MICROSCOPY RESEARCH AND TECHNIQUE 78:65–69 (2015)
Effect of Different Calcium Contents on the Microstructure and Mechanical Properties of Mg-5Al-1Bi-0.3Mn Magnesium Alloy LUO XIAO-PING,* DANG SU-E, AND ZHANG YA-QING School of Materials Science and Engineering, Taiyuan University of Science and Technology, Taiyuan 030024, Shanxi, China
KEY WORDS
magnesium alloy; Al2Ca phase; microstructure; mechanical properties
ABSTRACT The effect of different Ca contents on the microstructure and mechanical properties of Mg-5Al-1Bi-0.3Mn (AMB501) magnesium alloys was investigated by conventional melting and casting technique using different Ca contents (1.0, 2.0, and 3.0 wt %). Increasing the Ca content resulted in higher hardness and yield strength, but decreased elongation. The improved tensile properties of the AM50-1Bi-xCa alloys were due to the changes in AMB501 alloy microstructure when the Ca content increased, as demonstrated by scanning electron microscope, energy dispersive spectrum, and X-ray diffractometer. The alloy microstructure indicated that the amount of b-Mg17Al12 phase on grain boundaries decreased and the morphology of b-Mg17Al12 phase on grain boundaries changed from quasicontinuous-net shape to dispersed particles. The Mg17Al12 phase disappeared and a new secondary phase Al2Ca appeared after a 3.0 wt % Ca addition. Microsc. Res. Tech. 78:65–69, 2015. V 2014 Wiley Periodicals, Inc. C
INTRODUCTION Magnesium-based alloys, which are the lightest structural metallic materials, remain challenging to apply in structural parts because of their high strength-to-density ratio. Among commercial magnesium alloys, AM-series magnesium alloys are the most widely used because of their high energy absorption, adequate strength, and good castability. Meanwhile, AM-series alloys belonging to the Mg-Al system have poor thermal stability of the Mg17Al12 phase (with a eutectic temperature of 437 C), and their discontinuous precipitation can result in substantial grain boundary sliding at elevated temperatures. These make AM-series alloys generally unsuitable for use above 150 C, and even at this temperature, considerable losses in strength are evident, rendering them inadequate for major power train applications (Aljarrah et al., 2007; Faruk et al., 2013; Guan and Wang, 2003; Joonhong and Park, 2013; Ma et al., 2009; Xu et al., 2011; Zhang et al., 2012). One possible approach to improve thermal stability and creep resistance in these kinds of alloys involve introducing alloying elements with higher affinity to aluminum to suppress the formation of Mg17Al12 phase. Introducing secondary phase particles at grain boundaries to pin the sliding phenomenon is another such approach. One significant discovery emerged from considerable effort expended in the last few decades: it revealed the beneficial effect of rare earth (RE) or yttrium on mechanical properties, including strength and creep resistance of magnesium alloys (Khomamizadeh et al., 2005; Wang et al., 2000; Wang et al., 2002). However, currently developed REcontaining alloys are usually considered to be too expensive. The addition of low-cost calcium, which proC V
2014 WILEY PERIODICALS, INC.
vides finer solidified microstructure in the Mg alloy (Bai et al., 2006; Cheng et al., 2006; Ninomira et al., 1995; Wu et al., 2005), is also an effective way to improve the mechanical properties of magnesium alloys at elevated temperatures. Addition of other inexpensive elements such as bismuth could significantly increase the tensile strength and creep resistance, as reported by Yuan et al. (1999, 2001; Zhou et al., 2009). However, very few current reports have referred to the effects of the addition of calcium to AMB501 magnesium alloys in terms of the microstructures and mechanical properties. Hence, this work attempts to analyze the relationship between the microstructure and the properties with the addition of calcium into an AMB501 alloy to understand the function of new secondary phases in the microstructures on its properties. This can provide the basis for the application of high-temperature magnesium alloys. EXPERIMENTAL High-purity (99.5%) magnesium, Al (99.0%), Ca granules (99.5%), Bi granules (99.0%), and Al–10 wt % Mn master alloys were used to prepare the AMB501xCa alloy using resistance crucible furnace melting under the protection of a mixed gas atmosphere. The *Correspondence to: Luo Xiao-Ping; Taiyuan University of Science and Technology, Taiyuan 030024, Shanxi, China. E-mail: [email protected] Received 20 February 2014; accepted in revised form 30 September 2014 REVIEW EDITOR: Dr. Chuanbin Mao Contract grant sponsor: Natural Science Foundation of Shanxi Province, China; Contract grant number: 2014011015-3; Contract grant sponsor: Doctoral Foundation of Taiyuan University of Science and Technology; Contract grant number: 20132019. DOI 10.1002/jemt.22446 Published online 22 October 2014 in Wiley Online Library (wileyonlinelibrary. com).
