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Phase equilibria of the Al–Cr–Sn ternary system at 600 and 800 °C
Journal Pre-proof Phase equilibria of the Al–Cr–Sn ternary system at 600 and 800�°C Zhanpeng Zhao, Hao Tu, Ya Liu, Changjun Wu, Haoping Peng, Jianhua Wang, Xuping Su PII:
S0925-8388(19)34719-X
DOI:
https://doi.org/10.1016/j.jallcom.2019.153473
Reference:
JALCOM 153473
To appear in:
Journal of Alloys and Compounds
Received Date: 25 September 2019 Revised Date:
19 December 2019
Accepted Date: 19 December 2019
Please cite this article as: Z. Zhao, H. Tu, Y. Liu, C. Wu, H. Peng, J. Wang, X. Su, Phase equilibria of the Al–Cr–Sn ternary system at 600 and 800�°C, Journal of Alloys and Compounds (2020), doi: https:// doi.org/10.1016/j.jallcom.2019.153473. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
CRediT author statement Zhanpeng Zhao: Methodology, Investigation, Writing – Original draft. Hao Tu: Conceptualization, Supervision. Ya Liu: Writing - Review and Editing. Changjun Wu: Project. Haoping Peng: Resources. Jianhua Wang: Administration. Xuping Su: Funding acquisition.
Phase equilibria of the Al-Cr-Sn ternary system at 600 and 800 °C Zhanpeng Zhao1, Hao Tu1,2*, Ya Liu1,2, Changjun Wu1, Haoping Peng1, Jianhua Wang1,3,Xuping Su3,* 1. Key Laboratory of Materials Surface Science and Technology of Jiangsu Province, Changzhou University, Changzhou 213164, China 2. Jiangsu Collaborative Innovation Center of Photovoltaic Science and Engineering, Changzhou University, Changzhou 213164, China * Corresponding author: [email protected] 3. National Experimental Teaching Demonstration Center of Materials Science and Engineering, Changzhou University, Changzhou 213164, China * Corresponding author: [email protected]
Abstract: The 600 and 800 °C isothermal sections of the Al-Cr-Sn ternary system were determined based on examination of equilibrated alloys by X-ray diffraction and scanning electron microscopy coupled with energy dispersive spectroscopy. The experimental results indicate that seven and four three-phase regions exist in the Al-Cr-Sn system at 600 °C and 800 °C, respectively. The Al11Cr2 phase is found in both two isothermal sections. The Al11Cr4 Phase is confirmed, and is stable in the 600 °C isothermal section; its decomposition temperature is between 690 °C and 800 °C. The solubility of Sn in Al-Cr binary compounds is negligible. Six intermediate phases, i.e., Al7Cr, Al11Cr2, Al4Cr, Al11Cr4, Al8Cr5 and AlCr2, and no ternary compounds are found in this work. Key words: Al-Cr-Sn; Phase equilibria; Microstructure; Intermetallic compound; XRD
1. Introduction With the application and promotion of ferrite stainless steel, especially in the calorifier, tailpipe and other fields, the stress corrosion failure of ferrite stainless steel occurs frequently and its stress corrosion cracking sensitivity has attracted great interest [1-3]. To solve the stress corrosion problem, hot-dip aluminum plating was introduced into the process. Hot-dip aluminum plating is a protective method to improve the corrosion resistance property of steel by forming a stable and dense Al2O3 film to prevent contact between the steel substrate and the external
environment [4,5]. However, hot-dip aluminum plating usually works by cathodic deposition, and the protection of the substrate is lost when the coating is broken. Referring to the sacrificial anode protection characteristics of aluminum alloy, the alloy element Sn is introduced into the coating; Sn can enter the oxide layer in the form of Sn2+ and Sn4+, causing some cation and anion defects and thus reducing the resistance of the film and activating the aluminum anode [6,7]. Hence, to improve the hot-dip aluminum of austenitic stainless steel, knowledge of the phase equilibria in the Fe-Al-Si-Sn-Cr system and information on the interaction between the Fe-Cr alloy and the Al-Si-Sn pool are necessary. In this work, two isothermal sections of the Al-Cr-Sn ternary system at 600 and 800 °C were measured according to the hot-dip aluminum temperature, which will provide an important reference value for improving the stress corrosion of ferrite stainless steel by hot-dip aluminum plating.
2. Literature data 2.1 The Al-Cr system Investigation of the Al-Cr phase diagram has almost one hundred years of history and the first consistent version of this system was assessed by Bradley and Lu [8] in 1937 on the basis of earlier studies by Goto et al [9] and Fink et al [10]. Through the efforts of Cooper et al [11], Köster et al [12], Murray et al [13] and Grushko et al [14-17], the binary system has been deeply studied. In recent years, Chen et al [18], Tokunaga et al [19], Liang et al [20] and Khvan et al [21] reassessed the phase diagram, collated the relevant data, and provided new thinking for the study of the Al-Cr system. The Al-Cr binary phase diagram referred to in this work (shown in Fig. 1(a)) was assessed by Grushko et al [22] in 2008 and includes six intermediate phases, i.e., Al7Cr, Al11Cr2, Al4Cr, Al11Cr4, Al8Cr5 and AlCr2. Nevertheless, there are still many controversies about the Al-Cr binary system. First, two views exist on the reaction between Al7Cr, (Al) and the liquid phase, and their focus is whether this reaction is eutectic [9,23-25] or peritectic [10,13,26] and what is the accurate reaction temperature. Second, the temperature range of Al11Cr2 in which the phase is stable has not been settled [14,20,27-31]. Further studies are needed to determine whether the phase is stable at low temperature. The temperature of the peritectic reaction, at which Al7Cr decomposes to liquid and Al11Cr2 (detected at 790 °C in [12], at 835 °C in [32], at 799 °C in [28], at 795 °C in [22]),
remains controversial. The existence of Al11Cr4 is still a puzzle; Grushko affirmed the new phase in 2006 [15,33], while some researchers doubted his view and regarded Al11Cr4 as a ternary phase that was contaminated by quartz tubes [18,34,35]. Many studies have divided the phase region with a composition range between 30 and 42 at.% Cr into five different parts [8,12,13]; however, in recent studies, two solid solutions with large compositional ranges, one high- and one low-temperature sub-region were observed [14]. This view is adopted in this work for simplicity. The crystallographic data for the Al-Cr binary phases are summarized in Table 1.
