含有稀土镁合金

Materials Science and Engineering A 489 (2008) 113–119 Microstructures, tensile properties and nta ian , Jia ngchu un 13 nce, ber Abstract In order t –4RE developed. T een i of Mg–4Al– ry Al in Mg–4Al– 4Ce– resistance ar ertie containing fi d to t Al and Ce/La. © 2007 Elsevier B.V. All rights reserved. Keywords: Mg–Al–RE alloy; Microstructure; Tensile properties; Corrosion 1. Introdu As the very suitab vehicle we becoming consensus AM60B an ity and room the above-m applied to than 130 ◦C properties [ Thus, th performanc are based o earth elem and Mg–A ∗ Correspon E-mail ad 0921-5093/$ doi:10.1016/j ction lightest structural materials, magnesium alloys are le for the applications of automotive industry where ight reduction and consequently energy saving are the word focus. It is accepted widely by general that commercial magnesium alloys such as AZ91D, d AM50A offer a good balance between die castabil- temperature mechanical properties. Unfortunately, entioned alloys of AZ and AM series could not be power train parts operating at temperatures higher due to their poor elevated temperature mechanical 1–3]. e alloys with an improved elevated temperature e have been developed greatly and most of them n the Mg–Al–RE (RE represents mischmetal rare ents that is enriched in cerium element), Mg–Al–Si l–Ca/Sr systems [2–5]. The mechanical properties ding author. Tel.: +86 431 85262030; fax: +86 431 85698041. dress: jmeng@ciac.jl.cn (J. Meng). of AE42 (Mg–4Al–2RE) alloy developed in the late 1980s are improved due to the formation of relatively thermal stable Al11RE3 precipitates and the complete suppression of Mg17Al12 phase [3,6,7]. Recently, a new alloy named AE44 developed by hydro magnesium [8] has more excellent high temperature creep and strength performance than that of AE42 and has been successfully used for producing large structural magnesium cast- ings. At present, it is evident that most of the magnesium parts for vehicles are produced by high pressure die-casting, which is a high volume production technique. For this method, alloy cost takes a significant proportion of the component cost. RE used in AE alloys such as AE44 is Ce-rich mischmetal, which typically comprises cerium (Ce), lanthanum (La), neodymium (Nd) and praseodymium (Pr). However, due to the rising price of Nd and Pr, it inevitably causes the cost of mischmetal to increase quickly and Nd and Pr will be separated from it. It is well known that for new alloys one principal requirement is their price competitive ability with the existing commercial magnesium and aluminium alloys. So developing a new alloy with low cost and superior properties compared with commercial alloys is paid considerable attentions. Actually, at present, the – see front matter © 2007 Elsevier B.V. All rights reserved. .msea.2007.12.024 die-cast Mg–4Al-based alloys co Jinghuai Zhang a,b, Deping Zhang a, Zheng T Huayi Lu a, Dingxiang Tang a a Key Laboratory of Rare Earth Chemistry and Physics, Cha Chinese Academy of Sciences, Changch b Graduate School of the Chinese Academy of Scie Received 4 September 2007; received in revised form 2 Decem o study the properties of Mg–Al–RE (AE) series alloys, the Mg–4Al heir microstructures, tensile properties and corrosion behavior have b 4La–0.4Mn alloy consist of �-Mg and Al11La3 phases. While two bina 4Ce/La–0.4Mn alloy, and Al11Ce3 and Al2Ce are formed in Mg–4Al– e obtained in Mg–4Al–4Ce/La–0.4Mn alloy, the excellent tensile prop ne secondary phases, and the good corrosion resistance is partly relate corrosion behavior of ining La and/or Ce a , Jun Wang a,b, Ke Liu a,b, n Meng a,∗ n Institute of Applied Chemistry, 0022, China Beijing 100049, China 2007; accepted 14 December 2007 –0.4Mn (RE = La, Ce/La mischmetal or Ce) alloys were nvestigated. The results show that the phase compositions –RE (RE = Ce/La) phases, Al11RE3 and Al2RE, are formed 0.4Mn alloy. The optimal tensile properties and corrosion s are mainly attributed