JPEDAV DOI: 10.1007/s11669-015-0363-9 1547-7037 ASM International

The Phase Equilibria of the 75 at.% Al-Zn-Ce-Fe Quaternary System at 600 C Li Ji, Zhi Li, Yu Wu, Yongxiong Liu, Manxiu Zhao, and Fucheng Yin

(Submitted November 28, 2014; in revised form December 26, 2014) The 600 °C isothermal section of the Al-Zn-Ce-Fe quaternary system with Al being fixed at 75 at.% has been studied experimentally by means of scanning electron microscopy coupled with energy dispersive x-ray spectroscopy and x-ray powder diffraction. Four four-phase regions have been identified in this section, namely, s2+Liq.+a-Al+FeAl3, s2+W+a-Al+FeAl3, s2+W+aAl+aAl11Ce3, s2+W+aAl11Ce3+aAl3Ce. One Al-Ce-Fe ternary phase W (CeFe2Al10) and another Al-Zn-Ce ternary phase s2 (Al2Zn2Ce) has been found in this section but no quaternary compound has been found.

Keywords

Al-Zn-Ce-Fe quaternary system, phase diagram, SEM-EDS, x-ray diffraction

prior to dipping.[1] The temperature of the Galvalume (Zn-75 at.% Al) bath is near 600 C. Hence, the 600 C isothermal section of Al-Zn-Ce-Fe with Al being fixed at 75 at.% was studied using scanning electronic microscopy coupled with energy dispersive x-ray spectroscopy (SEM-EDS) and x-ray powder diffraction in present work.

1. Introduction The generation of zinc and zinc alloy coatings on steel is one of the commercially most important processing techniques used to protect steel components exposed to corrosive environments.[1] However, there are applications to develop an improved product for specific end-uses while improved corrosion resistance is desired.[2] The most common method is to add alloying elements to improve its performance. For this reason, a new generation of Al-Zn coating which has better corrosion resistance than pure Zn coating has been investigated, such as Galvalume (55 wt.% Al), Ganfan (5 wt.% Al) and Galvanized (<1 wt.% Al).[3] Galvalume sheet coating consists of nominally 55 wt.% Al (75 at.% Al), 43.4 wt.% Zn and 1.6 wt.% Si. Si is added to the bath to prevent a very strong exothermic reaction between the Al-Zn bath and the steel sheet. But problems still exists in Galvalume coating. Research has shown that addition of RE in Galvalume bath can reduce the viscosity and impurities in the bath to improve the quality of the coating. Successful application of the RE in Galfan (Zn-5 wt.% Al-RE) bath has proved the good effect of the RE.[4] For a deeper investigation of Al-Zn coating and the effect of the RE (such as Ce) in the bath, the phase equilibria in Al-Zn-Ce-Fe system related to galvanizing Al-Zn coating on steel is very important. The bath temperature depends upon the composition of the bath and the strip temperature

2. Literature Review The boundaries of the isothermal section of the quaternary system are constructed based on the knowledge of the following three ternary systems, i.e., Al-Ce-Fe, Al-Zn-Fe and Al-Zn-Ce system. The crystal structure data of the available intermetallic compounds in the three systems relevant to present work are summarized in Table 1. 2.1 Al-Ce-Fe Ternary System The first investigation of this ternary system was made in 1925 by Meissner[5] to describe the ‘‘clear-cross method’’ of Guertler in practical examples when Ce is added to Al-Fe alloys. Zarechnyuk[6] examined the ternary system with 106 alloys from 0 to 33.3 at.% Ce using x-ray structural analysis. Franceschini[7] published the lattice parameters and magnetic properties of these phases along CeFe2-CeAl2. The ternary phase CeFe4Al8 has been studied in detail.[6,8-11] Zarechnyuk[6] found additionally the ternary phases CeFe2Al10, CeFe2Al7, CeFe1 1.4Al1 0.6 and a solid solution of Al in the binary Ce2Fe17 compound (Th2Ni17 type structure) with a maximum Al content of about 60 at.%. CeFe2Al10 is iso-structural to YbFe2Al10.[12] 2.2 Al-Zn-Fe Ternary System

Li Ji, Zhi Li, Yu Wu, Yongxiong Liu, Manxiu Zhao, and Fucheng Yin, Key Laboratory of Materials Design and Preparation Technology of Hunan Province, School of Materials Science and Engineering, Xiangtan University, Hunan 411105, People’s Republic of China; and Zhi Li and Manxiu Zhao, National-Provincial Laboratory of Special Function Thin Film Materials, Xiangtan University, Hunan 411105, People’s Republic of China. Contact e-mail: [email protected].

