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Modification of Mg2Si morphology in squeeze cast Mg-Al-Zn-Si alloys by Ca or P addition
Scripta Materialia, Vol. 41, No. 3, pp. 333–340, 1999 Elsevier Science Ltd Copyright © 1999 Acta Metallurgica Inc. Printed in the USA. All rights reserved. 1359-6462/99/$–see front matter
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MODIFICATION OF Mg2Si MORPHOLOGY IN SQUEEZE CAST Mg-Al-Zn-Si ALLOYS BY Ca OR P ADDITION Jae Joong Kim, Do Hyang Kim*, K.S. Shin** and Nack J. Kim Center for Advanced Aerospace Materials, Pohang University of Science and Technology (POSTECH), San 31 Hyoja-Dong, Pohang, Kyungbuk 790-784, Korea * Department of Metallurgical Engineering, Yonsei University, Shinchon-dong, Seodaemun-ku, Seoul, Korea ** School of Materials Science and Engineering, Seoul National University, Seoul, Korea (Received November 16, 1998) (Accepted March 24, 1999)
1. Introduction Lightweight alloys have had varying degrees of importance in transportation systems. In aerospace industries, for example, the advances made in lightweight alloys have always provided the key to the improvements in performance. Moreover, recent growing demand for weight reduction of the vehicle has generated a considerable interest for lightweight alloys from the automotive industries. From these respects, Mg alloys, the lightest commercial alloys developed so far, have great potential for high performance aerospace and automotive applications [1– 4]. However, the proper combination of strength, toughness and corrosion resistance has been a major challenge for Mg alloys. Recent studies indicate that addition of alloying elements such as Si and rare earth elements to Mg-Al-Zn alloys can produce substantial improvements in strength, toughness and corrosion resistance [5–7]. Rare earth elements are very expensive and difficult to be alloyed homogeneously. On the other hand, Mg2Si formed by the addition of Si is the very useful intermetallic compound that exhibits high melting point, low density and high elastic modulus [8]. However, the addition of Si to Mg alloys often results in poor mechanical properties due to the undesirable morphology (size, shape and distribution) of Mg2Si particles. The present study is aimed at improving the mechanical properties of Mg2Si containing Mg alloys through the modification of morphology of Mg2Si particles by Ca or P addition.
2. Experimental Procedures Nominal chemical compositions of various Mg-Al-Zn-Si-(X) alloys used in the present study are Mg-5Al-1Zn-0.7Si, Mg-5Al-1Zn-0.7Si-0.2Ca and Mg-5Al-1Zn-0.7Si-0.03P. The ingots were re-melted in an electric resistance furnace followed by squeeze casting in a 110mm ⫻ 50mm size steel mold preheated at 250°C. The melt temperature was 750°C, approximately 100°C higher than the liquidus temperature of Mg. The pressure applied for squeeze casting of the alloys melt was 150MPa. The squeeze cast alloys were solution heat treated at 410°C for 10 hours, water quenched, and then aged at 190°C for 1 hour. Specimens for optical microscopy and scanning electron microscopy (SEM) were etched with a solution of 2 vol. % nitric acid ⫹ ethyl alcohol. Thin foils for transmission electron 333
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Figure 1. Optical micrographs showing the effect of P or Ca addition on the microstructure of squeeze cast alloys; a) Mg-5Al-1Zn-0.7Si alloy, (b) Mg-5Al-1Zn-0.7Si-0.2Ca alloy, and (c) Mg-5Al-1Zn-0.7Si-0.03P alloy.
