- Email: [email protected]
Effect of intermediate annealing on the microstructure and mechanical property of ZK60 magnesium alloy produced by twin roll casting and hot rolling
Materials Characterization 106 (2015) 437–441
Contents lists available at ScienceDirect
Materials Characterization journal homepage: www.elsevier.com/locate/matchar
Effect of intermediate annealing on the microstructure and mechanical property of ZK60 magnesium alloy produced by twin roll casting and hot rolling Hongmei Chen a,⁎, Qianhao Zang a, Hui Yu b, Jing Zhang c, Yunxue Jin a a b c
Provincial Key Lab of Advanced Welding Technology, Jiangsu University of Science and Technology, Zhenjiang 212003, China School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300132, China School of Metallurgical and Materials Engineering, Jiangsu University of Science and Technology, Zhang Jiagang 215600, China
a r t i c l e
i n f o
Article history: Received 29 January 2015 Received in revised form 13 May 2015 Accepted 8 June 2015 Available online 20 June 2015 Keywords: ZK60 magnesium alloy Intermediate annealing Microstructure Macrotexture
a b s t r a c t Twin roll cast (designated as TRC in short) ZK60 magnesium alloy strip with 3.5 mm thickness was used in this paper. The TRC ZK60 strip was multi-pass rolled at different temperatures, intermediate annealing heat treatment was performed when the thickness of the strip changed from 3.5 mm to 1 mm, and then continued to be rolled until the thickness reached to 0.5 mm. The effect of intermediate annealing during rolling process on microstructure, texture and room temperature mechanical properties of TRC ZK60 strip was studied by using OM, TEM, XRD and electronic universal testing machine. The introduction of intermediate annealing can contribute to recrystallization in the ZK60 sheet which was greatly deformed, and help to reduce the stress concentration generated in the rolling process. Microstructure uniformity and mechanical properties of the ZK60 alloy sheet were also improved; in particular, the room temperature elongation was greatly improved. When the TRC ZK60 strip was rolled at 300 °C and 350 °C, the room temperature elongation of the rolled sheet with 0.5 mm thickness which was intermediate annealed during the rolling process was increased by 95% and 72% than that of no intermediate annealing, respectively. © 2015 Elsevier Inc. All rights reserved.
1. Introduction Magnesium alloys are the lightest structural materials. They have attracted great interest due to their low density, high specific strength and specific stiffness, good damping capacity, excellent machinability and good castability [1]. How to improve the general properties of magnesium alloy, especially, its ductility, how to reduce the anisotropy and improve the plastic forming ability, how to reduce the production costs and expand the scope of application of magnesium alloy are the main direction of domestic and foreign researches. It shows that grain refinement is an effective means to improve the mechanical properties of magnesium alloys [2–5]. Fine-grained magnesium alloys have good ductility and low plastic brittle transition temperature, and have better formability at room temperature. The maximum mechanical properties are achieved through strain hardening and precipitation strengthening among most wrought magnesium alloys [5–8]. Significant hardening during deformation process is characterized by the accumulation of energy in the form of defects, such as dislocations. This will contribute to the occurrence of recrystallization and grain refinement. The deformation storage energy can be lowered, in general, by recovery and/or recrystallization. Recrystallization ⁎ Corresponding author. E-mail address: [email protected] (H. Chen).
http://dx.doi.org/10.1016/j.matchar.2015.06.015 1044-5803/© 2015 Elsevier Inc. All rights reserved.
may occur during straining (dynamic recrystallization, DRX) [9,10] or after subsequent annealing (static recrystallization, SRX) [11–13]. The researches [14,15] reported that the rolling deformation of twin roll cast ZK60 alloy at lower temperature can result in the forming of high density of shear bands, dislocations and twinning. And it is difficult to occur for dynamic recrystallization. And too higher rolling temperature is unfavorable for grain refinement of TRC ZK60 alloy. For the purpose of improving the plastic deformation, microstructure uniformity and mechanical properties of TRC ZK60 magnesium alloy, intermediate annealing was introduced in this study. The effect of intermediate annealing on the microstructure, texture and mechanical properties of hot rolled TRC ZK60 alloy was discussed in this paper. 2. Experimental material and procedure Twin roll cast ZK60 magnesium alloy strip with 3.5 mm thickness was used in this study. Hot rolling was carried out on TRC ZK60 alloy at 300 °C and 350 °C, respectively. Intermediate annealing heat treatment was performed when the thickness of the strip changed from 3.5 mm to 1 mm, and then continued to be rolled until the thickness reached to 0.5 mm. According to the previous research [16], when the alloy was rolled at 300 °C, the intermediate annealing parameter was 300 °C–1 h. And when the rolling temperature was 350 °C, that was 350 °C–0.5 h. The sheet was reheated at rolling temperature for
438
H. Chen et al. / Materials Characterization 106 (2015) 437–441
(a)
(b)
50µm
50µm (d)
(c)
RD
ND
50µm
50µm
Fig. 1. Microstructure comparison of ZK60 alloy sheet rolled at 300 °C. (a) (c) no intermediate annealing; (b) (d) intermediate annealing; (a) (b) as-rolled; (c) (d) as-annealed.