2.2 The Cr-Sn system There is no intermetallic phase reported in the Cr-Sn system. The Cr-Sn system (shown in Fig.1 (b)) exhibits a miscibility gap in the liquid phase, two invariant reactions including one monotectic invariant reaction, and very low solubilities in the solid state [36-38]. The critical points of the miscibility gap and the monotectic reaction (liquid↔A2(Cr)+liquid’) were reported to be 1485 and 1374 °C, respectively.
2.3 The Al-Sn system Al-Sn is a simple eutectic system with limited solid solubilities in the two terminal solid solutions, fcc (Al) and tetragonal (βSn). Pure Sn transforms from bct β-Sn to a diamond cubic structure α-Sn at low temperature [39-41]. McAlister and Kahan [39] indicated that the eutectic reaction (liquid↔(Al)+(βSn)) occurred at approximately 228.5 °C and 97.6 at. % Sn, and Balanovic et al proved this result in their thermal analysis of Al-Sn system [42]. The Al-Sn system can be seen in Fig.1 (c). There is no information about the phase relationships of the Al-Cr-Sn ternary system in the literature.
3. Experimental procedures More than fifty alloy samples were prepared to study the 600 °C and 800 °C isothermal sections of the Al-Cr-Sn ternary system. All equilibrated alloys were prepared by the method of alloy melting with the raw materials of high-purity Al particles (99.99 wt.%), Sn particles (99.99 wt.%) and Cr particles (99.99 wt.%). Each sample, with a weight of 3 g, was melted in an arc
furnace under a pure argon environment (99.999 vol. %), and both sides of the samples were melted three times. Zr particles were used in the melting process as the deoxidizer, which was replaced with each melting. Taking the reaction between aluminum and silicon in quartz cubes and the segregation of tin into consideration, all samples were packaged in tantalum foil, placed into quartz cubes and then sealed in evacuated tubes under an argon gas atmosphere. All samples were kept at 600 °C for 25 days and 800 °C for 20 days to ensure the equilibrium of alloys, and hightemperature samples were preserved by quenching in water. After quenching, the liquid phase present at high temperature exists as a solid solution. All annealed samples were divided into two parts: one part of the samples was prepared for microstructure examination, and the other part was prepared for X-ray powder diffraction tests. A JSM-6510 scanning electron microscope equipped with an OXFORD energy dispersive X-ray spectrometer (EDX) and an OXFORD INCA 500 wave dispersive X-ray spectrometer with a probe diameter of 1 µm and an accelerating voltage of 20 kV was used to examine the microstructure of annealed samples and analyze the microarea composition. To ensure the accuracy of the EDX data, the final data were averaged over at least five sets. X-ray powder diffraction was used to test the phases of each sample, in a D/max 2500 PC X-ray diffractometer with Cu Kα radiation and a step increase of 0.02° in the 2θ angle. Si powders were used as external calibration standards. Jade 5.0 was used to analyze and fit the X-ray diffraction patterns.
4. Results and discussion 4.1 Study on the Al11Cr4 phase The phases in the Al-Cr binary system have been basically confirmed, and only the phase Al11Cr4 has not been unanimously recognized by researchers. To confirm the existence of this phase, relevant experiments were designed. Two binary alloys that are very close to the composition of Al11Cr4 were prepared. The samples were melted in an arc furnace under a pure argon environment, and annealed at 600 °C for 30 days. Fig. 2(a) presents the microstructure of alloy Al76Cr24, and the SEM-EDX data indicates that this alloy includes two phases, i.e., Al4Cr and Al8Cr5. A similar microstructure was detected in alloy Al72Cr28, but it is noteworthy that a small amount of Al11Cr4 appears locally in this alloy (Fig. 2(b)). The Al11Cr4 phase is difficult to nucleate and develop during the normal cooling process, due to its slow growth kinetics. Thus, it
may take longer to achieve equilibrium. This observation is consistent with the result of Grushko et al [15].
4.2 Phase equilibria of the Al-Cr-Sn ternary system at 600 °C All the phases in the alloy samples can be clearly distinguished based on morphology, color and chemical composition. The nominal compositions of key alloys for assessing the isothermal section, different phases and related chemical compositions are all shown in Table 2. Seven key alloy samples (A1-A7) and related data were chosen to draw the Al-Cr-Sn ternary phase diagram at 600 °C. According to the experiment data and the relevant binary system, this isothermal section
can
be
divided
into
seven
three-phase
regions:
(1)
Liq.+α-Al+Al7Cr,
(2)
Liq.+Al7Cr+Al1Cr2, (3) Liq.+Al1Cr2+Al4Cr, (4) Liq.+Al4Cr+Al11Cr4, (5) Liq.+Al11Cr4+Al8Cr5, (6) Liq.+Al8Cr5+AlCr2, and (7) Liq.+AlCr2+A2(Cr). The experimental results show no ternary compound in the Al-Cr-Sn ternary system at 600 °C. Alloy A1 was designed to evaluate the phase relationship in the Al-rich corner of the Al-Cr-Sn ternary system at 600 °C. SEM-EDX analyses show that α-Al is in equilibrium with Al7Cr and Liquid-Sn. As shown in Table 2, the solubility of Sn in Al7Cr is approximately 0.3 at.%. The existence of this three-phase equilibrium is further verified by the XRD diffraction pattern (Fig. 3(A1)). The diffraction peak associated with the reported for the Al11Cr2 phase [17,48] is found in the two
alloys
A2
and
A3(shown
in
Fig.