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Test No.
Al
A B C D
5.0 5.0 5.0 5.0
TABLE 1. Chemical composition of experimental alloys (wt %) Bi Mn 1.0 1.0 1.0 1.0
0.3 0.3 0.3 0.3
Ca
Mg
— 1.0 2.0 3.0
surplus surplus surplus surplus
Fig. 1. Optical micrographs of the as-cast experimental alloys (a) AMB501; (b) AMB501-1Ca; (c) AMB501-2Ca; (d) AMB501-3Ca.
chemical composition of the experimental alloy had varying concentrations of Ca (0, 1, 2, and 3 wt %) content (Table 1). After the alloy completely melted, it was stirred for 5 min and kept at 760 C for 20 min before pouring poured into a steel mold preheated at 200 C. The as-cast samples were cross-sectioned and processed according to standard metallographic procedures (Tartaglia et al., 2001). To observe the morphology and distribution of the specimens, the as-cast specimens were etched with a mixture of 1 mL of nitric acid and 24 mL of ethylene glycol. The etching time was approximately 8 s. Material composition was confirmed with energy dispersive spectrum (EDS) attached to an S-4800 scanning electron microscope (SEM), whereas phase and structures were characterized using an X-ray diffractometer (XRD) (Rigakud/max-2500XRD). The hardness of each sample was measured using a HB3000 hardness tester. The hardness value was the average value obtained from the results of one test performed on five different parts of each specimen. Tensile strength of the tested alloys was determined using specimens with 5 mm diameter and 25 mm gauge length at a strain rate of 1 mm/min. The test results were determined by taking the average of at least five readings.
RESULTS AND DISCUSSION Microstructure The microstructure difference among the as-cast AMB501-xCa (x 5 0, 1.0, 2.0, and 3.0 wt %) material ingots after polishing and etching is shown in Figure 1. Alloy AMB501 is composed of a primary matrix and a secondary phase, and networks formed at grain boundaries, similar to the AMB501-1Ca alloy, as shown in Figure 1a. When the addition of Ca was increased, grain sizes were finer, and the secondary phase precipitates at the grain boundaries became noticeably thinner than that of alloy AMB501, as shown in Figures 1a and 1d. For investigation of the fine structure in the ally with better spatial resolution and chemical sensitivity, we perform SEM on the samples AMB501-xCa alloys, as shown in Figure 2. The two distinct regions presented appear as white and gray in each image. According to the EDS analysis result, the areas marked by an arrow are the secondary eutectic bMg17Al12 phase and a-Mg solid solution phase. Impurity inclusions were also observed marked by A in Figures 2a and 2b, M and N in Figure 2c, and P, Q, S in Figure 2d. The impurities have brighter contrast in Microscopy Research and Technique
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67
Fig. 2. SEM images and EDS of as-cast experimental alloys (a) AMB501; (b) AMB501-1Ca; (c) AMB501-2Ca; (d) AMB501-3Ca.
secondary electron SEM image compared with the a-phase Mg and secondary phase at the grain boundaries. The EDS and XRD (Fig. 3) analyses indicated that round particles marked by A in Figure 2b conMicroscopy Research and Technique
tained Mg-Al-Ca three elements, but no peak of Mg-AlCa phase was observed, which may be attributed to the small amount of Ca present in the b-Mg17Al12 phase. Round particles marked by A in Figure 2a
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contains Mg-O-Al three element oxide, but no peak of Mg3Bi2 phase was observed, which may be due to low Bi, Mn content, indicating that very few Bi, Mn were dissolved into the a-Mg during melting. Test point P and Q granular precipitates containing Al-Ca elements are new phase Al2Ca with higher melting point (1079 C) (Ninomira et al., 1995) and good thermal stability, which can improve the heat resistance of alloy. With the increased addition of Ca, coarse eutectic bMg17Al12 phase is refined and becomes discontinuous; a-phases are also refined to greater extent and new phases occur. The increase in the number of a-phases and the refinement of b-phases increased the total surface area of the grain boundary; thus surface energy becomes greater. During the solidification process, the addition of Ca element is expected to generate a constitutional under cooling diffusion layer above the solid/liquid
Fig. 3. XRD pattern of as-cast alloys.