10 min to maintain the sheet temperature at about the rolling temperature during other inter-pass rolling processes. The rolling reduction ratio per pass was 50% and roller surface temperature was 250 °C for the hot rolling. The hot rolling was performed on the twin roll mill with the roller diameter 300 mm.
Specimens for optical microstructure observation were made by mechanical polishing and revealed by subsequent etching with a solution of picric acid (3 g), acetic acid (20 ml), distilled water (20 ml), and ethanol (50 ml). Micromorphology was examined by using a JEM-2100F transmission electron microscope (TEM) operated at 200 kV. Thin foils
(b)
(a)
50µm
50µm (d)
(c)
RD
ND
50µm
50µm
Fig. 2. Microstructure comparison of ZK60 alloy sheet rolled at 350 °C. (a) (c) no intermediate annealing; (b) (d) intermediate annealing; (a) (b) as-rolled; (c) (d) as-annealed.
H. Chen et al. / Materials Characterization 106 (2015) 437–441
439
(b)
(a)
Recrystallized
500nm
500nm (d)
(c)
Recrystallized
500nm
500nm
Fig. 3. TEM images comparison of as-rolled ZK60 alloy sheet. (a) (c) no intermediate annealing; (b) (d) intermediate annealing; (a) (b) 300 °C; (c) (d) 350 °C.
(0001)
TD
(a)
(10-10)
TD
RD
RD
Max:13.3
Max:3.1
TD
TD
(b)
RD
Max:12.4
RD
Max:3.8
Fig. 4. Macrotexture comparison of ZK60 alloy sheet rolled at 300 °C. (a) no intermediate annealing; (b) intermediate annealing.
440
H. Chen et al. / Materials Characterization 106 (2015) 437–441
(0001)
TD
(10-10)
(a)
TD
RD
RD
Max:11.5
Max:5.1
TD
TD
(b)
RD
RD
Max:4.2
Max:11.3
Fig. 5. Macrotexture comparison of ZK60 alloy sheet rolled at 350 °C. (a) no intermediate annealing; (b) intermediate annealing.
examined by TEM paralleling to the rolling plane were prepared by a twin jet electro-polisher using a solution of HClO4 (5%), butanol (35%) and methanol (60%), and then ion-beam milled. The polishing voltage and temperature are 20 V–23 V and −10 °C ~ −15 °C, respectively. The texture of ZK60 Mg alloy was analyzed on rolling plane by using the specimens of 20 mm × 20 mm × 0.5 mm. The ð1010Þ, (0002), ð1011Þ and ð1 012Þ pole figures were measured by using X-ray diffraction with Cu-Kα radiation operated at 40 kV and 30 mA. Tensile test was conducted at an ambient temperature with 1.25 mm/min tensile speed on a standard universal testing machine (Instron 4206). The rolled sheets were machined to the subsize of ASTM E8 tensile specimen, whose dimensions were 3 mm in gauge width and 12.5 mm in gauge length. The specimens were prepared paralleling to the rolling direction. 3. Results and discussion 3.1. Microstructure of the hot rolled ZK60 alloy sheet Figs. 1 and 2 are the microstructures of as-rolled and as-annealed ZK60 alloy sheets with 0.5 mm thickness which were rolled at 300 °C Table 1 Tensile properties of ZK60 alloy sheet rolled by different rolling processes. Rolling temperature (°C)
Intermediate annealing parameter
σb (MPa)
σ0.2 (MPa)
δ (%)
300
– 300 °C–1 h – 350 °C–30 min
477 474 455 435
425 407 389 374
4.1 8.0 7.6 13.1
350
and 350 °C, respectively. The microstructure of the rolled ZK60 alloy sheet consisted of a fibrous structure with elongated grains and shear bands along the rolling direction (as shown in Figs. 1(a), (b) and 2(a), (b)). There was no obvious dynamic recrystallization during the rolling process without intermediate annealing according to Figs. 1(a) and 2(a). By contrast, a large number of fine recrystallized grains presented in the shear band area and the density of shear band decreased in the rolled sheet which was hot rolled and intermediate annealed during the rolling process, as shown in Figs. 1(b) and 2(b). The occurrence of static recrystallization during annealing process can contribute to grain refinement during further rolling process. Microstructures of asannealed ZK60 alloy sheets were shown in Figs. 1(c), (d) and 2(c), (d). The microstructure consisted of recrystallized grains and deformed grains, and a little shear band still remained in the annealed sheet. There were more recrystallized grains and lower density of shear band in the sheet which was rolled with intermediate annealing than that of the sheet which was rolled