to fine grains and grain boundaries he presence of compact corrosion product film containing 114 J. Zhang et al. / Materials Science and Engineering A 489 (2008) 113–119 mixture of Ce and La is abundant and its price is much lower than that of ordinary Ce-rich mischmetal in China. Thus it would have great significances to try to substitute Ce-rich mischmetal with the mixture of Ce and La in AE alloys. Herein, the current work mainly aimed to investigate the influences of Ce and/or La on the microstructures, mechanical properties and corrosion behavior of Mg–4Al–0.4Mn alloy, which would be beneficial for the research of AE alloys. 2. Experimental procedure The no Mg–4Al–4 Mg–4Al–4 about 60 w Al were us added in th ter alloys. S force cold sium alloy argon was was hand-l a melting normally u involving a 700 ◦C prio an oil heat heater was alloys were emission sp Table 1. The tens in gauge d 1121 tensil vated temp reported va Metallogra of the tens observed b with an ene compositio X-ray diffr (TEM). Th in alcohol. in an electr The corr grades of e performed tral 5 wt.% Table 1 Chemical com Alloys A B C Fig. 1. SEM images of the die-cast alloys. cimens in the experiment was done by dipping in a 400 ml s solution of 10% CrO3 + 1% AgNO3 in boiling condi- he extent of corrosion was given in weight loss per unit e area and time (mg/(cm2 day), or MCD). Electrochemical ation tests were carried out in a standard three-electrode lytic cell containing about 500 ml of Mg(OH)2 saturated wt.% NaCl solution. Specimens were immersed in the lution and a polarization scan was carried out at a rate of −1 . minal compositions of investigated alloys were La–0.4Mn (A), Mg–4Al–4Ce/La–0.4Mn (B), and Ce–0.4Mn (C). Ce/La represented the mixture of t.% Ce and 40 wt.% La. Commercial pure Mg and ed and Mn and RE (RE = Ce, Ce/La and Ce) were e form of Al–10 wt.% Mn and Mg–20 wt.% RE mas- pecimens were die casts using a 280 tonnes clamping chamber die-cast machine. About 20 kg of magne- ingots were melted in a mild steel crucible. Pure used as a protective gas and refined gas. The metal adled into the die-casting machine and this required temperature that was about 40 ◦C higher than that sed for casting with an automated metering system pump and heated tube, and the melt temperature of r to casting was used. The die was equipped with ing/cooling system and the temperature of the oil set to 240 ◦C. The chemical compositions of the determined by inductively coupled plasma atomic ectroscopy (ICP-AES) and the results were listed in ile samples were 75 mm in gauge length and 6.1 mm iameter. Tensile tests were performed using Instron e testing machine at room temperature (RT) and ele- eratures with a strain rate of 4.4 × 10−4 s−1. The lue was the average of at least four measurements. phic samples were cut from the middle segment ile bars and the microstructures of the alloys were y scanning electron microscope (SEM) equipped rgy dispersive X-ray spectrometer (EDS). The phase n, structure and morphology were characterized by action (XRD) and transmission electron microscopy e specimens for SEM were revealed by 4% nitric acid The TEM foils were prepared by twin-jet polishing olyte of 5 vol.% perchloric acid in alcohol. osion specimens were polished successively on finer mery papers up to 800 levels. Corrosion tests were using standard salt-spray corrosion chamber in neu- NaCl at different temperatures for 4 days. Cleaning of positions of the investigated alloys (wt.