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Effect of magnesium carbonate on microstructure and rolling behaviors of AZ31 alloy
Materials Science and Engineering A 528 (2011) 1485–1490
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Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Effect of magnesium carbonate on microstructure and rolling behaviors of AZ31 alloy Ling Wang a,b , Young Min Kim b,∗ , JeHyun Lee a , Bong Sun You b a b
School of Nano and Advanced Materials Engineering, Changwon National University, 9 Sarimdong, Changwon 641773, Republic of Korea Light Metals Research Group, Korea Institute of Materials Science, 66 Sangnam dong, Changwon 641831, Republic of Korea
a r t i c l e
i n f o
Article history: Received 16 September 2010 Received in revised form 18 October 2010 Accepted 18 October 2010
Keywords: Magnesium alloy Magnesium carbonate Rolling behavior Twinning Dynamic recrystallization
a b s t r a c t The present study aims at investigating grain refinement and rolling behavior of a magnesium carbonate (MgCO3 ) added AZ31 alloy. MgO particles produced by decomposition of MgCO3 in an AZ31 melt act as effective heterogeneous nuclei, and decrease the grain size of the ingot from 310 to 117 m. Grain refinement alters the hot deformation mechanism of AZ31 alloy from TDRX-dominance to CDRX-dominance. The twinned regions provide easy crack propagation routes and thus deteriorate workability. As a result, workability in hot rolling especially up to the intermediate stage is improved. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The use of magnesium alloys for engineering components in the automotive industry has increased significantly. Magnesium alloys are promising structural light materials due to their high specific strength and stiffness, although they exhibit poor ductility and workability. To date, therefore, a considerable body of research has been dedicated to improving the ductility and workability of magnesium alloys in order to broaden their applicability [1–4]. Polycrystalline materials with fine grains are believed to exhibit higher strength and ductility than those with coarse grains, because they have a larger total grain boundary area, which impedes dislocation motion and accommodates deformation. As clearly given by the Hall–Petch relation, y = 0 + ky d−1/2 , many materials with fine grains have high yield stress. In addition, improvement in formability as well as ductile–brittle transition temperature (DBTT) can be obtained through grain refinement of magnesium alloys. Chang et al. [5] demonstrated that the DBTT of a Mg alloy is 250 ◦ C when the grain size is 60 m, but it drops to room temperature after its grain size is refined to 2 m. During the past decade, a number of studies on grain refining methods for as-cast Mg–Al alloys have been carried out, placing particular focus on carbonbearing materials such as carbon, carbides, carbonates [6–9], master alloys such as Al–Ti–C and Mg–10Sr [10–12], and melt agi-
∗ Corresponding author. Tel.: +82 55 280 3537; fax: +82 55 280 3599. E-mail address: [email protected] (Y.M. Kim). 0921-5093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2010.10.053
tation processes such as magnetic stirring and ultrasonic vibration [13,14]. It is known that magnesium alloys undergo dynamic recrystallization (DRX) phenomena during hot working processes such as rolling and extrusion [15–20]. Many investigations on the effects of temperature and strain rate on the microstructural evolution including recrystallization behavior in magnesium alloys have been carried out. However, there have been few studies on the effects of as-cast microstructure with different initial grain sizes on the rolling behavior of magnesium alloys. Instead, fine grained as-extruded samples have generally been considered in most previous studies. The present research investigates the effects of MgCO3 on grain refinement of as-cast microstructure and subsequent hot rolling behaviors of AZ31 alloy. In addition, the recrystallization behaviors of AZ31 alloys with different initial grain sizes are examined. 2. Experimental procedures Commercial AZ31 alloys were melted in an electrical furnace using a mild steel crucible under a protecting gas (10% SF6 and 90% CO2 ). One weight percent of MgCO3 powder was enclosed in pure aluminum foil and plunged into the melt at 740 ◦ C. The melts were held for 30 min at 740 ◦ C, and then poured into a mild steel mold that has a size of 125 mm × 80 mm × 25 mm (l × w × t) and was coated with BN and preheated to 200 ◦ C. AZ31 and 1 wt.% MgCO3 -added AZ31 alloys were homogenized at 380 ◦ C for 8 h and then machined into plates with 15 mm thickness. Subsequently, the