without intermediate annealing. Comparison of Figs. 1 and 2 shows that the effect of intermediate annealing on the microstructure of ZK60 alloy sheet which was rolled at different temperatures is similar and recrystallized grains can be found in both sheets. Fig. 3 shows the TEM photograph of the ZK60 alloy sheet with 0.5 mm thickness which was rolled at different temperatures. High density of dislocations can be found in both sheets. Higher density of dislocation was found in the rolled sheet without intermediate annealing treatment (Fig. 3(a) (c)). According to Fig. 3(b) and (d), the introduction of intermediate annealing can contribute to recrystallization occurring in the ZK60 sheet, and help reduce the density of dislocation.
H. Chen et al. / Materials Characterization 106 (2015) 437–441
3.2. Macrotexture of as-rolled ZK60 magnesium alloy sheet Figs. 4 and 5 show the (0001) and ð1010Þ pole figures of as-rolled ZK60 alloy sheets with 0.5 mm thickness which were rolled at 300 °C and 350 °C, respectively. The sheets have strong (0001) basal texture after hot rolling. The maximum pole intensity of (0001) pole figure increased with the decrease in rolling temperature. Because of the existence of high density shear band, the basal pole tilted slightly to the transverse direction after hot rolling. There are a number of factors which may affect the deformation texture, including strain, rolling parameters, deformation temperature, grain size (here is the initial grain size), shear bands, dislocations and second-phase particles [17]. In this study, the hot rolled sheets were all rolled with the same rolling equipment and with the same initial grain size, so the factors which may cause the difference of deformation texture of the hot rolled ZK60 alloy sheet are rolling temperature, shear bands, dislocations and second-phase particles. As shown in Figs. 1 and 2, the density of shear bands decreased with the increase in the rolling temperature, and the introduction of intermediate annealing also decreased the density of shear bands. The formation of the dislocation (as shown in Fig. 3) during the deformation process tends to cause the rotations of single crystals and directly related to the crystallography of the deformation, and this may lead to intensity changing of the pole figures [17]. And the distribution of texture tends to parallel the rolling direction with the increase in rolling temperature. The introduction of intermediate annealing only decreased the maximum pole intensity of (0001) pole figure according to Figs. 4 and 5. 3.3. Room temperature mechanical properties of ZK60 alloy sheet Table 1 shows the room temperature tensile properties of as-rolled ZK60 alloy sheets with 0.5 mm thickness which were rolled at 300 °C and 350 °C, respectively. It indicates that the introduction of intermediate annealing has a little effect on the room temperature strength of ZK60 sheet, but the elongation increased significantly. When the TRC ZK60 strip was rolled at 300 °C and 350 °C, the room temperature elongation of the rolled sheets which were intermediate annealed during the rolling process increased by 95% and 72% than that of no intermediate annealing, respectively. According to the analysis of microstructure (shown in Figs. 1 and 2), lower density of shear band presented and dynamic recrystallization occurred in the sheet which was intermediate annealed during the rolling process; these all can increase the elongation of the sheet. The result of Table 1 also shows that the effect of intermediate annealing on the mechanical properties of the ZK60 sheet rolled at different temperatures is similar. The analysis above shows that the introduction of intermediate annealing can improve microstructure uniformity of TRC ZK60 magnesium alloy which was rolled at different temperatures, and further increase room temperature tensile properties, especially the room temperature elongation. 4. Conclusion (1) The introduction of intermediate annealing can contribute to recrystallization in the ZK60 sheet which was greatly deformed, and help reduce the stress concentration generated in the rolling process.
