JPEDAV (2014) 35:326–333 DOI: 10.1007/s11669-014-0310-1 1547-7037 ÓASM International

The Isothermal Section of the Fe-Si-Ti-93 at.% Zn Quaternary System at 450 °C Xinming Wang, Fucheng Yin, Pengfei He, Yongxiong Liu, Zhi Li, Manxiu Zhao, and Xuping Su

(Submitted February 20, 2013; in revised form February 4, 2014; published online May 1, 2014) The 450 °C isothermal section of the Fe-Si-Ti-Zn quaternary system with Zn being fixed at 93 at.% was determined experimentally by means of scanning electron microscopy coupled with wave dispersive x-ray spectroscopy, and x-ray powder diffraction. Fourteen four-phase regions were confirmed experimentally in this isothermal section. The Fe-Ti-Zn ternary phase T-TiFe2Zn22 was found to be in equilibrium with the liquid, f-FeZn13, s2-FeTiSi, and TiZn16 phases. The maximum solubility of Zn in s2-FeTiSi is 2.74 at.%, but less than 1.08 at.% in s1-FeTiSi2. Si solubility in T-TiFe2Zn22 is 0.31 at.%, but it is negligible in f-FeZn13. The solubility of Ti in the liquid phase is limited. The results of present work are consistent with the relevant ternary systems. No true quaternary compound was found in the isothermal section.

Keywords

Fe-Si-Ti-Zn quaternary system, phase diagram, scanning electron microscope, x-ray diffraction, zinc-based alloy

nary system at the Zn-rich corner was determined experimentally. The phase relationship of the Zn-rich corner at 450 °C will provide useful information to understand the behavior of Ti in zinc bath during hot-dip galvanizing of Sicontaining steels.

1. Introduction

2. Literature Data

Hot-dip galvanizing is one of the most popular techniques used to protect steel against corrosion. Although general galvanizing is a mature technology, galvanizing Si-containing steels is still a technical challenge.[1,2] The silicon dissolves in zinc bath from the steels and results in thick coating with poor adhesion. This phenomenon is commonly referred to as the silicon reactivity or Sandlin effect.[3] Some practical solutions to this problem have emerged as a result of extensive investigations, including high temperature galvanizing[4] and addition of other alloying elements such as Al, Ni, Mn, V, Co and Ti.[5-10] Among these solutions, Ti is very attractive as a potential alloying element used to control the Si reactivity,[10] Ti can improve the corrosion performance of the hot-dip galvanized coating in addition.[11] Virtually, galvanizing Sicontaining steels in a bath with Ti involves the Fe-Si-Ti-Zn quaternary system. So it is of much importance to know the complex phase relationship in this system. In the present work, the 450 °C isothermal section of Fe-Si-Ti-Zn quater-

The information supplied by the constituent ternary systems, especially those that include zinc, such as Fe-SiZn, Fe-Ti-Zn, Si-Ti-Zn, and Fe-Si-Ti systems, is important to construct the boundary of Fe-Si-Ti-Zn quaternary system. As it is important to explain the mechanism of the Si reactivity, the Fe-Si-Zn ternary system was well studied and reviewed by many researchers.[1,12,13] Ko¨ster[12] carried out a systematic study of the ternary system firstly, Foct et al.[1] proposed a revised 450 °C isotherm of the Fe-Si-Zn ternary system. The major issue is whether or not the f phase equilibrates with the FeSi compound. Su et al.[13] confirmed the phase relation published by Ko¨ster.[12] As shown in Fig. 1a, they found that the f phase is in equilibrium with the FeSi compound. The information about phase equilibria in the Zn-Fe-Ti ternary system is very important to understand the effect of Ti on Fe-Zn reaction kinetics during galvanizing. Gloriant et al.[10] proposed a metastable isothermal section at 450 °C. Recently Tang et al.[14] reported an isothermal section as shown in Fig. 1b, in which a true ternary phase T with an approximate formula of TiFe2Zn22 and a small homogeneity range has been identified, and the crystal structure of the T phase was investigated by Zhu et al.[15] The Fe-Si-Ti isothermal section at 800 °C was first presented by Markiv et al.[16] Loffler[17] investigated the Silean side and presented three partial isothermal sections at 800, 1000, and 1150 °C. Recently, the constitution between 900 and 1700 °C of this ternary system was reinvestigated over the entire composition range by Weitezer et al.,[18] as a part of his work, the isothermal section at 900 °C has been published, as shown in Fig. 1c.