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Table 1 Inter-atomic spacing misfit (%) along possible matching directions and d-value mismatch (%) between possible matching planes between Mg and MgO. Inter atomic spacing of matching directions 1 1 2¯ 0Mg /0 1 1MgO −7.2%
1 1 2¯ 0Mg /1 1 2MgO 60.6%
d-Value of matching planes 1 1 2¯ 0Mg /0 0 1MgO 31.2%
as-homogenized plates were rolled to sheets with 1 mm thickness by a total of five rolling passes at 400 ◦ C. The reduction rates (reduction per one pass) were 30%, 30%, 50%, 50%, and 50%, respectively, and total reduction was about 93%. The samples for rolling were heated at 400 ◦ C for 10 min prior to each pass of rolling. Twin rolls with a diameter of 210 mm were maintained at 200 ◦ C during hot rolling and the rolling speed was set at 5.7 mpm. The samples were heated to 400 ◦ C and held for 10 min before the first rolling pass, and afterwards the samples were reheated at 400 ◦ C and held for 5 min before subsequent rolling passes. Specimens for optical microstructure observation after casting and hot rolling were prepared by mechanical polishing with a polycrystalline diamond suspension glycol based solution. The microstructure was revealed by etching with a solution of picric acid (5 g), acetic acid (5 ml), distilled water (10 ml), and ethanol (100 ml). All optical microstructures of the rolled sheets were observed on the plane normal to the transverse direction.
{1 0 1¯ 0}Mg /{0 1 1}MgO 7.1%
{0 0 0 2}Mg /{1 1 1}MgO −6.7%
{1 0 1¯ 1}Mg /{1 1 1}MgO 0.89%
3. Results and discussion 3.1. Grain refinement of as-cast microstructure by MgCO3 addition The microstructures of as-cast AZ31 and 1 wt.% MgCO3 -added AZ31 alloys are shown in Fig. 1. After adding 1 wt.% MgCO3 to the melt, the average grain size of the as-cast AZ31 alloys remarkably decreased from 310 to 117 m, and fine and homogeneously distributed grains were obtained (Fig. 1). When MgCO3 powders were added into the AZ31 melt, they were decomposed by the following reaction: 94.7Mg + 3Al + 1Zn + 0.3Mn + MgCO3 → equilibrium phases (at 740 ◦ C)
(1)
According to thermodynamic calculations made using FactSage software with FTlite and Fact53 databases, the equilibrium phases consist of liquid (98.00 mass%), MgO (1.43 mass%) and Al4 C3
Fig. 1. Optical micrographs showing as-cast microstructures of (a) AZ31 and (b) 1 wt.% MgCO3 added AZ31 alloys. The initial grain sizes are 370 m (a) and 117 m (b), respectively.
Fig. 2. Outward appearances of the sheets hot-rolled with the total reduction of approximately 93%.
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Fig. 3. Optical microstructures of the as-rolled AZ31 alloy sheets after (a) the first pass, (b) the second pass, (c) the third pass, and (d) the fifth pass.
(0.57 mass%) at 740 ◦ C. It should be noted that the above results were calculated under a closed system, where excess Mg completely consumes CO2 gas released by MgCO3 decomposition. In a real process, however, a partially open system should be considered, where the kinetics of MgCO3 decomposition may be fast and the CO2 rising to the surface of the melt would not have enough time to react with the Mg melt. Therefore, the actually produced amounts of MgO and Al4 C3 in reaction (1) must be lower than the calculated values. Nevertheless, according to the thermodynamic calculation, the amount of MgO particles that can function as nuclei is more than two times larger than that of Al4 C3 , and thus MgO is expected to serve more of grain refinement in AZ31 alloy. MgO (FCC, a = 0.4200 nm) is stable in the AZ31 melt due to its high chemical stability. According to the edge-to-edge matching crystallographic model [21], the basic matching units, i.e. matching directions, have to be close-packed or nearly close-packed so as to maximize the atomic match across the interface. In addition, the d-value mismatch between the matching planes including the matching directions should be less than 6% [21]. Inter-atomic spacing misfit (%) along possible matching directions and d-value mismatch (%) between possible matching planes between Mg and MgO are listed in Table 1. The possible matching directions and matching planes are predicted as follows: 1 1 2¯ 0Mg /0 1 1MgO , {0 0 0 2}Mg /{1 1 1}MgO and 1 1 2¯ 0Mg /0 1 1MgO , {1 0 1¯ 1}Mg /{1 1 1}MgO . MgO can act as a heterogeneous nucleation site based on the theoretic calculations. In addition, as a product of reaction (1), Al4 C3 can be formed by the reaction of carbon, a product of CO2 released by MgCO3 decomposition, and aluminum solute atoms dissolved in the magnesium melt. According to the literature [21–23], Al4 C3 phase is well known as a good grain refiner due to its thermodynamic stability at melting temperature and similarity with the Mg matrix. Although unfortunately Al4 C3 could not be observed in this study likely due to the