441
(2) Microstructure uniformity and mechanical properties of TRC ZK60 alloy rolled at different temperatures with intermediate annealing were improved, in particular, the room temperature elongation. (3) The room temperature elongation of TRC ZK60 alloy which was rolled at 300 °C and 350 °C and intermediate annealed during the rolling process increased by 95% and 72% than that of no intermediate annealing, respectively. (4) The intermediate annealing during the rolling process decreased the maximum pole intensity of (0001) pole figure in the TRC ZK60 alloy sheet rolled at different temperatures.
Acknowledgment This work was supported by National Natural Science Foundation (Project No: 51301077), Natural Science Foundation of Jiangsu Province (Project No: BK20130470) and Priority Academic Program Development of Jiangsu Higher Education Institutions. References [1] B.L. Mordike, T. Ebert, Magnesium: properties–applications–potential, Mater. Sci. Eng. A 302 (1) (2001) 37–45. [2] R.W. Armstrong, Theory of the tensile ductile–brittle behavior of poly-crystalline h.c.p. materials, with application to beryllium, Acta Metall. 16 (3) (1968) 347–355. [3] L.W. Lu, C.M. Liu, J. Zhao, et al., Modification of grain refinement and texture in AZ31 Mg alloy by a new plastic deformation method, J. Alloys Compd. 628 (2015) 130–134. [4] A. Jain, O. Duygulu, D.W. Brown, et al., Grain size effects on the tensile properties and deformation mechanisms of a magnesium alloy, AZ31B, sheet [J], Mater. Sci. Eng. A 486 (1–2) (2007) 545–555. [5] M.G. Jiang, H. Yan, R.S. Chen, Enhanced mechanical properties due to grain refinement and texture modification in an AZ61 Mg alloy processed by small strain impact forging, Mater. Sci. Eng. A 621 (2015) 204–211. [6] M.M. Avedesian, B. Baker (Eds.), ASM Specialty Handbook Magnesium and Magnesium Alloys, ASM International, Materials Park, OH, 1999. [7] S.B. Kang, H.M. Chen, H.W. Kim, et al., Effect of reheating and warm rolling on microstructure and mechanical properties of twin roll strip cast Mg–4.5Al–1.0Zn–0.4Mn– 0.3Ca alloy sheet, Magnesium Technology 2008, TMS, 2008. 147–152. [8] H.M. Chen, S.B. Kang, H.S. Yu, et al., Microstructure and mechanical properties of Mg–4.5Al–1.0Zn alloy sheets produced by twin roll casting and sequential warm rolling, Mater. Sci. Eng. A 492 (1–2) (2008) 317–326. [9] M.M. Myshlyaev, H.J. McQueen, A. Mwembela, et al., Twinning, dynamic recovery and recrystallization in hot worked Mg–Al–Zn alloy, Mater. Sci. Eng. A 337 (1–2) (2002) 121–133. [10] J.C. Tan, M.J. Tan, Dynamic continuous recrystallization characteristics in two stage deformation of Mg–3Al–1Zn alloy sheet, Mater. Sci. Eng. A 339 (1–2) (2003) 124–132. [11] D. Sarker, J. Friedman, D.L. Chen, Influence of pre-deformation and subsequent annealing on strain hardening and anisotropy of AM30 magnesium alloy, J. Alloys Compd. 611 (2014) 341–350. [12] A. Jager, P. Lukac, V. Gartnerova, et al., Influence of annealing on the microstructure of commercial Mg alloy AZ31 after mechanical forming, Mater. Sci. Eng. A 432 (1–2) (2006) 20–25. [13] X.B. Gong, S.B. Kang, J.H. Cho, et al., Effect of annealing on microstructure and mechanical properties of ZK60 magnesium alloy sheets processed by twin-roll cast and differential speed rolling, Mater. Charact. 97 (2014) 183–188. [14] H.M. Chen, H.S. Yu, S.B. Kang, et al., Effect of forming process on microstructure and mechanical properties of ZK60 alloy sheet, Rare Metal Mater. Eng. 40 (10) (2011) 1708–1712. [15] H.M. Chen, H.S. Yu, S.B. Kang, et al., Effect of rolling temperature on microstructure and texture of twin roll cast ZK60 magnesium alloy, Trans. Nonferrous Metals Soc. China 20 (11) (2010) 2086–2091. [16] H.M. Chen, H.S. Yu, S.B. Kang, et al., Optimization of annealing treatment parameters in a twin roll cast and warm rolled ZK60 alloy sheet, Mater. Sci. Eng. A 527 (4–5) (2010) 1236–1242. [17] F.J. Humphreys, M. Hatherly, Recrystallization and Related Annealing Phenomena [M], Second ed. Elsevier Ltd., Kidlington, UK, 2004.