microscopy (TEM) were prepared by ion milling. Volume fraction of the constituent phases and the grain size were analyzed by using an image analyzer. Round tensile specimens with gauge length of 30-mm and gauge diameter of 6-mm were tested at room temperature with an initial strain rate of 6.67 ⫻ 10⫺4/s. Fractographic observation was conducted on the alloys to clarify the fracture process. 3. Results 3.1 Microstructure Fig. 1a shows the microstructure of squeeze cast Mg-5Al-1Zn-0.7Si alloy. Its microstructure can be described as consisting of Chinese script type Mg2Si particles with interdendritic Mg17Al12 particles in Mg matrix. These phases can be discerned as follows: bright area is Mg matrix, gray area is Mg17Al12 and dark area is Mg2Si. With the addition of Ca or P, however, the morphology of Mg2Si particles changes to polygonal type as shown in Fig. 1b and 1c. Ca containing alloy contains 3 ⬃ 6 m sized polygonal type Mg2Si and Mg17Al12 particles in the interdendritic region. Chinese script type Mg2Si particles are observed only in limited areas. As compared to the base alloy shown in Fig. 1a, considerable microstructural refinement is obtained by the addition of Ca. P has a similar effect as Ca,
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Figure 2. Scanning electron micrographs showing the various constituent phases present the squeeze cast alloys; a) Mg-5Al1Zn-0.7Si-0.2Ca alloy and b) Mg-5Al-1Zn-0.7Si-0.03P alloy.
as can be seen in Fig. 1c. Fig. 2 shows SEM micrographs of Mg-5Al-1Zn-0.7Si-0.2Ca and Mg-5Al1Zn-0.7Si-0.03P alloys. The microstructure of Mg-5Al-1Zn-0.7Si-0.2Ca alloy (Fig. 2a) is similar to that of Mg-5Al-1Zn-0.7Si-0.03P alloy (Fig. 2b), except that the former contains Ca-Si compound besides Mg2Si and Mg17Al12 particles. It is interesting to note that Mg2Si particles often contain small particles inside, which presumably act as nucleation sites for Mg2Si particles. Detailed TEM analysis has been conducted to identify the nature of these small particles. As shown in Fig. 3a, Mg2Si particles present in Mg-5Al-1Zn-0.7Si-0.2Ca alloy consist of several particles. Selected area diffraction pattern of the particle marked ‘␣’ in Fig. 3a shows double diffraction pattern indicating that the particle has 3-fold or 6-fold symmetry in this zone axis (Fig. 3b). Double diffraction occurs when a diffracted beam traveling through a crystal is rediffracted either within the same crystal or when it passes into a second crystal. Analysis of double diffraction pattern shows that the strong diffraction spots coincide with the diffraction spots of Mg2Si [111] zone. However, weak diffraction spots cannot unambiguously be identified since the information is available from only one
Figure 3. Transmission electron micrographs of Mg2Si particles in squeeze cast Mg-5Al-1Zn-0.7Si-0.2Ca alloy; a) bright field, b) diffraction pattern of the area ‘␣’ in a), and c) EDS spectrum from the area ‘␣’ in a).
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Figure 4. Microdiffraction pattern of CaMgSi compound.
zone axis. EDS spectrum obtained from this unidentified particle contains the Ca peak besides the Mg and Si peaks as shown in Fig. 3c. It is quite possible that some of the Mg and Si peaks are inevitably from the matrix and nearby located Mg2Si particles. However, combining the information from diffraction pattern and EDS spectrum shows that the nature of the particle is CaSi2. Orientation relationship between Mg2Si and CaSi2 deduced from the diffraction pattern is [111]Mg2Si//[001]CaSi2 and (⫺101)Mg2Si//(110)CaSi2. Fig. 3 clearly shows that polygonal type Mg2Si particles nucleate from CaSi2 particles, indicating the beneficial effect of Ca on modifying the morphology of Mg2Si particles. Microdiffraction analysis has also been conducted to identify the nature of Ca-Si compound as shown in Fig. 4. It shows that this compound is CaMgSi having orthorhombic structure. Its nature has also been confirmed by quantitative EDS analysis. In fact, it has the same characteristics as the one recently observed in Mg-Si-Ca alloys [9]. As shown in Fig. 2b, Mg2Si particles present in Mg-5Al-1Zn-0.7Si-0.03P alloy also contain small particles inside. Detailed TEM analysis has been conducted to identify the nature of this small particle. TEM micrograph of Mg2Si particle in Mg-5Al-1Zn-0.7Si-0.03P alloy shows that it consists of several subgrains (marked 1–5) with unidentified particle (marked ‘␣’) at the center as shown in Fig. 5a. Selected area diffraction pattern of the particle marked ‘␣’ in Fig. 5a shows a ring pattern indicating that the particle is actually a cluster of fine particles (Fig. 5b). EDS spectrum obtained from the clustered particle contains the P and O peaks besides the Mg and Si peaks as shown in Fig. 5c. As is the case of Ca containing alloy, it is not clear whether Mg and Si are present in the clustered particle since Mg and Si peaks can be obtained from nearby located Mg2Si particle and matrix. However, combination of information from diffraction pattern and EDS spectrum shows that the nature of clustered particle is Mg3(PO4)2, which has monoclinic structure. It has been clearly shown in Fig. 5 that the clustered Mg3(PO4)2 particles act as nucleation site for polygonal type Mg2Si particles.
