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High tensile ductility and strength in a gradient structured Zr
Accepted Manuscript High tensile ductility and strength in a gradient structured Zr Lina Wang, Yindong Shi, Yulong Zhang, Ming Li PII: DOI: Reference:
S0167-577X(18)30990-X https://doi.org/10.1016/j.matlet.2018.06.084 MLBLUE 24524
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
28 April 2018 10 June 2018 22 June 2018
Please cite this article as: L. Wang, Y. Shi, Y. Zhang, M. Li, High tensile ductility and strength in a gradient structured Zr, Materials Letters (2018), doi: https://doi.org/10.1016/j.matlet.2018.06.084
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
High tensile ductility and strength in a gradient structured Zr Lina Wang1, Yindong Shi1,*, Yulong Zhang1, Ming Li2 1
College of Materials Science and Engineering, Hebei University of Engineering, Handan
056038, PR China 2
State Key Laboratory of Metastable Materials Science and Technology, Yanshan University,
Qinhuangdao 066004, PR China
Abstract: We report that a gradient structured Zr processed by asymmetric rolling and partial annealing possesses an exceptional synergy of high strength (σy~550 MPa) and high tensile ductility (εu ~10.1%) much better than that (σy~250 MPa and εu~14.2%) of its coarse-grained (CG) counterpart. This gradient structure is characterized by a gradient dislocation density distribution from the surface to the core and composed of microscale and ultra-fined elongated grains connected by low angle grain boundaries (LAGBs). These findings can provide valuable insight for designing gradient structures with excellent mechanical properties. Keywords: Metals and alloys; Gradient structure; Microstructure; Mechanical properties; Asymmetric rolling; Zr.
*Author to whom correspondence should be addressed; e-mail: [email protected] (Yindong Shi).
1. Introduction The development of advanced materials with superior comprehensive properties is the everlasting theme. Nowadays, many materials scientists devote to heterogeneous nanostructures to produce novel functional or structural materials [1-3]. For most structural materials, there is a common trade-off between the strength and ductility [1]. In order to achieve the strengthductility synergy of metals, recently, a gradient nanostructure has been produced in many metals (e.g., Cu, Fe) by the use of surface mechanical grinding treatment (SMGT) or surface
1
mechanical attrition treatment (SMAT) [4-7]. This strategy can double the yield strength without sacrificing its tensile ductility [1,4-5]. Experimental and theoretical studies have confirmed that the improvement of strength is attributed to the grain size refinement at the sample surface and the extra synergetic strengthening induced by mechanical incompatibility between different layers [4,7-9]. The retention of high tensile ductility results from the work hardening capacity of coarse-grain (CG) matrix, extra work hardening of the gradient layers and back stress strengthening due to the gradient macroscopic stress and strain [4-5,9-10]. However, the processing of the gradient nanostructure through surface mechanical treatment confines the thickness of the gradient nanostructure layer and the enhancement of strength-ductility synergy is limited [1,10]. Therefore, seeking potential processing routes capable of producing thicker gradient microstructure layer and novel type of gradient microstructure is of significance to enhance the strength-ductility synergy of materials. Here, a gradient dislocation density distribution is produced in Zr by employing asymmetrical rolling and partial annealing and an exceptional synergy of high yield strength (σy~550 MPa) and high uniform tensile ductility (εu~10.1%) is achieved. 2.Materials and experimental procedure High-purity Zr plates (99.95 wt.%) with a thickness of ~5 mm was used as the starting materials. The plates were vacuum-annealed at 700 °C for 2 h to obtain a fully homogeneous CG microstructure with a mean grain size of ~30 µm. The annealed samples were subjected to asymmetrical rolling (AsR) at room temperature (RT) with a total reduction of ~75%. The roll velocity ratio was 1.05 and the rolling reduction was 0.25 mm per pass. The Zr sheet was flipped over between AsR passes. Following AsR, partial annealing under vacuum was performed at 450 °C for 1 h. Microstructures in the RD-TD plane of Zr subjected to AsR and annealing were determined by TEM measurements. Tensile tests of samples with a gauge dimension of 15 × 3.0 × 1.5 mm3 were performed at RT using an Instron 5966 machine under a constant cross-head speed (i.e., 0.45 mm/min) with an initial strain rate of 5 × 10-4 s-1. The strain was measured with a contacting extensometer with a gauge length of 12 mm. For each condition, at least three
