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Microstructure development during extrusion in a wrought Mg–Zn–Zr alloy
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Scripta Materialia 60 (2009) 536–538 www.elsevier.com/locate/scriptamat
Microstructure development during extrusion in a wrought Mg–Zn–Zr alloy Muhammad Shahzad* and Lothar Wagner Institute of Materials Science & Engineering, Clausthal University of Technology, Agricolastr. 6, Clausthal-Zellerfeld D-38678, Germany Received 16 October 2008; revised 26 November 2008; accepted 1 December 2008 Available online 13 December 2008
Microstructure development during extrusion has been studied in a direct chill-cast Mg–6%Zn–0.5%Zr alloy at two extrusion ratios. Upon extrusion, the characteristic Zr-rich cores either recrystallize and form pockets of very fine grains or remain unrecrystallized and give rise to the so-called ‘‘soft stringers”. Zn diffuses out of the Zn-rich rings at the grain boundaries and some of the metastable eutectic constituents undergo eutectoid decomposition. Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Magnesium alloy; ZK60; Extrusion ratio; Zr-rich cores
Magnesium is one of the lightest metals and its alloys offer great potential for future applications because of their high specific strengths. Mg alloys can be broadly classified as Al-bearing or Al-free alloys. Al-bearing Mg alloys are the most commonly used Mg alloys and are grain refined by superheating or inoculation [1]. On the other hand, Al-free Mg alloys mainly refer to a value-added class of the alloys, which are based on the exceptional grain refining ability of Zr [2]. The microstructure of the Al-bearing alloys cannot be refined by Zr addition, because Zr and Al form stable intermetallic compounds with each other. Zn is another important alloying element in Mg alloys. The solid solution strengthening effect of Zn is much higher than Al at higher concentrations [3]. In general, the alloying addition of Zn increases the response of Mg alloys towards precipitation hardening, and most precipitation-hardenable Mg alloys contain Zn. Binary Mg–Zn alloys, however, suffer from grain coarsening and microporosity [4]. These alloys are not susceptible to grain refinement by superheating and are generally grain refined by Zr addition [4]. Compared to cast Mg alloys, fine grain sizes attained after deformation in wrought alloys give a good combination of high strength and ductility at room temperature, and superplasticity at elevated temperatures [5]. Of the currently employed large-scale manufacturing processes, * Corresponding author. E-mail: [email protected]
extrusion is the best method for breaking a cast structure, because the billet is subjected to compressive stresses only [6]. Extrusion of Al-bearing Mg alloys has been extensively studied [7]; however, the literature on Mg–Zn–Zr alloys is limited. The characteristic Zr-rich cores present in Mg–Zn–Zr alloys [4] play an important role in the recrystallization behavior of the alloy, and the microstructure evolution is different than in Al-bearing alloys. Furthermore, these alloys contain many precipitates, which suppress the dynamic recrystallization (DRX) response of the alloy during deformation [8]. A previous study on Mg–6%Al–0.5%Zn alloy by the same authors has shown that the microstructure evolution is not greatly affected by variation of the extrusion ratio [7]; however, significant effects of extrusion ratio on microstructure development have been observed in Mg–6%Zn–0.5%Zr alloy processed under the same conditions. It has been observed that this difference stems from the Zr-rich cores and is discussed here. Direct chill (DC)-cast wrought Mg alloy with nominal composition Mg–6%Zn–0.5%Zr was received in the form of billets having a diameter of 70 mm. These billets and the extrusion die were heated to 250 °C for 1 h. Forward extrusion of the heated billets was done at a constant ram speed of 1 mm s 1 after applying MoS2 lubrication. Two extrusion dies with exit diameters of 10.5 and 20 mm were used, which correspond to the extrusion ratios of 44 and 12, respectively. After extrusion the alloys were cooled in still air. Both as-cast and as-extruded conditions were investigated for micro-
1359-6462/$ - see front matter Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2008.12.006
M. Shahzad, L. Wagner / Scripta Materialia 60 (2009) 536–538
Figure 1. Microstructure of the as-received alloy: (a) optical micrograph and (b) back-scattered electron micrograph. Representative regions A–D refer to the WDX analysis given in Table 1.