interface, which can hinder the diffusion of Mg and Al atoms, thereby lowering the growth rate of a-Mg. Therefore, Ca addition refined the grain. Mechanical Properties The hardness, yield strength, tensile strength, and percentage elongation of the alloy are given in Figure 4. Higher yield strength and hardness properties were obtained when the alloy had increased Ca content. The yield strength and the hardness of alloy AMB501-3Ca increased by 22 and 33%, respectively, compared to that of alloy AMB501; however, a reverse effect on the tensile strength and percentage elongation was observed. The change in the mechanical properties of an alloy may be caused by constitutes, fraction, and distribution of the secondary phases. The decreased elongation with increased Ca content is due to an increase in the high fraction area of secondary phases and the formation of a new secondary phase around grain boundaries. The addition of 3.0 wt % calcium led to the formation of a continuous network of the secondary phase of Al2Ca. Therefore, the presence of a continuous network in second phase with a brittle nature in the as-cast alloys is responsible for low elongation. Meanwhile, some hard and brittle Al2Ca phases adjacent to Mg17Al12 can act as barriers for dislocation slipping together with Mg17Al12 to improve yield strength and hardness. Tensile Fracture Analysis The SEM fractographs of the as-cast experimental alloys are shown in Figure 5. The fracture mechanism did not change after the addition of Ca. The fractographs consist of cleavage planes and some tear ridges. Some secondary cracks are also observed on the fracture surface, indicating that the fracture mechanism of AMB501-xCa alloys is a quasicleavage fracture mode. When the Ca addition is 3.0 wt %, a large quantity of flaky phases were observed on the fracture
Fig. 4. Mechanical properties of the experimental alloys.
Microscopy Research and Technique
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Fig. 5. SEM image crack morphology of experimental alloys (a) AMB501; (b) AMB501-1Ca; (c) AMB501-2Ca; (d) AMB501-3Ca.
surface, as shown in Figure 5d. The flaky phases have a positive role in the strengthening behavior rather than deteriorating the mechanical property due to its more refined microstructure.
CONCLUSIONS 1. Increasing the addition of Ca into AMB501 alloy decreased the grain size. The amount of Mg17Al12 phase was also decreased and dispersed. When the Ca content reached 3%; a new Al2Ca phase with higher thermal stability was formed. The phase in the high-temperature conditions of the grain boundary pinning effectively prevented the sliding and migration of grain boundary, thereby improving the high-temperature creep properties of the alloy. 2. The mechanical properties of AMB501-3.0Ca alloy improved significantly compared with that of the AMB501 alloy. The yield strength increased by 22%, hardness increased by 33%, and elongation slightly decreased. REFERENCES Aljarrah M, Medraj M, Wang X, Essadiqi E, Muntasar A, Denes G. 2007. Experimental investigation of the Mg-Al-Ca system. Alloys Compd 436:131–141. Bai J, Sun YS, Xue S, et al. 2006. Mater Sci Eng A 419:181–188. Cheng SL, Yang GC, Fan HF, Li YJ, Zhou YE. 2006. Microstructure and tensile creep behavior of Mg-4Al based magnesium alloys with alkaline earth elements Sr and Ca additions. Rare Met Mater Eng 35:1400–1403. Faruk M, Kainer OA, Ulrich K. 2013. Effect of Ca and Y on microstructure and mechanical properties of AZ91 alloy. Trans Nonferrous Met Soc China 23:66–72.