2
specimens were used for tests, and good reproduction was obtained. The tension direction was parallel to the rolling direction of the samples. 3.Results and Discussion The tensile engineering stress-strain curves of Zr subjected to AsR and annealing (AsRA) and its CG counterpart are shown in Fig. 1 (a). The CG sample (see curve 1) exhibits a low strength (i.e., yield strength σy~250 MPa and ultimate tensile strength σb~405 MPa) and a high ductility (e.g., uniform elongation εu~14.2%). After AsR of ~75% (see curve 2), the strength increases considerably and the ductility decreases. After the 450 °C/1 h partial annealing (see curve 3), an exceptional synergy of high strength (i.e., σy~550 MPa and σb ~712 MPa) and high tensile ductility (e.g., εu~10.1%) is achieved, which is much better than that (i.e., σy~250 MPa, σb~405 MPa and εu~14.2%) of its CG counterpart. In general, the strength and ductility are considered mutually exclusive and the relationship between the two properties is described as a trade-off [1,11], as shown by the gray region in Fig. 1 (b), which presents the ultimate tensile strength vs uniform elongation of Zr from this study and previously reported data [12-17]. However, there exist several data separated from the general trend (the gray region), and the AsRA sample exhibits an superior strength-ductility synergy among all the Zr samples (indicated by ★ in Fig. 1 (b)). According to the Considère criterion [18], the tensile uniform ductility of a metal is dominated by its work hardening capacity. Fig. 1 (c) indicates the normalized work hardening rate of the AsRA and CG samples. It is significant that the AsRA sample exhibits profound work hardening rate to large tensile strain comparable to that of the CG sample, which is responsible for the high uniform elongation of εu~10.1%. Previous studies demonstrate that a gradient microstructure is produced in materials processed by asymmetrical rolling [19]. In order to identify whether there exists a gradient microstructure in the AsRA specimen, the dependence of the microhardness on the depth from the sample surface is investigated at the RD-ND plane. As shown in Fig. 2, the microhardness increases gradually from the two surfaces to the core, indicating a gradient microstructure with a
3
thickness of ~0.7 mm is formed after AsRA treatments. The incomplete symmetry of the variation trend of the microhardness from two sides can be due to the small difference of reduction per pass. After tensile deformation, the microhardness of the overall sample increases evidently, which is consistent with the high work hardening capacity (see Fig. 1 (c)). The mechanical behaviour is controlled by a gradient dislocation density distribution. Fig. 3 (a)-(b) and (c)-(d) show the transmission electron microscope (TEM) images taken from the surface and the core of the AsRA sample, respectively. The microstructures at both the surface (see Fig. 3 (a)) and the core (see Fig. 3 (c)) are characterized by lamellar grains with length of a few micrometers and width of hundreds of nanometers. High-density dislocations are still retained and the lamellar grains are connected with wavy, curved or corrugated low-angle grainboundaries, which hints that the recrystallization during annealing is not completed. Furthermore, detailed observations demonstrate that some recrystallized dislocation-free grains (indicated by arrows in Fig. 3 (b)) with clear grain boundaries can be observed at the surface layer. By contrast, there exist partially recrystallized grains containing residual dislocations at the sample core and they are separated by non-equilibrium low-angle grain boundaries, which is confirmed by slightly elongated spots of the selected area electron diffraction (SAED) pattern (see the inset in Fig. 3 (d)) and these grains form lamellar bands (indicated by ellipse in Fig. 3 (d)). By comparing the microstructural difference between the surface and the core, it can be concluded that the dislocation density increases gradually with increasing the depth from the surface, which leads to the gradual increase in microhardness from the sample surface to the core (see Fig. 2). The gradient dislocation-structures rather than the heterogeneous lamellarstructure, which is achieved in Ref. [19] using similar processing steps, can be due to different asymmetrical rolling and partial recrystallization parameters. Based on the microstructural characterization, the formation of ultra-fine grains and the retention of high-density dislocations (see Fig. 3) contribute to the high strength of the AsRA sample (see Fig. 1). High tensile ductility is closely related with the specific deformation mechanism of the gradient microstructure. On the one hand, the gradient microstructure will induce macroscopic strain gradient and back stress under tensile test and change the stress state
