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Grain refinement and phase transition of commercial pure zirconium processed by cold rolling
Materials Characterization 129 (2017) 149–155
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Materials Characterization journal homepage: www.elsevier.com/locate/matchar
Grain refinement and phase transition of commercial pure zirconium processed by cold rolling
MARK
Xinyi Hu, Henglv Zhao, Song Ni⁎, Min Song⁎ State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Zirconium Grain refinement Phase transition Cold rolling
The grain refinement process and phase transition phenomena of a cold-rolled zirconium have been systematically investigated. The grain refinement was accomplished via a sequential series of dislocation behaviors, reaching a final grain size of < 100 nm. The refinement process can be described as: (1) dislocation nucleation, multiplication and tangling inside the original coarse-grains, (2) formation of microbands via dislocation rearrangement, (3) further division of microbands into thin laths, (4) split of microbands and thin laths into subgrains and then nanograins. Two different types of HCP-FCC phase transitions were also discovered during the cold rolling process, with the transition mechanisms and their influences on the mechanical properties being discussed.
1. Introduction The commercial pure (CP) zirconium (Zr), a typical hexagonal closepacked (HCP) metal, has lots of promising properties, such as high tensile strength, high density, and amazing corrosion resistance. Due to these superior properties, it has attracted lots of attention for applications in various fields such as automotive, aircraft engine and biomedical materials [1–4]. Nowadays, many researchers have been focusing on the HCP structural materials, such as titanium (Ti) [5,6], magnesium (Mg) [7,8], Zr [9–14] and hafnium (Hf) [15], in order to clarify the deformation mechanisms and structure–property relationship under various deformation processes. It is known that the independent slip systems of HCP structured metals don't reach the minimum number required for homogenous deformation. Therefore, twinning is taken as a very important supplemental deformation mechanism in HCP metals [16]. In addition to dislocation slip and twinning, phase transition can also be triggered under certain stress to effectively accommodate deformation [17–19]. As for Zr, Yang et al. reported that dislocation slip is the main way for Zr to accommodate strain during cold rolling [14]. Guo et al. [9] investigated the effect of rolling strain rate on the microstructure of Zr at a temperature range of 113–183 K, and found that besides dislocation slip, twinning can also be activated to accommodate the strain and to refine the microstructure during deformation. In the present study, the microstructural evolution of Zr deformed by cold rolling was systematically investigated using transmission electron microscopy (TEM) and high-resolution TEM (HRTEM).
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Corresponding authors. E-mail addresses: [email protected] (S. Ni), [email protected] (M. Song).
http://dx.doi.org/10.1016/j.matchar.2017.04.037 Received 23 March 2017; Received in revised form 30 April 2017; Accepted 30 April 2017 Available online 01 May 2017 1044-5803/ © 2017 Elsevier Inc. All rights reserved.
The results indicate that dislocation slip dominated the deformation process and surprisingly, two different kinds of HCP-FCC phase transitions in Zr were also activated to accommodate the strain. The transformation mechanisms and their influences on the mechanical properties were discussed. 2. Experimental Procedure Commercial pure Zr with a pure HCP phase was used as the raw material in this study. The chemical composition of impurities is listed in Table 1. The grains in the raw material have a mean size of 15 μm from optical microscopy analysis. Plates with a size of 30 mm × 10 mm × 5 mm (L × W × H) were rolled at room temperature with a thickness reduction of 1 mm per pass, to obtain the specimens with thickness reductions of 20%, 40%, 60% and 80%, respectively. The microstructures of un-deformed and deformed specimens were thoroughly investigated by TEM. The preparation procedure of the thin foils for TEM observation is using ion milling process: thin slices of the un-deformed and deformed specimens were cut perpendicular to the transverse direction (TD) plane with a thickness of 0.5 mm through wire-cutting, and were then mechanically thinned down to ~80 μm. Short strips with length of 3 mm were subsequently cut from the slice and were then mechanically thinned to 20 μm. By using Ganta PIPS system, these strips were ion-milled to perforation for the TEM characterization.
