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

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Materials and Design

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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 elevated 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 temperature 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 observed 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 morphologies exist there. Some parallel acicular compounds connecting 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. 2374 J. Zhang et al. / Materials and Design 30 (2009) 2372–2378

Table 2 alloy shows significant strain hardening behavior at RT. Typically, EDS analysis of the secondary phases in the die-cast AE44 and AlLa44 alloys. the strain hardening effect decreases when increasing testing Alloy Shapes Element (at.%) temperature, but no strain softening appears before necking until Al La Ce Pr Nd Mn Mga 200 °C. The curves indicate that the tensile properties of AlLa44 alloy can keep well until 200 °C. To compare the tensile proper- AE44 Polygon 47.97 5.02 14.06 1.51 4.52 0.89 26.04 Lamellar 53.95 3.35 7.62 1.31 2.04 1.23 30.50 ties of AE44 with AlLa44 alloys, the average ultimate tensile strength (UTS), yield strength (YS) and elongation to failure (e) AlLa44 Acicular 53.68 17.56 / / / 1.55 27.21 Lamellar 52.89 13.55 / / / 2.01 31.55 are listed in Table 3. AlLa44 exhibit more excellent tensile properties than AE44 alloy. The relevant strengthening mechanisms a Secondary phases containing Mg were not detected from the XRD result and Mg for the above alloys mainly have two factors to be considered. atoms were found in EDS analysis due to the interaction between the electron beam On the one hand, due to high cooling rate in the die casting pro- and the a-Mg matrix. cess, fine gains which could cause grain refinement strengthen- in Fig. 4. It is obvious that the grain boundary area contains almost ing for the alloys are formed. On the other hand, fine Al–RE all the alloying elements (i.e. Al, La and Mn). It also indicates that phases at the grain boundaries can effectively fortify grain boundary in both AE44 and AlLa44 alloys. In addition, the orderly significant amount of Mn dissolved into Al11La3 phase and this is further confirmed by results of EDS microanalyses of the phases pile of the Al11RE3 (Al11La3) phase can provide a considerable shown in table 2. deformation when the alloys undergo high stress. While the grain size is finer and the distribution of grain boundary phases 3.2. Mechanical properties is more uniform in AlLa44 alloy than those in AE44 alloy, consid- ering the two reasons above, substituting cerium-rich mischmet- Typical tensile stress vs strain cures of AlLa44 alloy tested al with lanthanum can further improve the tensile properties of from room temperature (RT) to 300 °C are shown in Fig. 5. The die-cast Mg–Al–RE alloy.

Fig. 3. SEM image (a) and SEM magniﬁed images of Al11La3 phase (b–f) of die-cast AlLa44 alloy. J. Zhang et al. / Materials and Design 30 (2009) 2372–2378 2375

Fig. 4. EDS elemental mapping showing distributions of Mg, Al, La, Mn in the die-cast AlLa44 alloy.

Fig. 6 shows the typical tensile creep curve of die-cast specimen 300 of the new AlLa44 alloy compared with that of die-cast AE44 alloy, which has been successfully used for producing powertrain com- 250 1 RT ponents [9]. The creep test was carried out at 200 °C under a load 2 120 of 70 MPa for 96 h. As shown in the figure, there is a significant 3 150 improvement in the creep resistance of the new AlLa44 alloy over 200 1 4 175 that of AE44 alloy. The minimum creep rate of AlLa44 alloy ob- 2 5 200 tained by measuring the slope of creep curves at the steady-state 150 6 250 stage is 1.17 109 s1, while the minimum creep rate of AE44 al- 3 7 300 loy is 3.42 109 s1, about three times higher than that of AlLa44 alloy. This implies that addition of lanthanum is more effective for Stress (MPa) 100 strengthening the alloy against creep deformation than that of Ce- 4 rich mischmetal. 6 7 5 50 The major differences in the microstructure of AE44 and AlLa44 alloys arise from the substituting Ce-rich mischmetal with lanthanum. In both alloys the predominant phase remains a-Mg, while 0 the grain boundary microstructure varies significantly. In AlLa44 0 4 8 12 16 20 24 28 32 36 alloy a large amount of Al La exists as the main Al–RE phase at Strain (%) 11 3 the gain boundary area, while in AE44 alloy a mixture of Al2RE Fig. 5. Typical stress–strain curves of the die-cast AlLa44 alloy. and Al11RE3 phases are included. Generally speaking, all the rare 2376 J. Zhang et al. / Materials and Design 30 (2009) 2372–2378

Table 3 Tensile properties of the die-cast AE44 and AlLa44 alloys at different temperatures.

