresponding SAED pattern (b), compression twining (c) and the corresponding SAED pattern (d), and (e) FCCeTi in compression twinning.
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temperature using a 90 die and drawing speeds of 10 and 20 mm s1. Simultaneously, the effect of drawing speed on the microstructural evolutions of CPeTi during ECAD was investigated. The following conclusions were obtained: CPeTi wires were successfully drawn up to 12 passes at 10 and 20 mm s1 without surface damage that typically occurred in ECAP. We successfully manufactured CPeTi wires through ECAD technology at room temperature. The drawing speed affected the deformation mode and microstructural evolution of CPeTi significantly. The grain size reduced from ~32 mm for the as-annealed samples to 700 nm for the 12 passes ECAD samples at the drawing speed of 10 mm s1, which were consistent with earlier reports of ECAP. While the FCC phase was obviously triggered to effectively accommodate the large shear strain induced by ECAD when the drawing speed increased to 20 mm s1, which was not reported in publications regarding ECAP. The thickness of the FCC phase increased gradually with the increase in drawing passes. The equiaxed grains were not present in CPeTi after 12 passes. Meanwhile, f1012g and f1122g twins were simultaneously detected during ECAD for the first time. Acknowledgements
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This work was supported by the National Natural Science Foundation of China (Grant No. 51525101), Fundamental Research Funds of the Central Universities (No. N170206003).
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Microstructural evolution of equal channel angular drawn purity titanium at room temperature Hong Zhao, Yuping Ren**, Bo Yang, Gaowu Qin* Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), School of Materials Science and Engineering, Northeastern University, Shenyang, 110819, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 June 2019 Received in revised form 29 July 2019 Accepted 24 August 2019 Available online 26 August 2019
Thin commercial-purity titanium (CPeTi) wire was successfully acquired by equal channel angular drawing (ECAD) at room temperature with route Bc using a 90 die at a relatively high drawing speed of 10 mm s1. The as-drawn CPeTi wires were of good quality free of cracks and segmentation on their surface. The grain size of CPeTi was reduced from ~32 mm for the as-annealed wire to ~700 nm for 12passes equal channel angular drawn wire. The grain experienced transition from microband to thin lath and to equiaxed subgrains with the increment in drawing passes. Face-centered cubic (FCC) phase was triggered obviously to accommodate the large shear strain induced by ECAD at the drawing rate of 20 mms1. The thickness of the FCC phase increased with an increase in drawing passes, and no equiaxed subgrains were formed in CPeTi. Accordingly, the drawing speed significantly affects the deformation mode and microstructural evolution of CPeTi during ECAD. A lower drawing speed provides a longer time for the structure recovery, thus resulting in the occurrence of dynamic recovery when ECAD was performed at room temperature. Additionally, f1012g tension twinning and f1122g compression twinning occurred simultaneously to accommodate ECAD shear deformation. The success in processing CPeTi rods at room temperature through multiple passes of ECAD provides a new perspective to efficiently fabricate ultrafine grained small-sized materials continuously. © 2019 Elsevier B.V. All rights reserved.
Keywords: ECAD Microbands FCC phase Small size Drawing speed
1. Introduction As one of the most typical severe plastic deformation (SPD) techniques [1], equal channel angular pressing (ECAP) is an efficient process to produce titanium with ultrafine grains. Consequently, improved mechanical properties of CPeTi are comparable to those of Ti-6Al-4V alloy and suitable for use as biomedical devices, such as bone pins and dental implants [2e5]. The exploitation of CPeTi is in coincidence with for the concept of compositional plainification and sustainability improvement [6]. In general, the processing of CPeTi by ECAP is conducted at elevated temperatures (473e873 K) [7e12] with a relatively slow ram speed of 0.25e8 mm s1 [7,12e15]. The grain size of CPeTi can be refined to 100e700 nm after four to eight passes of ECAP. However, CPeTi processed by ECAP at room temperature was inclined to split when it was pressed at a speed higher than 10 mm s1 despite using specially
* Corresponding author. ** Corresponding author. E-mail address: [email protected] (G. Qin). https://doi.org/10.1016/j.jallcom.2019.152002 0925-8388/© 2019 Elsevier B.V. All rights reserved.