Xinming Wang, Fucheng Yin, Pengfei He, Yongxiong Liu, Zhi Li, and Manxiu Zhao, School of Mechanical Engineering, Xiangtan University, Hunan 411105, P.R. China and Key Laboratory of Materials Design and Preparation Technology of Hunan Province, Xiangtan University, Hunan 411105, P.R. China; and Xuping Su, School of Mechanical Engineering, Xiangtan University, Hunan 411105, P.R. China; Key Laboratory of Materials Design and Preparation Technology of Hunan Province, Xiangtan University, Hunan 411105, P.R. China; and School of Material Science and Engineering, Changzhou University, Jiangsu 213164, P.R. China. Contact e-mail: [email protected].

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Fig. 1 The isothermal sections of: (a) Fe-Si-Zn system at 450 °C[13]; (b) Fe-Ti-Zn system at 450 °C[14]; (c) Fe-Si-Ti system at 900 °C[18]

The information of the Si-Ti-Zn system is not available in the literature at present, but it is significant for constructing the boundary of the Fe-Si-Ti-Zn quaternary system. In order to get some useful information about this ternary system, several specimens were prepared in the present work. The details will be introduced in ‘‘Results and Discussion’’ section. The available crystallographic data of intermetallic compounds involved in the present work is summarized in Table 1.

3. Experimental Methods The alloys were prepared by carefully weighing pure materials, 5 g in total for each sample. All alloys were

weighted to an accuracy of 0.0001 g. The designed nominal compositions of the alloys are listed in Table 2 and shown in Fig. 2. The purity of all raw materials is 99.99 wt.%. To expedite the dissolution and diffusion of the elements with high melting points, fine powders of Fe (200 mesh), Si (200 mesh) and Ti (50 mesh) were used. Ti react with the quartz tube at elevated temperature.[19] Therefore, the mixtures of these four elements were put into a corundum crucible which was sealed in an evacuated quartz tube. Each alloy mixture was heated to a temperature above its estimated liquidus temperature and kept for 24 h, followed by quenching in water using a bottom quenching technique to minimize Zn loss and reduce sample porosity.[20] The quaternary samples which contain more than 1 at.% Si were firstly annealed at 800 °C for 15 days, then

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Table 1

Crystallographic data of the intermetallic compounds relevant to the present work Compositions, at.%

Phase

Si

f-FeZn13 T-TiFe2Zn22 s1-FeSi2Ti s2-FeSiTi s7 s8 s9 Si2Ti SiTi Si4Ti5 Si3Ti5 FeSi FeSi2 TiZn8 TiZn16