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Figure 5. Transmission electron micrographs of Mg2Si particles in squeeze cast Mg-5Al-1Zn-0.7Si-0.03P alloy; a) bright field, b) diffraction pattern of the area ‘␣’ in a), and c) EDS spectrum from the area ‘␣’ in a).
3.2 Mechanical Properties and Fracture Behavior The above mentioned beneficial effect of Ca and P additions on modifying the morphology of Mg2Si particles should also be beneficial in improving the mechanical properties. Tensile properties and apparent fracture toughness of the base alloy and Ca and P containing alloys are shown in Table 1. It shows that the base alloy and Ca and P alloys have similar yield strength values. However, Ca and P containing alloys have improved ultimate tensile strength, elongation and apparent fracture toughness over the base alloy. Fracture surface analysis shows that there is a large difference in fracture behavior between the base alloy and the Ca and P containing alloys. Fractograph of the base alloy (Fig. 6a) shows that its fracture surface exhibits large cleavage-type facets, which presumably form along the interface between Mg matrix and Chinese script type Mg2Si particles. Fracture surface of the Ca containing alloy, on the other hand, exhibits fine scale dimples with no cleavage-type facet as shown in Fig. 6b. P containing alloy shows the similar fracture surface as the Ca containing alloy.
TABLE 1 Tensile Properties of Squeeze-Cast Mg-5A1-1Zn-0.7Si-X Alloys
YS (MPa) UTS (MPa) E1. (%) Apparent Fracture Toughness Kc (MPa 公m)
5A1-1Zn-0.7Si
5A1-1Zn-0.7Si-0.2Ca
5A1-1Zn-0.7Si-0.03P
116 194 5.6 18.4
116 213 7.3 21.9
112 208 6.9 22.1
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Figure 6. SEM fractographs; a) Mg-5Al-1Zn-0.7Si alloy and (b) Mg-5Al-1Zn-0.7Si-0.2Ca alloy.
4. Discussion The present study shows that the addition of Si to Mg-5Al-1Zn alloy results in the formation of Chinese script type Mg2Si particles. It has been shown in the previous studies [10 –12] that there are two types of Mg2Si particles, Chinese script type and polygonal type, forming in Mg-Al-Zn-Si alloy systems depending on the Al content. The Chinese script type Mg2Si particle forms as a result of a eutectic solidification with ␣-Mg. Therefore, the network of the Chinese script type eutectic Mg2Si particles observed in low Al containing Mg-Al-Zn-Si alloy results from the hypo-eutectic solidification. On the other hand, the polygonal type Mg2Si particle observed in high Al containing Mg-Al-Zn-Si alloy forms
Figure 7. Modified Mg-Si phase diagram.