designed die and composite lubricant [16e20]. Apart from restrictions in temperature and ram speed, the dimensions of billets for ECAP were confined to be no less than 10 mm in diameter and no more than 150 mm in length. Whereas with regard to biomaterials, the diameter of most bone pins and dental implants is below 2 mm and even is as small as 0.5 mm [21,22]. Therefore, medical pure titanium rod with ultrafine grains is difficult to be obtained directly by ECAP. Chakkingal et al. [23,24] found that the coarse grain size of 2000 mm could be reduced significantly to 1 mm by conducting six passes of ECAD on cast aluminum bars at room temperature. In comparison with ECAP, ECAD is a more innovative technique to draw samples through intersecting channels [23,24] and is more feasible to continuously process longer wires with smaller diameter. Therefore, ECAD will become a new processing method to produce ultrafine-grained metal wires. We aim to apply ECAD technology to process CPeTi rods and investigate the effect of high drawing speed on microstructural evolutions. This research provides a new approach to efficiently fabricate small-sized CPeTi with ultrafine grains continuously at room temperature.
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2. Material and methods The die for ECAD comprises two circular channels with diameters of 2 mm intersecting at an angle of 90 , as shown in Fig. 1(a), which is designed to yield an effective strain of 0.6 by a single drawing [25]. An annealed coarse-grained (30 mm, as shown in Fig. 1(b)) CPeTi (grade 1) wire with diameter of 2 mm and length over 1 m was drawn for 1e12 passes at room temperature through route Bc, with a clockwise rotation of 90 between adjacent passes. According to the manufacturer's specifications, the composition (in wt.%) was 0.11% O, 0.015% H, 0.001% N, 0.01% C, and 0.03% Fe, with the remainder is Ti. To provide a quantitative evaluation, the grain size was measured using the linear intercept method with a count of at least 200 grains for CPeTi sample. ECAD was performed at two constant drawing speeds of 10 and 20 mm s1 and graphite paste was carefully applied prior to and during drawing. Microstructural images were observed after ECAD using both a JSM-7001F field emission scanning electron microscope (SEM) and a JEM-2100F transmission electron microscopy (TEM) on polished samples cut parallel to the drawing direction. The samples used for SEM were etched with a solution of 16% hydrofluoric acid, 27% nitric acid and 57% water. Specimens for TEM analysis were prepared by mechanically polishing down to 40 mm thickness followed by double-jet polishing with an electrolyte of 5% perchloric, 35% butanol, and 60% methanol at 30 C with a constant voltage of 25 V. Selected area electron diffraction (SAED) patterns obtained at 200 kV were used to analyze the structure. 3. Results and discussion Fig. 2(a) shows a bright-field TEM image of starting material CPeTi. Few dislocations and twins are shown within the grains. Fig. 2(b) shows a series of wires drawn through 2, 5, 8, and 12 passes at 10 and 20 mm s1. It is apparent that all wires are in excellent quality with smooth and crack-free surfaces, while the billets were segmented at the same speeds for ECAP even at 598 K [26]. Our present experimental results demonstrate that thin CPeTi wires with excellent quality are produced successfully by ECAD at room temperature. Fig. 3 shows SEM images of as-drawn rods at low magnification. As shown in Fig. 3, compared with the image for the as-annealed wire (Fig. 1(b)), more twins and elongated grains distribute uniformly, as seen in these images for the as-drawn wires. This implies that the imposed strain during ECAD is partly accommodated by twinning and dislocation slip. To observe microstructures in more detail, TEM micrographs and SAED patterns of the CPeTi samples drawn at 10 mm s1 are