ÆÆÆ ÆÆÆ 49-50 33-35 40 40 43 ÆÆÆ ÆÆÆ ÆÆÆ ÆÆÆ 50 66.7 ÆÆÆ ÆÆÆ

Lattice parameter, nm

Ti

Fe

Zn

a

b

c

ÆÆÆ 4 25-26 33-35 50 40 40 33.3 50 55.6 60.5-64.5 ÆÆÆ ÆÆÆ 11.1 5.9

93-94 8 24-25 31-33 10 20 17 66.7 50 44.4 39.5-35.5 50 33.3 ÆÆÆ ÆÆÆ

6-7 88 ÆÆÆ ÆÆÆ ÆÆÆ ÆÆÆ ÆÆÆ ÆÆÆ ÆÆÆ ÆÆÆ ÆÆÆ ÆÆÆ ÆÆÆ 88.9 94.1

1.3424 13.9101 0.8612 0.6987 ÆÆÆ ÆÆÆ ÆÆÆ ÆÆÆ 0.3618 0.7133 0.7429 0.44891 0.98792 ÆÆÆ ÆÆÆ

0.7608 13.9101 0.9543 1.0827 ÆÆÆ ÆÆÆ ÆÆÆ ÆÆÆ 0.6492 ÆÆÆ ÆÆÆ ÆÆÆ 0.77991 ÆÆÆ ÆÆÆ

0.5061 13.9101 0.7631 0.6299 ÆÆÆ ÆÆÆ ÆÆÆ ÆÆÆ 0.4973 1.2977 0.5139 ÆÆÆ 0.78388 ÆÆÆ ÆÆÆ

followed cooling in furnace to 450 °C, and finally held at 450 °C for 30 days. Other samples were annealed at 450 °C for 30 days directly. All specimens were immediately quenched in water to preserve the equilibrium state at 450 °C. A JSM-6360LV scanning electron microscopy (SEM) equipped with an OXFORD INCA wave dispersive x-ray spectroscopy (WDS) was used to observe the morphology and determine the chemical composition of various phases in the samples, and x-ray diffraction (XRD) was employed for further determining the constituent phases in the alloys. The XRD analysis was performed using a Bruker D8 advanced x-ray diffractometer operating at 40 kV and 200 mA with Cu-Ka radiation.

4. Results and Discussion Based on the results of microstructure observation and phase analyses combined with the information of the relevant ternary systems in literatures, the 450 °C isothermal section of Fe-Si-Ti-Zn system with Zn being fixed at 93 at.% is proposed in Fig. 2. The identified phase fields are summarized in Table 2 together with the designed composition and the chemical composition of all the phases. The Zn-rich solid solution g-Zn phase is marked as ‘‘L’’ in all figures because it is in liquid state at 450 °C. As mentioned above, there is no information about the Si-Ti-Zn ternary system. Alloys B1-B5 are designed to determine the boundary of the quaternary system. Results indicate that they are all located in three-phase equilibrium fields which are L + (Si) + TiSi2, L + TiSi2 + TiSi, L + TiSi + Ti5Si4, L + Ti5Si4 + Ti5Si3, and L + Ti5Si3 + TiZn16, respectively. Two typical microstructures of three-phase regions are shown in Fig. 3. There are eight four-phase regions involved L and s2. The typical microstructures are shown in Fig. 4. Figure 4a is the microstructure of alloy 1 which clearly confirms that it

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Space group

Prototype

C2/m

CoZn13 ÆÆÆ MnSi2Ti FeSiTi ÆÆÆ ÆÆÆ ÆÆÆ ÆÆÆ ÆÆÆ Zr5Si4 Mn5Si3 FeSi FeSi2 ÆÆÆ TiZn16

ÆÆÆ Pbam Ima2 ÆÆÆ ÆÆÆ ÆÆÆ Fddd Pmm2 P412121 P63/mcm P213 Cmca ÆÆÆ Cmcm

Reference [21] [15] [18] [18] [18] [18] [18] [22] [22] [22] [22] [18] [18] [23] [23]

consists of four phases, L, FeSi, f, and s2. The f phase exists as blocky crystallites in the matrix of L, while blocky FeSi compound and s2 particles disperse around the f crystallites, and the color of FeSi is darker than that of s2. The result of WDS analyses showed that the f phase contains 0.10 at.% Si and 0.14 at.% Ti; the FeSi phase contains 0.18 at.% Ti and 1.12 at.% Zn. Alloy 2 corresponds to the L + f + s2 + T four-phase equilibrium as shown in Fig. 4b. The dark s2 particles disperse among the other phases. The T phase is more resistant to etchant than the f phase. As a result, it is easy to distinguish the T phase from the f phase based on their relief. WDS analyses indicate that the T phase is a quaternary extension of the T-TiFe2Zn22 ternary phase. The T phase contains 0.31 at.% Si, and the s2 phase contains 2.74 at.% Zn. Figure 4c indicates the co-existence of the L, s2, T, and TiZn16 phases in alloy 3. The morphology of this alloy is similar to that of alloy 2. The Si solubility in TiZn16 and L phases are 0.09, and 0.12 at.%, respectively. The Zn solubility in the s2 phase is 2.52 at.%, and Ti solubility in the L phase is 0.16 at.%. The microstructure of alloy 4 is shown in Fig. 4d, SEM-WDS analyses suggest that these phases are L, s2, Ti5Si3, and TiZn16. It can be seen that TiZn16 exists as blocky crystallites, while Ti5Si3 and s2 particles disperse in the matrix of L. The Fe solubility in the TiZn16 phase is 0.09 at.%, and that of Si is 0.10 at.%. The Zn solubility in the s2 phase is 2.45 at.%. The XRD patterns of alloy 1 to 4 are shown in Fig. 5, which confirmed the four-phase equilibrium states mentioned above. The typical microstructures of four-phase fields related to L and s1 are shown in Fig. 6. Figure 6a is the microstructure of alloy 6 which corresponds to the L + s1 + s2 + FeSi four-phase equilibrium state. The phases are similar in color except L, but s2 has a smooth surface, and FeSi locates on the surface of the blocky s1 phase. The Zn solubility in FeSi, s1, and s2, are 1.11, 1.08, and 2.51 at.%, respectively. As shown in Fig. 6b, the FeSi, FeSi2, and s1 phases co-exist in the L matrix in alloy 7. FeSi2 grows around FeSi and its color is much darker than that of FeSi. The s1 phase grows