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Figure 8. Optical micrographs of the squeeze cast alloys after solution treatment at 410°C for 10 hours; a) Mg-5Al-1Zn-0.7Si alloy, b) Mg-5Al-1Zn-0.7Si-0.2Ca alloy, and c) Mg-5Al-1Zn-0.7Si-0.03P alloy.
as a primary phase during solidification. This phenomenon, the change of Mg2Si shape due to Al content, can be explained experimentally by the change in the eutectic temperature with Al content of Mg-Al-Zn-Si alloys. From DTA analysis of various Mg alloys [13], modified Mg-Si phase diagram has been redrawn qualitatively as shown in Fig. 7. It shows that, with increasing Al content, the eutectic temperature decreases and the eutectic composition of the alloy shift to the left (direction to Mg in Fig. 7). These discussions suggest that the high Al containing Mg-Al-Zn-Si alloys such as Mg-9Al-1Zn0.7Si alloy can have the primary Mg2Si particles despite the hypo-eutectic composition in the Mg-Si system. As shown in Fig. 6a, Chinese script Mg2Si particles give a detrimental effect on mechanical properties of Mg-5Al-1Zn-0.7Si alloy since long cracks can easily nucleate along the interface between Chinese script Mg2Si particles and Mg matrix. Hence modification of morphology of Mg2Si particles from Chinese script type to polygonal type is needed to improve the mechanical properties of Mg-Al-Zn-Si alloys. However, the above-mentioned method, i.e., increasing the Al content, is not suitable since it increases the amount of Mg17Al12 and coarse polygonal type Mg2Si particles, which have a detrimental effect on mechanical properties. Therefore, to obtain the best combination of mechanical properties in Mg-Al-Zn-Si alloys, the morphology of Mg2Si particles should be modified by the other method, such as alloying addition of Ca or P. The present study shows that the addition
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of Ca or P modifies the morphology of Mg2Si particles from Chinese script type to refined polygonal type. Another beneficial effect of Ca or P addition on the microstructure of Mg-Al-Zn-Si alloys is the refinement of grain size. As shown in Fig. 8, the grain sizes of solution treated Ca or P containing alloys are about 27.5 m, while that of base alloy is about 52.2 m. This is undoubtedly due to the presence of finely distributed polygonal type Mg2Si particles in the former. 5. Summary The present study is aimed at improving the mechanical properties of squeeze cast Mg-Al-Zn-Si alloys by the modification of Mg2Si particles through alloying addition of Ca or P. It shows that the microstructure of squeeze cast Mg-5Al-1Zn-0.7Si alloy contains Chinese script Mg2Si particles. Chinese script type Mg2Si particles act as crack nucleation sites, resulting in poor mechanical properties in squeeze cast Mg-5Al-1Zn-0.7Si alloy. Addition of Ca or P promotes the formation of fine polygonal type Mg2Si particles by providing the nucleation sites for polygonal type Mg2Si particles. Moreover, the grain sizes of Ca or P modified Mg-Al-Zn-Si alloys are much finer than that of base alloy. Such improved microstructure of the modified alloys results in the large improvement in tensile properties and toughness as compared to the base alloy. Acknowledgment The authors appreciate the financial support of the Hyundai Motors Company and the Korea Ministry of Science and Technology. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
A. Luo, J. Renaud, I. Nakatsugawa, and J. Plourde, JOM. July, 28 (1995). D. M. Magers, in Light Materials for Transportation Systems, ed. N. J. Kim, p. 539, POSTECH, Pohang, Korea (1993). R. S. Busk, Magnesium Products Design, p. 149, Marcel Dekker Inc., New York (1987). H. Westengen, in Science and Engineering of Light Metals, ed. K. Hirano, H. Oikawa and K. Ikeda, p. 77, Japan Institute of Light Metals (1991). I. J. Polmear, Mater. Sci. Technol. 10, 1 (1994). G. L. Maker and J. Kruger, Inter. Mater. Rev. 38, 138 (1993). H. Gjestland, G. Nussbaum, G. Regazzoni, O. Lohne, and O. Bauger, Mater. Sci. Eng. A134, 1197 (1991). G. H. Li, H. S. Gill, and R. A. Varin, Metall. Trans. A. 24A, 2383 (1993). Y. Carbonneau, A. Couture, A. Van Neste, and R. Tremblay, Metall. Trans. A. 29A, 1759 (1998). J. J. Kim, D. H. Kim, S. H. Kim, and N. J. Kim, in Light Weight Alloys for Aerospace Applications IV, ed. E. W. Lee, W. E. Frazier, N. J. Kim, and K. V. Jata, p. 129, TMS, Warrendale, PA (1997). J. W. Kim, D. H. Kim, C. D. Yim, and K. S. Shin, J. Kor. Inst. Met. Mater. 35, 1446 (1997). J. J. Kim, D. H. Kim, S. J. Park, C. S. Shin, and N. J. Kim, J. Kor. Inst. Met. Mater. 34, 1558 (1996). A. Luo, in Proceedings of the 3rd International Magnesium Conference, ed. G. W. Lorimer, p. 449, Institute of Materials, UK (1997).