presented in Fig. 4, revealing the development of microbands after 2, 5, 8, and 12 passes of ECAD. Fig. 4(a) shows that many elongated parallel sub-microbands of width 700 nm are generated after 2 passes of ECAD. High-density dislocations appear within the microbands, which are similar to the slip bands formed in ECAPdeformed Ti [17,18]. The formation of the microbands was reported to result from slip on dominant slip systems and subsequently propagated by multiple-slip or cross-slip events owing to stress concentration [27]. After 5 passes, as shown in Fig. 4(b), the microbands continue to evolve. Some dislocation walls and irregular cell blocks are formed at some locations in microbands containing high-density dislocations, as indicated by the red arrows and white circle, respectively. This is the initiation of the grain refinement process. Fig. 4(c) shows that the dislocations within the microband tend to accumulate at certain locations (indicated by large arrows) that further divide the microband into several thin-lath structures with a width of approximately 500 nm. The microstructures are refined by further increasing the strain after 8 passes of ECAD. As shown, most of the lath boundaries are straight and remain perpendicular to the microband boundaries, as opposed to the results of longitudinal split that generally occur prior to an intense transverse breakdown in cold-rolled titanium [28]. Furthermore, high-density dislocations accumulate to break down the thin-lath into smaller blocks, as indicated by small white arrows. Fig. 4(d) shows more clearly defined equiaxed subgrains with grain size of approximately 700 nm when the wire is drawn up to 12 passes, and the dislocation density in the subgrains reduces prominently owing to the dynamic recovery. Fig. 4 clearly indicates that the dislocation slip dominates the ECAD process at 10 mm s1 and facilitates grain refinement, similar to that of CPeTi processed by ECAP [17,18] or Al processed by ECAD [23,24]. TEM micrographs and SAED patterns of the CPeTi samples drawn at 20 mm s1 are shown in Fig. 5. Fig. 5(a) and (b) show many elongated microbands with the width of approximately 0.5e2 mm after 2 and 5 passes of ECAD, and high-density dislocations appear in the band. It is noteworthy that elongated microbands are wider and their boundaries are more cambered compared with the results in Fig. 4(a) and (b). This phenomenon may be attributed to the rapid strain accumulation at the higher drawing rate. Moreover, some acicular lamellas in the microbands can be observed in Fig. 5(a) and (b), as indicated by yellow arrows and are defined as deformation microtwins in ECAP [29]. After 8 passes, as shown in Fig. 5(c), the acicular lamellas continue to increase. By increasing the number of passes to 12 (Fig. 5(d)), the interfaces of the band structure cannot be clearly identified owing
Fig. 1. (a) Schematic diagram of ECAD process, (b) optical micrograph of the as-annealed CPeTi.
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Fig. 2. (a) TEM images of the received CPeTi, (b) appearance of the wires after ECAD at room temperature, where the left-hand side was drawn at 10 mm s1 and the right-hand side was drawn at 20 mm s1.
Fig. 3. SEM images of the microstructures after ECAD at room temperature through (a1) 2 passes, (b1) 5 passes, (c1) 8 passes and (d1) 12 passes at 10 mm s1, and (a2) 2 passes, (b2) 5 passes, (c2) 8 passes and (d2) 12 passes at 20 mm s1.
Fig. 4. TEM micrographs and corresponding SAED patterns of CPeTi samples after different ECAD passes at constant drawing speed of 10 mm s1: (a) 2 passes, (b) 5 passes, (c) 8 passes, and (d) 12 passes.
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Fig. 5. TEM micrographs and corresponding SAED patterns of CPeTi samples after different ECAD passes at constant drawing speed of 20 mm s1: (a) 2 passes, (b) 5 passes, (c) 8 passes, (d) 12 passes, and (e) an HRTEM image of the FCC lamella and HCP matrix, viewed along the < 1213 > HCP// < 112 > FCC.
to the interference of increased acicular lamellas. Furthermore, dense acicular lamellas containing high-density dislocations intersect with each other. Based on Gatan Digital Micrograph software, the maximum thickness of lamellas after 2 passes of ECAD is no more than 30 nm, and it increases to 70 nm after 5 passes, then it increases further to 100 nm and 180 nm after 8 and 12 passes, respectively. While the length of the lamellas after 2, 5, 8 and 12 passes is 500 nm, 2000 nm, 3000 nm and even more than 4000 nm, respectively. It is evident the lamellas grow thicker and longer gradually with the increase of the drawing passes. For this condition, the acicular lamellas are morphologically similar to the phase in cold-rolled CPeTi that was demonstrated to be FCCstructured Ti [30,31]. To better understand the acicular structures, we performed an HRTEM and SAED analysis of the sample after 5 passes of ECAD at 20 mm s1. As shown in Fig. 6(a), the crystal structure of the acicular lamellar is indeed FCC accompanied by the proof of the fast Fourier transformation (FFT), differing from that of the matrix.