structure using optical microscopy and electron probe microanalysis (EPMA, Cameca SX 100). An optical micrograph of the alloy in the as-received condition (Fig. 1a) shows fairly equiaxed grains having an average size of 100 lm, and eutectic constituents at the grain boundaries. These constituents can be better appreciated in the back-scattered electron (BSE) image presented in Figure 1b. The wavelength dispersive X-ray (WDX) analysis, presented in Table 1, shows the chemical composition of the four representative regions marked with letters A–D in Figure 1b. Region A is rich in Zr and lean in Zn compared to region B, which represents Zn-rich rings at the grain boundaries. The eutectic constituents at the grain boundaries (C) are Mg–Zn based and the relative proportion of Mg and Zn is close to that of Mg7Zn3. The Zn–Zr particles (D) both inside and at the grain boundaries have Zn concentrations in the range 15–50%. The WDX line scan presented in Figure 2 shows that the concentration of Zr varies within the grain and shows a maximum at the core of the grain. In comparison, Zn concentration remains fairly consistent within the grain and has a maximum value at the grain boundary. Figure 3 shows the optical microstructure of the alloy after extrusion at both extrusion ratios. The recrystallized grains have two populations of grain sizes. The coarse grains are of 10–20 lm size while fine grains are 2–5 lm. In addition to the recrystallized grains, the optical micrographs show elongated regions which have not recrystallized. These regions are wide and hence can be easily seen in the case of the lower extrusion ratio. The EPMA (Fig. 4a and b and Table 1) reveals that these Table 1. Wavelength dispersive X-ray (WDX) analysis of the alloy in as-cast and as-extruded conditions. Letters A–D refer to the representative regions shown in Figs. 1 and 4 for as-cast and as-extruded conditions, respectively.
As-cast A B C D Extruded A B C D
Mg (wt.%)
Zn (wt.%)
Zr (wt.%)
94.0–95.0 88.0–90.0 63.0–66.0 5.0–10.0
1.8–2.0 4.0–7.0 25.0–30.0 15.0–50.0
1.0–1.4 0.1–0.3 0.02–0.04 20–50
94.0–95.0 92.0–94.0 63.0–66.0 25.0–30.0 5.0–10.0
2.0–2.5 4.5–5.5 25.0–30.0 50.0–65.0 15.0–50.0
1.0–1.4 0.1–0.3 0.02–0.04 0.02–0.04 20.0–50.0
537
Figure 2. WDX line scan of an individual grain in the as-received condition.
Figure 3. Optical micrograph of the alloy extruded at (a) ER 12 and (b) ER 44.
Figure 4. BSE image of the alloy extruded at (a) ER 12 and (b) ER 44. Representative regions A–D refer to the WDX analysis given in Table 1.
elongated regions are Zr-rich and Zn-lean, similar to the cores of the grains prior to extrusion. Furthermore it shows that upon extrusion, diffusion of Zn from Zn-rich rings takes place and the peak value of Zn concentration decreases from 7 to 5.5 wt.%. After unidirectional deformation during extrusion, the eutectic constituents form stringers in the extrusion direction (Fig. 4c) and some of the eutectic constituents decompose to a more Zn-bearing compound (Table 1). Zr is an effective grain refiner for Mg alloys that do not contain Al, Mn, Si and Fe, because these elements form stable compounds with Zr. An addition of Zr can readily reduce the average grain size from a few millimeters to about 50 lm at normal cooling rates [2,9]. In addition to the grain refinement, Zr addition gives a uniform microstructure because of the nearly round or nodular grains [10]. The refinement of grain size by Zr is generally attributed to the peritectic solidification, where Zr-rich Mg solidifies first when nucleation starts at the primary Zr-rich particles [4,2,11]. Not all the Zr-rich particles, but only those which separate from the Zr master alloy near the peritectic reaction temperature, give rise to such nucleation. Therefore Zr-rich cores are not seen around all Zr particles that are visible in the polished microstructure [4]. Similarly, the Zn–Zr
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M. Shahzad, L. Wagner / Scripta Materialia 60 (2009) 536–538