Microscopy Research and Technique
Guan S, Wang Y. 2003. Influence of Ce addition on microstructure and mechanical properties of high pressure die cast AM50magnesium alloy. Mater Mech Eng 27:23–29. Joonhong, Park. 2013. Research Progress and Application of Elevated Temperature Magnesium Alloys in Auto-mobile Industry. J Mech Sci Technol 27:2955–2960. Khomamizadeh F, Nami B, Khoshkhooei S. 2005. Microstructure and mechanical properties of AM50 alloy according to thickness and forming condition of the products by a high pressure die-casting process. Metall Mater Trans A Phys Metall Mater Sci 36A:3489– 3494. Ma Y, Zhang J, Yang M. 2009. Effect of rare-earth element additions on high-temperature mechanical properties of AZ91 magnesium alloy. J Alloys Compd 470:515–521. Ninomira R, Ojiro T, Kubota K. 1995. Research on microstructure and alloy phases of AM50 magnesium alloy. Acta Metall Mater 43: 669–674. Tartaglia JM, Swartz RE, Bentz RL, Howard JH. 2001. Improved heat resistance of Mg–Al alloys by the Ca addition. J Miner Met Mater Soc 53:16–19. Wang KK, Kang YL, Zhang K. 2002. Effects of rare earth elements on the microstructure and properties of magnesium alloy AZ91D. J Univ Sci Technol Beijing 9:363–366. Wang QD, Lu YZ, Zeng XQ, Ding WJ, Zhu YP. 2000. Effects of RE on microstructure and properties of AZ91 magnesium alloy. Trans Nonferrous Met Soc China 10:235–239. Wu GH, Fan Y, Gao HT, Zhai CQ, Zhu YP. 2005. The effect of Ca and rare earth elements on the microstructure, mechanical properties and corrosion behavior of AZ91D. Mater Sci Eng A 408:255–263. Xu Z, Smith C, Chen S, Sankar J. 2011. Development and microstructural characterizations of Mg–Zn–Ca alloys for biomedical applications. Mater Sci Eng B 176:1660–1665. Yuan G, Zhang W, Sun Y. 1999. Effect of bismuth addition on the mechanical properties of Mg-Al based alloys. J Southeast Univ 29: 115–119. (in Chinese) Yuan G-Y, Sun Y-S, Ding W-J. 2001. Effects of bismuth and antimony additions on the microstructure and mechanical properties of AZ91 magnesium alloy. Mater Sci Eng A 308:38–44. Zhang Q, Masoumi M, Hu H. 2012. Influence of Applied Pressure on Tensile Behaviour and Microstructure of Squeeze Cast Mg Alloy AM50 with Ca Addition. J Mater Eng Perform 21:38–46. Zhou W, Aung N, Sun Y. 2009. Effect of antimony, bismuth and calcium addition on corrosion and electrochemical behavior of AZ91 magnesium alloy. Corros Sci 51:403–408.
Effect of Different Calcium Contents on the Microstructure and Mechanical Properties of Mg-5Al-1Bi-0.3Mn Magnesium Alloy LUO XIAO-PING,* DANG SU-E, AND ZHANG YA-QING School of Materials Science and Engineering, Taiyuan University of Science and Technology, Taiyuan 030024, Shanxi, China
KEY WORDS
magnesium alloy; Al2Ca phase; microstructure; mechanical properties
ABSTRACT The effect of different Ca contents on the microstructure and mechanical properties of Mg-5Al-1Bi-0.3Mn (AMB501) magnesium alloys was investigated by conventional melting and casting technique using different Ca contents (1.0, 2.0, and 3.0 wt %). Increasing the Ca content resulted in higher hardness and yield strength, but decreased elongation. The improved tensile properties of the AM50-1Bi-xCa alloys were due to the changes in AMB501 alloy microstructure when the Ca content increased, as demonstrated by scanning electron microscope, energy dispersive spectrum, and X-ray diffractometer. The alloy microstructure indicated that the amount of b-Mg17Al12 phase on grain boundaries decreased and the morphology of b-Mg17Al12 phase on grain boundaries changed from quasicontinuous-net shape to dispersed particles. The Mg17Al12 phase disappeared and a new secondary phase Al2Ca appeared after a 3.0 wt % Ca addition. Microsc. Res. Tech. 78:65–69, 2015. V 2014 Wiley Periodicals, Inc. C
INTRODUCTION Magnesium-based alloys, which are the lightest structural metallic materials, remain challenging to apply in structural parts because of their high strength-to-density ratio. Among commercial magnesium alloys, AM-series magnesium alloys are the most widely used because of their high energy absorption, adequate strength, and good castability. Meanwhile, AM-series alloys belonging to the Mg-Al system have poor thermal stability of the Mg17Al12 phase (with a eutectic temperature of 437 C), and their discontinuous precipitation can result in substantial grain boundary sliding at elevated temperatures. These make AM-series alloys generally unsuitable for use above 150 C, and even at this temperature, considerable losses in strength are evident, rendering them inadequate for major power train applications (Aljarrah et al., 2007; Faruk et al., 2013; Guan and Wang, 2003; Joonhong and Park, 2013; Ma et al., 2009; Xu et al., 2011; Zhang et al., 2012). One possible approach to improve thermal stability and creep resistance in these kinds of alloys involve introducing alloying elements with higher affinity to aluminum to suppress the formation of Mg17Al12 phase. Introducing secondary phase particles at grain boundaries to pin the sliding phenomenon is another such approach. One significant discovery emerged from considerable effort expended in the last few decades: it revealed the beneficial effect of rare earth (RE) or yttrium on mechanical properties, including strength and creep resistance of magnesium alloys (Khomamizadeh et al., 2005; Wang et al., 2000; Wang et al., 2002). However, currently developed REcontaining alloys are usually considered to be too expensive. The addition of low-cost calcium, which proC V
