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Microstructure and mechanical properties of ZE10 magnesium alloy prepared by equal channel angular pressing
International Journal of Minerals, Metallurgy and Materials Volume 16, Number 5, October 2009, Page 559
Materials
Microstructure and mechanical properties of ZE10 magnesium alloy prepared by equal channel angular pressing Ying Liu1), Wei Li1), and Yuan-yuan Li2) 1) Department of Material Science and Engineering, Jinan University, Guangzhou 510632, China 2) School of Mechanical Engineering, South China University of Technology, Guangzhou 510640, China (Received 2008-09-28)
Abstract: ZE10 magnesium alloy was subjected to equal-channel angular pressing (ECAP) up to 12 passes in a die with an angle of 120q between the two channels at 250-300qC. An inhomogeneous microstructure of bimodal grains including fine grains of 1-2 Pm as well as coarse grains of about 20 Pm was obtained after the initial 1-4 ECAP passes. The grain size became increasingly homogeneous with further ECAP processing and the grains were significantly refined to 1-2 Pm after 8 passes and further refined to 0.5-1 Pm after 12 passes. The alloy’s yield strength changed slightly but the ductility improved greatly initially up to 4-6 passes corresponding to the bimodal grain microstructure. And after the subsequent pressing of more than 8 passes, the tensile strength including yield strength improved while the elongation decreased gradually. Key words: equal channel angular pressing; magnesium alloys; microstructure; mechanical properties
1. Introduction Magnesium alloys have received much attention in various engineering applications particularly in automotive, aerospace industries, as well as electronic devices due to their light density, high specific strength and stiffness, recyclability, etc. [1]. However, Mg alloys exhibit poor formability and possess only moderate strength compared to Al alloys. Mg alloys’ mechanical properties can be enhanced by grain refinement realized by extrusion, rolling, and equal channel angular pressing (ECAP). Recently, ECAP has attracted much interest because it is one of the most promising severe plastic deformation (SPD) techniques that can produce bulk ultra-fine-grained (UFG) materials with unique physical and mechanical properties [2]. Varieties of Mg alloys had been examined by the ECAP process and acquired improved mechanical properties such as strength, ductility, and superplasticity [3-11]. Yamashita et al. [3] reported that pure Mg and Mg-0.9Al had improved both strength and ductility significantly by ECAP. Mukai et al. [4] reported that the ECAP/annealed AZ31 alloy exhibited a large elongation-to-failure of 50% although the grain size was 15 Pm (which was almost equal to that Corresponding author: Ying Liu, E-mail: [email protected] © 2009 University of Science and Technology Beijing. All rights reserved.
of the extruded alloy), and suggested that the two- to three-times improvement in elongation compared with the conventionally extruded counterpart should be due to the randomization of the originally strong extrusion texture. Large ductility improvement and the change in texture were also observed in AZ61 alloy by ECAP as reported by Kim et al. [5]. The alloy composition and processing parameters in ECAP (such as die angles, routes, induced strain, and temperature) would strongly affect the mechanical properties and microstructure feature of magnesium alloys. The four basic processing routes in ECAP are route A (in which the sample is pressed without rotation), route BA (in which the sample is rotated by 90q in alternate directions between consecutive passes), route BC (in which the sample is rotated by 90q in one direction after each pass), and route C (in which the sample is rotated by 180q after each pass). Agnew et al. [6-7] reported that different textures were evolved among different magnesium alloys including AZ31, AZ80, and ZK60 deformed by ECAP by the processing route A or route BC. Li et al. [8] reported that the AZ31 alloy realized more effective grain refinement during ECAP by route BC in the 90q die than in the 120q die in one pass, but the role of the die angle was little after several passes Also available online at www.sciencedirect.com
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International Journal of Minerals, Metallurgy and Materials, Vol.16, No.5, Oct 2009
of processing and when the total strain reached 8, the AZ31 alloy samples deformed in two different dies acquired similar submicron grains of 1-5 Pm and similar mechanical properties with a large ductility of above 45% and a low yield strength of 100 MPa. AZ31 alloy had different mechanical properties when deformed by different routes such as A, BA, BC, or C, largely improved ductility and lowered yield strength were observed when processed by routes BC and BA, but a limited change happened in them by A and C routes [9]. In this presentation, a Mg-Zn series Mg alloy ZE10, which contains some rare earth (RE) and has moderate strength and good formability [13], was deformed by ECAP up to 12 passes in a die with an of angle 120q between channels by route BC at 250-300qC and the mechanical properties and microstructure were examined at room temperature.
