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Phase equilibria in the Mg-rich corner of the Mg-Zn-La system at 350°C
RARE METALS Vol. 25, No. 5, Oct 2006, p . 572
Phase equilibria in the Mg-rich corner of the Mg-Zn-La system at 350°C LI Hongxiao, REN Yuping, H U N G Mingli, CHEN Qin, and H A 0 Shiming School of Materials and Metallurgy, NortheasternUniversity, Shenyang 110004, China (Received 2006-06-19)
Abstract: The phase equilibria in the Mg-rich comer of the Mg-Zn-La system at 350°C have been investigated by scanning electron microscopy, X-ray diffraction, and electron probe microanalysis. It has been shown that the linear compound (Mg,Zn)17La2existed in the Mg-Zn-La system at 350°C. The linear compound (so-called Tphase) was with the C-centred orthorhombic crystal structure induced by the solution of significant quantities of the third element. The three-phase region a(Mg) + MgZn(La) + T and the two-phase region composed of the a(Mg) and the linear-compound T phase existed in the Mg-rich comer of the Mg-Zn-La system at 350°C. Key words: Mg-Zn-La system; phase diagram; linear compound
[This work isfinancially supported by the National Natural Science Foundation of China (No. 50471025).]
1. Introduction Mg-based alloys are attractive for applications because of the light weight and a high specific strength. Among the common alloying elements, the addition of zinc could make the magnesium alloys strengthen by precipitation [l-21. The further addition of the rare-earth elements to the Mg-Zn alloys is more effective on the precipitation-strengthening [3-41. But phase diagrams of the Mg-Zn-RE system, as the basis of the alloying, have hardly been built up [5-61. The phase equilibria and compositions between the magnesium solid solution and the intermetallic compounds in the Mg-Zn-La system at 350°C have been researched in this article, so that the essential knowledge for designing the alloys and heat-treatment could be provided.
2. Experimental procedure The experimental alloys with compositions (at.%) of M&5Z%LalO? MgS&n38Lal2, Mg79Zn20La1, and Mg7eZn2&a2were prepared by melting the high purity Mg (99.99%), zinc (99.999%), and lanthanum (99.8%)wrapped by the Ta foils in the vacuum ( lo4 Corresponding author: LI Hongxiao
Pa) quartz tubes. The quartz tubes hanged were heated to 800°C and kept for 1 h, and then cooled to room temperature in the furnace. The melting procedure was repeated again with the upside-down quartz tubes. During heating, the quartz tubes were shaken continuously to make the ingots homogeneous. All the as-cast samples were wrapped with Ta foils and sealed in a quartz tube with the vacuum of lo4 Pa, then kept at 350°C for 1440 h and quenched in cold water. X-ray diffraction analysis was carried out with powder samples on Siemens D5000 diffractometer with Cu K, radiation, a voltage of 40 kV and a current of 40 mA. The microstructure analysis was carried out on SSX-500 scanning electrical micro-analyzer with a voltage of 30 kV. The samples observed were polished with the MgO solution. The compositions of equilibrium phases in alloys were analyzed on EPMA-1600 electron probe microanalyzer with a beam size of 1 pm and a voltage of 15 kV. The high pure Mg, Zn, and La were served as standards to revise the characteristic radiations.
E-mail: hxli @rnail.ncu.du.cn
Li H.X. et al., Phase equilibria in the Mg-rich corner of the Mg-Zn-La system at 350OC
3. Results and discussion 3.1. Analysis of the linear compound The equilibrium microstructure of the Mg45Z~5Lalo alloy at 350°C was almost single phase. The equilibrium composition analyzed by EPMA was 46.7Mg-42.8Zn-lOSLa (at.%). The equilibrium structure in the Mg5~n38La12 alloy at 350°C was quasi-single phase, i.e. little second phases existed at the grain boundaries (Fig. 1). The composition of the matrix analyzed by EPMA was 55.4Mg-34.OZn-10.6La, and the composition of the second phase analyzed by EPMA was 33.4Mg42.5Zn-24.1La.