3(A2-A3)),
which
correspond
to
the
(Liquid-Sn+Al7Cr+Al11Cr2) and (Liquid-Sn+Al11Cr2+Al4Cr) three-phase regions, respectively. It can be easily deduced that the decomposition temperature is less than 600 °C. This outcome is different from the result of Mahdouk and Gachon [28], who claimed that this phase decomposed to Al7Cr and Al4Cr at 785 °C. Two three-phase coexistence zones are found in samples A4 and A5, namely, (Liquid-Sn +
Al4Cr + Al11Cr4) and (Liquid-Sn + Al11Cr4 + Al8Cr5) , respectively. The XRD results also confirm the existence of these two three-phase equilibria, as shown in Fig. 3(A4-A5). SEM-EDX analysis suggests that the compositional range of Al11Cr4 is 72.4-72.8 at.% Al. This range is slightly different from the results of Grushko et al [15] and Wu et al [33]. Interestingly, the addition of the third element seems to be more favorable to the equilibrium
of Al11Cr4. Hence, an Al76Cr24 alloy/Sn diffusion couple was prepared using a tin to Al-Cr alloy ratio of 3/4 and annealed at 600 °C for 4 hours. Remarkably, the Al11Cr4 phase was found in the microstructure of the diffusion couple top layer (Fig. 4), and the amount of the Al11Cr4 phase decreases gradually from the surface of the steel matrix to its interior. The experimental data obtained with EDX from the Al-Cr alloy and Al-Cr alloy/Sn diffusion couple are listed in Table 3. The composition of the metastable phase Al11Cr4 is Al73.6Cr26.3Sn0.1, very close to the composition of Al11Cr4 in the equilibrium alloys (A4 and A5). It may be inferred that the addition of tin promotes the nucleation of Al11Cr4 and accelerates its growth kinetics process. Fig. 3(A6) is the XRD pattern of alloy A6 in which three phases, namely, Al8Cr5, AlCr2 and Liquid-Sn, are observed. The isothermal section of the Al-Cr-Sn ternary system at 600 °C is assessed by integrating the related data, as shown in Fig. 5. In addition, several alloy samples ranging from alloy A9 to A14 were prepared to confirm the boundary of the Cr single-phase zone, which occupies the red area at the bottom right of Fig. 5.
4.3 Phase equilibria of the Al-Cr-Sn ternary system at 800 °C To further characterize the Al-Cr-Sn system, the 800 °C isothermal section was also experimentally determined in the present work. The summarized experimental data listed in Table 4 correspond to the isothermal section of the Al-Cr-Sn binary system at 800 °C, including four sets of three-phase equilibrium: Liq. +Al11Cr2+Al4Cr, Liq.+Al4Cr+Al8Cr5, Liq.+Al8Cr5+AlCr2 and Liq.+AlCr2+A2(Cr). Based on the experimental results, the 800 °C isothermal section of the Al-Cr-Sn system was constructed and is shown in Fig. 6. The alloys B1, B3 and B4 were annealed for 20 days. Three three-phase equilibrium states exist in these alloys, and the phase relationships at the 800 °C isothermal sections are in full agreement with that at 600 °C, i.e., Liq.+Al11Cr2+Al4Cr, Liq.+Al8Cr5+AlCr2 and Liq.+AlCr2+ A2(Cr). The microstructure and XRD pattern of alloy B2 (Figs. 7(a-b)) indicate that this alloy exists in the three-phase field with the Al4Cr, Al8Cr5 and Liquid-Sn phases. The long gray strip Al4Cr phase, the gray matrix Al8C5 phase and the white Liquid-Sn phase can coexist in this alloy. The detection of the Al11Cr2 phase in this isothermal section enlarges the range of its stable temperature to 800 °C, and this conclusion has been verified in Al-Cr-Ni [49], Al-Cr-Mn [16],
Al-Cr-Cu [17], Al-Cr-Mg [50] and Al-Cr-Sb [51] systems. This result is opposite those in references [28-31]. Otherwise, according to the information summarized in Table 5, the stability of the Al11Cr2 phase has also been determined at 450 °C. That is, the Al11Cr2 phase which is stable between 450 °C and 800 °C is confirmed. In this way, the Al11Cr4 phase is unstable at this temperature. Thus, the equilibrium B2 alloy was annealed again at 690 °C for 7 days, and the newly-formed gray metastable phase Al11Cr4 was discovered in the microstructure of B2, as shown in Fig. 8. Compared with Fig. 7(a), the amount of Al4Cr and Al8Cr5 are less, while that of Al11Cr4 is greater. This may be the result of the eutectoid reaction (Al4Cr+Al8Cr5↔Al11Cr4). These findings provide evidence that the decomposition temperature of Al11Cr4 is greater than 690 °C, and is consistent with the result of Grushko et al. [29].
5. Conclusions This work assessed the 600 and 800 °C isothermal sections of the Al-Cr-Sn ternary system by using the equilibrium alloy method to prepare samples. SEM-EDX and X-ray powder diffraction were used for analysis and detection. Experimental results illustrate that there exist seven three-phase regions in the 600 °C isothermal section, while only four three-phase regions can be detected at 800 °C. All of the Al-Cr binary compounds (i.e., Al7Cr, Al11Cr2, Al4Cr, Al11Cr4, Al8Cr5 and AlCr2) exist in the Al-Cr-Sn system at 600 °C. The Al11Cr2 phase is stable in the Al-Cr-Sn ternary system from 450 °C to 800 °C. The Al11Cr4 phase exists in only the Al-Cr and Al-Cr-Sn systems at 600 °C, and its decomposition temperature can be between 690 °C and 800 °C. The solubility of Sn in Al-Cr binary compounds is negligible. No ternary compounds were discovered in this work.
Acknowledgements Financial supports from the National Science Foundation of China (Grant Nos. 51671036 and 51871030) and a project funded by the Priority Academic Program Development of Jiangsu higher education institutions are greatly acknowledged. This work has also been sponsored by Qing Lan Project.