2. Experimental procedures The test alloy ZE10 was used with the following nominal composition: a Mg-0.9Zn-0.3RE-0.3Zr (wt%). The raw materials were pure magnesium (99.9wt% Mg) billets, pure zinc billets, Mg-20wt%Zr master alloy, and rare earth (RE, mainly composed of 50.4wt% Ce and 40.7wt% La). The alloy was melted in a resistance furnace with a mild steel crucible under the protective atmosphere of CO2 and SF6 mixed gas and at last it was cast into bars of 60 mm in diameter in a permanent mould. After homogenized at 420qC for 8 h, the ingots were firstly machined and hot extruded at 350qC into rods (with a cross-sectional size of 22.0 mmu22.0 mm) in an extrusion ratio of 4:1 and then used for the ECAP specimen (22.0 mmu22.0 mmu120 mm). The ECAP process was conducted at 300qC by route BC in the die with an angle of 120q between the two channels, the pressing velocity was 20 mm/s and the mixture of the graphite powder and engine oil (2:1) was used as lubricant. Before ECAP, the die and the specimens were preheated to 300qC and held above half an hour. The specimen was put into the die and the process could be carried out repeatedly during ECAP. The specimen could be deformed up to 12 passes without cracking and its temperature was cooled to 250qC. Tensile samples were machined to a gauge length of 25 mm and a diameter of 5 mm and tensile tests were performed on a universal test machine at a strain rate of 2 mm/min. The microstructure was examined by optical microscopy and the average grain size was measured by the linear intercept method. X-ray diffraction spectra were measured by a Philips X’pert MPD X-ray diffractometer (XRD) with monochromated Cu KĮ radiation at a scanning speed of 8.00q/min.
3. Experimental results 3.1. Microstructure of the ECAPed ZE10 Mg alloy Fig. 1 shows the optical photographs of the as-extruded and ECAPed ZE10 Mg alloy. It can be seen that, the as-extruded alloy has average sizes of 20-30 Pm. After ECAP, the alloy’s microstructure is effectively refined by the accumulated shear strain and dynamic recrystallization (DRX). In the initial stage, some coarse grains greater than 20 Pm exist although surrounding them there are many very fine grains with a size of 1-5 Pm. The grain structure is not homogeneous when the alloy deforms up to 2, 4, and 6 passes. With the increase in the number of deformation passes, more and more fine grains appear. After 8 passes, the microstructure becomes increasingly homogeneous and strongly refined and has an average grain size of 1-2 Pm. And after 12 passes, the grains are further refined to an average size of 0.5-1 Pm. 3.2. X-ray diffraction results Fig. 2 shows the XRD spectra of ZE10 Mg alloy. It can be seen that the ECAPed alloy has different diffraction peak magnitudes of the (10 1 0), (0002), and (101 1 ) planes compare to the as-extruded alloy. The as-extruded alloy has much big a peak magnitude of the (0002) plane parallel to the extrusion direction, it means that the distribution of most basal planes is inclined to the direction parallel to the extrusion direction. The case has changed some for the ECAPed alloy, the peak magnitude of the (0002) plane parallel to the extrusion direction is weakened a little, and the magnitude of the (10 1 0) and (101 1 ) plane diffraction parallel to the extrusion direction is strengthened obviously, it indicates that some basal planes rotate and depart from the extrusion direction. 3.3. Mechanical properties of the ECAPed ZE10 Mg alloy Fig. 3 shows the tensile behaviors of the ECAPed ZE10 Mg alloy. It can be seen that the mechanical properties of ZE10 Mg alloy are improved greatly by ECAP. The yield strength changes little from 2 up to 6 passes and then increases obviously up to 8 and 12 passes. The ductility, on the other hand, increases significantly from 12% in the as-extruded alloy to 26% corresponding to 4-6 passes of pressing, and decreases to 22% and 20% corresponding to 8 and 12 passes of pressing.