Contents lists available at ScienceDirect
Materials Characterization journal homepage: www.elsevier.com/locate/matchar
Effect of intermediate annealing on the microstructure and mechanical property of ZK60 magnesium alloy produced by twin roll casting and hot rolling Hongmei Chen a,⁎, Qianhao Zang a, Hui Yu b, Jing Zhang c, Yunxue Jin a a b c
Provincial Key Lab of Advanced Welding Technology, Jiangsu University of Science and Technology, Zhenjiang 212003, China School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300132, China School of Metallurgical and Materials Engineering, Jiangsu University of Science and Technology, Zhang Jiagang 215600, China
a r t i c l e
i n f o
Article history: Received 29 January 2015 Received in revised form 13 May 2015 Accepted 8 June 2015 Available online 20 June 2015 Keywords: ZK60 magnesium alloy Intermediate annealing Microstructure Macrotexture
a b s t r a c t Twin roll cast (designated as TRC in short) ZK60 magnesium alloy strip with 3.5 mm thickness was used in this paper. The TRC ZK60 strip was multi-pass rolled at different temperatures, intermediate annealing heat treatment was performed when the thickness of the strip changed from 3.5 mm to 1 mm, and then continued to be rolled until the thickness reached to 0.5 mm. The effect of intermediate annealing during rolling process on microstructure, texture and room temperature mechanical properties of TRC ZK60 strip was studied by using OM, TEM, XRD and electronic universal testing machine. The introduction of intermediate annealing can contribute to recrystallization in the ZK60 sheet which was greatly deformed, and help to reduce the stress concentration generated in the rolling process. Microstructure uniformity and mechanical properties of the ZK60 alloy sheet were also improved; in particular, the room temperature elongation was greatly improved. When the TRC ZK60 strip was rolled at 300 °C and 350 °C, the room temperature elongation of the rolled sheet with 0.5 mm thickness which was intermediate annealed during the rolling process was increased by 95% and 72% than that of no intermediate annealing, respectively. © 2015 Elsevier Inc. All rights reserved.
1. Introduction Magnesium alloys are the lightest structural materials. They have attracted great interest due to their low density, high specific strength and specific stiffness, good damping capacity, excellent machinability and good castability [1]. How to improve the general properties of magnesium alloy, especially, its ductility, how to reduce the anisotropy and improve the plastic forming ability, how to reduce the production costs and expand the scope of application of magnesium alloy are the main direction of domestic and foreign researches. It shows that grain refinement is an effective means to improve the mechanical properties of magnesium alloys [2–5]. Fine-grained magnesium alloys have good ductility and low plastic brittle transition temperature, and have better formability at room temperature. The maximum mechanical properties are achieved through strain hardening and precipitation strengthening among most wrought magnesium alloys [5–8]. Significant hardening during deformation process is characterized by the accumulation of energy in the form of defects, such as dislocations. This will contribute to the occurrence of recrystallization and grain refinement. The deformation storage energy can be lowered, in general, by recovery and/or recrystallization. Recrystallization ⁎ Corresponding author. E-mail address: [email protected] (H. Chen).
http://dx.doi.org/10.1016/j.matchar.2015.06.015 1044-5803/© 2015 Elsevier Inc. All rights reserved.
may occur during straining (dynamic recrystallization, DRX) [9,10] or after subsequent annealing (static recrystallization, SRX) [11–13]. The researches [14,15] reported that the rolling deformation of twin roll cast ZK60 alloy at lower temperature can result in the forming of high density of shear bands, dislocations and twinning. And it is difficult to occur for dynamic recrystallization. And too higher rolling temperature is unfavorable for grain refinement of TRC ZK60 alloy. For the purpose of improving the plastic deformation, microstructure uniformity and mechanical properties of TRC ZK60 magnesium alloy, intermediate annealing was introduced in this study. The effect of intermediate annealing on the microstructure, texture and mechanical properties of hot rolled TRC ZK60 alloy was discussed in this paper. 2. Experimental material and procedure Twin roll cast ZK60 magnesium alloy strip with 3.5 mm thickness was used in this study. Hot rolling was carried out on TRC ZK60 alloy at 300 °C and 350 °C, respectively. Intermediate annealing heat treatment was performed when the thickness of the strip changed from 3.5 mm to 1 mm, and then continued to be rolled until the thickness reached to 0.5 mm. According to the previous research [16], when the alloy was rolled at 300 °C, the intermediate annealing parameter was 300 °C–1 h. And when the rolling temperature was 350 °C, that was 350 °C–0.5 h. The sheet was reheated at rolling temperature for
438
H. Chen et al. / Materials Characterization 106 (2015) 437–441
(a)
(b)
50µm
50µm (d)
(c)
RD
ND
50µm
50µm
Fig. 1. Microstructure comparison of ZK60 alloy sheet rolled at 300 °C. (a) (c) no intermediate annealing; (b) (d) intermediate annealing; (a) (b) as-rolled; (c) (d) as-annealed.