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Table 2 The designed alloys and corresponding phases composition (at.%)

Table 2

continued Composition

Composition Sample Sample B1

B2

B3

B4

B5

1

2

3

4

5

6

7

8

9

Design composition 93Zn-6Si-1Ti-0Fe

Phase

L Si TiSi2 93Zn-4Si-3Ti-0Fe L TiSi2 TiSi 93Zn-3.3Si-3.7Ti-0Fe L TiSi Ti5Si4 93Zn-3Si-4Ti-0Fe L Ti5Si4 Ti5Si3 93Zn-2Si-5Ti-0Fe L Ti5Si3 TiZn16 93Zn-2Si-1Ti-4Fe L s2 f FeSi 93Zn-1Si-1.5Ti-4.5Fe L s2 f T 93Zn-1Si-3.5Ti-2.5Fe L s2 T TiZn16 93Zn-1.5Si-4.5Ti-1Fe L s2 TiZn16 Ti5Si3 93Zn-0.3Si-6.5Ti-0.2Fe T TiZn16 Ti5Si3 93Zn-3Si-1.5Ti-2.5Fe L s1 s2 FeSi 93Zn-4Si-0.5Ti-2.5Fe L s1 FeSi FeSi2 93Zn-5Si-0.5Ti-1.5Fe L s1 Si FeSi2 93Zn-5Si-1.5Ti-0.5Fe L s1 Si TiSi2

Si

Ti

Fe

Zn

0 99.56 66.13 0 65.1 49.62 0 49.54 43.16 0 43.13 35.37 0 35.18 0.08 0 32.66 0.1 48.28 0 31.14 0 0.31 0.12 31.09 0.24 0.09 0 31.52 0.1 36.28 0.04 0 36.08 0.1 49.31 32.67 49.74 0.11 49.24 49.88 64.92 0.1 50.02 99.25 66.73 0.12 50.16 99.14 66.01

0 0.09 33.5 0 34.45 49.48 0 49.45 55.69 0 55.67 63.12 0 63.55 5.71 0 34.14 0.14 0.18 0 34.76 0.15 4.41 0.16 35.84 4.58 5.52 0 34.85 5.57 60.45 4.63 5.63 62.31 0.11 24.86 34.15 0.2 0.1 24.22 0.01 0.01 0 25.63 0 0 0.13 24.54 0.15 32.96

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 31.63 7.3 50.42 0 31.36 7.35 8.04 0.34 30.55 8.02 0.14 0 31.18 0.09 0.71 8.13 0.09 0.35 0.14 24.75 30.67 48.95 0.1 25.85 48.93 34.14 0.25 23.79 0.1 32.09 0.21 24.47 0.18 0.02

100 0.35 0.37 100 0.45 0.9 100 1.01 1.15 100 1.2 1.51 100 1.27 94.21 100 1.57 92.46 1.12 100 2.74 92.5 87.24 99.38 2.52 87.16 94.25 100 2.45 94.24 2.56 87.2 94.28 1.26 99.65 1.08 2.51 1.11 99.69 0.69 1.18 0.93 99.65 0.56 0.65 1.18 99.54 0.83 0.53 1.01

Design composition

Phase

Si

Ti

Fe

Zn

10

93Zn-4Si-2.5Ti-0.5Fe

11

93Zn-2.9Si-2.3Ti-1.8Fe

12

93Zn-3.1Si-3Ti-0.9Fe

13

93Zn-3.4Si-3.3Ti-0.3Fe

14

93Zn-2.9Si-3.8Ti-0.3Fe

15

93Zn-2.7Si-3.6Ti-0.7Fe

L s1 TiSi TiSi2 L s1 s2 s8 L s1 s9 Ti5Si4 L s1 TiSi Ti5Si4 L s7 Ti5Si3 Ti5Si4 L s2 s7 Ti5Si3