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parts of M1 and M2). Fig. 3b indicates that small misorientation exists inside the microband M1 due to the formation of thin laths. Fig. 3c indicates that large misorientation exists between two adjacent microbands M1 and M2, in which two sets of patterns with zone axes of [1120] and [1010] were indicated. The angle between the two zone axes [1120] and [1010] is around 30°, showing a high angle misorientation, while the misorientations between the thin laths are usually < 15°, which is a low angle misorientation. Fig. 3d shows another TEM image taken at a different area of the rolled specimen with a thickness reduction of 60%, where thin laths were clearly observed inside a microband. The width of these laths is generally smaller than 100 nm, indicating that the thin laths were refined longitudinally during the deformation process. Meanwhile, due to the progressive accumulation of dislocations with increasing the strain, the former transverse dislocation walls at some locations evolved into low angle or high angle boundaries. It is therefore that the areas with different contrasts can be seen in a lath, as indicated by some yellow arrows in Fig. 3d. The formation of these boundaries further cut down the thin laths into elongated subgrains and therefore transversely refined the thin laths. The (sub)grain size distribution in this stage is listed in Fig. 3e. Fig. 4a and b shows the TEM images taken at different areas of the cold rolled specimen with a thickness reduction of 80%. In Fig. 4a, two short thin laths were indicated by dashed lines. It shows that the boundaries of these thin laths became turbulent, compared to the relatively straight ones in Fig. 3d. This configuration may be beneficial to the further transverse breakdown of the thin laths [5]. In Fig. 4b, some equiaxed grains were obviously recognizable with the average grain size of about 55 nm (see the grain size distribution diagram in Fig. 4c). An inserted SAED pattern taken from the area in Fig. 4b (upper right corner in Fig. 4b) shows diffuse arcing spots or ring shape, further confirming the formation of equiaxed nanocrystals. The grain refinement process can therefore be concluded as following: (1) microbands formed, (2) dislocation rearranged and the formation of thin-lath structure in the microbands, (3) thin-lath structure was refined longitudinally and transversely into elongated subgrains, (4) nano-grains were formed through the further transverse refinement of the subgrains. The dislocation slip dominated the deformation process and facilitated the grain refinement process. At the very beginning of deformation, dislocations are largely generated, as shown in Fig. 1b. They slipped on their dominant slip systems and then propagated by multiple-slip or cross-slip events [21–24]. The accumulation and tangling of these dislocations contributed to the formation of microbands. Hu et al. [25] calculated the stacking fault energy (SFE) of Zr using both local spin density approximation (LSDA) and the generalized gradient
Table 1 The chemical composition of impurities in the raw material. Impurity elements
Sn
Nb
Fe
Cr
Cu
Hf (ppm)
C (ppm)
Content (wt%)
1.4
0.4
0.3
0.1
0.05
< 60
< 50
3. Results and Discussion 3.1. Grain Refinement Fig. 1a shows a bright-field TEM image of the raw material. It can be seen that few dislocations and no twin can be observed inside the raw material. Fig. 1b shows a typical TEM image of the cold rolled sample with a thickness reduction of 20%. It shows that a large number of dislocations were generated, many of which tangled with each other, tending to rearrange and form dislocation walls, as indicated by red arrows in Fig. 1b. This is the very beginning of the grain refinement process. Fig. 2a shows a TEM image of the cold rolled sample with a thickness reduction of 40%. Microband boundaries (MB boundary), originated from the former dislocation walls were observed (some examples were indicated by white arrows). These microbands have widths ranging from ~750 nm to ~4 μm. Noting that there were a high density of dislocations tangling with each other and aligning parallel to form thin lath boundaries (TL boundary) inside the microbands. Some of the lath boundaries were indicated by yellow arrows in Fig. 2b, a magnified image of the yellow rectangular region in Fig. 2a. Fig. 2c shows a bright-field TEM image of the cold rolled sample with a thickness reduction of 40% in another area, from which thin lath structure within a microband can be clearly observed and the width of the lath is < 100 nm. Meanwhile, dislocation accumulation (white arrows in Fig. 2c) can be observed within the thin laths to form dislocation walls. These dislocation walls may transversely intersect the thin laths [20]. Fig. 3a shows a bright field TEM image of the deformed specimen with a thickness reduction of 60%. It shows that the microbands formed at former strain were totally split into thin-lath structure with increasing the strain. Within the microbands these thin laths have widths ranging from several dozens of nanometers to around 300 nm. Comparing the direction of thin-lath structure in microband M1 to that in microband M2, it's apparent that the orientations of the thin-lath structure in different microbands are different. Fig. 3b shows the selected area electron diffraction (SAED) pattern of M1 and Fig. 3c shows the SAED pattern of the circled area in Fig. 3a (contains both