reaction of Al4 C3 with water or humidity [22], it can be confirmed that both MgO and Al4 C3 can be the heterogeneous nucleation sites from crystallographic point of view. 3.2. Effect of initial grain size on rolling behavior The effective strain rate that the rolled sheets experienced can be calculated as follows:
¯˙ = −1102.66
[ln(1 − e) · V ] , [R · cos−1 (1 − eh0 /R)]
(2)
where e, ho , V and R denote the reduction rate (reduction per pass), the thickness of the sample before rolling, the roll speed (mpm), and the diameter of the roll, respectively. Regarding the actual rolling process conditions (ho : 15 mm, R: 210 mm, V: 5.74 m/min), the effective strain rate is 1.8/s and 2.7/s at reduction rates of 30% and 50%/pass, respectively. Such strain rates are considerably faster than those of a typical hot forming process for magnesium alloys, although they are below the hot rolling strain rates applied in industrial processes. Fig. 2(a) shows that the AZ31 alloy sheet rolled at a reduction rate of 30%/pass has numerous side cracks. By MgCO3 addition, however, the AZ31 alloy sheet shows remarkably improved surface quality and no side cracks (upper area of Fig. 2(b)). Although some side cracks can be seen in the lower area of Fig. 2(b), they were caused from cast defects in the upper part of the cast sample. This significant increase in rollability is mainly attributable to the large difference in initial grain size between the as-cast AZ31 alloy and the as-cast MgCO3 -added AZ31 alloy. Fig. 3 shows the microstructure changes of AZ31 alloy samples subjected to hot rolling at 400 ◦ C. In Fig. 3(a), twinning extensively occurred by the first rolling pass. The twins were considered to be {1 0 −1 2} twins, which are the most easily activated twins in magnesium [24–28]. Occasionally, dynamic recrystallized (DRXed)
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Fig. 4. Optical microstructures of the as-rolled MgCO3 -added AZ31 alloy sheets after (a) the first pass, (b) the second pass, (c) the third pass, (d) the fifth pass.
grains could be observed at the twinned regions (Fig. 3(a)). According to Al-Samman et al [25], twin domains possess much higher stored deformation energy compared to the matrix and are therefore expected to be favorable nucleation sites for DRX. After the second rolling pass (Fig. 3(b)), a greater number of DRXed grains continuously nucleated in the twinned regions. As the accumulated reduction increased, grains were further refined and new DRXed grains embedded into the remaining coarse grains. According to Liu et al. [29], twin-aided dynamic recrystallization (TDRX) normally occurs at low temperature and the development of TDRXed grains is shown to include three stages: (i) nucleation of recrystallization centers; (ii) transformation of special grain boundaries into random grain boundaries (of the general type); and (iii) subsequent growth of new grains. In Fig. 3(c), grain refinement by the TDRX mechanism can apparently be characterized as both nucleation of very small DRXed grains on twinned regions and subdivision of original grains by mutual intersection of primary twins [27]. On the other hand, Fig. 4 shows that there is no evident twins within the grains of the 1 wt.% MgCO3 -added AZ31 alloy samples subjected to hot rolling at 400 ◦ C. With the limited fraction of twins, as indicated in Fig. 4, compared to the AZ31 alloy, small DRXed grains less than 10 m in size were formed along the boundaries of slightly elongated original grains at the initial stage of rolling (Fig. 4(a) and (b)). The fraction of newly formed grains increases by increasing the accumulated reduction, as shown in Fig. 4(b) and (c), where most recrystallized sites were composed of a necklace structure that is strongly dependent on the crystallographic orientation of the grains. This necklace structure indicates that the rolling behavior of 1 wt.% MgCO3 -added AZ31 alloy with fine and equiaxed grains is related to a continuous dynamic recrystallization (CDRX) mechanism [25,27,30]. From a comparison of Figs. 3 and 4, the variation in the DRX mechanism between the normal AZ31 alloy and the MgCO3 -added AZ31 alloy originated from the difference in their grain size. According to Barnett et al.’s studies [24,31], the two deformation mechanisms of deformation twinning and dislocation glide exhibit different grain size dependencies in magnesium alloys. They also pointed out that the grain size has a greater impact on twinning
than it does on slip, and the number of twins found per grain increases markedly with grain size. In the present study, it was found that the number density of deformation twins in grain interiors is much higher for the coarse-grained AZ31 alloy than the
Fig. 5. The cross-sectional micrograph of the cracked region in the rolled AZ31 alloy showing a crack propagation path. The sample was taken at the side area of the plate after the first pass rolling.