Pergamon PII S1359-6462(99)00172-4
MODIFICATION OF Mg2Si MORPHOLOGY IN SQUEEZE CAST Mg-Al-Zn-Si ALLOYS BY Ca OR P ADDITION Jae Joong Kim, Do Hyang Kim*, K.S. Shin** and Nack J. Kim Center for Advanced Aerospace Materials, Pohang University of Science and Technology (POSTECH), San 31 Hyoja-Dong, Pohang, Kyungbuk 790-784, Korea * Department of Metallurgical Engineering, Yonsei University, Shinchon-dong, Seodaemun-ku, Seoul, Korea ** School of Materials Science and Engineering, Seoul National University, Seoul, Korea (Received November 16, 1998) (Accepted March 24, 1999)
1. Introduction Lightweight alloys have had varying degrees of importance in transportation systems. In aerospace industries, for example, the advances made in lightweight alloys have always provided the key to the improvements in performance. Moreover, recent growing demand for weight reduction of the vehicle has generated a considerable interest for lightweight alloys from the automotive industries. From these respects, Mg alloys, the lightest commercial alloys developed so far, have great potential for high performance aerospace and automotive applications [1– 4]. However, the proper combination of strength, toughness and corrosion resistance has been a major challenge for Mg alloys. Recent studies indicate that addition of alloying elements such as Si and rare earth elements to Mg-Al-Zn alloys can produce substantial improvements in strength, toughness and corrosion resistance [5–7]. Rare earth elements are very expensive and difficult to be alloyed homogeneously. On the other hand, Mg2Si formed by the addition of Si is the very useful intermetallic compound that exhibits high melting point, low density and high elastic modulus [8]. However, the addition of Si to Mg alloys often results in poor mechanical properties due to the undesirable morphology (size, shape and distribution) of Mg2Si particles. The present study is aimed at improving the mechanical properties of Mg2Si containing Mg alloys through the modification of morphology of Mg2Si particles by Ca or P addition.
2. Experimental Procedures Nominal chemical compositions of various Mg-Al-Zn-Si-(X) alloys used in the present study are Mg-5Al-1Zn-0.7Si, Mg-5Al-1Zn-0.7Si-0.2Ca and Mg-5Al-1Zn-0.7Si-0.03P. The ingots were re-melted in an electric resistance furnace followed by squeeze casting in a 110mm ⫻ 50mm size steel mold preheated at 250°C. The melt temperature was 750°C, approximately 100°C higher than the liquidus temperature of Mg. The pressure applied for squeeze casting of the alloys melt was 150MPa. The squeeze cast alloys were solution heat treated at 410°C for 10 hours, water quenched, and then aged at 190°C for 1 hour. Specimens for optical microscopy and scanning electron microscopy (SEM) were etched with a solution of 2 vol. % nitric acid ⫹ ethyl alcohol. Thin foils for transmission electron 333
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Figure 1. Optical micrographs showing the effect of P or Ca addition on the microstructure of squeeze cast alloys; a) Mg-5Al-1Zn-0.7Si alloy, (b) Mg-5Al-1Zn-0.7Si-0.2Ca alloy, and (c) Mg-5Al-1Zn-0.7Si-0.03P alloy.