573
Mg81.,Jn11.7MM7.3have been found in the as-cast Mg-8Zn-1SMM and Mg-5Zn-1OMM alloys by Wei et al. [7]. The T phase has a C-centred orthorhombic crystal structure, and the crystal parameters were a = 0.96 nm, b = 1.12 nm, c = 0.94 nm; a = 1.01 nm, b = 1.16 nm, c = 0.99 nm, respectively. The XRD patterns of Mg45Z~5Lalo and Mg50Zn38Lal~ alloys in this study were consistent with the C-centred orthorhombic crystal structure. Thus, it could be concluded that the linear compounds with the compositions Mg46.7Zn4~.~La10.5 and Mg55.&n34.&a10.6 in this study were with the C-centered orthorhombic crystal structure (so-called T phase), having not kept the hexagonal structure as Mg17La2 phase and the rhombohedral structure as Zn17La2phase because of the solution of the third element to the compound. 14000 12000 10000 8 8000 ,x 6000 4000 * 9 2000
3
. '# Fig. 1. Equilibrium microstructure in the Mg&n&alz alloy at 350OC.
20
40
60
80
2e/(7 The XRD patterns of the Mg45Zn45Lalo and Mg5OZn3pLalZalloys at 350°C are shown in Fig. 2. The diffraction peaks of these two alloys corresponded one by one, but the diffraction angles were different slightly. By combining with the EPMA results, these two alloys were thought to be the linear compound (Mg,Zn)17La2with different compositions and different crystal parameters, although these two X R D patterns could not agree with the PDF cards of Mg17La2phase and Zn17Lazphase. The reason was deduced as follows: with the solution of Zn to Mg17La2,the crystal parameters changed continuously; when the Zn content exceeded the extremum, the crystal structure did not keep the hexagonal structure as Mg17La2phase, and did not keep the rhombohedral structure as Zn17Lazphase either. The pseudo-ternary T phases with the compositions of Mg52.&n39.5MM(mi~~h meta1)7,9(at.%) and
Fig. 2. XRD patterns of the Mg45ZGalo and Mg&n3sLalz alloys at 350OC.
3.2. Determination of the three-phase region in the Mg-rich corner The equilibrium microstructures of the Mg79Zn2~Lal and Mg78Zn&a2 alloys at 350°C are shown in Fig. 3. It could be seen that both of the alloys consisted of three phases. In the Mg7$nZ&al alloy, the composition of the black phase was 96.4Mg-3.6Zn, nearly without La; the composition of the gray phase was 48.4Mg50.7Zn-0.9La; the white phase was with higher La, and the composition was 43.3Mg-49.5Zn-7.2La. In the Mg7xZn2&a2alloy, the composition of the black phase was 97.OMg-3.OZn, nearly without La either; the composition of the gray phase was 50.6Mg-
RARE METALS, Vol. 25, No. 5, Oct 2006
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47.8Zn-1.6La; the composition of the white phase was 47.1Mg-44.6Zn-8.3La. The white phases were surrounded by the gray phases and separated from black phases. This kind of microstructure was corresponding to the peritectic microstructure. The ternary compound with high melting point solidified from the liquid, then the liquid reacted with this ternary compound, and the gray phase formed by this kind of peritectic reaction.
2000 3 1500
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0
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60 281 (")
40
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h
34
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38
40 42 2 e / ("1
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Fig. 4. (a) XRD patterns of the Mg79Zn&al and Mg7&n&a2 alloys; (b) comparison with the XRD pattern of the linear compound.
The schematic of the isothermal section at 350°C in the Mg-Zn-La system is shown in Fig. 5.
Fig. 3. Structures of the Mg79Zn&al alloy (a) and Mg7&n&az alloy (b) at 35OOC.
The XRD patterns of the Mg79Zn2&al and Mg78Zn2&a2alloys are shown in Fig. 4(a). The diffraction angles of the peaks were almost the same. Besides the peaks originated from the Mg solid solution and MgZn compound, the additional peaks originated from the third phase existed. The characteristic peaks of the third phase in these two alloys were at the same position, and matched with the peaks of the liner compound with the approximate compositions (Fig. 4(b)). Therefore, it could be deduced that these two alloys were all in the three-phase region: a(Mg), MgZn phase with little La atoms, and Tphase.