Fig. 1. TEM images of (a) the un-deformed Zr and (b) the cold rolled Zr with a thickness reduction of 20%. Note that some of the dislocation walls are indicated by red arrows. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 2. (a) A bright-field TEM image of the cold rolled Zr with a thickness reduction of 40%, showing the formed microbands and thin laths, (b) the magnified image of the yellow rectangular area in (a), and (c) a TEM image of another area, showing fully formed thin-lath structure. Note that the white arrows in (c) showing the dislocation accumulation sites. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
approximation (GGA), which is about 210–230 mJ/m2. The SFE is relatively high and therefore the partials were closely packed, and thus it is easy for dislocations to cross slip. As the strain increased, the generated dislocations would move to the microband boundaries and annihilate, which can effectively increase the misorientation between two adjacent microbands. The dislocation density may decrease in this stage. Further, as the strain continued to increase different slip systems at different microbands were activated and the dislocation density rise again [26]. As a result, microbands further split into thin laths with different directions (as shown in Fig. 3a). It was reported that the direction of the thin-lath structure within certain microband is the same as that of the slip trace, e.g., the direction of the slip plane [24]. With further deformation, the dislocation density continued to rise and the thin lath boundaries continued to absorb dislocations until they eventually converted into high angle boundaries. It has been observed that the dislocations within the laths tend to accumulate at some locations to further break down the thin lath. Gray III and Xue [27] indicated that the split process can be enhanced by the interactions between the adjacent thin laths under heavy shear deformation. When the thin laths or the elongated segment boundaries became distorted or curved (as shown in the Fig. 4a), the adjacent one then put the curved part to split apart from the parent thin laths or elongated segments into fine elongated subgrains. Finally, those subgrains converted to the equiaxed nanosized grains (Fig. 4b) with the help of further lattice rotations [5]. The grain refinement process described above showed that the dislocation slip dominated the deformation while the twinning was suppressed in the cold-rolling process of Zr. The rolling speed in this study is relatively low, corresponding to a low strain rate (~ 1 s− 1), which is not ideal for the activation of twinning [28].
confirming the phase orientation relationship obtained from SAED pattern. The inset image is a Fourier-filtered image, where the left part is typical FCC sequence…ABCABC… and the right part is HCP sequence…ABAB…. According to the method that mentioned in our previous study [29], two 30° Shockley partial dislocations have been marked on every second (0001) plane in the phase interface. The existence of these partial dislocations confirms that this phase transformation was also triggered by the partial dislocations that glide on every second (0001) plane. Fig. 5d is also an HRTEM image of the HCPFCC phase interface. Interestingly, many full dislocations were formed in the FCC phase, as shown in the inset image in Fig. 5d. The burgers vectors of all these dislocations have a component parallel to the c-axis [0001] direction in HCP matrix, which means that they can effectively accommodate the c-axis strain in the HCP matrix and therefore enhance the deformability of materials. Fig. 6a shows a TEM image of the specimen with a thickness reduction of 40%, with the squared area being further magnified in Fig. 6b. A large number of the FCC lamellas were observed. The width of these lamellas is mainly < 100 nm and the existence of them can effectively refine the HCP matrix. So this HCPFCC phase transition facilitates the grain refinement process. Fig. 6c is a TEM image of the specimen with a thickness reduction of 60%. In this stage many grains were refined down to nanoscale. We also identified a FCC lamella as indicated by a yellow arrow in Fig. 6c, with the squared area being further magnified in Fig. 6d. This FCC lamella also tends to be refined because different diffraction contrasts can be observed within it. It means that FCC phase will still exist in the refined nanograins. Fig. 7a is a bright-field TEM image of the lamellas observed in another area, where two lamellas were indicated by red arrows. Fig. 7b is a SAED pattern of the area that contains both the HCP matrix and the lamellas. Surprisingly, the lamellas are also FCC structure, but with a totally different HCP-FCC phase orientation relationship compared to that observed in Fig. 5. From the SAED pattern, the orientation relationship between the HCP and FCC phases is [0001]HCP//[001]FCC and (1010)HCP (110)FCC . Fig. 7c is an HRTEM image of the HCP-FCC phase interface, further confirming this phase orientation relationship. Recently, Wu et al. [34] observed the same HCP-FCC phase transformation in pure Ti and proposed that the transformation occurred via a shear-shuffle mechanism through successive gliding of two-layer steps or a pure shuffle mechanism through gliding of four-layer steps. The HCP-FCC phase orientation relationship in pure Zr here is the same as that in Ti. In addition, many two-layer steps and four-layer steps (an example is shown in the inset image in Fig. 7c) were observed at the HCP-FCC interfacial regions. Therefore, it is proposed that the phase transformation might occur via the shear-shuffle and pure shuffle mechanisms.