10 min to maintain the sheet temperature at about the rolling temperature during other inter-pass rolling processes. The rolling reduction ratio per pass was 50% and roller surface temperature was 250 °C for the hot rolling. The hot rolling was performed on the twin roll mill with the roller diameter 300 mm.
Specimens for optical microstructure observation were made by mechanical polishing and revealed by subsequent etching with a solution of picric acid (3 g), acetic acid (20 ml), distilled water (20 ml), and ethanol (50 ml). Micromorphology was examined by using a JEM-2100F transmission electron microscope (TEM) operated at 200 kV. Thin foils
(b)
(a)
50µm
50µm (d)
(c)
RD
ND
50µm
50µm
Fig. 2. Microstructure comparison of ZK60 alloy sheet rolled at 350 °C. (a) (c) no intermediate annealing; (b) (d) intermediate annealing; (a) (b) as-rolled; (c) (d) as-annealed.
H. Chen et al. / Materials Characterization 106 (2015) 437–441
439
(b)
(a)
Recrystallized
500nm
500nm (d)
(c)
Recrystallized
500nm
500nm
Fig. 3. TEM images comparison of as-rolled ZK60 alloy sheet. (a) (c) no intermediate annealing; (b) (d) intermediate annealing; (a) (b) 300 °C; (c) (d) 350 °C.
(0001)
TD
(a)
(10-10)
TD
RD
RD
Max:13.3
Max:3.1
TD
TD
(b)
RD
Max:12.4
RD
Max:3.8
Fig. 4. Macrotexture comparison of ZK60 alloy sheet rolled at 300 °C. (a) no intermediate annealing; (b) intermediate annealing.
440
H. Chen et al. / Materials Characterization 106 (2015) 437–441
(0001)
TD
(10-10)
(a)
TD
RD
RD
Max:11.5
Max:5.1
TD
TD
(b)
RD
RD
Max:4.2
Max:11.3
Fig. 5. Macrotexture comparison of ZK60 alloy sheet rolled at 350 °C. (a) no intermediate annealing; (b) intermediate annealing.
examined by TEM paralleling to the rolling plane were prepared by a twin jet electro-polisher using a solution of HClO4 (5%), butanol (35%) and methanol (60%), and then ion-beam milled. The polishing voltage and temperature are 20 V–23 V and −10 °C ~ −15 °C, respectively. The texture of ZK60 Mg alloy was analyzed on rolling plane by using the specimens of 20 mm × 20 mm × 0.5 mm. The ð1010Þ, (0002), ð1011Þ and ð1 012Þ pole figures were measured by using X-ray diffraction with Cu-Kα radiation operated at 40 kV and 30 mA. Tensile test was conducted at an ambient temperature with 1.25 mm/min tensile speed on a standard universal testing machine (Instron 4206). The rolled sheets were machined to the subsize of ASTM E8 tensile specimen, whose dimensions were 3 mm in gauge width and 12.5 mm in gauge length. The specimens were prepared paralleling to the rolling direction. 3. Results and discussion 3.1. Microstructure of the hot rolled ZK60 alloy sheet Figs. 1 and 2 are the microstructures of as-rolled and as-annealed ZK60 alloy sheets with 0.5 mm thickness which were rolled at 300 °C Table 1 Tensile properties of ZK60 alloy sheet rolled by different rolling processes. Rolling temperature (°C)
Intermediate annealing parameter
σb (MPa)
σ0.2 (MPa)
δ (%)
300
– 300 °C–1 h – 350 °C–30 min
477 474 455 435
425 407 389 374
4.1 8.0 7.6 13.1
350
and 350 °C, respectively. The microstructure of the rolled ZK60 alloy sheet consisted of a fibrous structure with elongated grains and shear bands along the rolling direction (as shown in Figs. 1(a), (b) and 2(a), (b)). There was no obvious dynamic recrystallization during the rolling process without intermediate annealing according to Figs. 1(a) and 2(a). By contrast, a large number of fine recrystallized grains presented in the shear band area and the density of shear band decreased in the rolled sheet which was hot rolled and intermediate annealed during the rolling process, as shown in Figs. 1(b) and 2(b). The occurrence of static recrystallization during annealing process can contribute to grain refinement during further rolling process. Microstructures of asannealed ZK60 alloy sheets were shown in Figs. 1(c), (d) and 2(c), (d). The microstructure consisted of recrystallized grains and deformed grains, and a little shear band still remained in the annealed sheet. There were more recrystallized grains and lower density of shear band in the sheet which was rolled with intermediate annealing than that of the sheet which was rolled without intermediate annealing. Comparison of Figs. 1 and 2 shows that the effect of intermediate annealing on the microstructure of ZK60 alloy sheet which was rolled at different temperatures is similar and recrystallized grains can be found in both sheets. Fig. 3 shows the TEM photograph of the ZK60 alloy sheet with 0.5 mm thickness which was rolled at different temperatures. High density of dislocations can be found in both sheets. Higher density of dislocation was found in the rolled sheet without intermediate annealing treatment (Fig. 3(a) (c)). According to Fig. 3(b) and (d), the introduction of intermediate annealing can contribute to recrystallization occurring in the ZK60 sheet, and help reduce the density of dislocation.