4
from the uniaxial stress to multiaxial stress, which can promote the accumulation and interaction of dislocations, resulting in extra strain hardening to enhance the ductility [4-5,9-10]. On the other hand, lamellar bands with low-angle misorientation allow dislocations penetrate through the low-angle grain boundaries to continue slipping and the lamellar bands afford enough space to accommodate the dislocation interactions and accumulations [20-21], which devotes to provide a high uniform elongation. 4. Conclusions A gradient microstructure characterized by a gradient dislocation density distribution and composed of lamellar grains connected by low-angle grain boundaries has been prepared in Zr by using asymmetrical rolling and partial recrystallization. This specific structure results in an exceptional synergy of high strength (e.g., σb~712 MPa) and high uniform tensile ductility (εu~10.1%) much better than that (σb~405 MPa and εu~14.2%) of its coarse-grained (CG) counterpart. These observations provide guidance in designing advanced metals and alloys with high strength and ductility with gradient structures. Acknowledgements The authors gratefully acknowledge the financial support of Research Projects in Hebei Province (No. QN2017032) and State Key Laboratory of Metastable Materials Science and Technology (No. 201606). References [1] E. Ma, T. Zhu, Mater. Today 20 (2017) 323-331. [2] X. Li, L. Lou, W. Song, et al. Adv. Mater. 29 (2017) 1606430. [3] X. Li, L. Lou, W. Song, et al. Nano Lett. 17 (2017) 2985-2993. [4] T.H. Fang, W.L. Li, N.R. Tao, et al. Science 331 (2011) 1587-1590. [5] X.L. Wu, P. Jiang, L. Chen, et al. Proc. Natl. Acad. Sci. 111 (2014) 7197-7201. [6] J. Moering, X. Ma, J. Malkin, et al. Scr. Mater. 122 (2016) 106-109. [7] X. Yang, X. Ma, J. Moering, et al. Mater. Sci. Eng. A 645 (2015) 280-285. [8] X.L. Wu, P. Jiang, L. Chen, et al. Mater. Res. Lett. 2 (2014) 185-191. [9] M. Yang, Y. Pan, F. Yuan, et al. Mater. Res. Lett. 4 (2016) 145-151.
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[10] J. Li, G.J. Weng, S. Chen, et al. Int. J. Plast. 88 (2017) 89-107. [11] Y.T. Zhu, X. Liao. Nat. Mater. 3 (2004) 351-352. [12] D.S. Sarma, K.M. Al-Otaibi, K.L. Murty. Mater. Trans. JIM. 32 (1992) 596-603. [13] Y. Shi, M. Li, D. Guo, et al. Mater. Lett. 108 (2013) 228-230. [14] S. Care, T. Bretheau. J. de Phys. IV 3 (1993) 533-536. [15] L. Jiang, O.A. Ruano, M.E. Kassner, et al. J Met. 59 (2007) 42-45. [16] L. Jiang, M.T. Perez-Prado, P.A. Gruber, et al. Acta. Mater. 56 (2008) 1228-1242. [17] D. Guo, M. Li, Y. Shi, et al. Mater. Lett. 66 (2012) 305-307. [18] G.E. Dieter. Mechanical Metallurgy, 3rd ed. New York: McGraw-Hill; 1986, p. 289. [19] X. Wu, M. Yang, F. Yuan, et al. Proc. Natl. Acad. Sci. 112 (2015) 14501-14505. [20] H. Fujita, K. Toyoda, T. Mori, et al. Trans. Jpn. Inst. Metals 24 (1983) 195-204. [21] T. Hu, K. Ma, T.D. Topping, et al. Scr. Mater. 78 (2014) 25-28.
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Figure captions Fig. 1
(a) Tensile engineering stress-strain curves of Zr. Curve 1, CG sample; curve 2,
asymmetrical rolling; curve 3, asymmetrical rolling + 450 °C/1 h. (b) Representative tensile properties of Zr. Data CG, AsR and AsRA are yielded from the curves 1 to 3 in (a), respectively, and the others are previously reported data [12-17], (c) Normalized work hardening rate of the CG and AsRA Zr. Fig. 2 Microhardness change before (□■) and after (○●) tensile deformation of Zr subjected to asymmetrical rolling and partial recrystallization. Solid points indicate the mean Hv values of the experimentally measured values obtained by 2-3 time tests (indicated by open points). Fig. 3 TEM images and SAED pattern of Zr subjected to asymmetrical rolling and partial recrystallization. (a) (b) surface layer. Arrows in (b) indicate the recrystallized grains. (c) (d) core layer. Ellipse in (d) shows one lamellar band composed of grains connected by low-angle grain boundaries, and the inset in (d) indicates the SAED pattern taken from the one lamellar band region at the [0001] zone axis.