3.2. Phase Transition Apart from the grain refinement, the HCP-Zr also underwent phase transition during the deformation process. Two different types of phase transitions were observed. Fig. 5a shows a TEM image of the first kind of needle-like lamellas, which were indicated by red arrows. Fig. 5b is the SAED pattern of an area containing both the matrix and the lamellas, showing an HCP pattern with zone axis [1210] and a FCC pattern with zone axis [110]. From this SAED pattern, we can derive that the orientation relationship between the HCP matrix and the FCC lamellas is [1210]HCP [110]FCC and (0002)HCP (111)FCC . This HCP-FCC phase orientation has been discovered previously in metal Hf [29], Ti alloys [30,31], Co [32,33], et al. It was suggested that this phase transition can be triggered by Shockley partial dislocation gliding on every second basal plane (0001) [29,33]. Fig. 5c is an HRTEM image of the phase interface between the HCP and the FCC phases, further 151
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Fig. 3. (a) A bright-field TEM image of the cold rolled Zr with a thickness reduction of 60%, (b, c) corresponding SAED patterns of M1 and the circled area, (d) TEM image taken at a different area of the cold rolled Zr with a thickness reduction of 60%, and (e) the grain size distribution of the cold rolled Zr with a thickness reduction of 60%. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
that the relatively low strain rate (~ 1 s− 1) during rolling in this study might not be ideal for the activation of twinning. However, detailed reason why twinning is suppressed still needs further exploration. Though the effect of the FCC phase on the mechanical properties of Zr is unclear, it is reasonable to infer that the formation of FCC phase will improve the mechanical properties of Zr. Due to the incoherence of the slip system across the interface formed by the phase transition, the easy gliding of basal and prism < a > dislocations in the HCP matrix will be effectively blocked at the interface, which hinders the dislocation motions and thus enhances the strength. This view is supported by a recent investigation on the formation of FCC phase in bulk Ti, where the yield strength of bulk Ti increased from 381 MPa in the as-received state to 731 MPa after the formation of FCC phase [36]. Besides, FCC phase provides various slip systems for dislocations and some of the
Measuring the lattice parameters of the FCC phase in both types of phase transitions from HRTEM images (Figs. 5c and 7c), we found that the lattice parameter a for FCC-Zr is around 0.475 nm, while the lattice parameters for HCP-Zr are a = 0.323 nm and c = 0.515 nm. Therefore, as for the first kind of HCP-FCC phase transition, the formation of FCC phase will lead to a lattice expansion of around 6.5% along the c-axis ([0001]). For the second kind of HCP-FCC phase transition, the formation of FCC phase should lead to a lattice contraction of around 8.4% along the c-axis ([0001]). Lattice changes along the c-axis indicate that the formation of the FCC phase can effectively accommodate the caxis strain. Previous studies indicated that the c-axis strain in Zr is usually accommodated by the tensile or compressive twins [14,35]. However, in the present investigation, twins were not detected while the phase transition from HCP to FCC phase occurred. It is proposed 152
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Fig. 4. (a, b) TEM images of the cold rolled Zr with a thickness reduction of 80% and (c) the grain size distribution of the cold rolled Zr with a thickness reduction of 80%.
Fig. 5. (a) A bright-field TEM image of the first kind of lamellas detected in matrix, which were indicated by red arrows, (b) the SAED pattern of an area containing both HCP matrix and lamellas, (c) a HRTEM image of the phase interface, with the inset image showing two 30° partials existing in a FCC lamella front and (d) a HRTEM image of the phase interface in another area, where there are lots of full dislocations within the FCC phase. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
dislocation multiplication → dislocation rearrangement and the formation of microbands → the generation of thin-lath structure → the longitudinal and transverse breakdown of thin laths into elongated subgrains → the conversion of elongated subgrains into equiaxed nanosized grains with an average grain size of 55 nm. (2) Twining was suppressed while two different kinds of HCP-FCC phase transitions were detected during deformation. The formation of the FCC phase can accommodate the c-axis strain and is believed to improve the mechanical properties of the material.
dislocations can accommodate the c-axis strain as shown in Fig. 5d, which may increase the plasticity of the material. 4. Conclusions The grain refinement process and phase transition phenomena were studied on cold-rolled Zr. Conclusions are drawn as follows: (1) During the cold rolling process of pure Zr, the sequence of the grain refinement process follows: original equiaxed coarse grains → 153
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Fig. 6. (a) A bright-field TEM image of the first kind of lamellas detected in the specimen with a thickness reduction of 40%, (b) the HRTEM image of the yellow rectangular area in (a), showing both FCC and HCP phases, (c) a bright-field TEM image of the specimen with a thickness reduction of 60%, with a FCC lamella being indicated by a yellow arrow, and (d) the HRTEM image of the red rectangular area in (c), showing both FCC and HCP phases. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 7. (a) A bright-field image of the other kind of lamellas in matrix, indicated by red arrows, (b) the SAED pattern of an area containing both HCP matrix and the lamellas, and (c) a HRTEM image of the phase interface. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Acknowledgement [2]
The financial support from National Natural Science Foundation of China (51301207) is appreciated. One (M.S.) of the authors would also thank the financial support from State Key Laboratory of Powder Metallurgy at Central South University (11100-621020059).