4. Discussion The ZE10 Mg alloy ingots were subjected to conventional extrusion prior to ECAP in order to in-
Y. Liu et al., Microstructure and mechanical properties of ZE10 magnesium alloy prepared by…
561
Fig. 1. Microstructures of the ZE10 Mg alloy before and after ECAP: (a) 0 pass (as-extruded); (b) 2 passes; (c) 4 passes; (d) 6 passes; (e) 8 passes; (f) 12 passes.
Fig. 2. XRD patterns of ZE10 Mg alloy before and after ECAP: (a) 0 pass (as-extruded); (b) 4 passes; (c) 12 passes.
troduce grain refinement in the microstructure, en-
hance the formability of the cast alloy, and permit the
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International Journal of Minerals, Metallurgy and Materials, Vol.16, No.5, Oct 2009
use of multi-passed ECAP at lower temperatures. And the pre-extruded ZE10 alloy was successfully deformed by ECAP at 250-300qC up to 12 passes in a die with an angle of 120q between two connecting channels. The extruded ZE10 alloy had a relatively homogeneous structure of reasonably fine grains with the average sizes of 20-30 Pm. The grains could be exceptionally refined by subsequent ECAP. The alloy’s microstructure was inhomogeneous and consisted of bimodal grains including fine grains of 1-2 Pm as well as coarse grains of about 20 Pm after the initial 1-4 ECAP passes. After 8 passes and 12 passes, the increasingly homogeneous ultra-fine grain sizes of 1-2 Pm and 0.5-1 Pm were observed, respectively. The evolution of the microstructure from highly distorted grains to equiaxed grains at the end of extrusion and every additional ECAP pass indicates that dynamic recrystallization occurs during the deformation. As the main driving force of recrystallization is the stored energy of dislocations, and the dislocation density increases with the increase in the deformation amount, strain contributes much to grain refinement. As ECAP was conducted in the 120q die, the total strain HN after N passes can be estimated by the following equation [2, 14]:
HN
2 I N cot 2 3
(1)
where I is the die angle between the two channels. Since HN is in direct proportion to pass number N, the efficiency of the ECAP is closely correlative to N. Calculated by Eq. (1), the strain intensity in a single pass is 0.68, and after 12 passes of ECAP, the total strain is 8. As the homogeneous equiaxed grains are observed in the as-extruded alloy with a strain of 1.4, the strain of 0.68 in a single pass of ECAP may be too small to bring about enough dislocation density in all grains so that the inhomogeneous microstructure evolves in the initial stage, and the mixed grain structure including coarse grains and fine grains appear for ECAP even up to 4-6 passes. On the other hand, inhomogeneous deformation exists in the material with a multi-grain structure due to the neighbor grains need to assort with each other during deformation, so it is possible that the coarse grains with more favorable orientations will be deformed first since the slip systems in the Mg alloy are very limited. As dislocations generated during deformation are not distributed uniformly in the whole sample, dynamic recrystallization tends to take place at those zones where dislocation densities are high. Once dynamic recrystallization has taken place and new grains are formed, the dislocation density decreases sharply at these locations. With further ECAP processing, shear deformation takes place
in other shear planes and dislocations are accumulated in coarse grains and consequently induce dynamic recrystallization there. Therefore, the fraction of fine grains increases with the ECAP pass number increasing and eventually deformation spreads to all the grains and the microstructure becomes homogeneous and new recrystallized subgrains fill the whole volume of the specimen.
Fig. 3.
Tensile behaviors of the ECAPed ZE10 Mg alloy.