2014 WILEY PERIODICALS, INC.
vides finer solidified microstructure in the Mg alloy (Bai et al., 2006; Cheng et al., 2006; Ninomira et al., 1995; Wu et al., 2005), is also an effective way to improve the mechanical properties of magnesium alloys at elevated temperatures. Addition of other inexpensive elements such as bismuth could significantly increase the tensile strength and creep resistance, as reported by Yuan et al. (1999, 2001; Zhou et al., 2009). However, very few current reports have referred to the effects of the addition of calcium to AMB501 magnesium alloys in terms of the microstructures and mechanical properties. Hence, this work attempts to analyze the relationship between the microstructure and the properties with the addition of calcium into an AMB501 alloy to understand the function of new secondary phases in the microstructures on its properties. This can provide the basis for the application of high-temperature magnesium alloys. EXPERIMENTAL High-purity (99.5%) magnesium, Al (99.0%), Ca granules (99.5%), Bi granules (99.0%), and Al–10 wt % Mn master alloys were used to prepare the AMB501xCa alloy using resistance crucible furnace melting under the protection of a mixed gas atmosphere. The *Correspondence to: Luo Xiao-Ping; Taiyuan University of Science and Technology, Taiyuan 030024, Shanxi, China. E-mail: [email protected] Received 20 February 2014; accepted in revised form 30 September 2014 REVIEW EDITOR: Dr. Chuanbin Mao Contract grant sponsor: Natural Science Foundation of Shanxi Province, China; Contract grant number: 2014011015-3; Contract grant sponsor: Doctoral Foundation of Taiyuan University of Science and Technology; Contract grant number: 20132019. DOI 10.1002/jemt.22446 Published online 22 October 2014 in Wiley Online Library (wileyonlinelibrary. com).
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L. XIAO-PING ET AL.
Test No.
Al
A B C D
5.0 5.0 5.0 5.0
TABLE 1. Chemical composition of experimental alloys (wt %) Bi Mn 1.0 1.0 1.0 1.0
0.3 0.3 0.3 0.3
Ca
Mg
— 1.0 2.0 3.0
surplus surplus surplus surplus
Fig. 1. Optical micrographs of the as-cast experimental alloys (a) AMB501; (b) AMB501-1Ca; (c) AMB501-2Ca; (d) AMB501-3Ca.
chemical composition of the experimental alloy had varying concentrations of Ca (0, 1, 2, and 3 wt %) content (Table 1). After the alloy completely melted, it was stirred for 5 min and kept at 760 C for 20 min before pouring poured into a steel mold preheated at 200 C. The as-cast samples were cross-sectioned and processed according to standard metallographic procedures (Tartaglia et al., 2001). To observe the morphology and distribution of the specimens, the as-cast specimens were etched with a mixture of 1 mL of nitric acid and 24 mL of ethylene glycol. The etching time was approximately 8 s. Material composition was confirmed with energy dispersive spectrum (EDS) attached to an S-4800 scanning electron microscope (SEM), whereas phase and structures were characterized using an X-ray diffractometer (XRD) (Rigakud/max-2500XRD). The hardness of each sample was measured using a HB3000 hardness tester. The hardness value was the average value obtained from the results of one test performed on five different parts of each specimen. Tensile strength of the tested alloys was determined using specimens with 5 mm diameter and 25 mm gauge length at a strain rate of 1 mm/min. The test results were determined by taking the average of at least five readings.