H. Chen et al. / Materials Characterization 106 (2015) 437–441
3.2. Macrotexture of as-rolled ZK60 magnesium alloy sheet Figs. 4 and 5 show the (0001) and ð1010Þ pole figures of as-rolled ZK60 alloy sheets with 0.5 mm thickness which were rolled at 300 °C and 350 °C, respectively. The sheets have strong (0001) basal texture after hot rolling. The maximum pole intensity of (0001) pole figure increased with the decrease in rolling temperature. Because of the existence of high density shear band, the basal pole tilted slightly to the transverse direction after hot rolling. There are a number of factors which may affect the deformation texture, including strain, rolling parameters, deformation temperature, grain size (here is the initial grain size), shear bands, dislocations and second-phase particles [17]. In this study, the hot rolled sheets were all rolled with the same rolling equipment and with the same initial grain size, so the factors which may cause the difference of deformation texture of the hot rolled ZK60 alloy sheet are rolling temperature, shear bands, dislocations and second-phase particles. As shown in Figs. 1 and 2, the density of shear bands decreased with the increase in the rolling temperature, and the introduction of intermediate annealing also decreased the density of shear bands. The formation of the dislocation (as shown in Fig. 3) during the deformation process tends to cause the rotations of single crystals and directly related to the crystallography of the deformation, and this may lead to intensity changing of the pole figures [17]. And the distribution of texture tends to parallel the rolling direction with the increase in rolling temperature. The introduction of intermediate annealing only decreased the maximum pole intensity of (0001) pole figure according to Figs. 4 and 5. 3.3. Room temperature mechanical properties of ZK60 alloy sheet Table 1 shows the room temperature tensile properties of as-rolled ZK60 alloy sheets with 0.5 mm thickness which were rolled at 300 °C and 350 °C, respectively. It indicates that the introduction of intermediate annealing has a little effect on the room temperature strength of ZK60 sheet, but the elongation increased significantly. When the TRC ZK60 strip was rolled at 300 °C and 350 °C, the room temperature elongation of the rolled sheets which were intermediate annealed during the rolling process increased by 95% and 72% than that of no intermediate annealing, respectively. According to the analysis of microstructure (shown in Figs. 1 and 2), lower density of shear band presented and dynamic recrystallization occurred in the sheet which was intermediate annealed during the rolling process; these all can increase the elongation of the sheet. The result of Table 1 also shows that the effect of intermediate annealing on the mechanical properties of the ZK60 sheet rolled at different temperatures is similar. The analysis above shows that the introduction of intermediate annealing can improve microstructure uniformity of TRC ZK60 magnesium alloy which was rolled at different temperatures, and further increase room temperature tensile properties, especially the room temperature elongation. 4. Conclusion (1) The introduction of intermediate annealing can contribute to recrystallization in the ZK60 sheet which was greatly deformed, and help reduce the stress concentration generated in the rolling process.
441
(2) Microstructure uniformity and mechanical properties of TRC ZK60 alloy rolled at different temperatures with intermediate annealing were improved, in particular, the room temperature elongation. (3) The room temperature elongation of TRC ZK60 alloy which was rolled at 300 °C and 350 °C and intermediate annealed during the rolling process increased by 95% and 72% than that of no intermediate annealing, respectively. (4) The intermediate annealing during the rolling process decreased the maximum pole intensity of (0001) pole figure in the TRC ZK60 alloy sheet rolled at different temperatures.