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Highlights
• • •
Zr is processed by asymmetric rolling and partial annealing. A gradient structure is produced in Zr. An exceptional synergy of high strength and high tensile ductility is achieved.
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S0167-577X(18)30990-X https://doi.org/10.1016/j.matlet.2018.06.084 MLBLUE 24524
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
28 April 2018 10 June 2018 22 June 2018
Please cite this article as: L. Wang, Y. Shi, Y. Zhang, M. Li, High tensile ductility and strength in a gradient structured Zr, Materials Letters (2018), doi: https://doi.org/10.1016/j.matlet.2018.06.084
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
High tensile ductility and strength in a gradient structured Zr Lina Wang1, Yindong Shi1,*, Yulong Zhang1, Ming Li2 1
College of Materials Science and Engineering, Hebei University of Engineering, Handan
056038, PR China 2
State Key Laboratory of Metastable Materials Science and Technology, Yanshan University,
Qinhuangdao 066004, PR China
Abstract: We report that a gradient structured Zr processed by asymmetric rolling and partial annealing possesses an exceptional synergy of high strength (σy~550 MPa) and high tensile ductility (εu ~10.1%) much better than that (σy~250 MPa and εu~14.2%) of its coarse-grained (CG) counterpart. This gradient structure is characterized by a gradient dislocation density distribution from the surface to the core and composed of microscale and ultra-fined elongated grains connected by low angle grain boundaries (LAGBs). These findings can provide valuable insight for designing gradient structures with excellent mechanical properties. Keywords: Metals and alloys; Gradient structure; Microstructure; Mechanical properties; Asymmetric rolling; Zr.
*Author to whom correspondence should be addressed; e-mail: [email protected] (Yindong Shi).
1. Introduction The development of advanced materials with superior comprehensive properties is the everlasting theme. Nowadays, many materials scientists devote to heterogeneous nanostructures to produce novel functional or structural materials [1-3]. For most structural materials, there is a common trade-off between the strength and ductility [1]. In order to achieve the strengthductility synergy of metals, recently, a gradient nanostructure has been produced in many metals (e.g., Cu, Fe) by the use of surface mechanical grinding treatment (SMGT) or surface
1
mechanical attrition treatment (SMAT) [4-7]. This strategy can double the yield strength without sacrificing its tensile ductility [1,4-5]. Experimental and theoretical studies have confirmed that the improvement of strength is attributed to the grain size refinement at the sample surface and the extra synergetic strengthening induced by mechanical incompatibility between different layers [4,7-9]. The retention of high tensile ductility results from the work hardening capacity of coarse-grain (CG) matrix, extra work hardening of the gradient layers and back stress strengthening due to the gradient macroscopic stress and strain [4-5,9-10]. However, the processing of the gradient nanostructure through surface mechanical treatment confines the thickness of the gradient nanostructure layer and the enhancement of strength-ductility synergy is limited [1,10]. Therefore, seeking potential processing routes capable of producing thicker gradient microstructure layer and novel type of gradient microstructure is of significance to enhance the strength-ductility synergy of materials. Here, a gradient dislocation density distribution is produced in Zr by employing asymmetrical rolling and partial annealing and an exceptional synergy of high yield strength (σy~550 MPa) and high uniform tensile ductility (εu~10.1%) is achieved. 2.Materials and experimental procedure High-purity Zr plates (99.95 wt.%) with a thickness of ~5 mm was used as the starting materials. The plates were vacuum-annealed at 700 °C for 2 h to obtain a fully homogeneous CG microstructure with a mean grain size of ~30 µm. The annealed samples were subjected to asymmetrical rolling (AsR) at room temperature (RT) with a total reduction of ~75%. The roll velocity ratio was 1.05 and the rolling reduction was 0.25 mm per pass. The Zr sheet was flipped over between AsR passes. Following AsR, partial annealing under vacuum was performed at 450 °C for 1 h. Microstructures in the RD-TD plane of Zr subjected to AsR and annealing were determined by TEM measurements. Tensile tests of samples with a gauge dimension of 15 × 3.0 × 1.5 mm3 were performed at RT using an Instron 5966 machine under a constant cross-head speed (i.e., 0.45 mm/min) with an initial strain rate of 5 × 10-4 s-1. The strain was measured with a contacting extensometer with a gauge length of 12 mm. For each condition, at least three
