Effects of Zn content on as-cast microstructure and mechanical properties of Mg-xZn-4A1 alloys containing TiC and rare earth elements
BAI Liang(白 亮)1, PAN Fu-sheng(潘复生)1, YANG Ming-bo(杨明波)1,2
1. Chongqing Engineering Research Center for Magnesium Alloys, Chongqing University, Chongqing 400030, China;
2. Department of Materials Science and Engineering, Chongqing Institute of Technology, Chongqing 400050, China
Received 28 July 2006; accepted 15 September 2006
Abstract：The effects of Zn content on the as-cast microstructure and mechanical properties of Mg-xZn-4A1 alloys containing TiC and rare earth elements were investigated by optical microscopy(OM), scanning electron microscopy(SEM) analysis, X-ray diffraction (XRD) analysis and tensile test. The results show that Zn content which increased from 8% to 12% does not obviously influence on the alloy phase type of the Mg-xZn-4A1 experimental alloys containing 0.25%RE and 1%TiC, but with Zn content increasing from 8% to 12%, the amount of Mg32(Al,Zn)49 phase in the as-cast microstructure of the experimental alloys increases and its distribution becomes more continuous. In addition, the Mg-10Zn-4A1 alloy containing 0.25%RE and 1TiC has the highest ultimate tensile strength at room temperature and 150 ℃ and highest yield strength and elongation at 150 ℃. Furthermore, with Zn content increasing from 8% to 12%, the yield strength and elongation of Mg-xZn-4A1 experimental alloys containing 0.25%RE and 1%TiC increase and decrease at room temperature, respectively.
Key words: Mg-Zn-Al alloys; as-cast microstructure; Mg32(Al,Zn)49 phase ; mechanical properties
1 Introduction
Magnesium alloys have been more and more widely used as structural materials since 1990s. At present, the widely used magnesium alloys are Mg-Al series alloys, such as AZ91 and AM60 alloys, but they are unsuitable for manufacturing parts operating at temperatures higher than 120 ℃ because of their poor creep resistance[1-2]. Therefore, it is very necessary to develop some new magnesium alloys with good high temperature properties, acceptance castability and low cost. Recent investigation [3-6] indicated that the Zn element not only could contribute to solution strengthening but also had the tendency of age-hardening because of its solid solubility of 6.2% in magnesium alloys. Moreover, when Zn content reached a certain value, the Mg32(Zn, Al)49 phase with high melting point and thermal stability which can improve the high temperature properties of magnesium alloys, would occur in the Mg-Zn-Al based alloys. Therefore, Mg-Zn-Al based alloys are thought as a potential elevated temperature magnesium alloys. At present, the research and development about Mg-Zn-Al based alloys have received much attention all over the world, and consequently many researches, such as the effects of the Zn/Al value, Ca or RE elements on the as-cast microstructure and mechanical properties of the based alloys, have been carried out[7-16]. However, the investigations on the effects of Zn content on as-cast microstructure and mechanical properties of Mg-Zn-Al based alloys containing TiC and RE additions which are beneficial to this based alloys, has not been carried out.
The aim of the present work is to investigate the effects of Zn content, on the as-cast microstructure and mechanical properties of the Mg-xZn-4A1 alloys containing TiC and rare earth elements to obtain the theory basis of the microstructure and mechanical controlling for Mg-Zn-Al based alloys.
2 Experimental
The chemical composition of the experimental alloys is shown in Table.1, which were prepared from commercially AM60 alloy, pure Mg, Al, and Zn. The rare earth additions were made of Y-rich misch metal, and TiC were added in the form of powder. The experimental alloys were melted in a crucible resistance furnace and protected by a flux addition. After refined by C2Cl6, the melt was held at 740 ℃ for 20 min and then poured into a permanent mould which was coated and preheated to 200 ℃ in order to obtain specimens for structure analysis and tensile testing. Tensile specimens with a gage section of 15.0 mm×3.5 mm×2.0 mm were fabricated by electric spark machining, and then short time tensile testing of the as-cast specimens at room temperature and 150 ℃ were performed on SANS- CMT5105 materials testing machine, and short time tensile testing rates were 3 mm/min and 1 mm/min respectively at room temperature and 150 ℃. The structure analysis samples were etched with a solution of 8%(volume fraction) nitric acid+distilled water and then were examined by Olympus optical microscope and JOEL JSM-6460LV type scanning electron microscope equipped with Oxford energy dispersive X-ray spectrometer (EDS). The phases in the experimental alloys were also analyzed by D/Max-1200X type analyzer operated at 40 kV and 30 mA.