%) Al La Ce Mn Fe Mg 3.65 3.94 – 0.47 0.0059 Bal. 3.50 1.45 2.56 0.46 0.0050 Bal. 3.62 – 3.83 0.43 0.0087 Bal. the spe aqueou tion. T surfac polariz electro with 5 test so 1 mV s J. Zhang et al. / Materials Science and Engineering A 489 (2008) 113–119 115 3. Results and discussion 3.1. Microstructures Fig. 1 shows the SEM images of the die-cast alloys. It can be seen that all alloys are mainly composed of continuous or semi-continuous network grain boundaries, which comprised of a lot of secondary phases and granular phase of �-Mg. The average grain sizes of (A), (B) and (C) alloys, determined by linear-intercept method, are about 11, 7 and 16�m, respectively and the grain size of (B) alloy is more uniform than that of (A) and (C) alloys. Considering the formation of fine-grain magne- sium alloys, several factors as follows should be involved in this work. Under the condition of high pressure die casting, large thermal undercooling caused by high cooling rate may show a dominant effect on the formation of fine grains. According to the fundam of Mg–RE solute RE i tion proces liquid ahea in the const rate of atom the growth ment of so which mai grain grow brings certa seems that of Mg–4A exhibits be the followi and Ce tog reduce the consequent gated at so effective u Fig. 1(A), c of grain bo containing should be n the die-cas cooling bet F SEM micrographs of the secondary phases at grain boundary of the alloys. in size of the die-cast alloys. Furthermore, the area frac- f secondary phases in (A), (B) and (C) alloys decreased in his is because solid solubility of La in Mg (0.07 at.%) is r than that of Ce in Mg (0.13 at.%) and more La atoms are d in the formation of intermetallic compounds. The liter- 2] has reported that the formation of either Al11RE3 or is sensitive to individual rare earth element and a relation- also discovered between the relative amounts of Al11RE3 Al2RE and the La:Nd (below 0.7 Al2RE was seen and 0.7 Al11RE3 seemed to form). In the present work, XRD entals of solidification and the binary phase diagram (RE = Ce, La) system, the distribution coefficient of s less than 1 and consequently during the solidifica- s, solute atoms RE, as well as Al, are enriched in the d of the solid–liquid interface. And this could result itutional undercooling and the reduction of diffusion s, subsequently the number of nuclei is increased and of grains is restricted. On the other hand, the enrich- lute atoms leads to the formation of Al–RE phases, nly distribute in the grain boundary area, thus the th is further inhibited. In conclusion, addition of RE in effect on grain refinement. In the present work, it La has a finer effect than Ce on the grain refinement l-based alloy while addition of La and Ce together tter than the addition of La only. For this phenomenon ng factors may be considered. When addition of La ether to Mg–4Al-based alloy, they can interact and solid solubility in �-Mg matrix each other [9–11], ly more La and Ce atoms are enriched and aggre- lid–liquid interface, which would result in the more ndercooling refining effect. Moreover, as shown in onsiderable Al–La phases, which occupy a large area undary would also coarsen the grain size of the alloy La, determined by linear-intercept method. Also it oted that the molten Mg alloys are hand-ladled into ting machine, so small fluctuation of thermal under- ween the samples is inevitable, and this could affect ig. 2. X-ray diffraction pattern of the die-cast alloys. Fig. 3. die-cast the gra tions o turn. T smalle utilize ature [ Al2RE ship is versus above 116 J. Zhang et al. / Materials Science and Engineering A 489 (2008) 113–119 Fig. 4. TEM (e) petal-like analysis in solution an solid soluti (C) alloy in compounds Fig. 3 sh ing the seco (C) alloys. Fig. 5. Typica images showing (a) distribution of Al–RE phases at grain boundary; (b) polygonal A Al11RE3; (f) the corresponding SAED of (e). dicates that (A) alloy mainly consists of �-Mg solid d Al11La3 compound, (B) alloy is composed of�-Mg on and Al11(La/Ce)3 and Al2(La/Ce) compounds and cludes �-Mg solid solution and Al11Ce3 and Al2Ce (Fig. 2). ows the further magnified SEM micrographs reveal- ndary phases at the grain boundaries in (A), (B) and Acicular and lamellar phases congregating together l stress–strain curves of the (B) alloy at different temperatures. and connec and EDS an be seen tha is rod-like phase by t EDS sugge (RE = La/C that the RE Fig l2RE; (c) the corresponding SAED of (b); (d) rod-like Al11RE3; ted by crosswise branches are observed in (A) alloy alysis reveals that they are Al11La3 (Table 2). It can t there are two sorts of particles in (B) alloy. One shape and it is proved to be Al11RE3 (RE = La/Ce) he EDS analysis, the other is polygon shape and sts that the chemical formula for this phase is Al2RE e) (Table 2). In addition, further investigation shows compositions are La43±1Ce57±1 (at.