7, I at.'% Fig. 5. Schematic of the isothermal section at 35OOC in the Mg-Zn-La system.
4.
Conclusions
(1) The linear compound (Mg,Zn)17La2existed in the Mg-Zn-La system at 350°C. The linear compound (so-called T phase) was with the C-centred orthorhombic crystal structure as the solution of
Li H.X. et al., Phase equilibria in the Mg-rich corner of the Mg-Zn-La system at 35OOC
more third element. (2) The three-phase region a(Mg) + MgZn(La) + T existed in the Mg comer of the Mg-Zn-La system at 350"C,and there was the two-phase region composed of the a ( M g ) and the linear compound T phase.
References [I] Massalski T.B., Okamoto H., et al., Binary Alloy Phase Diagrams, Second Edition Plus Updates, CD-ROM, ASM International, 1996. [2] Maeng D.Y., KimTS., Lee J.H., Hong S.J., Seo S.K., and Chun B.S., Microstructure and strength of rapidly solidified and extruded Mg-Zn alloys, Scriptu Muter., 2000,43: 385.
575
[3] Wei L.Y., Dunlop G.L., and Westengen H., The intergranular microstructure of cast Mg-Zn and Mg-Zn-Rare Earth alloys, Metall. Muter. Trans. A, 1995,26A: 1947. [4] Wu W., Wang Y., Zeng X., Chen L.J., and Liu Z., Effect of neodymium on mechanical behavior of Mg-Zn-Zr magnesium alloy, J. Muter. Sci. Left., 2003,22: 445. [5] Villars P., Prince A., and Okamoto H., Handbook of Ternary Alloy Phase Diagrams, CD-ROM, ASM International, 1997. [6] Hao S.M., The alloying of magnesium alloys and phase diagram J. Muter. Metall., 2002,1(3): 166. [7] Wei L.Y., and Dunlop G.L., Crystal symmetry of the pseudo-ternary T-phases in Mg-Zn-rare earth alloys, J. Muter. Sci. Lett.,1996,lS (1): 4.
Phase equilibria in the Mg-rich corner of the Mg-Zn-La system at 350°C LI Hongxiao, REN Yuping, H U N G Mingli, CHEN Qin, and H A 0 Shiming School of Materials and Metallurgy, NortheasternUniversity, Shenyang 110004, China (Received 2006-06-19)
Abstract: The phase equilibria in the Mg-rich comer of the Mg-Zn-La system at 350°C have been investigated by scanning electron microscopy, X-ray diffraction, and electron probe microanalysis. It has been shown that the linear compound (Mg,Zn)17La2existed in the Mg-Zn-La system at 350°C. The linear compound (so-called Tphase) was with the C-centred orthorhombic crystal structure induced by the solution of significant quantities of the third element. The three-phase region a(Mg) + MgZn(La) + T and the two-phase region composed of the a(Mg) and the linear-compound T phase existed in the Mg-rich comer of the Mg-Zn-La system at 350°C. Key words: Mg-Zn-La system; phase diagram; linear compound
[This work isfinancially supported by the National Natural Science Foundation of China (No. 50471025).]
1. Introduction Mg-based alloys are attractive for applications because of the light weight and a high specific strength. Among the common alloying elements, the addition of zinc could make the magnesium alloys strengthen by precipitation [l-21. The further addition of the rare-earth elements to the Mg-Zn alloys is more effective on the precipitation-strengthening [3-41. But phase diagrams of the Mg-Zn-RE system, as the basis of the alloying, have hardly been built up [5-61. The phase equilibria and compositions between the magnesium solid solution and the intermetallic compounds in the Mg-Zn-La system at 350°C have been researched in this article, so that the essential knowledge for designing the alloys and heat-treatment could be provided.