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microstructure consists of fine DRXed grains with less than 3 m and a small fraction of remaining original grains with less than 10 m. In addition, a comparison of Figs. 3(d) and 4(d) indicates that, despite the large difference between the initial grain sizes of AZ31 and 1 wt.% MgCO3 -added AZ31 alloys, the grain sizes of the sheets after the final pass of hot rolling are similar regardless of MgCO3 addition. Based on the above results, the reduction of initial grain size by the addition of MgCO3 addition led to improvement in the rollability of AZ31 alloy. This enhancement is attributed to the DRX mechanism being changed from a TDRX-dominant to a CDRXdominant mechanism, especially at the initial stages of hot rolling. However, it did not affect the microstructure of the final product sheets. Consequently, it is anticipated that the mechanical properties of AZ31 alloy sheets would not be significantly improved by MgCO3 addition. 4. Conclusions The initial grain size of as-cast AZ31 alloy decreased from 310 to 117 m by the addition of 1 wt.% MgCO3 . According to thermodynamic calculations, Al4 C3 and MgO particles produced by decomposition of MgCO3 in the AZ31 melt act as effective heterogeneous nuclei for Mg grains. The AZ31 and 1 wt.% MgCO3 -added AZ31 alloys exhibited different deformation behaviors due to the difference of their initial grain size. It is thought that the reduction of the initial grain size by the addition of MgCO3 brought about improvement in rollability of the AZ31 alloy by changing the DRX mechanism from a TDRX-dominant to a CDRX-dominant mechanism at the initial stages of rolling. From an analysis of the fractured areas, it was confirmed that twinned regions formed in the AZ31 alloy with coarse initial grains, providing facile crack propagation routes. The results of the present study will be practically useful for effective production of thin magnesium sheets, leading to a reduction of production cost. Acknowledgement This study was financially supported by the R&D Program of Korea Institute of Materials Science, Republic of Korea. Fig. 6. The schematic illustration of grain refinement for the MgCO3 -added AZ31 alloy during hot rolling.
fine-grained AZ31 alloy. Therefore, the difference in rollability of the AZ31 and MgCO3 -added AZ31 alloys, as presented in Fig. 2, originated from the difference in the initial grain size and from the resultant presence of a large number density of deformation twins formed by a different DRX mechanism, in particular at the initial stage of rolling. This was confirmed by the experimental finding of conspicuous side cracks in the rolled AZ31 alloy, which began to form after the second rolling pass. Fig. 5 presents a crosssectional micrograph showing the cracked region beneath the side surface of a rolled AZ31 sheet after the first rolling pass. It is clearly seen that the crack easily propagated along twinned regions and TDRXed grains, and as a result more side cracks were observed in the TDRX-dominant AZ31 alloy, as shown in Fig. 2. Fig. 6 shows a schematic illustration of grain refinement of MgCO3 -added AZ31 alloy during hot rolling. At the initial stage of rolling, small DRXed grains are nucleated near or along the grain boundaries, and at the same time twinning occurs, followed by recrystallization (inside grains in Fig. 4). With increasing a total amount of reduction, the number of newly formed DRXed grains including those formed on twinned regions increases. These grains cluster and thereby consume most of the original grains. The final
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[28] M.H. Yoo, Met. Metall. Trans. 12A (1981) 409–418. [29] C. Liu, Z. Liu, X. Zhu, H. Zhou, Chin. J. Nonferr. Met. 16 (2006) 1–12. [30] C. Zhenhua, Wrought Mg Alloys, Chemical Industries Press, Beijing, China, 2005, p. 9. [31] M.R. Barnett, Scripta Mater. 59 (2008) 696–698.