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Grain refinement and phase transition of commercial pure zirconium processed by cold rolling
MARK
Xinyi Hu, Henglv Zhao, Song Ni⁎, Min Song⁎ State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Zirconium Grain refinement Phase transition Cold rolling
The grain refinement process and phase transition phenomena of a cold-rolled zirconium have been systematically investigated. The grain refinement was accomplished via a sequential series of dislocation behaviors, reaching a final grain size of < 100 nm. The refinement process can be described as: (1) dislocation nucleation, multiplication and tangling inside the original coarse-grains, (2) formation of microbands via dislocation rearrangement, (3) further division of microbands into thin laths, (4) split of microbands and thin laths into subgrains and then nanograins. Two different types of HCP-FCC phase transitions were also discovered during the cold rolling process, with the transition mechanisms and their influences on the mechanical properties being discussed.
1. Introduction The commercial pure (CP) zirconium (Zr), a typical hexagonal closepacked (HCP) metal, has lots of promising properties, such as high tensile strength, high density, and amazing corrosion resistance. Due to these superior properties, it has attracted lots of attention for applications in various fields such as automotive, aircraft engine and biomedical materials [1–4]. Nowadays, many researchers have been focusing on the HCP structural materials, such as titanium (Ti) [5,6], magnesium (Mg) [7,8], Zr [9–14] and hafnium (Hf) [15], in order to clarify the deformation mechanisms and structure–property relationship under various deformation processes. It is known that the independent slip systems of HCP structured metals don't reach the minimum number required for homogenous deformation. Therefore, twinning is taken as a very important supplemental deformation mechanism in HCP metals [16]. In addition to dislocation slip and twinning, phase transition can also be triggered under certain stress to effectively accommodate deformation [17–19]. As for Zr, Yang et al. reported that dislocation slip is the main way for Zr to accommodate strain during cold rolling [14]. Guo et al. [9] investigated the effect of rolling strain rate on the microstructure of Zr at a temperature range of 113–183 K, and found that besides dislocation slip, twinning can also be activated to accommodate the strain and to refine the microstructure during deformation. In the present study, the microstructural evolution of Zr deformed by cold rolling was systematically investigated using transmission electron microscopy (TEM) and high-resolution TEM (HRTEM).
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Corresponding authors. E-mail addresses: [email protected] (S. Ni), [email protected] (M. Song).
http://dx.doi.org/10.1016/j.matchar.2017.04.037 Received 23 March 2017; Received in revised form 30 April 2017; Accepted 30 April 2017 Available online 01 May 2017 1044-5803/ © 2017 Elsevier Inc. All rights reserved.
The results indicate that dislocation slip dominated the deformation process and surprisingly, two different kinds of HCP-FCC phase transitions in Zr were also activated to accommodate the strain. The transformation mechanisms and their influences on the mechanical properties were discussed. 2. Experimental Procedure Commercial pure Zr with a pure HCP phase was used as the raw material in this study. The chemical composition of impurities is listed in Table 1. The grains in the raw material have a mean size of 15 μm from optical microscopy analysis. Plates with a size of 30 mm × 10 mm × 5 mm (L × W × H) were rolled at room temperature with a thickness reduction of 1 mm per pass, to obtain the specimens with thickness reductions of 20%, 40%, 60% and 80%, respectively. The microstructures of un-deformed and deformed specimens were thoroughly investigated by TEM. The preparation procedure of the thin foils for TEM observation is using ion milling process: thin slices of the un-deformed and deformed specimens were cut perpendicular to the transverse direction (TD) plane with a thickness of 0.5 mm through wire-cutting, and were then mechanically thinned down to ~80 μm. Short strips with length of 3 mm were subsequently cut from the slice and were then mechanically thinned to 20 μm. By using Ganta PIPS system, these strips were ion-milled to perforation for the TEM characterization.