2. Experimental procedure The experimental alloys with compositions (at.%) of M&5Z%LalO? MgS&n38Lal2, Mg79Zn20La1, and Mg7eZn2&a2were prepared by melting the high purity Mg (99.99%), zinc (99.999%), and lanthanum (99.8%)wrapped by the Ta foils in the vacuum ( lo4 Corresponding author: LI Hongxiao
Pa) quartz tubes. The quartz tubes hanged were heated to 800°C and kept for 1 h, and then cooled to room temperature in the furnace. The melting procedure was repeated again with the upside-down quartz tubes. During heating, the quartz tubes were shaken continuously to make the ingots homogeneous. All the as-cast samples were wrapped with Ta foils and sealed in a quartz tube with the vacuum of lo4 Pa, then kept at 350°C for 1440 h and quenched in cold water. X-ray diffraction analysis was carried out with powder samples on Siemens D5000 diffractometer with Cu K, radiation, a voltage of 40 kV and a current of 40 mA. The microstructure analysis was carried out on SSX-500 scanning electrical micro-analyzer with a voltage of 30 kV. The samples observed were polished with the MgO solution. The compositions of equilibrium phases in alloys were analyzed on EPMA-1600 electron probe microanalyzer with a beam size of 1 pm and a voltage of 15 kV. The high pure Mg, Zn, and La were served as standards to revise the characteristic radiations.
E-mail: hxli @rnail.ncu.du.cn
Li H.X. et al., Phase equilibria in the Mg-rich corner of the Mg-Zn-La system at 350OC
3. Results and discussion 3.1. Analysis of the linear compound The equilibrium microstructure of the Mg45Z~5Lalo alloy at 350°C was almost single phase. The equilibrium composition analyzed by EPMA was 46.7Mg-42.8Zn-lOSLa (at.%). The equilibrium structure in the Mg5~n38La12 alloy at 350°C was quasi-single phase, i.e. little second phases existed at the grain boundaries (Fig. 1). The composition of the matrix analyzed by EPMA was 55.4Mg-34.OZn-10.6La, and the composition of the second phase analyzed by EPMA was 33.4Mg42.5Zn-24.1La.
573
Mg81.,Jn11.7MM7.3have been found in the as-cast Mg-8Zn-1SMM and Mg-5Zn-1OMM alloys by Wei et al. [7]. The T phase has a C-centred orthorhombic crystal structure, and the crystal parameters were a = 0.96 nm, b = 1.12 nm, c = 0.94 nm; a = 1.01 nm, b = 1.16 nm, c = 0.99 nm, respectively. The XRD patterns of Mg45Z~5Lalo and Mg50Zn38Lal~ alloys in this study were consistent with the C-centred orthorhombic crystal structure. Thus, it could be concluded that the linear compounds with the compositions Mg46.7Zn4~.~La10.5 and Mg55.&n34.&a10.6 in this study were with the C-centered orthorhombic crystal structure (so-called T phase), having not kept the hexagonal structure as Mg17La2 phase and the rhombohedral structure as Zn17La2phase because of the solution of the third element to the compound. 14000 12000 10000 8 8000 ,x 6000 4000 * 9 2000
3
. '# Fig. 1. Equilibrium microstructure in the Mg&n&alz alloy at 350OC.
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40
60
80
2e/(7 The XRD patterns of the Mg45Zn45Lalo and Mg5OZn3pLalZalloys at 350°C are shown in Fig. 2. The diffraction peaks of these two alloys corresponded one by one, but the diffraction angles were different slightly. By combining with the EPMA results, these two alloys were thought to be the linear compound (Mg,Zn)17La2with different compositions and different crystal parameters, although these two X R D patterns could not agree with the PDF cards of Mg17La2phase and Zn17Lazphase. The reason was deduced as follows: with the solution of Zn to Mg17La2,the crystal parameters changed continuously; when the Zn content exceeded the extremum, the crystal structure did not keep the hexagonal structure as Mg17La2phase, and did not keep the rhombohedral structure as Zn17Lazphase either. The pseudo-ternary T phases with the compositions of Mg52.&n39.5MM(mi~~h meta1)7,9(at.%) and
Fig. 2. XRD patterns of the Mg45ZGalo and Mg&n3sLalz alloys at 350OC.