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Effect of magnesium carbonate on microstructure and rolling behaviors of AZ31 alloy Ling Wang a,b , Young Min Kim b,∗ , JeHyun Lee a , Bong Sun You b a b
School of Nano and Advanced Materials Engineering, Changwon National University, 9 Sarimdong, Changwon 641773, Republic of Korea Light Metals Research Group, Korea Institute of Materials Science, 66 Sangnam dong, Changwon 641831, Republic of Korea
a r t i c l e
i n f o
Article history: Received 16 September 2010 Received in revised form 18 October 2010 Accepted 18 October 2010
Keywords: Magnesium alloy Magnesium carbonate Rolling behavior Twinning Dynamic recrystallization
a b s t r a c t The present study aims at investigating grain refinement and rolling behavior of a magnesium carbonate (MgCO3 ) added AZ31 alloy. MgO particles produced by decomposition of MgCO3 in an AZ31 melt act as effective heterogeneous nuclei, and decrease the grain size of the ingot from 310 to 117 m. Grain refinement alters the hot deformation mechanism of AZ31 alloy from TDRX-dominance to CDRX-dominance. The twinned regions provide easy crack propagation routes and thus deteriorate workability. As a result, workability in hot rolling especially up to the intermediate stage is improved. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The use of magnesium alloys for engineering components in the automotive industry has increased significantly. Magnesium alloys are promising structural light materials due to their high specific strength and stiffness, although they exhibit poor ductility and workability. To date, therefore, a considerable body of research has been dedicated to improving the ductility and workability of magnesium alloys in order to broaden their applicability [1–4]. Polycrystalline materials with fine grains are believed to exhibit higher strength and ductility than those with coarse grains, because they have a larger total grain boundary area, which impedes dislocation motion and accommodates deformation. As clearly given by the Hall–Petch relation, y = 0 + ky d−1/2 , many materials with fine grains have high yield stress. In addition, improvement in formability as well as ductile–brittle transition temperature (DBTT) can be obtained through grain refinement of magnesium alloys. Chang et al. [5] demonstrated that the DBTT of a Mg alloy is 250 ◦ C when the grain size is 60 m, but it drops to room temperature after its grain size is refined to 2 m. During the past decade, a number of studies on grain refining methods for as-cast Mg–Al alloys have been carried out, placing particular focus on carbonbearing materials such as carbon, carbides, carbonates [6–9], master alloys such as Al–Ti–C and Mg–10Sr [10–12], and melt agi-
∗ Corresponding author. Tel.: +82 55 280 3537; fax: +82 55 280 3599. E-mail address: [email protected] (Y.M. Kim). 0921-5093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2010.10.053
tation processes such as magnetic stirring and ultrasonic vibration [13,14]. It is known that magnesium alloys undergo dynamic recrystallization (DRX) phenomena during hot working processes such as rolling and extrusion [15–20]. Many investigations on the effects of temperature and strain rate on the microstructural evolution including recrystallization behavior in magnesium alloys have been carried out. However, there have been few studies on the effects of as-cast microstructure with different initial grain sizes on the rolling behavior of magnesium alloys. Instead, fine grained as-extruded samples have generally been considered in most previous studies. The present research investigates the effects of MgCO3 on grain refinement of as-cast microstructure and subsequent hot rolling behaviors of AZ31 alloy. In addition, the recrystallization behaviors of AZ31 alloys with different initial grain sizes are examined. 