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Effect of Substituting Cerium-Rich Mischmetal with Lanthanum on Microstructure and Mechanical Properties of Die-Cast Mg–Al–RE Alloys

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Effect of Substituting Cerium-Rich Mischmetal with Lanthanum on Microstructure and Mechanical Properties of Die-Cast Mg–Al–RE Alloys Materials and Design 30 (2009) 2372–2378 Contents lists available at ScienceDirect Materials and Design journal homepage: www.elsevier.com/locate/matdes Effect of substituting cerium-rich mischmetal with lanthanum on microstructure and mechanical properties of die-cast Mg–Al–RE alloys Jinghuai Zhang a,b,d, Peng Yu c, Ke. Liu a,b,d, Daqing Fang a,d, Dingxiang Tang a,d, Jian Meng a,* a State Key laboratory of Rare Earth Resources Application, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, China b Graduate School of the Chinese Academy of Science, Beijing 100049, China c School of Biological Engineering, Changchun University of Technology, Changchun 130012, China d Changchun Seemay Magnesium Co. Ltd., Changchun, China article info abstract Article history: Die-cast Mg–4Al–4RE–0.4Mn (RE = Ce-rich mischmetal) and Mg–4Al–4La–0.4Mn magnesium alloys were Received 11 September 2008 prepared successfully and their microstructure, tensile and creep properties have been investigated. The Accepted 30 October 2008 results show that two binary Al–RE phases, Al11RE3 and Al2RE, are formed along grain boundaries in Mg– Available online 7 November 2008 4Al–4RE–0.4Mn alloy, while the phase compositions of Mg–4Al–4La–0.4Mn alloy mainly consist of a-Mg phase and Al11La3 phase. And in Mg–4Al–4La–0.4Mn alloy the Al11La3 phase occupies a large grain Keywords: boundary area and grows with complicated morphologies, which is characterized by scanning electron Mg–Al–RE alloy microscopy in detail. Changing the rare earth content of the alloy from Ce-rich mischmetal to lanthanum Lanthanum gives a further improvement in the tensile and creep properties, and the later could be attributed to the Microstructure Mechanical properties better thermal stability of Al11La3 phase in Mg–4Al–4La–0.4Mn alloy than that of Al11RE3 phase in Mg– 4Al–4RE–0.4Mn alloy. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction boundary regions, is an important consideration in improving the elevated temperature properties of die-cast magnesium alloys. Re- Due to the low density of magnesium alloys more and more cently, it has been reported that a new alloy named AE44 (Mg– high-pressure die casts are being applied to automobile industry 4Al–4RE) developed by Hydro Magnesium [9] has more excellent [1]. Some commercial Mg–Al alloys, such as AZ91D, AM60B and high temperature creep and strength performance than that of AM50A, have already been introduced into certain automobile AE42. However, both RE used in AE42 and AE44 are Ce-rich misch- parts of instrument panel, seat frame, steering wheel and so on. metal, which typically composition is 52–55 wt.% Ce, 23–25 wt.% However, on account of their poor creep resistance above men- La, 16–20 wt.% Nd, and 5–6 wt.% Pr [8], for AE44, the causes of tioned AZ and AM series could not be applied to automotive pow- the decline of creep properties at high temperature has not been ertrain components operating at temperatures higher than 120 °C resolved completely. [2–3]. In the present work, a new alloy with improved microstructure Nowadays, enormous efforts have been contributed for explor- features which could offer higher resistance to creep deformation ing creep resistant magnesium alloys for die casting applications was developed. Herein, the rare earth content of Mg–Al–RE alloy and several alloy systems have been developed, such as Mg–Al– was changed from ordinary Ce-rich mischmetal to lanthanum RE (RE = rare earth), Mg–Al–Si and Mg–Al–Ca/Sr alloys [1,2,4–7]. and the microstructure and mechanical properties of the resulting Due to the formation of relatively thermally stable Al11RE3 precip- alloy, Mg–4Al–4La–0.4Mn alloy, were investigated. itates and the complete suppression of Mg17Al12 phase, the ele- vated temperature mechanical properties of AE42 (Mg–4Al–2RE, RE is rare earths added as misch metal) alloy are improved greatly 2. Experimental procedure [4]. Unfortunately, when the temperature surpasses 150 °C the decomposition of Al11RE3 phase distributed along the grain bound- The nominal composition of the studied alloy and the reference aries resulting in deteriorated creep property has also been re- alloy is Mg-4Al–4La–0.4Mn and Mg–4Al–4RE–0.4Mn (RE = Ce-rich ported in AE42 alloy [2,3,8]. It is apparent that alloy designed to mischmetal), respectively. The chemical compositions of the alloys increase microstructural stability, especially in the near-grain were determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES) and the results were listed in Table 1,in which the compositions of alloys named as AE44, AlLa44 were gi- * Corresponding author. Tel.: +86 431 85262030; fax: +86 431 85698041. E-mail address: [email protected] (J. Meng). ven. Commercial pure Mg and Al were used and Mn, La and Ce-rich 0261-3069/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2008.10.028 J. Zhang et al. / Materials and Design 30 (2009) 2372–2378 2373 mischmetal (La:Ce:Pr:Nd=23:55:6:16, wt.