Table 1 Chemical composition of experimental alloys(mass fraction, %)
3 Results and discussion
3.1 As-cast microstructure of experimental alloys
X-ray diffraction (XRD) patterns taken from the experimental alloys are shown in Fig.1. It is found that the as-cast microstructure of Mg-xZn-4A1 experimental alloys with 0.25%RE and 1%TiC and different Zn contents are composed of α-Mg matrix, Mg32(Al, Zn)49 phase and Al2Mg5Zn2 phase. Zn content does not obviously influence on the alloy phase type of the Mg-xZn-4A1 experimental alloys containing 0.25%RE and 1%TiC.
The as-cast microstructure of permanent mould cast Mg-xZn-4Al experimental alloys with 0.25%RE and 1%TiC and different Zn contents are shown in Fig.2. It is found that the as-cast microstructure of the experimental alloys with 8%, 10% and 12%Zn are mainly composed of white α-Mg matrix and black second phases, and the distribution of the second phases in the as-cast structure of the experimental alloys with 8%Zn, 10%Zn and 12% Zn shows different changing tendency. The results obtained by using XRD and EDS spectrum show that these black second phases are mainly Mg32(Al, Zn)49 phase and Al2Mg5Zn2 phase, and with the increasing Zn content, the amount of Mg32(Al, Zn)49 phase in the as- cast microstructure of the experimental alloys increases and becomes more continuous (Fig.2). This result might be related to the reason which the atomic radius of Zn element is more close to Al element than Mg element[3,7].
Fig.1 X-ray diffraction patterns of experimental alloys with different Zn contents: (a) 8%Zn; (b) 10%Zn; (c) 12%Zn
3.2 Mechanical properties of experimental alloys
The effects of Zn content on the as-cast mechanical properties of the Mg-xZn-4A1 experimental alloys containing 0.25%RE and 1%TiC are listed in Table 2. According to the testing results of mechanical properties, it is found that the mechanical properties of the experimental alloys at room temperature and 150 ℃ were influenced by Zn content. When Zn content is 10%, Mg-10Zn-4A1 experimental alloy(alloy 2) containing 0.25%RE and 1%TiC has the highest ultimate tensile strength(UTS) at room temperature and 150 ℃. Moreover, with the increasing Zn content from 8% to 12%, the yield strength(YS) and the elongation(d0.5) of Mg-xZn-4A1 experimental alloys containing 0.25%RE and 1%TiC increases and decreases respectively at room temperature. In addition, Mg-10Zn-4A1 experimental alloy containing 0.25%RE and 1%TiC has the highest yield strength and elongation at 150 ℃.
Fig.2 As-cast microstructures of experimental alloys with different Zn contents: (a) 8%Zn; (b) 10%Zn; (c) 12%Zn; (d) EDS spectrum from area A in (a)
Table 2 Mechanical properties of experimental alloys
4 Conclusions
1) Zn content which increased from 8% to 12% does not obviously influence on the alloy phase type of the Mg-xZn-4A1 experimental alloys containing 0.25%RE and 1%TiC, but with the Zn content increasing from 8% to 12%, the amount of Mg32(Al, Zn)49 phase in the as-cast microstructure of the experimental alloys increases and its distribution becomes more continuous.
2) Mg-10Zn-4A1 experimental alloy containing 0.25%RE and 1%TiC has the highest ultimate tensile strength at room temperature and 150 ℃ and the highest yield strength and elongation at 150 ℃. Furthermore, with Zn content increasing from 8% to 12%, the yield strength and elongation of Mg-xZn- 4A1experimental alloys containing 0.25%RE and 1%TiC increases and decreases at room temperature, respectively.
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(Edited by YANG Hua)
Foundation item: Projects(2001AA331050) supported by the National High-Tech Research and Development Program of China; Project (CSTC-2004AA4003) supported by Chongqing Science and Technology Commission of China
Corresponding author: BAI Liang; Tel: +86-23-65111520; E-mail: bailiang_wb@163.com