RESULTS AND DISCUSSION Microstructure The microstructure difference among the as-cast AMB501-xCa (x 5 0, 1.0, 2.0, and 3.0 wt %) material ingots after polishing and etching is shown in Figure 1. Alloy AMB501 is composed of a primary matrix and a secondary phase, and networks formed at grain boundaries, similar to the AMB501-1Ca alloy, as shown in Figure 1a. When the addition of Ca was increased, grain sizes were finer, and the secondary phase precipitates at the grain boundaries became noticeably thinner than that of alloy AMB501, as shown in Figures 1a and 1d. For investigation of the fine structure in the ally with better spatial resolution and chemical sensitivity, we perform SEM on the samples AMB501-xCa alloys, as shown in Figure 2. The two distinct regions presented appear as white and gray in each image. According to the EDS analysis result, the areas marked by an arrow are the secondary eutectic bMg17Al12 phase and a-Mg solid solution phase. Impurity inclusions were also observed marked by A in Figures 2a and 2b, M and N in Figure 2c, and P, Q, S in Figure 2d. The impurities have brighter contrast in Microscopy Research and Technique
EFFECT OF DIFFERENT CALCIUM CONTENTS
67
Fig. 2. SEM images and EDS of as-cast experimental alloys (a) AMB501; (b) AMB501-1Ca; (c) AMB501-2Ca; (d) AMB501-3Ca.
secondary electron SEM image compared with the a-phase Mg and secondary phase at the grain boundaries. The EDS and XRD (Fig. 3) analyses indicated that round particles marked by A in Figure 2b conMicroscopy Research and Technique
tained Mg-Al-Ca three elements, but no peak of Mg-AlCa phase was observed, which may be attributed to the small amount of Ca present in the b-Mg17Al12 phase. Round particles marked by A in Figure 2a
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contains Mg-O-Al three element oxide, but no peak of Mg3Bi2 phase was observed, which may be due to low Bi, Mn content, indicating that very few Bi, Mn were dissolved into the a-Mg during melting. Test point P and Q granular precipitates containing Al-Ca elements are new phase Al2Ca with higher melting point (1079 C) (Ninomira et al., 1995) and good thermal stability, which can improve the heat resistance of alloy. With the increased addition of Ca, coarse eutectic bMg17Al12 phase is refined and becomes discontinuous; a-phases are also refined to greater extent and new phases occur. The increase in the number of a-phases and the refinement of b-phases increased the total surface area of the grain boundary; thus surface energy becomes greater. During the solidification process, the addition of Ca element is expected to generate a constitutional under cooling diffusion layer above the solid/liquid
Fig. 3. XRD pattern of as-cast alloys.
interface, which can hinder the diffusion of Mg and Al atoms, thereby lowering the growth rate of a-Mg. Therefore, Ca addition refined the grain. Mechanical Properties The hardness, yield strength, tensile strength, and percentage elongation of the alloy are given in Figure 4. Higher yield strength and hardness properties were obtained when the alloy had increased Ca content. The yield strength and the hardness of alloy AMB501-3Ca increased by 22 and 33%, respectively, compared to that of alloy AMB501; however, a reverse effect on the tensile strength and percentage elongation was observed. The change in the mechanical properties of an alloy may be caused by constitutes, fraction, and distribution of the secondary phases. The decreased elongation with increased Ca content is due to an increase in the high fraction area of secondary phases and the formation of a new secondary phase around grain boundaries. The addition of 3.0 wt % calcium led to the formation of a continuous network of the secondary phase of Al2Ca. Therefore, the presence of a continuous network in second phase with a brittle nature in the as-cast alloys is responsible for low elongation. Meanwhile, some hard and brittle Al2Ca phases adjacent to Mg17Al12 can act as barriers for dislocation slipping together with Mg17Al12 to improve yield strength and hardness. Tensile Fracture Analysis The SEM fractographs of the as-cast experimental alloys are shown in Figure 5. The fracture mechanism did not change after the addition of Ca. The fractographs consist of cleavage planes and some tear ridges. Some secondary cracks are also observed on the fracture surface, indicating that the fracture mechanism of AMB501-xCa alloys is a quasicleavage fracture mode. When the Ca addition is 3.0 wt %, a large quantity of flaky phases were observed on the fracture
Fig. 4. Mechanical properties of the experimental alloys.
Microscopy Research and Technique
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69
Fig. 5. SEM image crack morphology of experimental alloys (a) AMB501; (b) AMB501-1Ca; (c) AMB501-2Ca; (d) AMB501-3Ca.
surface, as shown in Figure 5d. The flaky phases have a positive role in the strengthening behavior rather than deteriorating the mechanical property due to its more refined microstructure.