intermetallic phase that precipitates from saturated solid solution at low temperatures also does not contribute to the grain refinement. A Zn–Zr intermetallic phase is present in the Zr-bearing Mg alloys that have Zn concentration in excess of 4% [12]. As seen in Figures 1b, 4a and b, the contrast in the BSE image is based on Zn, and Zr-rich cores are not visible in the way that they were in the absence of Zn [11]. Therefore it is not possible to differentiate between the Zr-rich particles associated with Zr-rich cores and other Zn–Zr particles on the BSE image. The WDX analysis performed on various Zn–Zr particles shows Zn concentration in the range 15–50 wt.%. An earlier study of Mg–4.5%Zn–0.7%Zr alloy has shown that the Zr-rich particles and intermetallic phases have similar compositions [4]. Mg alloys undergo DRX during extrusion and the recrystallized grains are much finer than in the as-cast condition (Figs. 1a and 3). The Zr-rich cores present in the alloy play an important role in the recrystallization behavior of the alloy. These cores give rise to the conditions that restrict grain growth after deformation [4], so when recrystallized, these cores result in pockets of very fine grains (Fig. 3). The Zr-rich cores that do not recrystallize form ‘‘soft stringers” or striations, where the amount of Zr in solid solution is considerably higher than the immediate neighborhood [4]. It seems that the recrystallization of these cores is strongly affected by the mode of deformation. In the grains where slip dominates, new grains nucleate at the previous grain boundaries and a thick necklace region recrystallizes, leaving the centre with only basal slip; however, the grains which undergo deformation twinning are likely to be replaced with new grains because of the nucleation at the twin boundaries [13]. The higher extrusion ratio (ER44) not only deforms the Zr-rich cores to fine bands ( Fig. 4) but also affects the mode of DRX. At lower strains, a conventional DRX involving bulging out of the part of serrated grain boundaries operates, while at higher strains a continuous DRX inside the grains becomes active [14]. This means that a thick layer of recrystallized grains is formed at ER44, and the ‘‘soft stringers” are reduced to a thin cross-section (Fig. 4b). Moreover a higher extrusion ratio probably also enhances the contribution of deformation twinning and thereby improves the recrystallization behavior by providing additional nucleation sites at the twin boundaries. Large ‘‘soft stringers” in the lower ER extrusions have significant effects on mechanical properties of the alloy. These affects will be reported elsewhere. In addition to fine and nodular grains, another advantage of Zr addition is a reduction in the amount of Mg–Zn compound at the grain boundaries, so that more Zn can go in the solid solution [4]. The WDX analysis (Table 1) suggests that the eutectic constituents present at the grain boundaries in the as-cast condition are Mg7Zn3, which is a primary metastable intermetallic phase and during post-casting heat treatment transforms to an equilibrium phase MgZn. This transformation, however, proceeds stepwise, and an intermediate phase, Mg4Zn2 [15] or MgZn2 [3,4], is formed because of thermodynamic preference. This transformation is affected by the alloying elements and their relative proportion [16]. In Mg–Zn alloys, this transformation starts
after 2 h of isothermal aging at 200 °C; however, it takes long time before this decomposition is complete [15]. Since the holding time (1 h) at the extrusion temperature (250 °C) before extrusion was relatively short, only some of the eutectic constituents could undergo eutectoid decomposition. In summary, the microstructure of a DC-cast Mg– 6%Zn–0.5%Zr alloy consists of Zr-rich cores associated with the Zr-rich particles, Zn-rich rings and Mg7Zn3 eutectic constituents at the grain boundaries, and Zn–Zr precipitates which are formed during cooling of the saturated solid solution. Upon extrusion, the Zr-rich cores are elongated and either recrystallize to form pockets of very fine grains (2–5 lm) compared to the other recrystallized grains (10–20 lm), or remain unrecrystallized and give rise to the so-called Zr-rich ‘‘soft stringers”. More twinning contribution and activation of the continuous DRX mode at the higher extrusion ratio improves the recrystallization behavior of these cores. During heating and holding at the extrusion temperature, diffusion of Zn from