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Figure captions: Fig. 1 The binary phase diagrams, (a) Al-Cr system [43], (b) Cr-Sn system [36], (c) Al-Sn system [44] Fig. 2 The BSE images of the binary alloy samples, (a) Al76Cr24, (b)Al72Cr28 Fig. 3 XRD patterns of typical ternary alloys, A1: (Sn) + α-Al + Al7Cr, A2: (Sn) + Al7Cr +Al11Cr2, A3: (Sn) + Al11Cr2 +Al4Cr, A4: (Sn) + Al4Cr + Al11Cr4, A5: (Sn) + Al11Cr4 + Al8Cr5, A6: (Sn) + Al8Cr5 +AlCr2 Fig. 4 The BSE image of the top layer of Al76Cr24 alloy/Sn diffusion couple sample Fig. 5 The isothermal section of the Al-Cr-Sn system at 600 °C Fig. 6 The isothermal section of the Al-Cr-Sn system at 800 °C Fig. 7 Phase coexisting in alloy B2, (a) BSE image and (b) XRD pattern Fig. 8 The BSE image of equilibria alloy sample after secondary annealing at 690 °C
Table captions: Table 1 Crystallographic data of the phases in the Al-Cr system Table 2 Summarized experimental data for key specimens annealed at 600 °C Table 3 Summarized experimental data of Al-Cr alloy and Al-Cr alloy/Sn diffusion couple Table 4 Summarized experimental data for key specimens annealed at 800 °C Table 5 Summarized experimental data for key specimens annealed at 450 °C
Table 1. Crystallographic data of the phases in the Al-Cr system Phase
Al7Cr (Al45Cr7, Al13Cr2)
Pearson symbol
Referenc e
a
b
c
α,β,γ
C2/m
21.596
7.574
10.949
128°43′
[11]
mC104
C2/m C2/c
20.595 17.6
7.574 30.5
10.949 17.6
107°34′ ≈90°
[31] [45]
mC616
C2/c
17.735
30.456
17.737
91.052°
[46]
P63/mmc P63/mmc
20.0 20.191
-
24.7 24.854
-
[43] [46]
Cmcm
12.52
34.705
20.223
-
[47]
P-1
5.089
9.033
5.044
[17]
R3m I4/mmm
12.818 2.999
-
7.945 8.630
α=91.84° β=100.77° γ=107.59° -
Al4Cr hp574-6. 32 oC584-2 0.48
Al11Cr4
Al8Cr5 AlCr2
Lattice parameter(nm)
mC104
Al11Cr2 (Al5Cr)
Al4Cr
Space Group
hR26 tI26
[14] [8]
Table 2. Summarized experimental data for key specimens annealed at 600°C No.
Nominal composition (at.%)
A1
91Al-7Cr-2Sn
A2
83Al-16Cr-1Sn
A3
80Al-18Cr-2Sn
A4
75Al-24Cr-1Sn
A5
70Al-28Cr-2Sn
A6
43Al-54Cr-3Sn
A7
25Al-73Cr-2Sn
A8
57Al-37Cr-6Sn
A9
18Al-71Cr-11Sn
A10
10Al-50Cr-40Sn
A11
11Al-63Cr-26Sn
A12
6Al-44Cr-50Sn
A13
7Al-73Cr-20Sn
A14
3Al-62Cr-35Sn
Phase Liq. α-Al Al7Cr Liq. Al7Cr Al1Cr2 Liq. Al1Cr2 Al4Cr Liq. Al4Cr Al11Cr4 Liq. Al11Cr4 Al8Cr5 Liq. Al8Cr5 AlCr2 Liq. AlCr2 A2(Cr) Liq. Al8Cr5 Liq. A2(Cr) Liq. A2(Cr) Liq. A2(Cr) Liq. A2(Cr) Liq. A2(Cr) Liq. A2(Cr)
Composition (at.%) Al
Cr
Sn
21.3 99.3 86.0 15.7 85.3 82.1 13.5 82.3 79.8 8.6 77.1 72.8 5.1 72.4 68.9 1.9 56.8 33.0 1.6 29.3 19.6 2.4 61.4 0.3 20.0 0.3 17.2 0.3 15.1 0.2 12.0 0.2 8.8 0.1 5.2
1.6 0.6 13.7 2.7 14.6 17.6 3.1 17.6 20.0 4.5 22.6 27.1 2.9 27.5 31.0 2.2 42.6 66.8 1.8 70.6 80.2 2.6 38.3 1.2 79.5 1.3 82.2 1.4 84.2 1.2 87.1 1.2 90.2 1.1 93.5
77.1 0.1 0.3 81.6 0.1 0.3 83.4 0.1 0.2 86.9 0.3 0.1 92.0 0.1 0.1 95.9 0.6 0.2 96.6 0.1 0.2 95.0 0.3 98.5 0.5 98.4 0.6 98.3 0.7 98.6 0.9 98.6 1.0 98.8 1.3
Table 3. Summarized experimental data of Al-Cr alloy and Al-Cr alloy/Sn diffusion couple No.
Nominal composition (at.%)
1
Al76Cr24
2
Al72Cr28
3
Al76Cr24/Sn diffusion
Phase Al4Cr Al9Cr4 Al4Cr Al9Cr4 Al11Cr4 Al4Cr Al9Cr4 Al11Cr4
Composition (at.%) Al
Cr
Sn
78.2 68.4 77.5 68.6 73.1 77.4 68.9 73.6
21.8 31.6 22.5 31.4 26.9 22.4 31.0 26.3
0.2 0.1 0.1
Table 4. Summarized experimental data for key specimens annealed at 800°C No.
Nominal composition (at.%)
B1
79Al-18Cr-3Sn
B2
75Al-24Cr-1Sn
B3
41Al-54Cr-5Sn
B4
25Al-71Cr-4Sn
Phase Liq. Al1Cr2 Al4Cr Liq. Al4Cr Al8Cr5 Liq. Al8Cr5 AlCr2 Liq. AlCr2 A2(Cr)
Composition (at.%) Al
Cr
Sn
11.7 81.8 80.3 3.1 76.8 68.6 0.4 56.2 33.3 0.3 30.8 23.2
1.9 17.9 19.5 2.0 22.9 31.3 2.1 43.2 66.5 2.5 69.8 76.7
86.4 0.3 0.2 94.9 0.3 0.1 97.5 0.6 0.2 97.2 0.1 0.1
Table 5. Summarized experimental data for key specimens annealed at 450°C No.
Nominal composition (at.%)
C1
83Al-16Cr-1Sn
C2
80Al-18Cr-2Sn
Phase Liq. A7Cr Al11Cr2 Liq. Al11Cr2 Al4Cr
Composition (at.%) Al
Cr
Sn
4.5 85.9 83.2 4.7 83.1 78.3
2.7 13.9 16.7 2.1 16.7 21.4
92.8 0.2 0.1 93.3 0.2 0.3
Highlights Phase equilibria of the Al-Cr-Sn ternary system at 600°C and 800 °C were determined. The Al11Cr2 phase is stable in the Al-Cr-Sn ternary system from 450°C to 800 °C. The Al11Cr4 phase exists in Al-Cr and Al-Cr-Sn systems at 600°C, its decomposition temperature is from 690°C to 800°C.
Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0925-8388(19)34719-X
DOI:
https://doi.org/10.1016/j.jallcom.2019.153473
Reference:
JALCOM 153473
To appear in:
Journal of Alloys and Compounds
Received Date: 25 September 2019 Revised Date:
19 December 2019
Accepted Date: 19 December 2019
Please cite this article as: Z. Zhao, H. Tu, Y. Liu, C. Wu, H. Peng, J. Wang, X. Su, Phase equilibria of the Al–Cr–Sn ternary system at 600 and 800�°C, Journal of Alloys and Compounds (2020), doi: https:// doi.org/10.1016/j.jallcom.2019.153473. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
CRediT author statement Zhanpeng Zhao: Methodology, Investigation, Writing – Original draft. Hao Tu: Conceptualization, Supervision. Ya Liu: Writing - Review and Editing. Changjun Wu: Project. Haoping Peng: Resources. Jianhua Wang: Administration. Xuping Su: Funding acquisition.
Phase equilibria of the Al-Cr-Sn ternary system at 600 and 800 °C Zhanpeng Zhao1, Hao Tu1,2*, Ya Liu1,2, Changjun Wu1, Haoping Peng1, Jianhua Wang1,3,Xuping Su3,* 1. Key Laboratory of Materials Surface Science and Technology of Jiangsu Province, Changzhou University, Changzhou 213164, China 2. Jiangsu Collaborative Innovation Center of Photovoltaic Science and Engineering, Changzhou University, Changzhou 213164, China * Corresponding author: [email protected] 3. National Experimental Teaching Demonstration Center of Materials Science and Engineering, Changzhou University, Changzhou 213164, China * Corresponding author: [email protected]
Abstract: The 600 and 800 °C isothermal sections of the Al-Cr-Sn ternary system were determined based on examination of equilibrated alloys by X-ray diffraction and scanning electron microscopy coupled with energy dispersive spectroscopy. The experimental results indicate that seven and four three-phase regions exist in the Al-Cr-Sn system at 600 °C and 800 °C, respectively. The Al11Cr2 phase is found in both two isothermal sections. The Al11Cr4 Phase is confirmed, and is stable in the 600 °C isothermal section; its decomposition temperature is between 690 °C and 800 °C. The solubility of Sn in Al-Cr binary compounds is negligible. Six intermediate phases, i.e., Al7Cr, Al11Cr2, Al4Cr, Al11Cr4, Al8Cr5 and AlCr2, and no ternary compounds are found in this work. Key words: Al-Cr-Sn; Phase equilibria; Microstructure; Intermetallic compound; XRD
1. Introduction With the application and promotion of ferrite stainless steel, especially in the calorifier, tailpipe and other fields, the stress corrosion failure of ferrite stainless steel occurs frequently and its stress corrosion cracking sensitivity has attracted great interest [1-3]. To solve the stress corrosion problem, hot-dip aluminum plating was introduced into the process. Hot-dip aluminum plating is a protective method to improve the corrosion resistance property of steel by forming a stable and dense Al2O3 film to prevent contact between the steel substrate and the external
environment [4,5]. However, hot-dip aluminum plating usually works by cathodic deposition, and the protection of the substrate is lost when the coating is broken. Referring to the sacrificial anode protection characteristics of aluminum alloy, the alloy element Sn is introduced into the coating; Sn can enter the oxide layer in the form of Sn2+ and Sn4+, causing some cation and anion defects and thus reducing the resistance of the film and activating the aluminum anode [6,7]. Hence, to improve the hot-dip aluminum of austenitic stainless steel, knowledge of the phase equilibria in the Fe-Al-Si-Sn-Cr system and information on the interaction between the Fe-Cr alloy and the Al-Si-Sn pool are necessary. In this work, two isothermal sections of the Al-Cr-Sn ternary system at 600 and 800 °C were measured according to the hot-dip aluminum temperature, which will provide an important reference value for improving the stress corrosion of ferrite stainless steel by hot-dip aluminum plating.
2. Literature data 2.1 The Al-Cr system Investigation of the Al-Cr phase diagram has almost one hundred years of history and the first consistent version of this system was assessed by Bradley and Lu [8] in 1937 on the basis of earlier studies by Goto et al [9] and Fink et al [10]. Through the efforts of Cooper et al [11], Köster et al [12], Murray et al [13] and Grushko et al [14-17], the binary system has been deeply studied. In recent years, Chen et al [18], Tokunaga et al [19], Liang et al [20] and Khvan et al [21] reassessed the phase diagram, collated the relevant data, and provided new thinking for the study of the Al-Cr system. The Al-Cr binary phase diagram referred to in this work (shown in Fig. 1(a)) was assessed by Grushko et al [22] in 2008 and includes six intermediate phases, i.e., Al7Cr, Al11Cr2, Al4Cr, Al11Cr4, Al8Cr5 and AlCr2. Nevertheless, there are still many controversies about the Al-Cr binary system. First, two views exist on the reaction between Al7Cr, (Al) and the liquid phase, and their focus is whether this reaction is eutectic [9,23-25] or peritectic [10,13,26] and what is the accurate reaction temperature. Second, the temperature range of Al11Cr2 in which the phase is stable has not been settled [14,20,27-31]. Further studies are needed to determine whether the phase is stable at low temperature. The temperature of the peritectic reaction, at which Al7Cr decomposes to liquid and Al11Cr2 (detected at 790 °C in [12], at 835 °C in [32], at 799 °C in [28], at 795 °C in [22]),
remains controversial. The existence of Al11Cr4 is still a puzzle; Grushko affirmed the new phase in 2006 [15,33], while some researchers doubted his view and regarded Al11Cr4 as a ternary phase that was contaminated by quartz tubes [18,34,35]. Many studies have divided the phase region with a composition range between 30 and 42 at.% Cr into five different parts [8,12,13]; however, in recent studies, two solid solutions with large compositional ranges, one high- and one low-temperature sub-region were observed [14]. This view is adopted in this work for simplicity. The crystallographic data for the Al-Cr binary phases are summarized in Table 1.