Due to the importance to galvanizing, many researchers investigated the Al-Zn-Fe ternary system at 450 C;[13-15] and Nakano et al. gave a full thermodynamic optimization of the Zn-Fe-Al system within the 420-500 C temperature range.[16] Uwakweh and Liu[17] investigated the isothermal section at 575 C which was adopted in the present work.

Journal of Phase Equilibria and Diffusion

Table 1

Crystal structure data for binary and ternary phases relevant to present study Lattice parameters(pm)

Compound

Pearson symbol

Space group

Structure type

a

FeAl3 aAl11Ce3 aAl3Ce s2(Al2Zn2Ce) W(CeFe2Al10)

mC102 oI28 hP8 tI10 oC52

C2/m Immm P63/mmc I4/mmm Cmcm

Fe4Al13 aAl11La3 Ni3Sn CeAl2Ga2 YbFe2Al10

1552.7 439.5 654.7 424.4 900.0

The Zn-rich corner of this system at 600 C was determined by Rennhack.[18] The Al-Fe rich side of the Al-Zn-Fe system at 300 and 500 C was studied by Luo et al.[19] The phase equilibria of the Al-Zn-Fe at different temperatures have a little difference, but are similar as the Al content is about 75 at .% Al. 2.3 Al-Zn-Ce Ternary System Chen et al.[20] calculated the isothermal section at 320 C in the Al-Zn-Ce ternary system with the experimental data of Ikromov’s.[21] In this system, the complete thermodynamic assessment cannot be carried out because only a part of the experiment data of the 320 C isothermal section is available. No more information about the Al-Zn-Ce ternary system could be obtained from the literatures.

3. Experimental Procedures The isothermal section in the Al-rich corner of the Al-ZnCe-Fe system at 600 C was determined using equilibrated alloys. The purity of all the elemental powder was 99.99%. Samples were prepared by carefully weighing the Zn, Al, Fe and Ce powder, 2 g in total for each sample. Due to the fact that Al will react with quartz, the mixed powder was first put into a corundum crucible and then sealed in an evacuated quartz tube. The samples were heated to 1100 C and kept for 3 days, followed by quenching in water. The quenched samples were re-sealed in evacuated quartz tube, and then annealed at 600 C for 30 days to make sure reaching equilibrium state. The specimens were prepared in the conventional way for microstructure examination. A nital solution was used to reveal the microstructure of the samples. A JSM-6360LV scanning electron microscope equipped with SEM-EDS was utilized to study the morphology and chemical composition of various phases in the samples. The phase identities were further confirmed by analyzing their x-ray powder diffraction patterns generated by a D/max-rA x-ray diffractometer, operating at 40 kV and 100 mA with Cu Ka-radiation.

4. Results and Discussion A series of alloys were prepared for determining the 600 C isothermal section in 75 at.% Al-Zn-Ce-Fe. The

b

c

b

Ref.

803.5 1302.0

1244.9 1009.0 461.0 1098.6 907.3

107.7

[22] [23] [23] [21] [12]