The mechanical properties of ZE10 Mg alloy after ECAP are influenced by many factors such as grain refinement, texture development, grain boundary structure, residual stress distribution, etc. For the conventionally extruded ZE10 alloy, the distribution of most basal planes is aligned with the extrusion direction according to the XRD diffraction. This implies that most grains are in hard orientation relative to the tensile loading direction, so the yield strength is high and the ductility is limited. After ECAP, the distribution of basal planes in many grains is modified to some extent according to the XRD diffraction. Unlike the great decrease in yield strength happened in AZ31 alloy dominated by the texture softening after ECAP [4, 7-8], the yield strength of ZE10 alloy changes little from 2 up to 6 passes and then increases greatly up to 8 and 12 passes corresponding to the prominent grain refinement. As the basal plane in some grains can be rotated and inclined to the shear plane in ECAP, the basal plane will tilt less than 30° to the extrusion axis when the alloy is deformed by ECAP in the 120q die. Perhaps small number of grains has the texture modified in one pass in the 120q die and the texture softening is limited, so a lot of grains still have hard orientation after multi-passed pressing, and the large decrease in yield strength can not happen. On the other hand, the texture evolution during ECAP could be affected strongly by alloy addition. Agnew et al. [6] reported that ZK60 had a different texture evolution from AZ31 by ECAP. It was suggested that while AZ alloys tended to develop textures with <0001> highly inclined (about 55q) to the extrusion axis, the ZK alloy
Y. Liu et al., Microstructure and mechanical properties of ZE10 magnesium alloy prepared by…
developed a primary fiber closer to 90q and along with secondary <0001> fiber(s), either normal to the flow plane or within the flow plane during the ECAP process [7]. For the ECAPed AZ31 [4] and AZ61 alloys [6] deformed by the 90q die, the basal planes were rearranged and paralleled to the shear plane tilted 45q to the longitudinal direction, the enhanced mechanical properties were mainly due to the texture softening. In this presentation, the texture softening in ZE10 alloy was weak after ECAP. The bimodal grain microstructure existed in the first stage initially up to 4-6 passes; the ZE10 alloy had the improved ductility and this should be due to the integrated effect of grain refinement and texture modification in some grains. In the subsequent passes, the tensile strength including yield strength improved while the elongation decreased gradually, and the grains were greatly refined. The strengthening effects should be dominated by grain refinement.
[3]
[4]
[5]
[6]
[7]
[8]
5. Summary The ZE10 alloy was deformed continuously by ECAP using the die with an angle of 120q between two channels through route BC up to 12 passes at 250-300qC successfully and the grains were significantly refined to 1-2 Pm after 8 passes and further refined to 0.5-1 Pm after 12 passes, although the microstructure was initially not uniform with a bimodal grain size distribution but became increasingly homogeneous and strongly refined when deformed up to 8-12 passes. The ductility of the ZE10 alloy was improved greatly but the yield strength changed slightly initially up to 4 passes mainly due to the integrated effect of grain refinement and texture modification. In the subsequent passes, the alloy had improved the tensile strength including yield strength dominated by the effects of grain refinement.
[9]
[10]
[11]
[12]
[13]
References [1] B.L. Mordike and T. Ebert, Magnesium: properties-applications-potential, Mater. Sci. Eng. A, 302(2001), p.37. [2] R.Z. Valiev, R.K. Islamgaliev, and I.V. Alexandrov, Bulk
[14]