2.2 The Cr-Sn system There is no intermetallic phase reported in the Cr-Sn system. The Cr-Sn system (shown in Fig.1 (b)) exhibits a miscibility gap in the liquid phase, two invariant reactions including one monotectic invariant reaction, and very low solubilities in the solid state [36-38]. The critical points of the miscibility gap and the monotectic reaction (liquid↔A2(Cr)+liquid’) were reported to be 1485 and 1374 °C, respectively.
2.3 The Al-Sn system Al-Sn is a simple eutectic system with limited solid solubilities in the two terminal solid solutions, fcc (Al) and tetragonal (βSn). Pure Sn transforms from bct β-Sn to a diamond cubic structure α-Sn at low temperature [39-41]. McAlister and Kahan [39] indicated that the eutectic reaction (liquid↔(Al)+(βSn)) occurred at approximately 228.5 °C and 97.6 at. % Sn, and Balanovic et al proved this result in their thermal analysis of Al-Sn system [42]. The Al-Sn system can be seen in Fig.1 (c). There is no information about the phase relationships of the Al-Cr-Sn ternary system in the literature.
3. Experimental procedures More than fifty alloy samples were prepared to study the 600 °C and 800 °C isothermal sections of the Al-Cr-Sn ternary system. All equilibrated alloys were prepared by the method of alloy melting with the raw materials of high-purity Al particles (99.99 wt.%), Sn particles (99.99 wt.%) and Cr particles (99.99 wt.%). Each sample, with a weight of 3 g, was melted in an arc
furnace under a pure argon environment (99.999 vol. %), and both sides of the samples were melted three times. Zr particles were used in the melting process as the deoxidizer, which was replaced with each melting. Taking the reaction between aluminum and silicon in quartz cubes and the segregation of tin into consideration, all samples were packaged in tantalum foil, placed into quartz cubes and then sealed in evacuated tubes under an argon gas atmosphere. All samples were kept at 600 °C for 25 days and 800 °C for 20 days to ensure the equilibrium of alloys, and hightemperature samples were preserved by quenching in water. After quenching, the liquid phase present at high temperature exists as a solid solution. All annealed samples were divided into two parts: one part of the samples was prepared for microstructure examination, and the other part was prepared for X-ray powder diffraction tests. A JSM-6510 scanning electron microscope equipped with an OXFORD energy dispersive X-ray spectrometer (EDX) and an OXFORD INCA 500 wave dispersive X-ray spectrometer with a probe diameter of 1 µm and an accelerating voltage of 20 kV was used to examine the microstructure of annealed samples and analyze the microarea composition. To ensure the accuracy of the EDX data, the final data were averaged over at least five sets. X-ray powder diffraction was used to test the phases of each sample, in a D/max 2500 PC X-ray diffractometer with Cu Kα radiation and a step increase of 0.02° in the 2θ angle. Si powders were used as external calibration standards. Jade 5.0 was used to analyze and fit the X-ray diffraction patterns.
4. Results and discussion 4.1 Study on the Al11Cr4 phase The phases in the Al-Cr binary system have been basically confirmed, and only the phase Al11Cr4 has not been unanimously recognized by researchers. To confirm the existence of this phase, relevant experiments were designed. Two binary alloys that are very close to the composition of Al11Cr4 were prepared. The samples were melted in an arc furnace under a pure argon environment, and annealed at 600 °C for 30 days. Fig. 2(a) presents the microstructure of alloy Al76Cr24, and the SEM-EDX data indicates that this alloy includes two phases, i.e., Al4Cr and Al8Cr5. A similar microstructure was detected in alloy Al72Cr28, but it is noteworthy that a small amount of Al11Cr4 appears locally in this alloy (Fig. 2(b)). The Al11Cr4 phase is difficult to nucleate and develop during the normal cooling process, due to its slow growth kinetics. Thus, it
may take longer to achieve equilibrium. This observation is consistent with the result of Grushko et al [15].
4.2 Phase equilibria of the Al-Cr-Sn ternary system at 600 °C All the phases in the alloy samples can be clearly distinguished based on morphology, color and chemical composition. The nominal compositions of key alloys for assessing the isothermal section, different phases and related chemical compositions are all shown in Table 2. Seven key alloy samples (A1-A7) and related data were chosen to draw the Al-Cr-Sn ternary phase diagram at 600 °C. According to the experiment data and the relevant binary system, this isothermal section
can
be
divided
into
seven
three-phase
regions:
(1)
Liq.+α-Al+Al7Cr,
(2)
Liq.+Al7Cr+Al1Cr2, (3) Liq.+Al1Cr2+Al4Cr, (4) Liq.+Al4Cr+Al11Cr4, (5) Liq.+Al11Cr4+Al8Cr5, (6) Liq.+Al8Cr5+AlCr2, and (7) Liq.+AlCr2+A2(Cr). The experimental results show no ternary compound in the Al-Cr-Sn ternary system at 600 °C. Alloy A1 was designed to evaluate the phase relationship in the Al-rich corner of the Al-Cr-Sn ternary system at 600 °C. SEM-EDX analyses show that α-Al is in equilibrium with Al7Cr and Liquid-Sn. As shown in Table 2, the solubility of Sn in Al7Cr is approximately 0.3 at.%. The existence of this three-phase equilibrium is further verified by the XRD diffraction pattern (Fig. 3(A1)). The diffraction peak associated with the reported for the Al11Cr2 phase [17,48] is found in the two
alloys
A2
and
A3(shown
in
Fig.