CONCLUSIONS 1. Increasing the addition of Ca into AMB501 alloy decreased the grain size. The amount of Mg17Al12 phase was also decreased and dispersed. When the Ca content reached 3%; a new Al2Ca phase with higher thermal stability was formed. The phase in the high-temperature conditions of the grain boundary pinning effectively prevented the sliding and migration of grain boundary, thereby improving the high-temperature creep properties of the alloy. 2. The mechanical properties of AMB501-3.0Ca alloy improved significantly compared with that of the AMB501 alloy. The yield strength increased by 22%, hardness increased by 33%, and elongation slightly decreased. REFERENCES Aljarrah M, Medraj M, Wang X, Essadiqi E, Muntasar A, Denes G. 2007. Experimental investigation of the Mg-Al-Ca system. Alloys Compd 436:131–141. Bai J, Sun YS, Xue S, et al. 2006. Mater Sci Eng A 419:181–188. Cheng SL, Yang GC, Fan HF, Li YJ, Zhou YE. 2006. Microstructure and tensile creep behavior of Mg-4Al based magnesium alloys with alkaline earth elements Sr and Ca additions. Rare Met Mater Eng 35:1400–1403. Faruk M, Kainer OA, Ulrich K. 2013. Effect of Ca and Y on microstructure and mechanical properties of AZ91 alloy. Trans Nonferrous Met Soc China 23:66–72.
Microscopy Research and Technique
Guan S, Wang Y. 2003. Influence of Ce addition on microstructure and mechanical properties of high pressure die cast AM50magnesium alloy. Mater Mech Eng 27:23–29. Joonhong, Park. 2013. Research Progress and Application of Elevated Temperature Magnesium Alloys in Auto-mobile Industry. J Mech Sci Technol 27:2955–2960. Khomamizadeh F, Nami B, Khoshkhooei S. 2005. Microstructure and mechanical properties of AM50 alloy according to thickness and forming condition of the products by a high pressure die-casting process. Metall Mater Trans A Phys Metall Mater Sci 36A:3489– 3494. Ma Y, Zhang J, Yang M. 2009. Effect of rare-earth element additions on high-temperature mechanical properties of AZ91 magnesium alloy. J Alloys Compd 470:515–521. Ninomira R, Ojiro T, Kubota K. 1995. Research on microstructure and alloy phases of AM50 magnesium alloy. Acta Metall Mater 43: 669–674. Tartaglia JM, Swartz RE, Bentz RL, Howard JH. 2001. Improved heat resistance of Mg–Al alloys by the Ca addition. J Miner Met Mater Soc 53:16–19. Wang KK, Kang YL, Zhang K. 2002. Effects of rare earth elements on the microstructure and properties of magnesium alloy AZ91D. J Univ Sci Technol Beijing 9:363–366. Wang QD, Lu YZ, Zeng XQ, Ding WJ, Zhu YP. 2000. Effects of RE on microstructure and properties of AZ91 magnesium alloy. Trans Nonferrous Met Soc China 10:235–239. Wu GH, Fan Y, Gao HT, Zhai CQ, Zhu YP. 2005. The effect of Ca and rare earth elements on the microstructure, mechanical properties and corrosion behavior of AZ91D. Mater Sci Eng A 408:255–263. Xu Z, Smith C, Chen S, Sankar J. 2011. Development and microstructural characterizations of Mg–Zn–Ca alloys for biomedical applications. Mater Sci Eng B 176:1660–1665. Yuan G, Zhang W, Sun Y. 1999. Effect of bismuth addition on the mechanical properties of Mg-Al based alloys. J Southeast Univ 29: 115–119. (in Chinese) Yuan G-Y, Sun Y-S, Ding W-J. 2001. Effects of bismuth and antimony additions on the microstructure and mechanical properties of AZ91 magnesium alloy. Mater Sci Eng A 308:38–44. Zhang Q, Masoumi M, Hu H. 2012. Influence of Applied Pressure on Tensile Behaviour and Microstructure of Squeeze Cast Mg Alloy AM50 with Ca Addition. J Mater Eng Perform 21:38–46. Zhou W, Aung N, Sun Y. 2009. Effect of antimony, bismuth and calcium addition on corrosion and electrochemical behavior of AZ91 magnesium alloy. Corros Sci 51:403–408.