the Zn-rich rings takes place and some of the primary eutectic constituents decompose into a more Zn-bearing compound. One of the authors (M.S.) gratefully acknowledges an award of scholarship by the Higher Education Commission of Pakistan. Authors would also like to acknowledge the help of Mr. Klaus Hermann, IELF, TU Clausthal, for performing EPMA. [1] I.J. Polmear, Light Alloys, third ed., Arnold, London, 1995. [2] D.H. St. John, M. Qian, M.A. Easton, P. Cao, Z. Hildebrand, Metall. Mater. Trans. A 36A (2005) 1669– 1679. [3] C.H. Caceres, A. Blake, Phys. Stat. Sol. 194 (2002) 147– 158. [4] E.F. Emley, Principles of Magnesium Technology, Pergamon Press, Oxford, 1966. [5] K. Kubota, M. Mabuchi, K. Higashi, J. Mater. Sci. 34 (1999) 2225–2262. [6] P.K. Saha, Aluminum Extrusion Technology, ASM, Materials Park, OH, 2000. [7] M. Shahzad, L. Wagner, Mater. Sci. Eng. A (2008), doi:10.1016/j.msea.2008.11.038. [8] Z.P. Luo, D.Y. Song, S.Q. Zhang, J. Alloys Compd. 230 (1995) 109–114. [9] H. Okamoto, J. Phase Equil. 23 (2002) 198–199. [10] M. Qian, D.H. St. John, M.T. Frost, in: K.U. Kainer (Ed.), Mangesium Alloys and Their Applications, WileyVCH, Wolfsburg, 2003, pp. 706–712. [11] M. Qian, D.H. St. John, M.T. Frost, Scripta Mater. 46 (2002) 649–654. [12] Z. Hildebrand, M. Qian, D.H. St. John, M.T. Frost, in: A.A. Luo (Ed.), Magnesium Technology 2004, TMS, Warrendale, PA, 2004, pp. 241–245. [13] H.J. McQueen, M.M. Myshlyaev, A. Mwembela, Can. Metall. Quart. 42 (2003) 97–112. [14] Y. Zhang, X. Zeng, C. Lu, W. Ding, Mater. Sci. Eng. A. 428 (2006) 91–97. [15] X. Gao, J.F. Nie, Scripta Mater. 57 (2007) 655–658. [16] Z. Liu, X. Liu, H. Guo, B, Liu, P. Shen, International Materials Research Conference 2008, 9–12 June 2008, Chongqing, China.
Scripta Materialia 60 (2009) 536–538 www.elsevier.com/locate/scriptamat
Microstructure development during extrusion in a wrought Mg–Zn–Zr alloy Muhammad Shahzad* and Lothar Wagner Institute of Materials Science & Engineering, Clausthal University of Technology, Agricolastr. 6, Clausthal-Zellerfeld D-38678, Germany Received 16 October 2008; revised 26 November 2008; accepted 1 December 2008 Available online 13 December 2008
Microstructure development during extrusion has been studied in a direct chill-cast Mg–6%Zn–0.5%Zr alloy at two extrusion ratios. Upon extrusion, the characteristic Zr-rich cores either recrystallize and form pockets of very fine grains or remain unrecrystallized and give rise to the so-called ‘‘soft stringers”. Zn diffuses out of the Zn-rich rings at the grain boundaries and some of the metastable eutectic constituents undergo eutectoid decomposition. Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Magnesium alloy; ZK60; Extrusion ratio; Zr-rich cores
Magnesium is one of the lightest metals and its alloys offer great potential for future applications because of their high specific strengths. Mg alloys can be broadly classified as Al-bearing or Al-free alloys. Al-bearing Mg alloys are the most commonly used Mg alloys and are grain refined by superheating or inoculation [1]. On the other hand, Al-free Mg alloys mainly refer to a value-added class of the alloys, which are based on the exceptional grain refining ability of Zr [2]. The microstructure of the Al-bearing alloys cannot be refined by Zr addition, because Zr and Al form stable intermetallic compounds with each other. Zn is another important alloying element in Mg alloys. The solid solution strengthening effect of Zn is much higher than Al at higher concentrations [3]. In general, the alloying addition of Zn increases the response of Mg alloys towards precipitation hardening, and most precipitation-hardenable Mg alloys contain Zn. Binary Mg–Zn alloys, however, suffer from grain coarsening and microporosity [4]. These alloys are not susceptible to grain refinement by superheating and are generally grain refined by Zr addition [4]. Compared to cast Mg alloys, fine grain sizes attained after deformation in wrought alloys give a good combination of high strength and ductility at room temperature, and superplasticity at elevated temperatures [5]. Of the currently employed large-scale manufacturing processes, * Corresponding author. E-mail: [email protected]