%) were added in the form of Al–10 wt.% Mn, Mg–20 wt.% La and Mg–20 wt.% Ce-rich mischmetal master alloys, respectively. Specimens were die casts using a 280 ton clamping force cold chamber die-cast machine. About 20 kg of alloy ingots were melted in a mild steel crucible. Mg Pure argon was used as a protective gas and refined gas. The metal Al11RE3 was hand-ladled into the die casting machine at 700 °C, which was Al RE about 40 °C higher than that of normally casting using an auto- 2 mated metering system involving a pump and heated tube. The die was equipped with an oil heating/cooling system and the tem- perature of the oil heater was set to 240 °C. The tensile samples were 75 mm in gauge length and 6.1 mm in gauge diameter and the value in this study was the average of at least 4 measurements. The tensile creep tests were carried out AE44 Intensity (arb. unit) Intensity (arb. on the specimens of cylindrical geometry with a 100 mm gauge length and 10 mm diameter cross section. Metallographic samples were cut from the middle segments of the tensile or creep bars. To AlLa44 reveal microstructure, the specimen surfaces were etched with 4% nitric acid solution and the microstructures of the alloys were ob- served by scanning electron microscopy (SEM) equipped with an 20 30 40 50 60 70 energy dispersive X-ray spectrometer (EDS) and transmission elec- 2 (degree) tron microscopy (TEM). The phase identification was confirmed by X-ray diffractometry (XRD). Fig. 1. XRD patterns of the die-cast AE44 and AlLa44 alloys. abundant and arranges in layers, and EDS analysis suggests that 3. Results and discussion it is Al11RE3 phase. The other is polygon phase which has only a few, and EDS analysis shows that it is Al RE phase. All the EDS re- 3.1. Analysis of microstructures 2 sults are shown in table 2. Further investigation of the Al–RE phases shows that La has the higher atom percentage of RE in The XRD patterns of the die-cast alloys are shown in Fig. 1. It re- Al RE than in Al RE. Similar results have been reported in die- veals that both the AE44 and AlLa44 mainly consist of a-Mg solid 11 3 2 cast Mg–6%Al–0.5%Zn–1%Ca–3%RE alloy [10]. solution and Al11RE3 phase and at 31.57° also the characteristic Fig. 3 shows the microstructure of die-cast specimen of the new peak of Al2RE phase exists in AE44. It indicates that in AE44 a small alloy that contains lanthanum instead of cerium-rich mischmetal. quantity of Al2RE phase is formed and coexists with a-Mg solid As shown in Fig. 3(a), the microstructure of AlLa44 alloy seems solution and Al11RE3 phase. Fig. 2 shows the microstructures of die-cast specimen of AE44 similar to that of AE44 shown in Fig. 2(a), but the grain size of alloy. As shown in Fig. 2(a), eutectic phases distributed along grain AlLa44 alloy is finer than that of AE44 alloy. Furthermore, the dis- boundary area and a-Mg together constitute the microstructure, tribution of the eutectic phases along the grain boundaries is more and the grain size is about 10–20 lm. The magnified image of uniform. Fig. 3(b) and (f) show the different morphologies of the Fig. 2(b) further shows that the near-grain boundary area is occu- secondary phase, Al11La3, identified by EDS (table 2), from different pied by two secondary phases. One is lamellar phase which is angles. Observed from that in Fig. 3 mainly three complicated mor- phologies exist there. Some parallel acicular compounds connect- ing with lamellar compounds by crosswise branches shown in Table 1 Fig. 3(b) and (d) and some parallel acicular compounds connecting Chemical compositions of the investigated alloys (wt.%). with dendritic compounds shown in Fig. 3(c) can be seen clearly. Alloys Al RE Mn Mg Of the compounds distributed along grain boundary area some AE44 3.84 4.02a 0.41 Balance are parallel with a-Mg grain (Fig. 3b and d) and some are vertical ALa44 3.65 3.94b 0.47 Balance with that (Fig. 3e). The parallel acicular compounds are typically 100 nm in diameter (Fig. 3f) with a length of 2–3 m(Fig. 1d). a Ce-rich mischmetal. l b Lanthanum. Map distributions of the elements in AlLa44 alloy are presented Fig. 2. SEM image (a) and SEM magnified image of the Al–RE phases (b) of the die-cast AE44 alloy.
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