Acknowledgment This work was supported by National Natural Science Foundation (Project No: 51301077), Natural Science Foundation of Jiangsu Province (Project No: BK20130470) and Priority Academic Program Development of Jiangsu Higher Education Institutions. References [1] B.L. Mordike, T. Ebert, Magnesium: properties–applications–potential, Mater. Sci. Eng. A 302 (1) (2001) 37–45. [2] R.W. Armstrong, Theory of the tensile ductile–brittle behavior of poly-crystalline h.c.p. materials, with application to beryllium, Acta Metall. 16 (3) (1968) 347–355. [3] L.W. Lu, C.M. Liu, J. Zhao, et al., Modification of grain refinement and texture in AZ31 Mg alloy by a new plastic deformation method, J. Alloys Compd. 628 (2015) 130–134. [4] A. Jain, O. Duygulu, D.W. Brown, et al., Grain size effects on the tensile properties and deformation mechanisms of a magnesium alloy, AZ31B, sheet [J], Mater. Sci. Eng. A 486 (1–2) (2007) 545–555. [5] M.G. Jiang, H. Yan, R.S. Chen, Enhanced mechanical properties due to grain refinement and texture modification in an AZ61 Mg alloy processed by small strain impact forging, Mater. Sci. Eng. A 621 (2015) 204–211. [6] M.M. Avedesian, B. Baker (Eds.), ASM Specialty Handbook Magnesium and Magnesium Alloys, ASM International, Materials Park, OH, 1999. [7] S.B. Kang, H.M. Chen, H.W. Kim, et al., Effect of reheating and warm rolling on microstructure and mechanical properties of twin roll strip cast Mg–4.5Al–1.0Zn–0.4Mn– 0.3Ca alloy sheet, Magnesium Technology 2008, TMS, 2008. 147–152. [8] H.M. Chen, S.B. Kang, H.S. Yu, et al., Microstructure and mechanical properties of Mg–4.5Al–1.0Zn alloy sheets produced by twin roll casting and sequential warm rolling, Mater. Sci. Eng. A 492 (1–2) (2008) 317–326. [9] M.M. Myshlyaev, H.J. McQueen, A. Mwembela, et al., Twinning, dynamic recovery and recrystallization in hot worked Mg–Al–Zn alloy, Mater. Sci. Eng. A 337 (1–2) (2002) 121–133. [10] J.C. Tan, M.J. Tan, Dynamic continuous recrystallization characteristics in two stage deformation of Mg–3Al–1Zn alloy sheet, Mater. Sci. Eng. A 339 (1–2) (2003) 124–132. [11] D. Sarker, J. Friedman, D.L. Chen, Influence of pre-deformation and subsequent annealing on strain hardening and anisotropy of AM30 magnesium alloy, J. Alloys Compd. 611 (2014) 341–350. [12] A. Jager, P. Lukac, V. Gartnerova, et al., Influence of annealing on the microstructure of commercial Mg alloy AZ31 after mechanical forming, Mater. Sci. Eng. A 432 (1–2) (2006) 20–25. [13] X.B. Gong, S.B. Kang, J.H. Cho, et al., Effect of annealing on microstructure and mechanical properties of ZK60 magnesium alloy sheets processed by twin-roll cast and differential speed rolling, Mater. Charact. 97 (2014) 183–188. [14] H.M. Chen, H.S. Yu, S.B. Kang, et al., Effect of forming process on microstructure and mechanical properties of ZK60 alloy sheet, Rare Metal Mater. Eng. 40 (10) (2011) 1708–1712. [15] H.M. Chen, H.S. Yu, S.B. Kang, et al., Effect of rolling temperature on microstructure and texture of twin roll cast ZK60 magnesium alloy, Trans. Nonferrous Metals Soc. China 20 (11) (2010) 2086–2091. [16] H.M. Chen, H.S. Yu, S.B. Kang, et al., Optimization of annealing treatment parameters in a twin roll cast and warm rolled ZK60 alloy sheet, Mater. Sci. Eng. A 527 (4–5) (2010) 1236–1242. [17] F.J. Humphreys, M. Hatherly, Recrystallization and Related Annealing Phenomena [M], Second ed. Elsevier Ltd., Kidlington, UK, 2004.