extrusion is the best method for breaking a cast structure, because the billet is subjected to compressive stresses only [6]. Extrusion of Al-bearing Mg alloys has been extensively studied [7]; however, the literature on Mg–Zn–Zr alloys is limited. The characteristic Zr-rich cores present in Mg–Zn–Zr alloys [4] play an important role in the recrystallization behavior of the alloy, and the microstructure evolution is different than in Al-bearing alloys. Furthermore, these alloys contain many precipitates, which suppress the dynamic recrystallization (DRX) response of the alloy during deformation [8]. A previous study on Mg–6%Al–0.5%Zn alloy by the same authors has shown that the microstructure evolution is not greatly affected by variation of the extrusion ratio [7]; however, significant effects of extrusion ratio on microstructure development have been observed in Mg–6%Zn–0.5%Zr alloy processed under the same conditions. It has been observed that this difference stems from the Zr-rich cores and is discussed here. Direct chill (DC)-cast wrought Mg alloy with nominal composition Mg–6%Zn–0.5%Zr was received in the form of billets having a diameter of 70 mm. These billets and the extrusion die were heated to 250 °C for 1 h. Forward extrusion of the heated billets was done at a constant ram speed of 1 mm s 1 after applying MoS2 lubrication. Two extrusion dies with exit diameters of 10.5 and 20 mm were used, which correspond to the extrusion ratios of 44 and 12, respectively. After extrusion the alloys were cooled in still air. Both as-cast and as-extruded conditions were investigated for micro-
1359-6462/$ - see front matter Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2008.12.006
M. Shahzad, L. Wagner / Scripta Materialia 60 (2009) 536–538
Figure 1. Microstructure of the as-received alloy: (a) optical micrograph and (b) back-scattered electron micrograph. Representative regions A–D refer to the WDX analysis given in Table 1.
structure using optical microscopy and electron probe microanalysis (EPMA, Cameca SX 100). An optical micrograph of the alloy in the as-received condition (Fig. 1a) shows fairly equiaxed grains having an average size of 100 lm, and eutectic constituents at the grain boundaries. These constituents can be better appreciated in the back-scattered electron (BSE) image presented in Figure 1b. The wavelength dispersive X-ray (WDX) analysis, presented in Table 1, shows the chemical composition of the four representative regions marked with letters A–D in Figure 1b. Region A is rich in Zr and lean in Zn compared to region B, which represents Zn-rich rings at the grain boundaries. The eutectic constituents at the grain boundaries (C) are Mg–Zn based and the relative proportion of Mg and Zn is close to that of Mg7Zn3. The Zn–Zr particles (D) both inside and at the grain boundaries have Zn concentrations in the range 15–50%. The WDX line scan presented in Figure 2 shows that the concentration of Zr varies within the grain and shows a maximum at the core of the grain. In comparison, Zn concentration remains fairly consistent within the grain and has a maximum value at the grain boundary. Figure 3 shows the optical microstructure of the alloy after extrusion at both extrusion ratios. The recrystallized grains have two populations of grain sizes. The coarse grains are of 10–20 lm size while fine grains are 2–5 lm. In addition to the recrystallized grains, the optical micrographs show elongated regions which have not recrystallized. These regions are wide and hence can be easily seen in the case of the lower extrusion ratio. The EPMA (Fig. 4a and b and Table 1) reveals that these Table 1. Wavelength dispersive X-ray (WDX) analysis of the alloy in as-cast and as-extruded conditions. Letters A–D refer to the representative regions shown in Figs. 1 and 4 for as-cast and as-extruded conditions, respectively.
As-cast A B C D Extruded A B C D
Mg (wt.%)
Zn (wt.%)
Zr (wt.%)
94.0–95.0 88.0–90.0 63.0–66.0 5.0–10.0
1.8–2.0 4.0–7.0 25.0–30.0 15.0–50.0
1.0–1.4 0.1–0.3 0.02–0.04 20–50
94.0–95.0 92.0–94.0 63.0–66.0 25.0–30.0 5.0–10.0
2.0–2.5 4.5–5.5 25.0–30.0 50.0–65.0 15.0–50.0
1.0–1.4 0.1–0.3 0.02–0.04 0.02–0.04 20.0–50.0
537
Figure 2. WDX line scan of an individual grain in the as-received condition.