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nanostructured materials from severe plastic deformation, Prog. Mater. Sci., 45(2000), p.103. A. Yamashita, Z. Horita, and T.G. Langdon, Improving the mechanical properties of magnesium and a magnesium alloy through severe plastic deformation, Mater. Sci. Eng. A, 300(2001), p.142. T. Mukai, M. Yamanoi, H. Watanabe, et al., Ductility enhancement in AZ31 magnesium alloy by controlling its grain structure, Scripta Mater., 45(2001), p.89. W.J. Kim, C.W. An, Y.S. Kim, et al., Mechanical properties and microstructures of an AZ61 Mg alloy produced by equal channel angular pressing, Scripta Mater., 47(2002), p.39. S.R. Agnew, P. Mehrotra, T.M. Lillo, et al., Texture evolution of five wrought magnesium alloys during route A equal channel angular extrusion: experiments and simulations, Acta Mater., 53(2005), p.3135. S.R. Agnew, P. Mehrotra, T.M. Lillo, et al., Crystallographic texture evolution of three wrought magnesium alloys during equal channel angular extrusion, Mater. Sci. Eng. A, 408(2005), p.72. Y.Y. Li, Y. Liu, T.W.L. Ngai, et al., Effects of die angle on microstructures and mechanical properties of AZ31 magnesium alloy processed by equal channel angular pressing, Trans. Nonferrous Met. Soc. China, 14(2004), No.1, p.53. Y. Liu, W.P. Chen, D.T. Zhang, et al., Structure and mechanical property of the magnesium alloy prepared by the equal-channel angular pressing via different processing routes, J. South China Univ. Technol. (in Chinese), 32(2004), No.10, p.10. K. Matsubara, Y. Miyahara, Z. Horita, and T.G. Langdon, Developing superplasticity in a magnesium alloy through a combination of extrusion and ECAP, Acta Mater., 51(2003), p.3073. R.B. Figueiredo and T.G. Langdon, The development of superplastic ductilities and microstructural homogeneity in a magnesium ZK60 alloy processed by ECAP, Mater. Sci. Eng. A, 430(2006), p.151. K. Xia, J.T. Wang, X. Wu, et al., Equal channel angular pressing of magnesium alloy AZ31, Mater. Sci. Eng. A, 410-411(2005), p.324. Y. Liu, Y.Y. Li, and W. Li, Stamping formability of ZE10 magnesium alloy sheets, J. Rare Earths, 25(2007), No.4, p.480. V.M. Segal, Materials processing by simple shear, Mater. Sci. Eng. A, 197(1995), p.157.
Materials
Microstructure and mechanical properties of ZE10 magnesium alloy prepared by equal channel angular pressing Ying Liu1), Wei Li1), and Yuan-yuan Li2) 1) Department of Material Science and Engineering, Jinan University, Guangzhou 510632, China 2) School of Mechanical Engineering, South China University of Technology, Guangzhou 510640, China (Received 2008-09-28)
Abstract: ZE10 magnesium alloy was subjected to equal-channel angular pressing (ECAP) up to 12 passes in a die with an angle of 120q between the two channels at 250-300qC. An inhomogeneous microstructure of bimodal grains including fine grains of 1-2 Pm as well as coarse grains of about 20 Pm was obtained after the initial 1-4 ECAP passes. The grain size became increasingly homogeneous with further ECAP processing and the grains were significantly refined to 1-2 Pm after 8 passes and further refined to 0.5-1 Pm after 12 passes. The alloy’s yield strength changed slightly but the ductility improved greatly initially up to 4-6 passes corresponding to the bimodal grain microstructure. And after the subsequent pressing of more than 8 passes, the tensile strength including yield strength improved while the elongation decreased gradually. Key words: equal channel angular pressing; magnesium alloys; microstructure; mechanical properties
1. Introduction Magnesium alloys have received much attention in various engineering applications particularly in automotive, aerospace industries, as well as electronic devices due to their light density, high specific strength and stiffness, recyclability, etc. [1]. However, Mg alloys exhibit poor formability and possess only moderate strength compared to Al alloys. Mg alloys’ mechanical properties can be enhanced by grain refinement realized by extrusion, rolling, and equal channel angular pressing (ECAP). Recently, ECAP has attracted much interest because it is one of the most promising severe plastic deformation (SPD) techniques that can produce bulk ultra-fine-grained (UFG) materials with unique physical and mechanical properties [2]. Varieties of Mg alloys had been examined by the ECAP process and acquired improved mechanical properties such as strength, ductility, and superplasticity [3-11]. Yamashita et al. [3] reported that pure Mg and Mg-0.9Al had improved both strength and ductility significantly by ECAP. Mukai et al. [4] reported that the ECAP/annealed AZ31 alloy exhibited a large elongation-to-failure of 50% although the grain size was 15 Pm (which was almost equal to that Corresponding author: Ying Liu, E-mail: [email protected] © 2009 University of Science and Technology Beijing. All rights reserved.