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parts of M1 and M2). Fig. 3b indicates that small misorientation exists inside the microband M1 due to the formation of thin laths. Fig. 3c indicates that large misorientation exists between two adjacent microbands M1 and M2, in which two sets of patterns with zone axes of [1120] and [1010] were indicated. The angle between the two zone axes [1120] and [1010] is around 30°, showing a high angle misorientation, while the misorientations between the thin laths are usually < 15°, which is a low angle misorientation. Fig. 3d shows another TEM image taken at a different area of the rolled specimen with a thickness reduction of 60%, where thin laths were clearly observed inside a microband. The width of these laths is generally smaller than 100 nm, indicating that the thin laths were refined longitudinally during the deformation process. Meanwhile, due to the progressive accumulation of dislocations with increasing the strain, the former transverse dislocation walls at some locations evolved into low angle or high angle boundaries. It is therefore that the areas with different contrasts can be seen in a lath, as indicated by some yellow arrows in Fig. 3d. The formation of these boundaries further cut down the thin laths into elongated subgrains and therefore transversely refined the thin laths. The (sub)grain size distribution in this stage is listed in Fig. 3e. Fig. 4a and b shows the TEM images taken at different areas of the cold rolled specimen with a thickness reduction of 80%. In Fig. 4a, two short thin laths were indicated by dashed lines. It shows that the boundaries of these thin laths became turbulent, compared to the relatively straight ones in Fig. 3d. This configuration may be beneficial to the further transverse breakdown of the thin laths [5]. In Fig. 4b, some equiaxed grains were obviously recognizable with the average grain size of about 55 nm (see the grain size distribution diagram in Fig. 4c). An inserted SAED pattern taken from the area in Fig. 4b (upper right corner in Fig. 4b) shows diffuse arcing spots or ring shape, further confirming the formation of equiaxed nanocrystals. The grain refinement process can therefore be concluded as following: (1) microbands formed, (2) dislocation rearranged and the formation of thin-lath structure in the microbands, (3) thin-lath structure was refined longitudinally and transversely into elongated subgrains, (4) nano-grains were formed through the further transverse refinement of the subgrains. The dislocation slip dominated the deformation process and facilitated the grain refinement process. At the very beginning of deformation, dislocations are largely generated, as shown in Fig. 1b. They slipped on their dominant slip systems and then propagated by multiple-slip or cross-slip events [21–24]. The accumulation and tangling of these dislocations contributed to the formation of microbands. Hu et al. [25] calculated the stacking fault energy (SFE) of Zr using both local spin density approximation (LSDA) and the generalized gradient
Table 1 The chemical composition of impurities in the raw material. Impurity elements
Sn
Nb
Fe
Cr
Cu
Hf (ppm)
C (ppm)
Content (wt%)
1.4
0.4
0.3
0.1
0.05
< 60
< 50
3. Results and Discussion 3.1. Grain Refinement Fig. 1a shows a bright-field TEM image of the raw material. It can be seen that few dislocations and no twin can be observed inside the raw material. Fig. 1b shows a typical TEM image of the cold rolled sample with a thickness reduction of 20%. It shows that a large number of dislocations were generated, many of which tangled with each other, tending to rearrange and form dislocation walls, as indicated by red arrows in Fig. 1b. This is the very beginning of the grain refinement process. Fig. 2a shows a TEM image of the cold rolled sample with a thickness reduction of 40%. Microband boundaries (MB boundary), originated from the former dislocation walls were observed (some examples were indicated by white arrows). These microbands have widths ranging from ~750 nm to ~4 μm. Noting that there were a high density of dislocations tangling with each other and aligning parallel to form thin lath boundaries (TL boundary) inside the microbands. Some of the lath boundaries were indicated by yellow arrows in Fig. 2b, a magnified image of the yellow rectangular region in Fig. 2a. Fig. 2c shows a bright-field TEM image of the cold rolled sample with a thickness reduction of 40% in another area, from which thin lath structure within a microband can be clearly observed and the width of the lath is < 100 nm. Meanwhile, dislocation accumulation (white arrows in Fig. 2c) can be observed within the thin laths to form dislocation walls. These dislocation walls may transversely intersect the thin laths [20]. Fig. 3a shows a bright field TEM image of the deformed specimen with a thickness reduction of 60%. It shows that the microbands formed at former strain were totally split into thin-lath structure with increasing the strain. Within the microbands these thin laths have widths ranging from several dozens of nanometers to around 300 nm. Comparing the direction of thin-lath structure in microband M1 to that in microband M2, it's apparent that the orientations of the thin-lath structure in different microbands are different. Fig. 3b shows the selected area electron diffraction (SAED) pattern of M1 and Fig. 3c shows the SAED pattern of the circled area in Fig. 3a (contains both
Fig. 1. TEM images of (a) the un-deformed Zr and (b) the cold rolled Zr with a thickness reduction of 20%. Note that some of the dislocation walls are indicated by red arrows. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 2. (a) A bright-field TEM image of the cold rolled Zr with a thickness reduction of 40%, showing the formed microbands and thin laths, (b) the magnified image of the yellow rectangular area in (a), and (c) a TEM image of another area, showing fully formed thin-lath structure. Note that the white arrows in (c) showing the dislocation accumulation sites. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
approximation (GGA), which is about 210–230 mJ/m2. The SFE is relatively high and therefore the partials were closely packed, and thus it is easy for dislocations to cross slip. As the strain increased, the generated dislocations would move to the microband boundaries and annihilate, which can effectively increase the misorientation between two adjacent microbands. The dislocation density may decrease in this stage. Further, as the strain continued to increase different slip systems at different microbands were activated and the dislocation density rise again [26]. As a result, microbands further split into thin laths with different directions (as shown in Fig. 3a). It was reported that the direction of the thin-lath structure within certain microband is the same as that of the slip trace, e.g., the direction of the slip plane [24]. With further deformation, the dislocation density continued to rise and the thin lath boundaries continued to absorb dislocations until they eventually converted into high angle boundaries. It has been observed that the dislocations within the laths tend to accumulate at some locations to further break down the thin lath. Gray III and Xue [27] indicated that the split process can be enhanced by the interactions between the adjacent thin laths under heavy shear deformation. When the thin laths or the elongated segment boundaries became distorted or curved (as shown in the Fig. 4a), the adjacent one then put the curved part to split apart from the parent thin laths or elongated segments into fine elongated subgrains. Finally, those subgrains converted to the equiaxed nanosized grains (Fig. 4b) with the help of further lattice rotations [5]. The grain refinement process described above showed that the dislocation slip dominated the deformation while the twinning was suppressed in the cold-rolling process of Zr. The rolling speed in this study is relatively low, corresponding to a low strain rate (~ 1 s− 1), which is not ideal for the activation of twinning [28].