2
specimens were used for tests, and good reproduction was obtained. The tension direction was parallel to the rolling direction of the samples. 3.Results and Discussion The tensile engineering stress-strain curves of Zr subjected to AsR and annealing (AsRA) and its CG counterpart are shown in Fig. 1 (a). The CG sample (see curve 1) exhibits a low strength (i.e., yield strength σy~250 MPa and ultimate tensile strength σb~405 MPa) and a high ductility (e.g., uniform elongation εu~14.2%). After AsR of ~75% (see curve 2), the strength increases considerably and the ductility decreases. After the 450 °C/1 h partial annealing (see curve 3), an exceptional synergy of high strength (i.e., σy~550 MPa and σb ~712 MPa) and high tensile ductility (e.g., εu~10.1%) is achieved, which is much better than that (i.e., σy~250 MPa, σb~405 MPa and εu~14.2%) of its CG counterpart. In general, the strength and ductility are considered mutually exclusive and the relationship between the two properties is described as a trade-off [1,11], as shown by the gray region in Fig. 1 (b), which presents the ultimate tensile strength vs uniform elongation of Zr from this study and previously reported data [12-17]. However, there exist several data separated from the general trend (the gray region), and the AsRA sample exhibits an superior strength-ductility synergy among all the Zr samples (indicated by ★ in Fig. 1 (b)). According to the Considère criterion [18], the tensile uniform ductility of a metal is dominated by its work hardening capacity. Fig. 1 (c) indicates the normalized work hardening rate of the AsRA and CG samples. It is significant that the AsRA sample exhibits profound work hardening rate to large tensile strain comparable to that of the CG sample, which is responsible for the high uniform elongation of εu~10.1%. Previous studies demonstrate that a gradient microstructure is produced in materials processed by asymmetrical rolling [19]. In order to identify whether there exists a gradient microstructure in the AsRA specimen, the dependence of the microhardness on the depth from the sample surface is investigated at the RD-ND plane. As shown in Fig. 2, the microhardness increases gradually from the two surfaces to the core, indicating a gradient microstructure with a
3
thickness of ~0.7 mm is formed after AsRA treatments. The incomplete symmetry of the variation trend of the microhardness from two sides can be due to the small difference of reduction per pass. After tensile deformation, the microhardness of the overall sample increases evidently, which is consistent with the high work hardening capacity (see Fig. 1 (c)). The mechanical behaviour is controlled by a gradient dislocation density distribution. Fig. 3 (a)-(b) and (c)-(d) show the transmission electron microscope (TEM) images taken from the surface and the core of the AsRA sample, respectively. The microstructures at both the surface (see Fig. 3 (a)) and the core (see Fig. 3 (c)) are characterized by lamellar grains with length of a few micrometers and width of hundreds of nanometers. High-density dislocations are still retained and the lamellar grains are connected with wavy, curved or corrugated low-angle grainboundaries, which hints that the recrystallization during annealing is not completed. Furthermore, detailed observations demonstrate that some recrystallized dislocation-free grains (indicated by arrows in Fig. 3 (b)) with clear grain boundaries can be observed at the surface layer. By contrast, there exist partially recrystallized grains containing residual dislocations at the sample core and they are separated by non-equilibrium low-angle grain boundaries, which is confirmed by slightly elongated spots of the selected area electron diffraction (SAED) pattern (see the inset in Fig. 3 (d)) and these grains form lamellar bands (indicated by ellipse in Fig. 3 (d)). By comparing the microstructural difference between the surface and the core, it can be concluded that the dislocation density increases gradually with increasing the depth from the surface, which leads to the gradual increase in microhardness from the sample surface to the core (see Fig. 2). The gradient dislocation-structures rather than the heterogeneous lamellarstructure, which is achieved in Ref. [19] using similar processing steps, can be due to different asymmetrical rolling and partial recrystallization parameters. Based on the microstructural characterization, the formation of ultra-fine grains and the retention of high-density dislocations (see Fig. 3) contribute to the high strength of the AsRA sample (see Fig. 1). High tensile ductility is closely related with the specific deformation mechanism of the gradient microstructure. On the one hand, the gradient microstructure will induce macroscopic strain gradient and back stress under tensile test and change the stress state