Meanwhile, the orientation relation between hexagonal closepacked (HCP) matrix and FCC phase is [0001]HCP//[001]FCC, f0110g HCP//{110}FCC and f2110g HCP// f110g FCC, which is consistent with the phase relationship in literatures [30,31]. Previous studies in ECAP failed to examine crystal structures under highresolution transmission conditions. Therefore, the FCC structure had not been detected exactly and effectively [29,32]. It is clear that several steps exist along the interface between the FCC phase and HCP matrix (Fig. 6(a)), lying on f2110g HCP, as indicated by green lines. The SAED pattern of the interface in Fig. 6(b) indicates that in addition to the fundamental spots corresponding to the HCP matrix (red solid circle) and FCC phase (yellow hollow circle), independent spots between the fundamental spots appear along f2110g HCP (green box), corresponding to the steps in Fig. 6(a). Moreover, it is noteworthy that the steps typically correspond to stacking faults (Fig. 6(c)), indicating that the steps are ordered and coherent to a high degree. Meanwhile, the FCCeTi can grow longer and thicker (Fig. 5(c) and (d)) through the ledge slipping along the interface,
Fig. 6. (a) HRTEM image of the FCC lamella after 5 passes of ECAD at 20 mm s1, viewed along the <0001>HCP//<001>FCC, (b) SAED pattern of an area containing both HCP matrix and FCC lamella, and (c) FCC lamella and enlarged area in green ellipse showing the stacking faults in interphase. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
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thus strongly implying the shearing characteristic of the FCC phase growth [30,33,34]. It can be concluded that shear-induced HCP to FCC transformation can produce numerous defects and dislocation gliding, which can accommodate the large plastic strain when the drawing speed increases. At low temperature, the FCC phase is reported as stable as the HCP phase [33,35]. Thus, the boundaries of the FCC phase can effectively segment the microstructure and facilitate grain refinement. Additionally, Fig. 7 shows more details about lenticular f1012g tension twinning and f1122g compression twinning developed after 5 passes of ECAD at 20 mm s1, as seen in Fig. 7 (a) and (c). Their corresponding SAED patterns were illustrated in Fig. 7(b) and (d). It is noteworthy that both twins are observed under other ECAD passes drawn at 10 and 20 mm s1, which is consistent with Fig. 3, indicating that twinning accommodates deformation during the
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entire processing owing to the relatively high strain rate and low temperature. This result is in agreement with those of previous investigations, where the f1012g < 1011 > tension twins and the f1122g < 1123 > compression twins are the most active twins during other conventional deformation at ambient temperature [36,37]. However, it is noticeable that deformation twinning is typically identified as either f1011g < 1012 > type or f1012g < 1011 > type in most ECAPedeformed CPeTi at room temperature [12,18], instead of being observed simultaneously. Apart from in the matrix, the FCC phase develops in the deformation twins to accommodate the plastic strain, as shown in Fig. 7(e). 4. Conclusions In this study, ECAD process was applied to commercial CPeTi wires to evaluate the feasibility of continuous processing at room
Fig. 7. TEM image of CPeTi sample after 5 passes of ECAD at constant drawing speed of 20 mm s1: Tension twining (a) and the corresponding SAED pattern (b), compression twining (c) and the corresponding SAED pattern (d), and (e) FCCeTi in compression twinning.
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temperature using a 90 die and drawing speeds of 10 and 20 mm s1. Simultaneously, the effect of drawing speed on the microstructural evolutions of CPeTi during ECAD was investigated. The following conclusions were obtained: CPeTi wires were successfully drawn up to 12 passes at 10 and 20 mm s1 without surface damage that typically occurred in ECAP. We successfully manufactured CPeTi wires through ECAD technology at room temperature. The drawing speed affected the deformation mode and microstructural evolution of CPeTi significantly. The grain size reduced from ~32 mm for the as-annealed samples to 700 nm for the 12 passes ECAD samples at the drawing speed of 10 mm s1, which were consistent with earlier reports of ECAP. While the FCC phase was obviously triggered to effectively accommodate the large shear strain induced by ECAD when the drawing speed increased to 20 mm s1, which was not reported in publications regarding ECAP. The thickness of the FCC phase increased gradually with the increase in drawing passes. The equiaxed grains were not present in CPeTi after 12 passes. Meanwhile, f1012g and f1122g twins were simultaneously detected during ECAD for the first time. Acknowledgements
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This work was supported by the National Natural Science Foundation of China (Grant No. 51525101), Fundamental Research Funds of the Central Universities (No. N170206003).
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