%) in Al11RE3 . 6. Weight loss corrosion rate for the die-cast alloys. J. Zhang et al. / Materials Science and Engineering A 489 (2008) 113–119 117 Fig. 7. Corroded surface photographs of the die-cast alloys after the salt-spray test at 35 ◦C for 4 days. Table 2 The results of EDS analysis of Fig. 3 (at.%) Location nAl:nREa a 65.2:17.6 b 62.0:16.6 c 62.0:29.6 d 72.4:19.1 e 64.6:29.3 a RE: La, Ce/La or Ce. phase and L the averag reported in [12]. Comp ferred to fo also two ph like phase a are Al11Ce ing the thre microstruc Fig. 4 s diffraction rod-like Al at grain boundary area are observed in (B) alloy (Fig. 4(a)); polygonal Al2RE phase with size of 300–500 nm (Fig. 4(b)) and a small amount of petal-like particle (Fig. 4(e)) which does not appear in SEM image distribute around the network. SAED anal- yses further confirm that both rod-like and petal-like particles are Al11RE3 phase, which belongs to body-centered orthorhom- bic structure, and polygonal particles are Al2RE phase which belongs to face-centered cubic structure [13]. ensil typ per sile see allo h (U the s alloy s tha atur a32±1Ce68±1 (at.%) in Al2RE phase determined by e values of four points. Similar results have been die-cast Mg–6% Al–0.5% Zn–1% Ca–3% RE alloy aring the two results, it is confirmed that La is pre- rm Al11La3 phase in Mg–4Al-based alloy. There are ases with different morphologies in (C) alloy, rod- nd polygon phase, and EDS analysis shows that they 3 and Al2Ce phases, respectively (Table 2). Contrast- e micrographs it is found that (B) alloy has the finer ture of grain boundary than (A) and (C) alloys. hows the TEM images and selected area electron 3.2. T The ent tem the ten clearly of the strengt (ε) of AE44 It show temper (SAED) of (B) alloy. Fine network composed by fine 11RE3 phase with the width of 100–200 nm (Fig. 4(d)) (B) alloy ar 27% at 200 Fig. 8. SEM images of the corrosion product film of the die-cast alloys: (a)– e properties ical stress–strain curves of (B) alloy tested at differ- atures are shown in Fig. 5. The curves indicate that properties of (B) alloy can keep well until 200 ◦C. To the influence of different RE on the tensile properties ys, the related tensile properties of ultimate tensile TS), yield strength (YS) and elongation to failure tudied alloys and AE44 alloy are listed in Table 3. was prepared in the same condition for comparison. t the optimal tensile properties both at room and high es are obtained in the (B) alloy. The UTS, YS and ε of e 271, 160 MPa and 14% at RT, and 120, 107 MPa and ◦C. The (C) alloy and AE44 alloy exhibit the similar (c) (B) alloy, (d) (A) alloy, and (e) (C) alloy. 118 J. Zhang et al. / Materials Science and Engineering A 489 (2008) 113–119 Table 3 Tensile properties of the die-cast alloys (S.D. is given in parentheses) Alloys UTS (MPa) YS (MPa) RT 150 ◦C 200 ◦C RT 150 ◦C A 264 (3.4) 148 (4.5) 118 (4.8) 146 (3.1) 112 (3.4) B 271 (2.8) 150 (3.6) 120 (2.4) 160 (3.7) 121 (2.8) C 250 (4.4) 147 (2.1) 113 (3.6) 141 (4.7) 109 (3.8) AE44 247 (4.7) 145 (3.0) 115 (6.9) 140 (4.9) 110 (3.7) test results and the tensile properties of (A) alloy are better than that of (C) alloy and AE44 alloy in the most condition. The above results suggest that Ce/La mischmetal can improve the die-cast Mg–4Al-based alloy more effectively than La, Ce as well as Ce-rich mischmetal. The following two factors should be considered. First, the grain size of (B) alloy is finer than that of (A) and (C) alloys (Fig. 1), so the tensile strength and elonga- tion