424.4 1022.2

nominal composition of each sample in present work is listed in Table 2. All phases formed in the specimens are listed in Table 2 along with their measured composition. Based on the results of SEM-EDS and XRD analyses of the equilibrated phases, there are 4 four-phase regions in the 600 C isothermal section of 75 at.% Al: s2+Liq.+aAl+FeAl3, s2+W+a-Al+FeAl3, s2+W+a-Al+aAl11Ce3, s2+W+aAl11Ce3+aAl3Ce. Following is a brief discussion according to the experimental results. Alloy 1 consists of the FeAl3, s2 and Al-Zn solid solution phases as shown in Fig. 1(a). The Al-Zn solid solution is marked as ‘‘Liq.’’ because it is in the liquid state at 600 C which is different from a-Al in composition and morphology. Figure 1(c) shows the microstructure of alloy 3, i.e., the FeAl3 phase, Al-rich solid solution a-Al phase and s2 phase. The coexistence of s2, Liq., a-Al, FeAl3 is found by examining of the microstructure of alloy 2 as shown in Fig. 1(e). The s2 phase exists as gray-white block which is a ternary compound of Al2Zn2Ce. EDS analyses indicate that the solubility of Zn in the FeAl3 phase is 0.7 at.% and that of Ce is 0.2 at.%. The maximum solubility of Zn in a-Al is 7.9 at.% but in Liq. reaches 18.9 at.%.. The a-Al and Liq. phases are confirmed according to the XRD pattern of (Al) and the Al-Zn solid solution phase, respectively. The XRD pattern of the two phases are similar. Figure 1(b), (d) and (f) are the XRD results of the alloys which further confirm the equilibria state. Figure 2(a) represents a three-phase equilibrium state in alloy 5. According to the analyses of EDS and the XRD pattern shown in Fig. 2(b), the s2 and a-Al phases can be determined. The composition of the other phase which is named as W is similar to the CeFe2Al10 phase,[12] but no XRD pattern of the CeFe2Al10 phase can be found. EDS analyses reveal that the W phase contains 73.8 at.% Al, 16.2 at.% Fe and 8.9 at.% Ce in alloy 5. The composition of W phase is different from that of the FeAl3 phase, which contains 74.9 at.% Al, 24.3 at.% Fe and 0.1 at.% Ce. We have tried many times to prepare the single W phase but failed because of its limited composition range. The XRD peaks of the W phase could only be determined preliminarily by wiping off the peaks of the s2 and a-Al phase in alloy 5. There exists the W phase in all samples except alloy 1 to 3 according the composition and XRD patterns. As can be seen in Fig. 2(c) and (d), alloy 8 contains the W, a-Al and FeAl3 phase, and alloy 4 consists of the s2, W, a-Al and FeAl3 phases according to Fig. 2(e) and (f). The dark gray block phase is FeAl3 phase, the gray W phase and the light

Journal of Phase Equilibria and Diffusion

Table 2 Composition of samples and phases in the 600 °C isothermal section of Al-Zn-Ce-Fe quaternary system with Al being fixed at 75 at.% (at.%) Specimen

Nominal composition

1

Al75Zn18Fe3Ce4

2

Al75Zn17Fe3Ce5

3

Al75Zn6Fe15Ce4

4

Al75Zn8Fe10Ce7

5

Al75Zn2Fe3Ce10

6

Al75Zn8Fe3Ce14

7

Al75Zn2Fe5Ce18

8

Al75Zn1Fe20Ce4

9

Al75Zn1Fe5Ce19

Phase

Al

Zn

Fe

Ce

Liq. s2 FeAl3 Liq. s2 FeAl3 a-Al a-Al FeAl3 s2 a-Al FeAl3 s2 W a-Al s2 W a-Al s2 W aAl11Ce3 s2 W aAl11Ce3 aAl3Ce a-Al FeAl3 W aAl11Ce3 aAl3Ce W

81.1 49.5 74.2 80.4 51.3 74.8 92.1 96.7 74.1 52.9 93.3 74.4 48.9 74.9 96.7 55.1 73.8 96.8 52.2 74.6 78.8 47.8 74.4 78.8 74.3 95.2 74.9 74.8 79.1 75.4 74.6

18.9 29.4 0.7 17.4 27.2 0.7 7.9 3.3 0.6 26.1 6.1 0.6 29.5 0 3.3 22.6 1.0 3.2 25.7 0.7 1.5 25.8 0.9 1.8 0.6 4.7 0.7 0.7 1.5 1.2 0

0 0 25.0 0.6 0.1 24.3 0 0 25.2 0.1 0.6 24.9 0 16.6 0 0.1 16.2 0 0 15.8 0.6 0 15.9 0.1 0 0.1 24.3 15.9 0.6 0 16.5