microscopy (TEM) were prepared by ion milling. Volume fraction of the constituent phases and the grain size were analyzed by using an image analyzer. Round tensile specimens with gauge length of 30-mm and gauge diameter of 6-mm were tested at room temperature with an initial strain rate of 6.67 ⫻ 10⫺4/s. Fractographic observation was conducted on the alloys to clarify the fracture process. 3. Results 3.1 Microstructure Fig. 1a shows the microstructure of squeeze cast Mg-5Al-1Zn-0.7Si alloy. Its microstructure can be described as consisting of Chinese script type Mg2Si particles with interdendritic Mg17Al12 particles in Mg matrix. These phases can be discerned as follows: bright area is Mg matrix, gray area is Mg17Al12 and dark area is Mg2Si. With the addition of Ca or P, however, the morphology of Mg2Si particles changes to polygonal type as shown in Fig. 1b and 1c. Ca containing alloy contains 3 ⬃ 6 m sized polygonal type Mg2Si and Mg17Al12 particles in the interdendritic region. Chinese script type Mg2Si particles are observed only in limited areas. As compared to the base alloy shown in Fig. 1a, considerable microstructural refinement is obtained by the addition of Ca. P has a similar effect as Ca,
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Figure 2. Scanning electron micrographs showing the various constituent phases present the squeeze cast alloys; a) Mg-5Al1Zn-0.7Si-0.2Ca alloy and b) Mg-5Al-1Zn-0.7Si-0.03P alloy.
as can be seen in Fig. 1c. Fig. 2 shows SEM micrographs of Mg-5Al-1Zn-0.7Si-0.2Ca and Mg-5Al1Zn-0.7Si-0.03P alloys. The microstructure of Mg-5Al-1Zn-0.7Si-0.2Ca alloy (Fig. 2a) is similar to that of Mg-5Al-1Zn-0.7Si-0.03P alloy (Fig. 2b), except that the former contains Ca-Si compound besides Mg2Si and Mg17Al12 particles. It is interesting to note that Mg2Si particles often contain small particles inside, which presumably act as nucleation sites for Mg2Si particles. Detailed TEM analysis has been conducted to identify the nature of these small particles. As shown in Fig. 3a, Mg2Si particles present in Mg-5Al-1Zn-0.7Si-0.2Ca alloy consist of several particles. Selected area diffraction pattern of the particle marked ‘␣’ in Fig. 3a shows double diffraction pattern indicating that the particle has 3-fold or 6-fold symmetry in this zone axis (Fig. 3b). Double diffraction occurs when a diffracted beam traveling through a crystal is rediffracted either within the same crystal or when it passes into a second crystal. Analysis of double diffraction pattern shows that the strong diffraction spots coincide with the diffraction spots of Mg2Si [111] zone. However, weak diffraction spots cannot unambiguously be identified since the information is available from only one
Figure 3. Transmission electron micrographs of Mg2Si particles in squeeze cast Mg-5Al-1Zn-0.7Si-0.2Ca alloy; a) bright field, b) diffraction pattern of the area ‘␣’ in a), and c) EDS spectrum from the area ‘␣’ in a).