2. Experimental procedures Commercial AZ31 alloys were melted in an electrical furnace using a mild steel crucible under a protecting gas (10% SF6 and 90% CO2 ). One weight percent of MgCO3 powder was enclosed in pure aluminum foil and plunged into the melt at 740 ◦ C. The melts were held for 30 min at 740 ◦ C, and then poured into a mild steel mold that has a size of 125 mm × 80 mm × 25 mm (l × w × t) and was coated with BN and preheated to 200 ◦ C. AZ31 and 1 wt.% MgCO3 -added AZ31 alloys were homogenized at 380 ◦ C for 8 h and then machined into plates with 15 mm thickness. Subsequently, the
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Table 1 Inter-atomic spacing misfit (%) along possible matching directions and d-value mismatch (%) between possible matching planes between Mg and MgO. Inter atomic spacing of matching directions 1 1 2¯ 0Mg /0 1 1MgO −7.2%
1 1 2¯ 0Mg /1 1 2MgO 60.6%
d-Value of matching planes 1 1 2¯ 0Mg /0 0 1MgO 31.2%
as-homogenized plates were rolled to sheets with 1 mm thickness by a total of five rolling passes at 400 ◦ C. The reduction rates (reduction per one pass) were 30%, 30%, 50%, 50%, and 50%, respectively, and total reduction was about 93%. The samples for rolling were heated at 400 ◦ C for 10 min prior to each pass of rolling. Twin rolls with a diameter of 210 mm were maintained at 200 ◦ C during hot rolling and the rolling speed was set at 5.7 mpm. The samples were heated to 400 ◦ C and held for 10 min before the first rolling pass, and afterwards the samples were reheated at 400 ◦ C and held for 5 min before subsequent rolling passes. Specimens for optical microstructure observation after casting and hot rolling were prepared by mechanical polishing with a polycrystalline diamond suspension glycol based solution. The microstructure was revealed by etching with a solution of picric acid (5 g), acetic acid (5 ml), distilled water (10 ml), and ethanol (100 ml). All optical microstructures of the rolled sheets were observed on the plane normal to the transverse direction.
{1 0 1¯ 0}Mg /{0 1 1}MgO 7.1%
{0 0 0 2}Mg /{1 1 1}MgO −6.7%
{1 0 1¯ 1}Mg /{1 1 1}MgO 0.89%
3. Results and discussion 3.1. Grain refinement of as-cast microstructure by MgCO3 addition The microstructures of as-cast AZ31 and 1 wt.% MgCO3 -added AZ31 alloys are shown in Fig. 1. After adding 1 wt.% MgCO3 to the melt, the average grain size of the as-cast AZ31 alloys remarkably decreased from 310 to 117 m, and fine and homogeneously distributed grains were obtained (Fig. 1). When MgCO3 powders were added into the AZ31 melt, they were decomposed by the following reaction: 94.7Mg + 3Al + 1Zn + 0.3Mn + MgCO3 → equilibrium phases (at 740 ◦ C)
(1)
According to thermodynamic calculations made using FactSage software with FTlite and Fact53 databases, the equilibrium phases consist of liquid (98.00 mass%), MgO (1.43 mass%) and Al4 C3
Fig. 1. Optical micrographs showing as-cast microstructures of (a) AZ31 and (b) 1 wt.% MgCO3 added AZ31 alloys. The initial grain sizes are 370 m (a) and 117 m (b), respectively.
Fig. 2. Outward appearances of the sheets hot-rolled with the total reduction of approximately 93%.
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Fig. 3. Optical microstructures of the as-rolled AZ31 alloy sheets after (a) the first pass, (b) the second pass, (c) the third pass, and (d) the fifth pass.