confirming the phase orientation relationship obtained from SAED pattern. The inset image is a Fourier-filtered image, where the left part is typical FCC sequence…ABCABC… and the right part is HCP sequence…ABAB…. According to the method that mentioned in our previous study [29], two 30° Shockley partial dislocations have been marked on every second (0001) plane in the phase interface. The existence of these partial dislocations confirms that this phase transformation was also triggered by the partial dislocations that glide on every second (0001) plane. Fig. 5d is also an HRTEM image of the HCPFCC phase interface. Interestingly, many full dislocations were formed in the FCC phase, as shown in the inset image in Fig. 5d. The burgers vectors of all these dislocations have a component parallel to the c-axis [0001] direction in HCP matrix, which means that they can effectively accommodate the c-axis strain in the HCP matrix and therefore enhance the deformability of materials. Fig. 6a shows a TEM image of the specimen with a thickness reduction of 40%, with the squared area being further magnified in Fig. 6b. A large number of the FCC lamellas were observed. The width of these lamellas is mainly < 100 nm and the existence of them can effectively refine the HCP matrix. So this HCPFCC phase transition facilitates the grain refinement process. Fig. 6c is a TEM image of the specimen with a thickness reduction of 60%. In this stage many grains were refined down to nanoscale. We also identified a FCC lamella as indicated by a yellow arrow in Fig. 6c, with the squared area being further magnified in Fig. 6d. This FCC lamella also tends to be refined because different diffraction contrasts can be observed within it. It means that FCC phase will still exist in the refined nanograins. Fig. 7a is a bright-field TEM image of the lamellas observed in another area, where two lamellas were indicated by red arrows. Fig. 7b is a SAED pattern of the area that contains both the HCP matrix and the lamellas. Surprisingly, the lamellas are also FCC structure, but with a totally different HCP-FCC phase orientation relationship compared to that observed in Fig. 5. From the SAED pattern, the orientation relationship between the HCP and FCC phases is [0001]HCP//[001]FCC and (1010)HCP (110)FCC . Fig. 7c is an HRTEM image of the HCP-FCC phase interface, further confirming this phase orientation relationship. Recently, Wu et al. [34] observed the same HCP-FCC phase transformation in pure Ti and proposed that the transformation occurred via a shear-shuffle mechanism through successive gliding of two-layer steps or a pure shuffle mechanism through gliding of four-layer steps. The HCP-FCC phase orientation relationship in pure Zr here is the same as that in Ti. In addition, many two-layer steps and four-layer steps (an example is shown in the inset image in Fig. 7c) were observed at the HCP-FCC interfacial regions. Therefore, it is proposed that the phase transformation might occur via the shear-shuffle and pure shuffle mechanisms.