0.2 50.28 50.58 64.15 0.1 50.02 33.67 39.21 0 49.87 43.16 44.98 0.21 49.95 49.26 44.27 0 40.54 36.98 44.09 0 32.74 40.37 37.56

0.1 25.97 48.48 34.13 0.28 24.55 33.7 41.39 0.11 24.97 39.57 54 0.14 25.66 49.48 54.77 0 49.34 59.84 54.53 0.18 33.92 49.7 58.96

0.13 22.99 0.08 0.2 0.16 24.59 31.3 19.03 0.05 24.19 16.66 0.24 0.13 24.11 0.22 0.23 0 9.46 1.08 0.25 0 31.02 9.15 1.43

99.57 0.76 0.86 1.52 99.46 0.84 1.33 0.37 99.84 0.97 0.61 0.78 99.52 0.28 1.04 0.73 100 0.66 2.1 1.13 99.82 2.32 0.78 2.05

totally alone and is more resistant to the etchant than FeSi2. The Zn solubility in FeSi, FeSi2, and s1 are 1.18, 0.93, and 0.69 at.%, respectively. The Ti solubility in FeSi and FeSi2 are hard to detect by WDS. The SEM image of alloy 9 shown in Fig. 6c suggests that TiSi2, s1, and (Si) co-exist in the L matrix. (Si) is the darkest phase of the three (except the L matrix), while the lightest one is s1. The Zn solubility in (Si), TiSi2, and s1 are 0.53, 1.01, and 0.83 at.%, respectively. Alloy 8 corresponds to the FeSi2 + (Si) + s1 + L four-phase equilibrium, and the metallographic appearance of this alloy is similar to that of alloy 9. The microstructure of alloy 10 is shown in Fig. 6d. WDS analyses indicate that it corresponds to the L + s1 +TiSi + TiSi2 four-phase equilibrium state. The dark TiSi2 phase grows around the grey TiSi phase, and light grey s1 phase disperses in the matrix. The Zn solubility in TiSi2, TiSi, and s1 are 1.52, 0.86, and 0.76 at.%, respectively. The Fe solubility in both TiSi and TiSi2 are limited. Figure 6e is the microstructure of alloy 13, WDS analyses show that alloy 13 consists of the TiSi, Ti5Si4, s1, and L phases. The darkness of each phase is in line with the content of Si in each compound. The Zn solubility in TiSi, Ti5Si4, and s1 are 1.04, 0.73, and 0.28 at.%, respectively. The Fe solubility in TiSi and Ti5Si4 are 0.22 and 0.23 at.%. The microstructure of alloy 11 is shown in Fig. 6f. Experimental results indicate that alloy 11 corresponds to the L + s1 + s2 + s8 four-phase

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Fig. 2 The 450 °C isothermal section of the Fe-Si-Ti-Zn quaternary system with Zn composition being fixed at 93 at.%. Solid circles indicate the designed alloy compositions, which are numbered according to Table 2

Fig. 3 The typical microstructures corresponding to different three-phase regions:(a) dark grey TiSi and light grey Ti5Si4 exist in the matrix of L in alloy B3; (b) light grey Ti5Si3 and blocky TiZn16 co-exist with L in alloy B5

equilibrium state. The color of s1 is darker than that of s2, and s8 exists in shape of bamboo leaf. The Zn solubility in s8 is 0.37 at.%. Alloy 12, 14, and 15 correspond to the L + s1 + s9 + Ti5Si4, L + s7 + Ti5Si4 + Ti5Si3, and L + Ti5Si3 + s7 + s2 four-phase equilibrium, respectively. In these alloys, the maximum solubility of Zn in Ti5Si3 is 2.10 at.%, while that is no more than 0.80 at.% in s7 and s9.