Figure 3. Optical micrograph of the alloy extruded at (a) ER 12 and (b) ER 44.
Figure 4. BSE image of the alloy extruded at (a) ER 12 and (b) ER 44. Representative regions A–D refer to the WDX analysis given in Table 1.
elongated regions are Zr-rich and Zn-lean, similar to the cores of the grains prior to extrusion. Furthermore it shows that upon extrusion, diffusion of Zn from Zn-rich rings takes place and the peak value of Zn concentration decreases from 7 to 5.5 wt.%. After unidirectional deformation during extrusion, the eutectic constituents form stringers in the extrusion direction (Fig. 4c) and some of the eutectic constituents decompose to a more Zn-bearing compound (Table 1). Zr is an effective grain refiner for Mg alloys that do not contain Al, Mn, Si and Fe, because these elements form stable compounds with Zr. An addition of Zr can readily reduce the average grain size from a few millimeters to about 50 lm at normal cooling rates [2,9]. In addition to the grain refinement, Zr addition gives a uniform microstructure because of the nearly round or nodular grains [10]. The refinement of grain size by Zr is generally attributed to the peritectic solidification, where Zr-rich Mg solidifies first when nucleation starts at the primary Zr-rich particles [4,2,11]. Not all the Zr-rich particles, but only those which separate from the Zr master alloy near the peritectic reaction temperature, give rise to such nucleation. Therefore Zr-rich cores are not seen around all Zr particles that are visible in the polished microstructure [4]. Similarly, the Zn–Zr
538
M. Shahzad, L. Wagner / Scripta Materialia 60 (2009) 536–538
intermetallic phase that precipitates from saturated solid solution at low temperatures also does not contribute to the grain refinement. A Zn–Zr intermetallic phase is present in the Zr-bearing Mg alloys that have Zn concentration in excess of 4% [12]. As seen in Figures 1b, 4a and b, the contrast in the BSE image is based on Zn, and Zr-rich cores are not visible in the way that they were in the absence of Zn [11]. Therefore it is not possible to differentiate between the Zr-rich particles associated with Zr-rich cores and other Zn–Zr particles on the BSE image. The WDX analysis performed on various Zn–Zr particles shows Zn concentration in the range 15–50 wt.%. An earlier study of Mg–4.5%Zn–0.7%Zr alloy has shown that the Zr-rich particles and intermetallic phases have similar compositions [4]. Mg alloys undergo DRX during extrusion and the recrystallized grains are much finer than in the as-cast condition (Figs. 1a and 3). The Zr-rich cores present in the alloy play an important role in the recrystallization behavior of the alloy. These cores give rise to the conditions that restrict grain growth after deformation [4], so when recrystallized, these cores result in pockets of very fine grains (Fig. 3). The Zr-rich cores that do not recrystallize form ‘‘soft stringers” or striations, where the amount of Zr in solid solution is considerably higher than the immediate neighborhood [4]. It seems that the recrystallization of these cores is strongly affected by the mode of deformation. In the grains where slip dominates, new grains nucleate at the previous grain boundaries and a thick necklace region recrystallizes, leaving the centre with only basal slip; however, the grains which undergo deformation twinning are likely to be replaced with new grains because of the nucleation at the twin boundaries [13]. The higher extrusion ratio (ER44) not only deforms the Zr-rich cores to fine bands ( Fig. 4) but also affects the mode of DRX. At lower strains, a conventional DRX involving bulging out of the part of serrated grain boundaries operates, while at higher strains a continuous DRX inside the grains becomes active [14]. This means that a thick layer of recrystallized grains is formed at ER44, and the ‘‘soft stringers” are reduced to a thin cross-section (Fig. 4b). Moreover