(0.57 mass%) at 740 ◦ C. It should be noted that the above results were calculated under a closed system, where excess Mg completely consumes CO2 gas released by MgCO3 decomposition. In a real process, however, a partially open system should be considered, where the kinetics of MgCO3 decomposition may be fast and the CO2 rising to the surface of the melt would not have enough time to react with the Mg melt. Therefore, the actually produced amounts of MgO and Al4 C3 in reaction (1) must be lower than the calculated values. Nevertheless, according to the thermodynamic calculation, the amount of MgO particles that can function as nuclei is more than two times larger than that of Al4 C3 , and thus MgO is expected to serve more of grain refinement in AZ31 alloy. MgO (FCC, a = 0.4200 nm) is stable in the AZ31 melt due to its high chemical stability. According to the edge-to-edge matching crystallographic model [21], the basic matching units, i.e. matching directions, have to be close-packed or nearly close-packed so as to maximize the atomic match across the interface. In addition, the d-value mismatch between the matching planes including the matching directions should be less than 6% [21]. Inter-atomic spacing misfit (%) along possible matching directions and d-value mismatch (%) between possible matching planes between Mg and MgO are listed in Table 1. The possible matching directions and matching planes are predicted as follows: 1 1 2¯ 0Mg /0 1 1MgO , {0 0 0 2}Mg /{1 1 1}MgO and 1 1 2¯ 0Mg /0 1 1MgO , {1 0 1¯ 1}Mg /{1 1 1}MgO . MgO can act as a heterogeneous nucleation site based on the theoretic calculations. In addition, as a product of reaction (1), Al4 C3 can be formed by the reaction of carbon, a product of CO2 released by MgCO3 decomposition, and aluminum solute atoms dissolved in the magnesium melt. According to the literature [21–23], Al4 C3 phase is well known as a good grain refiner due to its thermodynamic stability at melting temperature and similarity with the Mg matrix. Although unfortunately Al4 C3 could not be observed in this study likely due to the
reaction of Al4 C3 with water or humidity [22], it can be confirmed that both MgO and Al4 C3 can be the heterogeneous nucleation sites from crystallographic point of view. 3.2. Effect of initial grain size on rolling behavior The effective strain rate that the rolled sheets experienced can be calculated as follows:
¯˙ = −1102.66
[ln(1 − e) · V ] , [R · cos−1 (1 − eh0 /R)]
(2)
where e, ho , V and R denote the reduction rate (reduction per pass), the thickness of the sample before rolling, the roll speed (mpm), and the diameter of the roll, respectively. Regarding the actual rolling process conditions (ho : 15 mm, R: 210 mm, V: 5.74 m/min), the effective strain rate is 1.8/s and 2.7/s at reduction rates of 30% and 50%/pass, respectively. Such strain rates are considerably faster than those of a typical hot forming process for magnesium alloys, although they are below the hot rolling strain rates applied in industrial processes. Fig. 2(a) shows that the AZ31 alloy sheet rolled at a reduction rate of 30%/pass has numerous side cracks. By MgCO3 addition, however, the AZ31 alloy sheet shows remarkably improved surface quality and no side cracks (upper area of Fig. 2(b)). Although some side cracks can be seen in the lower area of Fig. 2(b), they were caused from cast defects in the upper part of the cast sample. This significant increase in rollability is mainly attributable to the large difference in initial grain size between the as-cast AZ31 alloy and the as-cast MgCO3 -added AZ31 alloy. Fig. 3 shows the microstructure changes of AZ31 alloy samples subjected to hot rolling at 400 ◦ C. In Fig. 3(a), twinning extensively occurred by the first rolling pass. The twins were considered to be {1 0 −1 2} twins, which are the most easily activated twins in magnesium [24–28]. Occasionally, dynamic recrystallized (DRXed)
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Fig. 4. Optical microstructures of the as-rolled MgCO3 -added AZ31 alloy sheets after (a) the first pass, (b) the second pass, (c) the third pass, (d) the fifth pass.
grains could be observed at the twinned regions (Fig. 3(a)). According to Al-Samman et al [25], twin domains possess much higher stored deformation energy compared to the matrix and are therefore expected to be favorable nucleation sites for DRX. After the second rolling pass (Fig. 3(b)), a greater number of DRXed grains continuously nucleated in the twinned regions. As the accumulated reduction increased, grains were further refined and new DRXed grains embedded into the remaining coarse grains. According to Liu et al. [29], twin-aided dynamic recrystallization (TDRX) normally occurs at low temperature and the development of TDRXed grains is shown to include three stages: (i) nucleation of recrystallization centers; (ii) transformation of special grain boundaries into random grain boundaries (of the general type); and (iii) subsequent growth of new grains. In Fig. 3(c), grain refinement by the TDRX mechanism can apparently be characterized as both nucleation of very small DRXed grains on twinned regions and subdivision of original grains by mutual intersection of primary twins [27]. On the other hand, Fig. 4 shows that there is no evident twins within the grains of the 1 wt.% MgCO3 -added AZ31 alloy samples subjected to hot rolling at 400 ◦ C. With the limited fraction of twins, as indicated in Fig. 4, compared to the AZ31 alloy, small DRXed grains less than 10 m in size were formed along the boundaries of slightly elongated original grains at the initial stage of rolling (Fig. 4(a) and (b)). The fraction of newly formed grains increases by increasing the accumulated reduction, as shown in Fig. 4(b) and (c), where most recrystallized sites were composed of a necklace structure that is strongly dependent on the crystallographic orientation of the grains. This necklace structure indicates that the rolling behavior of 1 wt.% MgCO3 -added AZ31 alloy with fine and equiaxed grains is related to a continuous dynamic recrystallization (CDRX) mechanism [25,27,30]. From a comparison of Figs. 3 and 4, the variation in the DRX mechanism between the normal AZ31 alloy and the MgCO3 -added AZ31 alloy originated from the difference in their grain size. According to Barnett et al.’s studies [24,31], the two deformation mechanisms of deformation twinning and dislocation glide exhibit different grain size dependencies in magnesium alloys. They also pointed out that the grain size has a greater impact on twinning
than it does on slip, and the number of twins found per grain increases markedly with grain size. In the present study, it was found that the number density of deformation twins in grain interiors is much higher for the coarse-grained AZ31 alloy than the
Fig. 5. The cross-sectional micrograph of the cracked region in the rolled AZ31 alloy showing a crack propagation path. The sample was taken at the side area of the plate after the first pass rolling.