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For the alloys discussed above, only the equilibrium state of alloy 7 is confirmed by XRD as shown in Fig. 7. For the other alloys, the volume fraction of the s1 phase is too low to be detected by the XRD technique. Because the differences of compositions and morphologies among the ternary phases in Fe-Si-Ti system are extremely small, the phase relations around s7, s8, and s9 are

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Fig. 4 The typical microstructures corresponding to different four-phase regions involved with s2. (a) alloy 1, L + f + s2 + FeSi; (b) alloy 2, L + f + s2 + T; (c) alloy 3, L + s2 + T + TiZn16; (d) alloy 4, L + s2 + Ti5Si3 + TiZn16

Fig. 5 The XRD patterns of alloys 1, 2, 3, and 4

difficult to determine experimentally. 4 four-phase fields have been deduced from the results of alloys 11, 12, 14, and 15. Alloy 5 corresponds to Ti5Si3 + TiZn16 + T three-phase equilibrium which is shown in Fig. 8. So the four-phase equilibriums, Ti5Si3 + TiZn16 + T + TiZn8 and Ti5Si3 + TiZn16 + T +s2 can be deduced reasonably. Due to the absence of the liquids phase in these two regions, the phases

in the alloys are hard to reach equilibrium state. All the deduced phase regions are drawn in dotted lines on the isothermal section. As mentioned above, the isothermal section of 93 at.% Fe-Si-Ti-Zn system involves 20 four-phase regions. Figure 2 shows that the L and f phases, which are the typical phases during hot-dipping, can co-exist with the s2 and T phases. In this system, all of the compounds except the TiZn8 phase co-exist with liquid Zn. The maximum solubility of Si in the T phase is 0.31 at.%. In the Zn-Fe-NiSi system [21,22], a Zn-Fe-Ni ternary compound, which is also designated as T phase, contains almost no Si at 450 °C. The solubility of Si in the T phase in the Fe-Si-Ti-Zn system is much greater than that in the Fe-Si-Ni-Zn system at 450 °C. The results obtained in the present work could help us to understand the effect of Ti on the Si reactivity during hot-dip galvanizing of Si-containing steels. The multilayer interface (g-Zn, f, d, and C + C1 phase) is formatted between the iron substrate and the molten Zn alloy during hot-dipping. The Si solubility in the f phase is almost zero, Si in the galvanizing coating trends to accumulate at the d/f interface, and it caused the Silicon reactivity.[7,8,13,23] When sufficient amount of Ti was added to the zinc bath, the ternary phase T formed at the interface of f and liquid. The Si solubility in the T phase is up to 0.31 at.%, which avoids the enriching of silicon at the boundary of f phase and the silicon reactivity during the hot-dip galvanizing of the Si-containing steel would be restrained.

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Fig. 6 The typical microstructures corresponding to different four-phase regions involved with s1. (a) alloy 6, L + s1 + s2 + FeSi; (b) alloy 7, L + s1 + FeSi + FeSi2; (c) alloy 9, L + s1 + TiSi2 + Si; (d) alloy 10, L + s1 + TiSi + TiSi2; (e) alloy 13, L + s1 + TiSi + Ti5Si4; (f) alloy 11, L + s1 + s2 + s8

Fig. 8 Microstructure image of alloy 5, T, TiZn16, and Ti5Si3 coexist in this alloy

Fig. 7 The XRD pattern of alloy 7 reveals the existence of L, FeSi, FeSi2, and s1

2.

5. Conclusion

3.

The 450 °C isothermal section of the Fe-Si-Ti-Zn quaternary phase diagram with Zn being fixed at 93 at.% was determined by SEM-WDS and XRD. The main findings are listed as follows:

4.

1.

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Fourteen four-phase regions have been confirmed experimentally in this system.

All the compounds can co-exist with L except the TiZn8 phase. The solubility of Si in the T phase is 0.31 at.%, but negligible in the f phase. Ti solubility in L is limited. The maximum solubility of Zn in s2 is 2.74 at.%, but limited in other Fe-Ti-Si ternary phases. No true quaternary compound was found in the 450 °C isothermal section of the Fe-Si-Ti-Zn quaternary system with the Zn being fixed at 93 at.%.

Journal of Phase Equilibria and Diffusion Vol. 35 No. 3 2014

Acknowledgments This investigation was supported by National Natural Science Foundation of China (No. 51071135), the Ph. D. Programs Foundation of Ministry of Education of China (No. 20114301110005) and Scientific Research Fund of Hunan Provincial Educational Department (No. 12A128).

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