a higher extrusion ratio probably also enhances the contribution of deformation twinning and thereby improves the recrystallization behavior by providing additional nucleation sites at the twin boundaries. Large ‘‘soft stringers” in the lower ER extrusions have significant effects on mechanical properties of the alloy. These affects will be reported elsewhere. In addition to fine and nodular grains, another advantage of Zr addition is a reduction in the amount of Mg–Zn compound at the grain boundaries, so that more Zn can go in the solid solution [4]. The WDX analysis (Table 1) suggests that the eutectic constituents present at the grain boundaries in the as-cast condition are Mg7Zn3, which is a primary metastable intermetallic phase and during post-casting heat treatment transforms to an equilibrium phase MgZn. This transformation, however, proceeds stepwise, and an intermediate phase, Mg4Zn2 [15] or MgZn2 [3,4], is formed because of thermodynamic preference. This transformation is affected by the alloying elements and their relative proportion [16]. In Mg–Zn alloys, this transformation starts
after 2 h of isothermal aging at 200 °C; however, it takes long time before this decomposition is complete [15]. Since the holding time (1 h) at the extrusion temperature (250 °C) before extrusion was relatively short, only some of the eutectic constituents could undergo eutectoid decomposition. In summary, the microstructure of a DC-cast Mg– 6%Zn–0.5%Zr alloy consists of Zr-rich cores associated with the Zr-rich particles, Zn-rich rings and Mg7Zn3 eutectic constituents at the grain boundaries, and Zn–Zr precipitates which are formed during cooling of the saturated solid solution. Upon extrusion, the Zr-rich cores are elongated and either recrystallize to form pockets of very fine grains (2–5 lm) compared to the other recrystallized grains (10–20 lm), or remain unrecrystallized and give rise to the so-called Zr-rich ‘‘soft stringers”. More twinning contribution and activation of the continuous DRX mode at the higher extrusion ratio improves the recrystallization behavior of these cores. During heating and holding at the extrusion temperature, diffusion of Zn from the Zn-rich rings takes place and some of the primary eutectic constituents decompose into a more Zn-bearing compound. One of the authors (M.S.) gratefully acknowledges an award of scholarship by the Higher Education Commission of Pakistan. Authors would also like to acknowledge the help of Mr. Klaus Hermann, IELF, TU Clausthal, for performing EPMA. [1] I.J. Polmear, Light Alloys, third ed., Arnold, London, 1995. [2] D.H. St. John, M. Qian, M.A. Easton, P. Cao, Z. Hildebrand, Metall. Mater. Trans. A 36A (2005) 1669– 1679. [3] C.H. Caceres, A. Blake, Phys. Stat. Sol. 194 (2002) 147– 158. [4] E.F. Emley, Principles of Magnesium Technology, Pergamon Press, Oxford, 1966. [5] K. Kubota, M. Mabuchi, K. Higashi, J. Mater. Sci. 34 (1999) 2225–2262. [6] P.K. Saha, Aluminum Extrusion Technology, ASM, Materials Park, OH, 2000. [7] M. Shahzad, L. Wagner, Mater. Sci. Eng. A (2008), doi:10.1016/j.msea.2008.11.038. [8] Z.P. Luo, D.Y. Song, S.Q. Zhang, J. Alloys Compd. 230 (1995) 109–114. [9] H. Okamoto, J. Phase Equil. 23 (2002) 198–199. [10] M. Qian, D.H. St. John, M.T. Frost, in: K.U. Kainer (Ed.), Mangesium Alloys and Their Applications, WileyVCH, Wolfsburg, 2003, pp. 706–712. [11] M. Qian, D.H. St. John, M.T. Frost, Scripta Mater. 46 (2002) 649–654. [12] Z. Hildebrand, M. Qian, D.H. St. John, M.T. Frost, in: A.A. Luo (Ed.), Magnesium Technology 2004, TMS, Warrendale, PA, 2004, pp. 241–245. [13] H.J. McQueen, M.M. Myshlyaev, A. Mwembela, Can. Metall. Quart. 42 (2003) 97–112. [14] Y. Zhang, X. Zeng, C. Lu, W. Ding, Mater. Sci. Eng. A. 428 (2006) 91–97. [15] X. Gao, J.F. Nie, Scripta Mater. 57 (2007) 655–658. [16] Z. Liu, X. Liu, H. Guo, B, Liu, P. Shen, International Materials Research Conference 2008, 9–12 June 2008, Chongqing, China.