0 21.1 0.1 1.6 21.4 0.2 0 0 0.1 20.9 0 0.1 21.6 8.5 0 22.2 8.9 0 22.1 8.9 19.4 26.4 8.8 19.3 25.1 0 0.1 8.6 18.8 23.4 8.9

gray s2 phase are embedded in the a-Al matrix. The Zn solubility in the a-Al phase is 6.1 at.%. The maximum solubility of Fe in a-Al is 0.6 at.%, but Ce could not dissolve in the a-Al phase. Figure 3(a) indicates the co-existence of the s2, a-Al, W and aAl11Ce3 four phases in alloy 6. The gray block belongs to the W phase. The aAl11Ce3 phase is different from s2 in its composition and morphology. The solubility of Zn and Fe in the aAl11Ce3 phase is 1.5 and 0.6 at.%, respectively. Figure 3(b) is the XRD result of the alloy which further confirms the four-phase equilibrium state. Alloy 9 consists of the W, aAl11Ce3 and aAl3Ce phases as shown in Fig. 4(a). and the s2, W, aAl11Ce3 and aAl3Ce phases are consisted in alloy 7 as shown in Fig. 4(c). The XRD results of alloy 9 and alloy 7 are shown in Fig. 4(b) and (d), respectively. The phase equilibria in 75 at.% Al-Zn-Fe are based on the results of Uwakweh[16] and Rennhack[17] et.al, and the phase relationships in the Al-Ce-Fe and Al-Zn-Ce ternary systems at 600 C determined by the present authors in another manuscript.

Based on the experimental results obtained in this study and the information of relevant ternary systems, isothermal section of the Al-Zn-Ce-Fe quaternary system at 600 C with Al being fixed 75 at.% is constructed in Fig. 5. The section at 75 at.% Al in Fig. 5 contains two observed four-phase equilibria in form of triangle and another two four-phase equilibria in form of quadrilateral. When one of the phase composition lies ‘‘above’’ the selected section (75 at.% Al) and three other phase composition are located ‘‘below’’ this section, the four-phase region will be a triangle in the section, while two equilibrium phases are on one side and two equilibrium phases on the other side of the selected section, one would obtain a quadrilateral as a representation of four-phase field.

5. Conclusions Based on the SEM-EDS analyses and x-ray diffraction studies, isothermal section of the Al-Zn-Ce-Fe quaternary

Journal of Phase Equilibria and Diffusion

Fig.1 The microstructure (a) and XRD pattern (b) of alloy 1. The microstructure (c) and XRD pattern (d) of alloy 3. The microstructure (e) and XRD pattern (f) of alloy 2

Journal of Phase Equilibria and Diffusion

Fig.2 The microstructure (a) and XRD pattern (b) of alloy 5. The microstructure (c) and XRD pattern (d) of alloy 8. The microstructure (e) and XRD pattern (f) of alloy 4

Journal of Phase Equilibria and Diffusion

Fig.3

The microstructure (a) and XRD pattern (b) of alloy 6

Fig.4

The microstructure (a) and XRD pattern (b) of alloy 9. The microstructure (c) and XRD pattern (d) of alloy 7

Journal of Phase Equilibria and Diffusion

Fig.5 The 600 C isothermal section of the Al-Zn-Ce-Fe quaternary system with Al being fixed at 75 at.%. The phase boundaries are drawn tentatively based on the experimental results

system at 600 C with Al being fixed 75 at.% is determined in present work. The main conclusions are listed below: 1.

2.

Four four-phase regions exist in the isothermal section of the Al-Zn-Ce-Fe quaternary system at 600 C with Al being fixed 75 at.%. No quaternary compound is found in present work. The maximum solubility of Zn in Liq., a-Al, FeAl3, W, aAl11Ce3 and aAl3Ce are 18.9, 7.9, 0.7, 1.0, 1.8, 1.2 at.%, respectively. The solubility of Fe in s2 is very limited.

Acknowledgments This investigation is supported by the National Science Foundation of the China (No. 51471140), the Ph.D. Programs Foundation of Ministry of Education of China (No. 20124301110006) and Scientific Research Found of Hunan Provincial Education Department (No. 14A143).

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