of (B) alloy are enhanced by grain refinement strengthening effect. Second, generally the size, shape, quantity and distri- bution of secondary phases can also influence the mechanical properties phases occ sidered to and disloca Although th than that in finer than t strengthen lent tensile refinement 3.3. Corro Fig. 6 sh tests. The M lowing ord corrosion r for all the a accelerate Table 4 Electrochemi Alloys A − B − C − Fig. 7 sh after the sa corrosion p allo its di oes the S es of nt th rpho S a 68 w ple (c)) 1.36 oduc cor with tio o truc oxi nd 1 cont cont that tter c in Fig. 8(a) is more protective against corrosion to oy substrate than that formed on the area marked with in Fig. 8(a). All the analyses also indicate that the area d with letter b in Fig. 8(a) should correspond to the rel- deep corrosion pit in Fig. 7(B). The corrosion products on most of the surfaces of (A) alloy and (C) alloy are in Fig. 8(d) and (e). Compared with that in Fig. 8(c), of the alloy [14–16]. As shown in Fig. 3, the Al–RE upies a large grain boundary area and this is con- be an effective obstacle to grain boundary sliding tion motion in the vicinity of the grain boundaries. e area fraction of Al–RE phases in (A) alloy is higher (B) and (C) alloys, the Al–RE phases in (B) alloy are hat in (A) and (C) alloys, which can more effectively the grain boundaries. As a consequence, the excel- properties of (B) alloy can be mainly ascribed to the of the microstructure. sion behavior ows the corrosion rates obtained from the salt-spray CD values of corrosion rates increased in the fol- er: (B) < (A) < (C) both at 25 and 35 ◦C. In addition, ate tested at 35 ◦C is higher than that tested at 25 ◦C lloys and it indicates that increase of temperature can corrosion of the alloys. very sh sion p underg shows surfac appare ent mo and ED and 0. the sam (Fig. 8 RE is sion pr further related ume ra micros minum 1.10 a higher lower cluded with le the all letter b marke atively formed shown Fig. 9. Polarization curves of the die-cast alloys. they appea partly contr The polariz alloys are seen that t from corro (B) < (A) < corrosion r ε (%) 200 ◦C RT 150 ◦C 200 ◦C 102 (5.1) 13 (0.9) 27 (1.5) 20 (1.8) 107 (3.9) 14 (1.2) 31 (1.8) 27 (2.8) 98 (4.1) 10 (1.1) 25 (3.7) 17 (2.6) 105 (6.1) 11 (1.6) 25 (2.7) 23 (4.6) cal data of the die-cast alloys Ecorr (V) Icorr (mA cm−2) Corrosion rate (g m−2 h−1) 1.613 0.103 0.0461 1.611 0.084 0.0376 1.623 0.118 0.0529 ows the surface features of the corroded specimens lt-spray test at 35 ◦C for 4 days and removal of the roducts. It can be seen that all the specimens suffer w surface corrosion with some relatively deep corro- stributing on it. Obviously, the surface of (B) alloy milder corrosion than that of (A) and (C) alloys. Fig. 8 EM images of the corrosion products formed on the the die-cast alloys after the 4 days salt-spray test. It is at the corrosion products of (B) alloy have two differ- logies (Fig. 8(a)). One is loose and cracked (Fig. 8(b)) nalysis shows that the content of Al and RE is 0.35 t.%, respectively; the other, which occupies most of surface, is smooth, uniform as well as more compact and EDS analysis shows that the content of Al and and 1.66 wt.%, respectively. As expected, the corro- t film formed during the corrosion process obstructs rosion underneath. This protective characteristic is the so-called Pilling–Bedworth ratio (PBR, the vol- f metal elements and their oxide) criterion and the ture of film [17]. The PBR of magnesium oxide, alu- de, lanthanum oxide and cerium oxide is 0.81, 1.28, .16 [18], respectively. It means that the film with ent of Al and RE is more protective than that of with ent. According to the analysis above, it can be con- the corrosion product film formed on the area marked r coarse and less uniform relatively and these could ibute to the high corrosion rates of (A) and (C) alloys. ation curves and electrochemical data of the die-cast shown in Fig. 9 and Tab