4
from the uniaxial stress to multiaxial stress, which can promote the accumulation and interaction of dislocations, resulting in extra strain hardening to enhance the ductility [4-5,9-10]. On the other hand, lamellar bands with low-angle misorientation allow dislocations penetrate through the low-angle grain boundaries to continue slipping and the lamellar bands afford enough space to accommodate the dislocation interactions and accumulations [20-21], which devotes to provide a high uniform elongation. 4. Conclusions A gradient microstructure characterized by a gradient dislocation density distribution and composed of lamellar grains connected by low-angle grain boundaries has been prepared in Zr by using asymmetrical rolling and partial recrystallization. This specific structure results in an exceptional synergy of high strength (e.g., σb~712 MPa) and high uniform tensile ductility (εu~10.1%) much better than that (σb~405 MPa and εu~14.2%) of its coarse-grained (CG) counterpart. These observations provide guidance in designing advanced metals and alloys with high strength and ductility with gradient structures. Acknowledgements The authors gratefully acknowledge the financial support of Research Projects in Hebei Province (No. QN2017032) and State Key Laboratory of Metastable Materials Science and Technology (No. 201606). References [1] E. Ma, T. Zhu, Mater. Today 20 (2017) 323-331. [2] X. Li, L. Lou, W. Song, et al. Adv. Mater. 29 (2017) 1606430. [3] X. Li, L. Lou, W. Song, et al. Nano Lett. 17 (2017) 2985-2993. [4] T.H. Fang, W.L. Li, N.R. Tao, et al. Science 331 (2011) 1587-1590. [5] X.L. Wu, P. Jiang, L. Chen, et al. Proc. Natl. Acad. Sci. 111 (2014) 7197-7201. [6] J. Moering, X. Ma, J. Malkin, et al. Scr. Mater. 122 (2016) 106-109. [7] X. Yang, X. Ma, J. Moering, et al. Mater. Sci. Eng. A 645 (2015) 280-285. [8] X.L. Wu, P. Jiang, L. Chen, et al. Mater. Res. Lett. 2 (2014) 185-191. [9] M. Yang, Y. Pan, F. Yuan, et al. Mater. Res. Lett. 4 (2016) 145-151.
5
[10] J. Li, G.J. Weng, S. Chen, et al. Int. J. Plast. 88 (2017) 89-107. [11] Y.T. Zhu, X. Liao. Nat. Mater. 3 (2004) 351-352. [12] D.S. Sarma, K.M. Al-Otaibi, K.L. Murty. Mater. Trans. JIM. 32 (1992) 596-603. [13] Y. Shi, M. Li, D. Guo, et al. Mater. Lett. 108 (2013) 228-230. [14] S. Care, T. Bretheau. J. de Phys. IV 3 (1993) 533-536. [15] L. Jiang, O.A. Ruano, M.E. Kassner, et al. J Met. 59 (2007) 42-45. [16] L. Jiang, M.T. Perez-Prado, P.A. Gruber, et al. Acta. Mater. 56 (2008) 1228-1242. [17] D. Guo, M. Li, Y. Shi, et al. Mater. Lett. 66 (2012) 305-307. [18] G.E. Dieter. Mechanical Metallurgy, 3rd ed. New York: McGraw-Hill; 1986, p. 289. [19] X. Wu, M. Yang, F. Yuan, et al. Proc. Natl. Acad. Sci. 112 (2015) 14501-14505. [20] H. Fujita, K. Toyoda, T. Mori, et al. Trans. Jpn. Inst. Metals 24 (1983) 195-204. [21] T. Hu, K. Ma, T.D. Topping, et al. Scr. Mater. 78 (2014) 25-28.
6
Figure captions Fig. 1
(a) Tensile engineering stress-strain curves of Zr. Curve 1, CG sample; curve 2,
asymmetrical rolling; curve 3, asymmetrical rolling + 450 °C/1 h. (b) Representative tensile properties of Zr. Data CG, AsR and AsRA are yielded from the curves 1 to 3 in (a), respectively, and the others are previously reported data [12-17], (c) Normalized work hardening rate of the CG and AsRA Zr. Fig. 2 Microhardness change before (□■) and after (○●) tensile deformation of Zr subjected to asymmetrical rolling and partial recrystallization. Solid points indicate the mean Hv values of the experimentally measured values obtained by 2-3 time tests (indicated by open points). Fig. 3 TEM images and SAED pattern of Zr subjected to asymmetrical rolling and partial recrystallization. (a) (b) surface layer. Arrows in (b) indicate the recrystallized grains. (c) (d) core layer. Ellipse in (d) shows one lamellar band composed of grains connected by low-angle grain boundaries, and the inset in (d) indicates the SAED pattern taken from the one lamellar band region at the [0001] zone axis.
7
8
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10
Highlights
• • •
Zr is processed by asymmetric rolling and partial annealing. A gradient structure is produced in Zr. An exceptional synergy of high strength and high tensile ductility is achieved.
11