3(A2-A3)),
which
correspond
to
the
(Liquid-Sn+Al7Cr+Al11Cr2) and (Liquid-Sn+Al11Cr2+Al4Cr) three-phase regions, respectively. It can be easily deduced that the decomposition temperature is less than 600 °C. This outcome is different from the result of Mahdouk and Gachon [28], who claimed that this phase decomposed to Al7Cr and Al4Cr at 785 °C. Two three-phase coexistence zones are found in samples A4 and A5, namely, (Liquid-Sn +
Al4Cr + Al11Cr4) and (Liquid-Sn + Al11Cr4 + Al8Cr5) , respectively. The XRD results also confirm the existence of these two three-phase equilibria, as shown in Fig. 3(A4-A5). SEM-EDX analysis suggests that the compositional range of Al11Cr4 is 72.4-72.8 at.% Al. This range is slightly different from the results of Grushko et al [15] and Wu et al [33]. Interestingly, the addition of the third element seems to be more favorable to the equilibrium
of Al11Cr4. Hence, an Al76Cr24 alloy/Sn diffusion couple was prepared using a tin to Al-Cr alloy ratio of 3/4 and annealed at 600 °C for 4 hours. Remarkably, the Al11Cr4 phase was found in the microstructure of the diffusion couple top layer (Fig. 4), and the amount of the Al11Cr4 phase decreases gradually from the surface of the steel matrix to its interior. The experimental data obtained with EDX from the Al-Cr alloy and Al-Cr alloy/Sn diffusion couple are listed in Table 3. The composition of the metastable phase Al11Cr4 is Al73.6Cr26.3Sn0.1, very close to the composition of Al11Cr4 in the equilibrium alloys (A4 and A5). It may be inferred that the addition of tin promotes the nucleation of Al11Cr4 and accelerates its growth kinetics process. Fig. 3(A6) is the XRD pattern of alloy A6 in which three phases, namely, Al8Cr5, AlCr2 and Liquid-Sn, are observed. The isothermal section of the Al-Cr-Sn ternary system at 600 °C is assessed by integrating the related data, as shown in Fig. 5. In addition, several alloy samples ranging from alloy A9 to A14 were prepared to confirm the boundary of the Cr single-phase zone, which occupies the red area at the bottom right of Fig. 5.
4.3 Phase equilibria of the Al-Cr-Sn ternary system at 800 °C To further characterize the Al-Cr-Sn system, the 800 °C isothermal section was also experimentally determined in the present work. The summarized experimental data listed in Table 4 correspond to the isothermal section of the Al-Cr-Sn binary system at 800 °C, including four sets of three-phase equilibrium: Liq. +Al11Cr2+Al4Cr, Liq.+Al4Cr+Al8Cr5, Liq.+Al8Cr5+AlCr2 and Liq.+AlCr2+A2(Cr). Based on the experimental results, the 800 °C isothermal section of the Al-Cr-Sn system was constructed and is shown in Fig. 6. The alloys B1, B3 and B4 were annealed for 20 days. Three three-phase equilibrium states exist in these alloys, and the phase relationships at the 800 °C isothermal sections are in full agreement with that at 600 °C, i.e., Liq.+Al11Cr2+Al4Cr, Liq.+Al8Cr5+AlCr2 and Liq.+AlCr2+ A2(Cr). The microstructure and XRD pattern of alloy B2 (Figs. 7(a-b)) indicate that this alloy exists in the three-phase field with the Al4Cr, Al8Cr5 and Liquid-Sn phases. The long gray strip Al4Cr phase, the gray matrix Al8C5 phase and the white Liquid-Sn phase can coexist in this alloy. The detection of the Al11Cr2 phase in this isothermal section enlarges the range of its stable temperature to 800 °C, and this conclusion has been verified in Al-Cr-Ni [49], Al-Cr-Mn [16],
Al-Cr-Cu [17], Al-Cr-Mg [50] and Al-Cr-Sb [51] systems. This result is opposite those in references [28-31]. Otherwise, according to the information summarized in Table 5, the stability of the Al11Cr2 phase has also been determined at 450 °C. That is, the Al11Cr2 phase which is stable between 450 °C and 800 °C is confirmed. In this way, the Al11Cr4 phase is unstable at this temperature. Thus, the equilibrium B2 alloy was annealed again at 690 °C for 7 days, and the newly-formed gray metastable phase Al11Cr4 was discovered in the microstructure of B2, as shown in Fig. 8. Compared with Fig. 7(a), the amount of Al4Cr and Al8Cr5 are less, while that of Al11Cr4 is greater. This may be the result of the eutectoid reaction (Al4Cr+Al8Cr5↔Al11Cr4). These findings provide evidence that the decomposition temperature of Al11Cr4 is greater than 690 °C, and is consistent with the result of Grushko et al. [29].
5. Conclusions This work assessed the 600 and 800 °C isothermal sections of the Al-Cr-Sn ternary system by using the equilibrium alloy method to prepare samples. SEM-EDX and X-ray powder diffraction were used for analysis and detection. Experimental results illustrate that there exist seven three-phase regions in the 600 °C isothermal section, while only four three-phase regions can be detected at 800 °C. All of the Al-Cr binary compounds (i.e., Al7Cr, Al11Cr2, Al4Cr, Al11Cr4, Al8Cr5 and AlCr2) exist in the Al-Cr-Sn system at 600 °C. The Al11Cr2 phase is stable in the Al-Cr-Sn ternary system from 450 °C to 800 °C. The Al11Cr4 phase exists in only the Al-Cr and Al-Cr-Sn systems at 600 °C, and its decomposition temperature can be between 690 °C and 800 °C. The solubility of Sn in Al-Cr binary compounds is negligible. No ternary compounds were discovered in this work.
Acknowledgements Financial supports from the National Science Foundation of China (Grant Nos. 51671036 and 51871030) and a project funded by the Priority Academic Program Development of Jiangsu higher education institutions are greatly acknowledged. This work has also been sponsored by Qing Lan Project.