of the extruded alloy), and suggested that the two- to three-times improvement in elongation compared with the conventionally extruded counterpart should be due to the randomization of the originally strong extrusion texture. Large ductility improvement and the change in texture were also observed in AZ61 alloy by ECAP as reported by Kim et al. [5]. The alloy composition and processing parameters in ECAP (such as die angles, routes, induced strain, and temperature) would strongly affect the mechanical properties and microstructure feature of magnesium alloys. The four basic processing routes in ECAP are route A (in which the sample is pressed without rotation), route BA (in which the sample is rotated by 90q in alternate directions between consecutive passes), route BC (in which the sample is rotated by 90q in one direction after each pass), and route C (in which the sample is rotated by 180q after each pass). Agnew et al. [6-7] reported that different textures were evolved among different magnesium alloys including AZ31, AZ80, and ZK60 deformed by ECAP by the processing route A or route BC. Li et al. [8] reported that the AZ31 alloy realized more effective grain refinement during ECAP by route BC in the 90q die than in the 120q die in one pass, but the role of the die angle was little after several passes Also available online at www.sciencedirect.com
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International Journal of Minerals, Metallurgy and Materials, Vol.16, No.5, Oct 2009
of processing and when the total strain reached 8, the AZ31 alloy samples deformed in two different dies acquired similar submicron grains of 1-5 Pm and similar mechanical properties with a large ductility of above 45% and a low yield strength of 100 MPa. AZ31 alloy had different mechanical properties when deformed by different routes such as A, BA, BC, or C, largely improved ductility and lowered yield strength were observed when processed by routes BC and BA, but a limited change happened in them by A and C routes [9]. In this presentation, a Mg-Zn series Mg alloy ZE10, which contains some rare earth (RE) and has moderate strength and good formability [13], was deformed by ECAP up to 12 passes in a die with an of angle 120q between channels by route BC at 250-300qC and the mechanical properties and microstructure were examined at room temperature.
2. Experimental procedures The test alloy ZE10 was used with the following nominal composition: a Mg-0.9Zn-0.3RE-0.3Zr (wt%). The raw materials were pure magnesium (99.9wt% Mg) billets, pure zinc billets, Mg-20wt%Zr master alloy, and rare earth (RE, mainly composed of 50.4wt% Ce and 40.7wt% La). The alloy was melted in a resistance furnace with a mild steel crucible under the protective atmosphere of CO2 and SF6 mixed gas and at last it was cast into bars of 60 mm in diameter in a permanent mould. After homogenized at 420qC for 8 h, the ingots were firstly machined and hot extruded at 350qC into rods (with a cross-sectional size of 22.0 mmu22.0 mm) in an extrusion ratio of 4:1 and then used for the ECAP specimen (22.0 mmu22.0 mmu120 mm). The ECAP process was conducted at 300qC by route BC in the die with an angle of 120q between the two channels, the pressing velocity was 20 mm/s and the mixture of the graphite powder and engine oil (2:1) was used as lubricant. Before ECAP, the die and the specimens were preheated to 300qC and held above half an hour. The specimen was put into the die and the process could be carried out repeatedly during ECAP. The specimen could be deformed up to 12 passes without cracking and its temperature was cooled to 250qC. Tensile samples were machined to a gauge length of 25 mm and a diameter of 5 mm and tensile tests were performed on a universal test machine at a strain rate of 2 mm/min. The microstructure was examined by optical microscopy and the average grain size was measured by the linear intercept method. X-ray diffraction spectra were measured by a Philips X’pert MPD X-ray diffractometer (XRD) with monochromated Cu KĮ radiation at a scanning speed of 8.00q/min.
3. Experimental results 3.1. Microstructure of the ECAPed ZE10 Mg alloy Fig. 1 shows the optical photographs of the as-extruded and ECAPed ZE10 Mg alloy. It can be seen that, the as-extruded alloy has average sizes of 20-30 Pm. After ECAP, the alloy’s microstructure is effectively refined by the accumulated shear strain and dynamic recrystallization (DRX). In the initial stage, some coarse grains greater than 20 Pm exist although surrounding them there are many very fine grains with a size of 1-5 Pm. The grain structure is not homogeneous when the alloy deforms up to 2, 4, and 6 passes. With the increase in the number of deformation passes, more and more fine grains appear. After 8 passes, the microstructure becomes increasingly homogeneous and strongly refined and has an average grain size of 1-2 Pm. And after 12 passes, the grains are further refined to an average size of 0.5-1 Pm. 3.2. X-ray diffraction results Fig. 2 shows the XRD spectra of ZE10 Mg alloy. It can be seen that the ECAPed alloy has different diffraction peak magnitudes of the (10 1 0), (0002), and (101 1 ) planes compare to the as-extruded alloy. The as-extruded alloy has much big a peak magnitude of the (0002) plane parallel to the extrusion direction, it means that the distribution of most basal planes is inclined to the direction parallel to the extrusion direction. The case has changed some for the ECAPed alloy, the peak magnitude of the (0002) plane parallel to the extrusion direction is weakened a little, and the magnitude of the (10 1 0) and (101 1 ) plane diffraction parallel to the extrusion direction is strengthened obviously, it indicates that some basal planes rotate and depart from the extrusion direction. 3.3. Mechanical properties of the ECAPed ZE10 Mg alloy Fig. 3 shows the tensile behaviors of the ECAPed ZE10 Mg alloy. It can be seen that the mechanical properties of ZE10 Mg alloy are improved greatly by ECAP. The yield strength changes little from 2 up to 6 passes and then increases obviously up to 8 and 12 passes. The ductility, on the other hand, increases significantly from 12% in the as-extruded alloy to 26% corresponding to 4-6 passes of pressing, and decreases to 22% and 20% corresponding to 8 and 12 passes of pressing.
4. Discussion The ZE10 Mg alloy ingots were subjected to conventional extrusion prior to ECAP in order to in-
Y. Liu et al., Microstructure and mechanical properties of ZE10 magnesium alloy prepared by…
561
Fig. 1. Microstructures of the ZE10 Mg alloy before and after ECAP: (a) 0 pass (as-extruded); (b) 2 passes; (c) 4 passes; (d) 6 passes; (e) 8 passes; (f) 12 passes.
Fig. 2. XRD patterns of ZE10 Mg alloy before and after ECAP: (a) 0 pass (as-extruded); (b) 4 passes; (c) 12 passes.
troduce grain refinement in the microstructure, en-
hance the formability of the cast alloy, and permit the
562
International Journal of Minerals, Metallurgy and Materials, Vol.16, No.5, Oct 2009
use of multi-passed ECAP at lower temperatures. And the pre-extruded ZE10 alloy was successfully deformed by ECAP at 250-300qC up to 12 passes in a die with an angle of 120q between two connecting channels. The extruded ZE10 alloy had a relatively homogeneous structure of reasonably fine grains with the average sizes of 20-30 Pm. The grains could be exceptionally refined by subsequent ECAP. The alloy’s microstructure was inhomogeneous and consisted of bimodal grains including fine grains of 1-2 Pm as well as coarse grains of about 20 Pm after the initial 1-4 ECAP passes. After 8 passes and 12 passes, the increasingly homogeneous ultra-fine grain sizes of 1-2 Pm and 0.5-1 Pm were observed, respectively. The evolution of the microstructure from highly distorted grains to equiaxed grains at the end of extrusion and every additional ECAP pass indicates that dynamic recrystallization occurs during the deformation. As the main driving force of recrystallization is the stored energy of dislocations, and the dislocation density increases with the increase in the deformation amount, strain contributes much to grain refinement. As ECAP was conducted in the 120q die, the total strain HN after N passes can be estimated by the following equation [2, 14]:
HN
2 I N cot 2 3
(1)
where I is the die angle between the two channels. Since HN is in direct proportion to pass number N, the efficiency of the ECAP is closely correlative to N. Calculated by Eq. (1), the strain intensity in a single pass is 0.68, and after 12 passes of ECAP, the total strain is 8. As the homogeneous equiaxed grains are observed in the as-extruded alloy with a strain of 1.4, the strain of 0.68 in a single pass of ECAP may be too small to bring about enough dislocation density in all grains so that the inhomogeneous microstructure evolves in the initial stage, and the mixed grain structure including coarse grains and fine grains appear for ECAP even up to 4-6 passes. On the other hand, inhomogeneous deformation exists in the material with a multi-grain structure due to the neighbor grains need to assort with each other during deformation, so it is possible that the coarse grains with more favorable orientations will be deformed first since the slip systems in the Mg alloy are very limited. As dislocations generated during deformation are not distributed uniformly in the whole sample, dynamic recrystallization tends to take place at those zones where dislocation densities are high. Once dynamic recrystallization has taken place and new grains are formed, the dislocation density decreases sharply at these locations. With further ECAP processing, shear deformation takes place
in other shear planes and dislocations are accumulated in coarse grains and consequently induce dynamic recrystallization there. Therefore, the fraction of fine grains increases with the ECAP pass number increasing and eventually deformation spreads to all the grains and the microstructure becomes homogeneous and new recrystallized subgrains fill the whole volume of the specimen.