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Figure 4. Microdiffraction pattern of CaMgSi compound.
zone axis. EDS spectrum obtained from this unidentified particle contains the Ca peak besides the Mg and Si peaks as shown in Fig. 3c. It is quite possible that some of the Mg and Si peaks are inevitably from the matrix and nearby located Mg2Si particles. However, combining the information from diffraction pattern and EDS spectrum shows that the nature of the particle is CaSi2. Orientation relationship between Mg2Si and CaSi2 deduced from the diffraction pattern is [111]Mg2Si//[001]CaSi2 and (⫺101)Mg2Si//(110)CaSi2. Fig. 3 clearly shows that polygonal type Mg2Si particles nucleate from CaSi2 particles, indicating the beneficial effect of Ca on modifying the morphology of Mg2Si particles. Microdiffraction analysis has also been conducted to identify the nature of Ca-Si compound as shown in Fig. 4. It shows that this compound is CaMgSi having orthorhombic structure. Its nature has also been confirmed by quantitative EDS analysis. In fact, it has the same characteristics as the one recently observed in Mg-Si-Ca alloys [9]. As shown in Fig. 2b, Mg2Si particles present in Mg-5Al-1Zn-0.7Si-0.03P alloy also contain small particles inside. Detailed TEM analysis has been conducted to identify the nature of this small particle. TEM micrograph of Mg2Si particle in Mg-5Al-1Zn-0.7Si-0.03P alloy shows that it consists of several subgrains (marked 1–5) with unidentified particle (marked ‘␣’) at the center as shown in Fig. 5a. Selected area diffraction pattern of the particle marked ‘␣’ in Fig. 5a shows a ring pattern indicating that the particle is actually a cluster of fine particles (Fig. 5b). EDS spectrum obtained from the clustered particle contains the P and O peaks besides the Mg and Si peaks as shown in Fig. 5c. As is the case of Ca containing alloy, it is not clear whether Mg and Si are present in the clustered particle since Mg and Si peaks can be obtained from nearby located Mg2Si particle and matrix. However, combination of information from diffraction pattern and EDS spectrum shows that the nature of clustered particle is Mg3(PO4)2, which has monoclinic structure. It has been clearly shown in Fig. 5 that the clustered Mg3(PO4)2 particles act as nucleation site for polygonal type Mg2Si particles.
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Figure 5. Transmission electron micrographs of Mg2Si particles in squeeze cast Mg-5Al-1Zn-0.7Si-0.03P alloy; a) bright field, b) diffraction pattern of the area ‘␣’ in a), and c) EDS spectrum from the area ‘␣’ in a).
3.2 Mechanical Properties and Fracture Behavior The above mentioned beneficial effect of Ca and P additions on modifying the morphology of Mg2Si particles should also be beneficial in improving the mechanical properties. Tensile properties and apparent fracture toughness of the base alloy and Ca and P containing alloys are shown in Table 1. It shows that the base alloy and Ca and P alloys have similar yield strength values. However, Ca and P containing alloys have improved ultimate tensile strength, elongation and apparent fracture toughness over the base alloy. Fracture surface analysis shows that there is a large difference in fracture behavior between the base alloy and the Ca and P containing alloys. Fractograph of the base alloy (Fig. 6a) shows that its fracture surface exhibits large cleavage-type facets, which presumably form along the interface between Mg matrix and Chinese script type Mg2Si particles. Fracture surface of the Ca containing alloy, on the other hand, exhibits fine scale dimples with no cleavage-type facet as shown in Fig. 6b. P containing alloy shows the similar fracture surface as the Ca containing alloy.
TABLE 1 Tensile Properties of Squeeze-Cast Mg-5A1-1Zn-0.7Si-X Alloys
YS (MPa) UTS (MPa) E1. (%) Apparent Fracture Toughness Kc (MPa 公m)
5A1-1Zn-0.7Si
5A1-1Zn-0.7Si-0.2Ca
5A1-1Zn-0.7Si-0.03P
116 194 5.6 18.4
116 213 7.3 21.9
112 208 6.9 22.1
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Figure 6. SEM fractographs; a) Mg-5Al-1Zn-0.7Si alloy and (b) Mg-5Al-1Zn-0.7Si-0.2Ca alloy.
4. Discussion The present study shows that the addition of Si to Mg-5Al-1Zn alloy results in the formation of Chinese script type Mg2Si particles. It has been shown in the previous studies [10 –12] that there are two types of Mg2Si particles, Chinese script type and polygonal type, forming in Mg-Al-Zn-Si alloy systems depending on the Al content. The Chinese script type Mg2Si particle forms as a result of a eutectic solidification with ␣-Mg. Therefore, the network of the Chinese script type eutectic Mg2Si particles observed in low Al containing Mg-Al-Zn-Si alloy results from the hypo-eutectic solidification. On the other hand, the polygonal type Mg2Si particle observed in high Al containing Mg-Al-Zn-Si alloy forms
Figure 7. Modified Mg-Si phase diagram.
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Figure 8. Optical micrographs of the squeeze cast alloys after solution treatment at 410°C for 10 hours; a) Mg-5Al-1Zn-0.7Si alloy, b) Mg-5Al-1Zn-0.7Si-0.2Ca alloy, and c) Mg-5Al-1Zn-0.7Si-0.03P alloy.
as a primary phase during solidification. This phenomenon, the change of Mg2Si shape due to Al content, can be explained experimentally by the change in the eutectic temperature with Al content of Mg-Al-Zn-Si alloys. From DTA analysis of various Mg alloys [13], modified Mg-Si phase diagram has been redrawn qualitatively as shown in Fig. 7. It shows that, with increasing Al content, the eutectic temperature decreases and the eutectic composition of the alloy shift to the left (direction to Mg in Fig. 7). These discussions suggest that the high Al containing Mg-Al-Zn-Si alloys such as Mg-9Al-1Zn0.7Si alloy can have the primary Mg2Si particles despite the hypo-eutectic composition in the Mg-Si system. As shown in Fig. 6a, Chinese script Mg2Si particles give a detrimental effect on mechanical properties of Mg-5Al-1Zn-0.7Si alloy since long cracks can easily nucleate along the interface between Chinese script Mg2Si particles and Mg matrix. Hence modification of morphology of Mg2Si particles from Chinese script type to polygonal type is needed to improve the mechanical properties of Mg-Al-Zn-Si alloys. However, the above-mentioned method, i.e., increasing the Al content, is not suitable since it increases the amount of Mg17Al12 and coarse polygonal type Mg2Si particles, which have a detrimental effect on mechanical properties. Therefore, to obtain the best combination of mechanical properties in Mg-Al-Zn-Si alloys, the morphology of Mg2Si particles should be modified by the other method, such as alloying addition of Ca or P. The present study shows that the addition
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of Ca or P modifies the morphology of Mg2Si particles from Chinese script type to refined polygonal type. Another beneficial effect of Ca or P addition on the microstructure of Mg-Al-Zn-Si alloys is the refinement of grain size. As shown in Fig. 8, the grain sizes of solution treated Ca or P containing alloys are about 27.5 m, while that of base alloy is about 52.2 m. This is undoubtedly due to the presence of finely distributed polygonal type Mg2Si particles in the former. 5. Summary The present study is aimed at improving the mechanical properties of squeeze cast Mg-Al-Zn-Si alloys by the modification of Mg2Si particles through alloying addition of Ca or P. It shows that the microstructure of squeeze cast Mg-5Al-1Zn-0.7Si alloy contains Chinese script Mg2Si particles. Chinese script type Mg2Si particles act as crack nucleation sites, resulting in poor mechanical properties in squeeze cast Mg-5Al-1Zn-0.7Si alloy. Addition of Ca or P promotes the formation of fine polygonal type Mg2Si particles by providing the nucleation sites for polygonal type Mg2Si particles. Moreover, the grain sizes of Ca or P modified Mg-Al-Zn-Si alloys are much finer than that of base alloy. Such improved microstructure of the modified alloys results in the large improvement in tensile properties and toughness as compared to the base alloy. Acknowledgment The authors appreciate the financial support of the Hyundai Motors Company and the Korea Ministry of Science and Technology. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
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