Temperature (°C) AE44 AlLa44 UTS (MPa) YS (MPa) e (%) UTS (MPa) YS (MPa) e (%) RT 247 140 11 264 146 13 120 172 126 22 182 128 23 150 145 110 25 148 112 27 175 123 107 26 135 108 25 200 115 105 23 118 102 20

earth-containing phases could be effective for improving the creep phase in AlLa44 alloy appears to be responsible for its higher creep resistance of magnesium alloys [11,12]. But the nature of Al11La3 resistant performance as discussed below. Fig. 7 shows the SEM images taken from the longitudinal section in the stress direction of AE44 specimens before (Fig. 7a and 0.20 b) and after (Fig. 7c and d) creep test at 200 °C under an applied 0.18 stress of 70 MPa for 96 h. The changes of morphology in near-grain boundary are observed obviously. The ﬁne eutectic which consists 0.16 AE44 200 /70MPa of Al11RE3 phase and a-Mg at the grain boundaries becomes 0.14 incompact and coarse, and more polygonal Al2RE phase appears AlLa44 (analyzed by EDS). The similar results have been reported in die- 0.12 cast AE42 alloy that contains Ce-rich mischmetal [8,11]. Results 0.10 of this study suggest that Al11RE3 in AE44 alloy is unstable and 0.08 partly decomposes into Al2RE and aluminum, consequently, vol- Alloy Minimum Creep Rate (s-1) ume fraction of Al11RE3 phase decreases while that of Al2RE in- Creep Strain (%) Strain Creep 0.06 creases through phase transformations during creep test at AE44 3.42× 10-9 0.04 200 °C under applied stress of 70 MPa. Fig. 8 shows XRD patterns AlLa44 1.17× 10-9 of AE44 alloy before and after creep test for 96 h. The intensity of 0.02 Al11RE3 peaks decrease and that of Al2RE peaks increase obviously. 0.00 These changes are consistent with the deduction which Al11RE3 0 20 40 60 80 100 decomposes to Al2RE and free aluminum. But Mg17Al12 is not found Time (h) in this study as the results in AE42 [8], the reason may be that due

Fig. 6. Creep properties of the die-cast AE44 and AlLa44 alloys. to the higher content of rare earth elements in AE44, most Al is

Fig. 7. SEM images of the die-cast AE44 alloy before (a, b) and after (c, d) creep test. J. Zhang et al. / Materials and Design 30 (2009) 2372–2378 2377

Mg Mg Al11RE3 Al11La3 Al2RE

After creep test After creep test Intensity (arb.unit) Intensity (arb.unit)

Before creep test Before creep test

16 18 20 22 24 26 28 30 32 34 36 38 16 18 20 22 24 26 28 30 32 34 36 38 θ 2θ (degree) 2 (degree)

Fig. 8. XRD patterns of AE44 alloy before creep and creep for 96 h. Fig. 10. XRD patterns of AlLa44 alloy before creep and creep for 96 h.

Al11RE3 phase is sensitive to rare earth elements, and Al11La3 phase present in the form of Al11RE3 and Al2RE, with little Al sequestered as solute in the a-Mg matrix. This is also supported by TEM obser- in AlLa44 alloy is apparently more thermally stable than Al11RE3 in AE44 alloy, which contains different rare earth elements like cer- vations that no discernible Mg17Al12 forms after ageing at elevated temperatures [13]. Fig. 9 shows the SEM images taken from the ium or neodymium that may readily form Al2RE phase under se- longitudinal section in the stress direction of AlLa44 specimens be- vere creep conditions. Owing to the Al11La3 phase existing in the fore (Fig. 9a and b) and after (Fig. 9c and d) creep test at the same grain boundary area, a very effective hindrance is provided to im- condition as AE44. As shown in Fig. 9, after creep exposure no pede grain boundary sliding and dislocation motion in the vicinity apparent changes of morphology in microstructure are observed of the grain boundaries (Fig. 11). And also the conclusion above im- in AlLa44 alloy. Also, it can not be found discernible changes from plies the importance of the stability of microstructure in the near- XRD patterns of AlLa44 alloy before and after creep test (Fig. 10), grain boundary on creep resistance in these ﬁne-grained die-cast which supports the SEM images. These indicate that the micro- alloys. structure of the new alloy remains stable in the creep test. Consid- At the same time, we turn to theoretic calculation to support ering the results above it is concluded that the thermal stability of the results of creep experiment. The calculations were performed

Fig. 9. SEM images of the die-cast AlLa44 alloy before (a, b) and after (c, d) creep test. 2378 J. Zhang et al. / Materials and Design 30 (2009) 2372–2378

Fig. 11. TEM images of dislocation pileups at grain boundaries in AlLa44 alloy after creep test at 200 °C under applied stress of 70 MPa for 96 h.

Table 4 Acknowledgements

Decomposition energy of Al11RE3 (RE = La, Ce, Pr, Nd). This project was supported by the Ministry of Science and Tech- Kinds of RE EAl11 RE3 (eV/ EAl2 RE (eV/ EAl (eV/atom) 4E (eV/atom) atom) atom) nology of China (2006AA03Z520, 2008DFR50160), Chinese Acad- RE = La 3235.5337 982.2516 57.1592 2.9828 emy of Sciences and Jilin Province. The authors also would like to RE = Ce 3835.0802 1182.4250 57.1592 2.0092 thank Shanxi Wenxi Yinguang Magnesium Group for the assistance RE = Pr 4519.7980 1410.3775 57.1592 2.8695 of preparation of samples. RE = Nd 5307.0863 1673.7845 57.1592 0.0632

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