Fig. 3.
Tensile behaviors of the ECAPed ZE10 Mg alloy.
The mechanical properties of ZE10 Mg alloy after ECAP are influenced by many factors such as grain refinement, texture development, grain boundary structure, residual stress distribution, etc. For the conventionally extruded ZE10 alloy, the distribution of most basal planes is aligned with the extrusion direction according to the XRD diffraction. This implies that most grains are in hard orientation relative to the tensile loading direction, so the yield strength is high and the ductility is limited. After ECAP, the distribution of basal planes in many grains is modified to some extent according to the XRD diffraction. Unlike the great decrease in yield strength happened in AZ31 alloy dominated by the texture softening after ECAP [4, 7-8], the yield strength of ZE10 alloy changes little from 2 up to 6 passes and then increases greatly up to 8 and 12 passes corresponding to the prominent grain refinement. As the basal plane in some grains can be rotated and inclined to the shear plane in ECAP, the basal plane will tilt less than 30° to the extrusion axis when the alloy is deformed by ECAP in the 120q die. Perhaps small number of grains has the texture modified in one pass in the 120q die and the texture softening is limited, so a lot of grains still have hard orientation after multi-passed pressing, and the large decrease in yield strength can not happen. On the other hand, the texture evolution during ECAP could be affected strongly by alloy addition. Agnew et al. [6] reported that ZK60 had a different texture evolution from AZ31 by ECAP. It was suggested that while AZ alloys tended to develop textures with <0001> highly inclined (about 55q) to the extrusion axis, the ZK alloy
Y. Liu et al., Microstructure and mechanical properties of ZE10 magnesium alloy prepared by…
developed a primary fiber closer to 90q and along with secondary <0001> fiber(s), either normal to the flow plane or within the flow plane during the ECAP process [7]. For the ECAPed AZ31 [4] and AZ61 alloys [6] deformed by the 90q die, the basal planes were rearranged and paralleled to the shear plane tilted 45q to the longitudinal direction, the enhanced mechanical properties were mainly due to the texture softening. In this presentation, the texture softening in ZE10 alloy was weak after ECAP. The bimodal grain microstructure existed in the first stage initially up to 4-6 passes; the ZE10 alloy had the improved ductility and this should be due to the integrated effect of grain refinement and texture modification in some grains. In the subsequent passes, the tensile strength including yield strength improved while the elongation decreased gradually, and the grains were greatly refined. The strengthening effects should be dominated by grain refinement.
[3]
[4]
[5]
[6]
[7]
[8]
5. Summary The ZE10 alloy was deformed continuously by ECAP using the die with an angle of 120q between two channels through route BC up to 12 passes at 250-300qC successfully and the grains were significantly refined to 1-2 Pm after 8 passes and further refined to 0.5-1 Pm after 12 passes, although the microstructure was initially not uniform with a bimodal grain size distribution but became increasingly homogeneous and strongly refined when deformed up to 8-12 passes. The ductility of the ZE10 alloy was improved greatly but the yield strength changed slightly initially up to 4 passes mainly due to the integrated effect of grain refinement and texture modification. In the subsequent passes, the alloy had improved the tensile strength including yield strength dominated by the effects of grain refinement.
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