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microstructure consists of fine DRXed grains with less than 3 m and a small fraction of remaining original grains with less than 10 m. In addition, a comparison of Figs. 3(d) and 4(d) indicates that, despite the large difference between the initial grain sizes of AZ31 and 1 wt.% MgCO3 -added AZ31 alloys, the grain sizes of the sheets after the final pass of hot rolling are similar regardless of MgCO3 addition. Based on the above results, the reduction of initial grain size by the addition of MgCO3 addition led to improvement in the rollability of AZ31 alloy. This enhancement is attributed to the DRX mechanism being changed from a TDRX-dominant to a CDRXdominant mechanism, especially at the initial stages of hot rolling. However, it did not affect the microstructure of the final product sheets. Consequently, it is anticipated that the mechanical properties of AZ31 alloy sheets would not be significantly improved by MgCO3 addition. 4. Conclusions The initial grain size of as-cast AZ31 alloy decreased from 310 to 117 m by the addition of 1 wt.% MgCO3 . According to thermodynamic calculations, Al4 C3 and MgO particles produced by decomposition of MgCO3 in the AZ31 melt act as effective heterogeneous nuclei for Mg grains. The AZ31 and 1 wt.% MgCO3 -added AZ31 alloys exhibited different deformation behaviors due to the difference of their initial grain size. It is thought that the reduction of the initial grain size by the addition of MgCO3 brought about improvement in rollability of the AZ31 alloy by changing the DRX mechanism from a TDRX-dominant to a CDRX-dominant mechanism at the initial stages of rolling. From an analysis of the fractured areas, it was confirmed that twinned regions formed in the AZ31 alloy with coarse initial grains, providing facile crack propagation routes. The results of the present study will be practically useful for effective production of thin magnesium sheets, leading to a reduction of production cost. Acknowledgement This study was financially supported by the R&D Program of Korea Institute of Materials Science, Republic of Korea. Fig. 6. The schematic illustration of grain refinement for the MgCO3 -added AZ31 alloy during hot rolling.
fine-grained AZ31 alloy. Therefore, the difference in rollability of the AZ31 and MgCO3 -added AZ31 alloys, as presented in Fig. 2, originated from the difference in the initial grain size and from the resultant presence of a large number density of deformation twins formed by a different DRX mechanism, in particular at the initial stage of rolling. This was confirmed by the experimental finding of conspicuous side cracks in the rolled AZ31 alloy, which began to form after the second rolling pass. Fig. 5 presents a crosssectional micrograph showing the cracked region beneath the side surface of a rolled AZ31 sheet after the first rolling pass. It is clearly seen that the crack easily propagated along twinned regions and TDRXed grains, and as a result more side cracks were observed in the TDRX-dominant AZ31 alloy, as shown in Fig. 2. Fig. 6 shows a schematic illustration of grain refinement of MgCO3 -added AZ31 alloy during hot rolling. At the initial stage of rolling, small DRXed grains are nucleated near or along the grain boundaries, and at the same time twinning occurs, followed by recrystallization (inside grains in Fig. 4). With increasing a total amount of reduction, the number of newly formed DRXed grains including those formed on twinned regions increases. These grains cluster and thereby consume most of the original grains. The final
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