3.2. Phase Transition Apart from the grain refinement, the HCP-Zr also underwent phase transition during the deformation process. Two different types of phase transitions were observed. Fig. 5a shows a TEM image of the first kind of needle-like lamellas, which were indicated by red arrows. Fig. 5b is the SAED pattern of an area containing both the matrix and the lamellas, showing an HCP pattern with zone axis [1210] and a FCC pattern with zone axis [110]. From this SAED pattern, we can derive that the orientation relationship between the HCP matrix and the FCC lamellas is [1210]HCP [110]FCC and (0002)HCP (111)FCC . This HCP-FCC phase orientation has been discovered previously in metal Hf [29], Ti alloys [30,31], Co [32,33], et al. It was suggested that this phase transition can be triggered by Shockley partial dislocation gliding on every second basal plane (0001) [29,33]. Fig. 5c is an HRTEM image of the phase interface between the HCP and the FCC phases, further 151
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Fig. 3. (a) A bright-field TEM image of the cold rolled Zr with a thickness reduction of 60%, (b, c) corresponding SAED patterns of M1 and the circled area, (d) TEM image taken at a different area of the cold rolled Zr with a thickness reduction of 60%, and (e) the grain size distribution of the cold rolled Zr with a thickness reduction of 60%. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
that the relatively low strain rate (~ 1 s− 1) during rolling in this study might not be ideal for the activation of twinning. However, detailed reason why twinning is suppressed still needs further exploration. Though the effect of the FCC phase on the mechanical properties of Zr is unclear, it is reasonable to infer that the formation of FCC phase will improve the mechanical properties of Zr. Due to the incoherence of the slip system across the interface formed by the phase transition, the easy gliding of basal and prism < a > dislocations in the HCP matrix will be effectively blocked at the interface, which hinders the dislocation motions and thus enhances the strength. This view is supported by a recent investigation on the formation of FCC phase in bulk Ti, where the yield strength of bulk Ti increased from 381 MPa in the as-received state to 731 MPa after the formation of FCC phase [36]. Besides, FCC phase provides various slip systems for dislocations and some of the
Measuring the lattice parameters of the FCC phase in both types of phase transitions from HRTEM images (Figs. 5c and 7c), we found that the lattice parameter a for FCC-Zr is around 0.475 nm, while the lattice parameters for HCP-Zr are a = 0.323 nm and c = 0.515 nm. Therefore, as for the first kind of HCP-FCC phase transition, the formation of FCC phase will lead to a lattice expansion of around 6.5% along the c-axis ([0001]). For the second kind of HCP-FCC phase transition, the formation of FCC phase should lead to a lattice contraction of around 8.4% along the c-axis ([0001]). Lattice changes along the c-axis indicate that the formation of the FCC phase can effectively accommodate the caxis strain. Previous studies indicated that the c-axis strain in Zr is usually accommodated by the tensile or compressive twins [14,35]. However, in the present investigation, twins were not detected while the phase transition from HCP to FCC phase occurred. It is proposed 152
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Fig. 4. (a, b) TEM images of the cold rolled Zr with a thickness reduction of 80% and (c) the grain size distribution of the cold rolled Zr with a thickness reduction of 80%.
Fig. 5. (a) A bright-field TEM image of the first kind of lamellas detected in matrix, which were indicated by red arrows, (b) the SAED pattern of an area containing both HCP matrix and lamellas, (c) a HRTEM image of the phase interface, with the inset image showing two 30° partials existing in a FCC lamella front and (d) a HRTEM image of the phase interface in another area, where there are lots of full dislocations within the FCC phase. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
dislocation multiplication → dislocation rearrangement and the formation of microbands → the generation of thin-lath structure → the longitudinal and transverse breakdown of thin laths into elongated subgrains → the conversion of elongated subgrains into equiaxed nanosized grains with an average grain size of 55 nm. (2) Twining was suppressed while two different kinds of HCP-FCC phase transitions were detected during deformation. The formation of the FCC phase can accommodate the c-axis strain and is believed to improve the mechanical properties of the material.
dislocations can accommodate the c-axis strain as shown in Fig. 5d, which may increase the plasticity of the material. 4. Conclusions The grain refinement process and phase transition phenomena were studied on cold-rolled Zr. Conclusions are drawn as follows: (1) During the cold rolling process of pure Zr, the sequence of the grain refinement process follows: original equiaxed coarse grains → 153
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Fig. 6. (a) A bright-field TEM image of the first kind of lamellas detected in the specimen with a thickness reduction of 40%, (b) the HRTEM image of the yellow rectangular area in (a), showing both FCC and HCP phases, (c) a bright-field TEM image of the specimen with a thickness reduction of 60%, with a FCC lamella being indicated by a yellow arrow, and (d) the HRTEM image of the red rectangular area in (c), showing both FCC and HCP phases. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 7. (a) A bright-field image of the other kind of lamellas in matrix, indicated by red arrows, (b) the SAED pattern of an area containing both HCP matrix and the lamellas, and (c) a HRTEM image of the phase interface. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Acknowledgement [2]
The financial support from National Natural Science Foundation of China (51301207) is appreciated. One (M.S.) of the authors would also thank the financial support from State Key Laboratory of Powder Metallurgy at Central South University (11100-621020059).
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