3.2. Determination of the three-phase region in the Mg-rich corner The equilibrium microstructures of the Mg79Zn2~Lal and Mg78Zn&a2 alloys at 350°C are shown in Fig. 3. It could be seen that both of the alloys consisted of three phases. In the Mg7$nZ&al alloy, the composition of the black phase was 96.4Mg-3.6Zn, nearly without La; the composition of the gray phase was 48.4Mg50.7Zn-0.9La; the white phase was with higher La, and the composition was 43.3Mg-49.5Zn-7.2La. In the Mg7xZn2&a2alloy, the composition of the black phase was 97.OMg-3.OZn, nearly without La either; the composition of the gray phase was 50.6Mg-
RARE METALS, Vol. 25, No. 5, Oct 2006
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47.8Zn-1.6La; the composition of the white phase was 47.1Mg-44.6Zn-8.3La. The white phases were surrounded by the gray phases and separated from black phases. This kind of microstructure was corresponding to the peritectic microstructure. The ternary compound with high melting point solidified from the liquid, then the liquid reacted with this ternary compound, and the gray phase formed by this kind of peritectic reaction.
2000 3 1500
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3 . ,x .-
1000
v)
S
0
Y
S
500 0 20
60 281 (")
40
80
100
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h
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40 42 2 e / ("1
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Fig. 4. (a) XRD patterns of the Mg79Zn&al and Mg7&n&a2 alloys; (b) comparison with the XRD pattern of the linear compound.
The schematic of the isothermal section at 350°C in the Mg-Zn-La system is shown in Fig. 5.
Fig. 3. Structures of the Mg79Zn&al alloy (a) and Mg7&n&az alloy (b) at 35OOC.
The XRD patterns of the Mg79Zn2&al and Mg78Zn2&a2alloys are shown in Fig. 4(a). The diffraction angles of the peaks were almost the same. Besides the peaks originated from the Mg solid solution and MgZn compound, the additional peaks originated from the third phase existed. The characteristic peaks of the third phase in these two alloys were at the same position, and matched with the peaks of the liner compound with the approximate compositions (Fig. 4(b)). Therefore, it could be deduced that these two alloys were all in the three-phase region: a(Mg), MgZn phase with little La atoms, and Tphase.
7, I at.'% Fig. 5. Schematic of the isothermal section at 35OOC in the Mg-Zn-La system.
4.
Conclusions
(1) The linear compound (Mg,Zn)17La2existed in the Mg-Zn-La system at 350°C. The linear compound (so-called T phase) was with the C-centred orthorhombic crystal structure as the solution of
Li H.X. et al., Phase equilibria in the Mg-rich corner of the Mg-Zn-La system at 35OOC
more third element. (2) The three-phase region a(Mg) + MgZn(La) + T existed in the Mg comer of the Mg-Zn-La system at 350"C,and there was the two-phase region composed of the a ( M g ) and the linear compound T phase.
References [I] Massalski T.B., Okamoto H., et al., Binary Alloy Phase Diagrams, Second Edition Plus Updates, CD-ROM, ASM International, 1996. [2] Maeng D.Y., KimTS., Lee J.H., Hong S.J., Seo S.K., and Chun B.S., Microstructure and strength of rapidly solidified and extruded Mg-Zn alloys, Scriptu Muter., 2000,43: 385.
575
[3] Wei L.Y., Dunlop G.L., and Westengen H., The intergranular microstructure of cast Mg-Zn and Mg-Zn-Rare Earth alloys, Metall. Muter. Trans. A, 1995,26A: 1947. [4] Wu W., Wang Y., Zeng X., Chen L.J., and Liu Z., Effect of neodymium on mechanical behavior of Mg-Zn-Zr magnesium alloy, J. Muter. Sci. Left., 2003,22: 445. [5] Villars P., Prince A., and Okamoto H., Handbook of Ternary Alloy Phase Diagrams, CD-ROM, ASM International, 1997. [6] Hao S.M., The alloying of magnesium alloys and phase diagram J. Muter. Metall., 2002,1(3): 166. [7] Wei L.Y., and Dunlop G.L., Crystal symmetry of the pseudo-ternary T-phases in Mg-Zn-rare earth alloys, J. Muter. Sci. Lett.,1996,lS (1): 4.