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Figure captions: Fig. 1 The binary phase diagrams, (a) Al-Cr system [43], (b) Cr-Sn system [36], (c) Al-Sn system [44] Fig. 2 The BSE images of the binary alloy samples, (a) Al76Cr24, (b)Al72Cr28 Fig. 3 XRD patterns of typical ternary alloys, A1: (Sn) + α-Al + Al7Cr, A2: (Sn) + Al7Cr +Al11Cr2, A3: (Sn) + Al11Cr2 +Al4Cr, A4: (Sn) + Al4Cr + Al11Cr4, A5: (Sn) + Al11Cr4 + Al8Cr5, A6: (Sn) + Al8Cr5 +AlCr2 Fig. 4 The BSE image of the top layer of Al76Cr24 alloy/Sn diffusion couple sample Fig. 5 The isothermal section of the Al-Cr-Sn system at 600 °C Fig. 6 The isothermal section of the Al-Cr-Sn system at 800 °C Fig. 7 Phase coexisting in alloy B2, (a) BSE image and (b) XRD pattern Fig. 8 The BSE image of equilibria alloy sample after secondary annealing at 690 °C
Table captions: Table 1 Crystallographic data of the phases in the Al-Cr system Table 2 Summarized experimental data for key specimens annealed at 600 °C Table 3 Summarized experimental data of Al-Cr alloy and Al-Cr alloy/Sn diffusion couple Table 4 Summarized experimental data for key specimens annealed at 800 °C Table 5 Summarized experimental data for key specimens annealed at 450 °C
Table 1. Crystallographic data of the phases in the Al-Cr system Phase
Al7Cr (Al45Cr7, Al13Cr2)
Pearson symbol
Referenc e
a
b
c
α,β,γ
C2/m
21.596
7.574
10.949
128°43′
[11]
mC104
C2/m C2/c
20.595 17.6
7.574 30.5
10.949 17.6
107°34′ ≈90°
[31] [45]
mC616
C2/c
17.735
30.456
17.737
91.052°
[46]
P63/mmc P63/mmc
20.0 20.191
-
24.7 24.854
-
[43] [46]
Cmcm
12.52
34.705
20.223
-
[47]
P-1
5.089
9.033
5.044
[17]
R3m I4/mmm
12.818 2.999
-
7.945 8.630
α=91.84° β=100.77° γ=107.59° -
Al4Cr hp574-6. 32 oC584-2 0.48
Al11Cr4
Al8Cr5 AlCr2
Lattice parameter(nm)
mC104
Al11Cr2 (Al5Cr)
Al4Cr
Space Group
hR26 tI26
[14] [8]
Table 2. Summarized experimental data for key specimens annealed at 600°C No.
Nominal composition (at.%)
A1
91Al-7Cr-2Sn
A2
83Al-16Cr-1Sn
A3
80Al-18Cr-2Sn
A4
75Al-24Cr-1Sn
A5
70Al-28Cr-2Sn
A6
43Al-54Cr-3Sn
A7
25Al-73Cr-2Sn
A8
57Al-37Cr-6Sn
A9
18Al-71Cr-11Sn
A10
10Al-50Cr-40Sn
A11
11Al-63Cr-26Sn
A12
6Al-44Cr-50Sn
A13
7Al-73Cr-20Sn
A14
3Al-62Cr-35Sn
Phase Liq. α-Al Al7Cr Liq. Al7Cr Al1Cr2 Liq. Al1Cr2 Al4Cr Liq. Al4Cr Al11Cr4 Liq. Al11Cr4 Al8Cr5 Liq. Al8Cr5 AlCr2 Liq. AlCr2 A2(Cr) Liq. Al8Cr5 Liq. A2(Cr) Liq. A2(Cr) Liq. A2(Cr) Liq. A2(Cr) Liq. A2(Cr) Liq. A2(Cr)
Composition (at.%) Al
Cr
Sn
21.3 99.3 86.0 15.7 85.3 82.1 13.5 82.3 79.8 8.6 77.1 72.8 5.1 72.4 68.9 1.9 56.8 33.0 1.6 29.3 19.6 2.4 61.4 0.3 20.0 0.3 17.2 0.3 15.1 0.2 12.0 0.2 8.8 0.1 5.2
1.6 0.6 13.7 2.7 14.6 17.6 3.1 17.6 20.0 4.5 22.6 27.1 2.9 27.5 31.0 2.2 42.6 66.8 1.8 70.6 80.2 2.6 38.3 1.2 79.5 1.3 82.2 1.4 84.2 1.2 87.1 1.2 90.2 1.1 93.5
77.1 0.1 0.3 81.6 0.1 0.3 83.4 0.1 0.2 86.9 0.3 0.1 92.0 0.1 0.1 95.9 0.6 0.2 96.6 0.1 0.2 95.0 0.3 98.5 0.5 98.4 0.6 98.3 0.7 98.6 0.9 98.6 1.0 98.8 1.3
Table 3. Summarized experimental data of Al-Cr alloy and Al-Cr alloy/Sn diffusion couple No.
Nominal composition (at.%)
1
Al76Cr24
2
Al72Cr28
3
Al76Cr24/Sn diffusion
Phase Al4Cr Al9Cr4 Al4Cr Al9Cr4 Al11Cr4 Al4Cr Al9Cr4 Al11Cr4
Composition (at.%) Al
Cr
Sn
78.2 68.4 77.5 68.6 73.1 77.4 68.9 73.6
21.8 31.6 22.5 31.4 26.9 22.4 31.0 26.3
0.2 0.1 0.1
Table 4. Summarized experimental data for key specimens annealed at 800°C No.
Nominal composition (at.%)
B1
79Al-18Cr-3Sn
B2
75Al-24Cr-1Sn
B3
41Al-54Cr-5Sn
B4
25Al-71Cr-4Sn
Phase Liq. Al1Cr2 Al4Cr Liq. Al4Cr Al8Cr5 Liq. Al8Cr5 AlCr2 Liq. AlCr2 A2(Cr)
Composition (at.%) Al
Cr
Sn
11.7 81.8 80.3 3.1 76.8 68.6 0.4 56.2 33.3 0.3 30.8 23.2
1.9 17.9 19.5 2.0 22.9 31.3 2.1 43.2 66.5 2.5 69.8 76.7
86.4 0.3 0.2 94.9 0.3 0.1 97.5 0.6 0.2 97.2 0.1 0.1
Table 5. Summarized experimental data for key specimens annealed at 450°C No.
Nominal composition (at.%)
C1
83Al-16Cr-1Sn
C2
80Al-18Cr-2Sn
Phase Liq. A7Cr Al11Cr2 Liq. Al11Cr2 Al4Cr
Composition (at.%) Al
Cr
Sn
4.5 85.9 83.2 4.7 83.1 78.3
2.7 13.9 16.7 2.1 16.7 21.4
92.8 0.2 0.1 93.3 0.2 0.3
Highlights Phase equilibria of the Al-Cr-Sn ternary system at 600°C and 800 °C were determined. The Al11Cr2 phase is stable in the Al-Cr-Sn ternary system from 450°C to 800 °C. The Al11Cr4 phase exists in Al-Cr and Al-Cr-Sn systems at 600°C, its decomposition temperature is from 690°C to 800°C.
Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: