Effect of Zn content on microstructure, mechanical properties and fracture behavior of Mg-Mn alloy

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1 Effect of Zn content on microstructure, mechanical properties and fracture behavior of Mg-Mn alloy *Yin Dongsong 1, Zhang Erlin 2 and Zeng Songyan 1 (1. School of Materials Science and Engineering, Harbin Institute of Technology, Harbin , China; 2. Institute of Metal Research Chinese Academy of Science, Shenyang , China) Abstract: The optical microscope, scanning electron microscope and universal testing machine are used to investigate the effect of Zn content on the microstructure, mechanical properties and fracture behavior of Mg- Mn-Zn alloy. The results indicate that fi ne (Mg, Mn, Al)-containing phases are distributed uniformly in the Mg-Mn alloy matrix, while small amount of (Mg, Zn)-containing phases are formed in the matrix and the grain boundary becomes coarse when 1wt.% Zn is added. As the Zn content increases, the amount of (Mg, Zn)-containing phases increases, and the grain boundary becomes coarser. When the Zn content is between 3wt.%-5wt.%, slender (Mg, Zn)-containing phases precipitate at the grain boundary. The addition of Zn could reduce the grain size and enhance the mechanical properties of the alloy matrix, and both of the effects can be enhanced by increasing the Zn content further more. When the Zn content is more than 3wt.%, grain size stops decrease, the strength cannot be improved any more and elongation decreases significantly. The fracture behavior of Mg-Mn alloy appears to be cleavage fracture, and transforms into quasi-cleavage fracture as Zn is added. When Zn content exceeded 3wt.%, large amount of (Mg, Zn)-containing phases appear on the fracture face, and act as the crack sources. Key words: zinc; magnesium alloy; microstructure; mechanical properties; fracture behavior CLC number: TG Document code: A Article ID: (2009) Magnesium alloy is a newly developed environmental friendly material, which could be applied in many fields such as automobile, aeronautic, communication, electric and sports equipment etc. The growth rate of magnesium alloy exceeds 20% per year. Due to its light weight ( g/cm 3 ), high specific strength and excellent dimensional stability, magnesium alloy will be used widely in the future [1-3]. However, low plastic yield and strength are inherited problems for alloys having hexagonal close-packed structure. Therefore, it is very necessary to resolve these problems. It has been found that mechanical properties of magnesium could be enhanced by alloying method. Experimental results proved that fine grain strengthening, solution strengthening and second-phase strengthening can be achieved by proper adding of alloying elements. Mg-Mn alloy is a kind of anti-corrosion magnesium alloy with excellent corrosion resistance and weldability. Meanwhile, stress corrosion will never occur on Mg-Mn alloy. However, its strength and plasticity need to be improved. [4] Zn is generally used as alloying element for magnesium *Yin Dongsong Male, born in 1974, Ph.D, graduated from Jiamusi University in 1998, majored in Foundry Equipment and Casting Technology. He got his master s and doctor s degree in 2004 and 2008, respectively from Harbin Institute of Technology. His research interest is mainly focused on biomedical magnesium alloy. dongsongyin@126.com Received: ; Accepted: alloy to enhance room temperature strength [5,6]. Small amount of Zn can be dissolved in Mg, as solution strengthening element, while excess Zn will react with Mg to form (Mg, Zn)-containing phases [6]. However, there is no report on the optimal content of the Zn addition and there is no report in depth about the strengthening mechanism of Zn and the influence of Zn on fracture behavior of Mg-Mn alloy. In order to improve the mechanical properties of as-cast Mg- Mn alloy, this paper focuses on the effects of Zn (1wt.%-5wt.%) on the microstructure, mechanical properties and fracture behavior of Mg-Mn alloy. Strengthening mechanisms and fracture behavior of Mg-Mn alloy were also analyzed, which can provide helpful information for the development of high strength anti-corrosion magnesium alloy. 1 Experimental methods Pure magnesium ingot (99.99 wt.%), pure zinc ingot (99.99 wt.%), analytically pure MnCl 2 and pure aluminum (99.99 wt.%) were used in this experiment. Nominal composition of the alloys is shown in Table 1. Melting process was carried out in a crucible under the protection of (CO %SF 6 ) gases. As magnesium metal was melted, MnCl 2, pure zinc ingot and pure aluminum were added. After all these materials were melted completely, the melt at a specified temperature was then cast into a mold. Composition of the cast alloy was analyzed by chemical method. Tensile samples with gauge length of 10 mm and thickness of 43

2 CHINA FOUNDRY Vol.6 No.1 Table 1 Nominal chemical composition of the Mg-Mn-Zn alloys (wt.%) Composition Zn Mn Al Fe Ni Cu Mg Mg-Mn Mg-Mn-1Zn Mg-Mn-2Zn Mg-Mn-3Zn Mg-Mn-4Zn Mg-Mn-5Zn Balance 2 mm were cut from the casting ingot. Tensile tests were carried out on an INSTRON Series IX automatic testing machine with a tensile speed of 1 mm/min. Meanwhile, extensometer was used to measure the elongation. Data reported in this paper were average value of 3 separated measurements. All samples used for microstructure observation were cut from the same position of the ingots. Samples were ground and polished, then etched by 0.5% nital solution. Node method was used to measure the grain size. Microstructure and tensile fracture surfaces were studied using scanning electron microscope, and the phase composition were determined by energy dispersive spectroscopy (EDS). 2 Results and discussion 2.1 Microstructure Microstructures of as-cast Mg-Mn-xZn (x=0, 1, 2, 3, 4, 5) alloys are shown in Fig.1. It can be seen that the microstructure of Mg-Mn alloy is much coarser than those of Mg-Mn-Zn alloys. The mean grain sizes of Mg-Mn-Zn alloys as function of Zn content are shown in Fig. 2. The results show that grain size of Mg-Mn alloy is ranging from 700 to 900 µm. The grain size of the Mg-Mn-Zn alloys decreases with increasing of Zn content. When the Zn content is more than 3wt%, the grain size keeps in the range of 50 to 80 µm. (a) Mg-Mn alloy (b) Mg-Mn-1Zn alloy (c) Mg-Mn-2Zn alloy (d) Mg-Mn-3Zn alloy (e) Mg-Mn-4Zn alloy (f) Mg-Mn-5Zn alloy Fig. 2 Relationship between Zn content and grain size of the alloys Fig. 1 Microstructure of the alloys Figure 3 shows the SEM micrographs of Mg-Mn-Zn alloys with different Zn content. The fine second-phases with size of 1-2 µm can be seen in the Mg-Mn alloy (Fig. 3(a) and 3(b)). The EDS results of the second-phases are listed in Table 2. Zn concentration can be found near the grain boundary of Mg-Mn-xZn (x=1,2,3,4,5) and its richness increase with the Zn content increase (Fig. 3(c)- (g)). When x 3, the (Mg, Zn)-containing phases appear at the grain boundary and the amount of (Mg, Zn)-containing phases increase with the Zn content increasing (see Fig. 3(e), 3(f) and 3(g)). These (Mg, Zn) containing phases have the strip, rod-like shape and elliptical shape (Fig. 3(h) and 3(i)). The EDS results of (Mg, Zn)-containing phases are shown in Table 2. 44

3 (a) Mg-Mn alloy (b) (Mg, Mn, Al)-containing phase (c) Mg-Mn-1Zn alloy (d) Mg-Mn-2Zn alloy (e) Mg-Mn-3Zn alloy (f) Mg-Mn-4Zn alloy (g) Mg-Mn-5Zn alloy (h) (Mg, Zn)-containing phase in the grain (i) (Mg, Zn)-containing phase at the grain boundary Fig. 3 SEM morphologies of Mg-Mn-Zn alloys Table 2 EDS results of the second-phase (at.%) Alloys Phase Mg Mn Zn Al No. of measurements Mg-Mn (Mg, Mn, Al)-containing phase (Mg, Mn, Al)-containing phase Mg-Mn-xZn (x=1,2,3,4,5) (Mg, Zn)-containing phase (intragranular) (Mg, Zn)-containing phase (at grain boundary) Mechanical properties It can be concluded that the mechanical properties of Mg- Mn-xZn alloy, which are listed in Table 3, were improved obviously by adding Zn. When the Zn content is 3wt.%, the tensile strength and yield strength are enhanced by nearly 130 MPa and 40 MPa, respectively, and the elongation percentage is also doubled. This improvement is due to the grain refinement, which improves the strength and plasticity of the alloy significantly. When the Zn content is 4wt.%-5wt.%, the excess Zn will react with Mg and form large amount of (Mg, Zn)-containing phases in the matrix and grain boundary. These phases dissever the matrix and increase the number of crack sources. Therefore, the tensile strength of the alloy will not be improved and meanwhile, the toughness decreases slightly also. 45

4 CHINA FOUNDRY Vol.6 No.1 Table 3 Mechanical properties of Mg-Mn-Zn alloys Alloys Tensile strength (MPa) Yield strength (MPa) Elongation (%) Mg-Mn 89.2 ± ± ± 1.0 Mg-Mn-1Zn ± ± ± 1.1 Mg-Mn-2Zn ± ± ± 1.0 Mg-Mn-3Zn ± ± ± 2.0 Mg-Mn-4Zn ± ± ± 1.8 Mg-Mn-5Zn ± ± ± Fracture behavior Figure 4 shows the SEM micrographs of room temperature tensile fracture of Mg-Mn-xZn alloys. These figures indicate that fracture behavior changes with the alloy composition. Figure 4(a) shows the cleavage fracture of Mg-Mn alloy. Figures 4(b), 4(c) and 4(d) are tensile fracture surfaces of Mg- Mn-Zn, Mg-Mn-2Zn and Mg-Mn-3Zn alloys, respectively. From these figures, it can be seen that tear ridges appear as 1wt.% and 2wt.% of Zn was added. The cleavage facet connects with each other by some tear ridges, and some river patterns are formed by tear ridges. As a result, tensile strength and yield strength of Mg-Mn-1Zn and Mg-Mn-2Zn alloys are much higher than those of Mg-Mn alloy. As the Zn content reaches 3wt.%, the amount of tear ridges increase significantly and the toughness is also improved. River patterns formed by cleavage stage can be seen on small cleavage facet. (a) Mg-Mn alloy (b) Mg-Mn-1Zn alloy (c) Mg-Mn-2Zn alloy (d) Mg-Mn-3Zn alloy (e) Mg-Mn-4Zn alloy (f) Mg-Mn-5Zn alloy Fig. 4 SEM morphology of fracture surfaces of Mg-Mn-Zn alloys Figure 4(e) shows the tensile fracture surface of Mg-Mn-4Zn alloy. It is known that the fracture behavior of the alloy is quasicleavage fracture, and some tear ridges still retain. However, the amount of tear ridges is much smaller than that of Mg-Mn-3Zn alloy, and the amount of (Mg, Zn)-containing phases increases. For Mg-Mn-5Zn alloy, the amount of tear ridges decreases and (Mg, Zn)-containing phase increases compared with the Mg- Mn-4Zn alloy. It can be seen that brittle failure occurs during the fracture process. The hard brittle (Mg, Zn) containing phase fractured first might become the crack sources. Cleavage fracture, quasi-cleavage fracture and intergranular fracture are the main fracture characteristics of as-cast magnesium alloy [7]. The fracture behavior of Mg-Mn alloy in this study is cleavage fracture. As for cleavage fracture, microcrack extends on the specified crystal face, which is (0001) for magnesium alloy. Two parallel cleavage cracks at different height connect with each other by secondary cleavage to form stage, and transform into quasi-cleavage fracture [7, 8]. Many tear ridges appear on small quasi-cleavage fracture faces, which are formed by the connection of cracks that nucleate independently [8]. As the Zn content increases, the tearing phenomenon enhances. When the Zn content exceeds 3wt.%, large amount of (Mg, Zn)-containing phases appear in the matrix, and become crack sources. 46

5 3 Conclusions (1) Fine block-like (Mg, Mn, Al)-containing phase are formed uniformly in the Mg-Mn alloy matrix. Small amount of fine (Mg, Zn)-containing phases appear in the grains when proper amount of Zn content is added. As the Zn content increases, the amount of (Mg, Zn)-containing phases increases in the Mg-Mn-xZn alloys. When the Zn content is 3wt.%, (Mg, Zn)-containing phases precipitate near the grain boundary, and their amount, both in the matrix and at the grain boundary, increases accordingly as the Zn content increases further. (2) Grain size refinement can be achieved by adding proper amount of Zn in the Mg-Mn alloy. As the Zn content increases, the microstructure of the alloy will be refined. When the Zn content is 3wt.%, grain size decreases from µm to µm. As the Zn content increases further, the grain size cannot be refined any more. (3) Tensile strength, yield strength and elongation are improved significantly by adding proper amount of Zn. When the Zn content is 3wt.%, tensile strength, yield strength and elongation can be enhanced by nearly 130 MPa, 40 MPa and 100%, respectively. As the Zn content increases further, tensile strength and elongation decrease slightly, while the yield strength almost remain unchanged. (4) The Mg-Mn alloy shows cleavage fracture behavior. Tearing ridges appear when proper amount of Zn is added, however, the tearing phenomenon will be enhanced by further adding Zn. When the Zn content exceeds 3wt.%, large amount of (Mg, Zn) containing phases appear on the fracture surface, and act as crack sources. References [1] Mordike B L, Ebert T. Magnesium properties, applications, potential. Materials Science and Engineering A, 2001, 302: [2] Cahn R W, Haasen P, Kramer E J. MateriaIs Science and Technology. New York: VCH Publishers Inc, [3] Chen Zhenhua. Magnesium Alloy. Beijing: Chemical Industry Press, 2003: (in Chinese) [4] Lunder O, Aune T K, Nisancioglu K. Effect of Mn additions on the corrosion behavior of mould-cast magnesium ASTM AZ91. Corrosion Science, 1987, 43(5): [5] Wang Yeshuan. Effects of Zn on the solidifi cation behavior of Mg-9Al alloys. Special Casting & Nonferrous Alloys, 2002(5): (in Chinese) [6] Yao jiusan, Liu Weihua, Chen Riyue. High Zn-Mg alloy. Special Casting & Nonferrous Alloys, 2002(s1): (in Chinese) [7] Song Yulai, Liu Yaohui, Zhu Xianyong, et al. Influence of neodymium on the corrosion of AZ91 magnesium alloy. Journal of Jilin University (Engineering and Technology Edition), 2006, 36 (3): (in Chinese) [8] Zhong Peidao. Fracture failure analysis. Physical Testing and Chemical Analysis Part A: Physical Testing, 2005, 41(8): (in Chinese) 47