Sulfide Capacities of Solid Oxides in Calcium-Aluminate Systems

Abstract

To clarify the mechanism of CaS formation on the oxide inclusion of the CaO–Al₂O₃ system, the sulfide capacities of solid oxides was measured in the present study. The results show that the sulfide capacity of (12CaO·7Al₂O₃; C₁₂A₇) was much larger than of other compounds, and increased with temperature. The value for C₁₂A₇ was larger than that measured for the liquid oxide of the same composition. Furthermore, the diffusion behavior of sulfur in solid steel to the inclusion of the CaO–Al₂O₃ system, was investigated using a diffusion couple. After heating at 1473 K for 72 h, in the case of C₁₂A₇, the intensity of sulfur in the oxide was high, but the formation of CaS was not detected. This suggests that the formation of CaS was suppressed around the C₁₂A₇ particles by the diffusion of sulfur in the solid oxide.

1. Introduction

In steel melts, various oxides are entrapped as non-metallic inclusions. These inclusions are formed by the deoxidation reaction, entrapment of slag or mold fluxes, erosion of refractories etc. and affect the steel properties. The CaO–Al₂O₃ system inclusion is usually formed by the addition of Ca to Al-killed steel, for modifying Al₂O₃ to a less harmful oxide with a low melting point. However, CaS precipitation is observed on this inclusion^1,2,3,4) and causes pitting corrosion in ferritic stainless steel.^5,6) Several studies have been conducted for determining the mechanism of CaS precipitation on the CaO–Al₂O₃ system inclusions. For example, Karino et al.¹⁾ reported that CaS precipitated after the oxide formation, based on the relationship between the oxide composition and the Ca content in the precipitated sulfide. Takenouchi et al.²⁾ concluded that the decreased solubility of sulfur in the oxide of the CaO–Al₂O₃ system at lower temperature caused the formation of CaS. On the other hand, in some cases, CaS was observed without Ca addition. Kitamura et al.⁷⁾ observed CaS in ultra-low sulfur cold-rolled steel, whereas CaS was never observed in molten steels or slabs. They predicted that CaS precipitation was due to the diffusion of sulfur in the solid steel toward the CaO–Al₂O₃ system inclusion, during the heating of the slab.

Many studies have been conducted to measure the sulfide capacity of the liquid CaO–Al₂O₃ system at various temperature.^{8,9,10,11,12,13)} Hino et al.⁸⁾ have summarized these data with their experimental results and showed the effects of temperature and composition. Ban-ya et al.⁹⁾ have investigated the solubility of CaS in the liquid CaO–Al₂O₃ system and found that its solubility was less than 0.05 (mol fraction) and increased slightly with increasing CaO content. These data are important for predicting the precipitation of CaS for temperature above the melting point of the oxide, i.e., during the secondary refining or the solidification of steel.

However, in the case of steels with low sulfur content, the precipitation of CaS was not observed around the oxide which sampled from molten steel and from slab after the solidification. Therefore, the solubility of CaS and the sulfide capacity for the solid CaO–Al₂O₃ system have to be clarified. In the previous studies,^14,15) the authors measured the sulfur content in the CaS-saturated solid CaO–Al₂O₃ oxide with various compositions. The results showed that among the CaO·Al₂O₃, 12CaO·7Al₂O₃, and 3CaO·Al₂O₃, sulfur was dissolved only in 12CaO·7Al₂O₃, at approximately 1.2 mass%. Based on this result, the importance of 12CaO·7Al₂O₃ for dissolution of sulfur in the solid state, for preventing CaS formation, was suggested. In the present work, to evaluate the capacity of sulfur dissolution for each oxide compound in the solid CaO–Al₂O₃ system, the sulfide capacity of the solid oxide was measured. In addition, the diffusion behavior of sulfur in solid steel to the inclusion of the CaO–Al₂O₃ system was investigated using a diffusion couple.

2. Experimental

2.1. Sulfide Capacity

In the CaO–Al₂O₃ system, various line compounds are known. Among them, in this experiment, 3CaO·Al₂O₃(C₃A), 12CaO·7Al₂O₃(C₁₂A₇), CaO·Al₂O₃(CA), CaO·2Al₂O₃(CA₂), CaO·6Al₂O₃(CA₆) were synthesized by sintering. Reagent-grade CaCO₃, Al₂O₃, were used, and CaO was produced by calcining CaCO₃ at 1373 K for 12 h or longer. The reagents were mixed to the target composition and sintered at 1573 K for 48 h or longer under air atmosphere in a Pt crucible. After sintering, the tablet was crushed into powders, with particles smaller than 52 μm, for the X-ray diffraction analysis (XRD; Rigaku, RINT2200).

The heat treatment of the sample was conducted using a vertical-type, electric-resistance furnace, shown in Fig. 1. The inner diameter and the length of the reaction tube made by alumina (SSA-S) were 42 mm and 1 m, respectively. The sample powders were compressed, to form pellet in the size of about 4 mm in diameter and about 5 mm in height. The five Pt crucibles with the different-composition pellets were set in a holder made by refractory, and located in the heating zone of the furnace. The samples were inserted at room temperature and heated to the target temperature (1373, 1473, 1573 and 1673 K) at a heating rate of about 3 K/min, under an Ar atmosphere. Then, the gas mixture of the CO–CO₂–SO₂–Ar system was flown into the reaction tube. To control the gas composition, pure CO, pure CO₂, and Ar+1%SO₂ gas were used. The partial pressures of O₂ (P_O2) and S₂ (P_S2) were determined using FactSage to avoid the formation of CaS and CaSO₄. Gas compositions did not vary with temperature, and the values of P_O2 and P_S2 are shown in Table 1. The heating time at each target temperature was 24 h. After the heating, the samples were withdrawn and quenched by He gas.

Fig. 1.

Schematic of the experimental setup for measuring the sulfide capacity.

Table 1. Partial pressures of oxygen and sulfur used at each temperature.

Temperature/K	PO2	PS2
1373	1.18×10−11	1.18×10−5
1473	1.15×10−10	1.00×10−5
1573	1.11×10−9	5.56×10−6
1673	1.09×10−8	1.87×10−6

The heat-treated samples were crushed again into powders, with particles smaller than 52 μm. The sulfur contents of the samples were analyzed using the combustion method (Horiba, EMIA-220S). The elemental mapping was conducted using electron probe micro-analyzer (EPMA) (JEOL: JXA-8200), and the structure of the oxides was analyzed using micro Raman spectroscopy (Renishaw: inVia Reflex).

2.2. Diffusion Couple

The experimental setup is shown schematically in Fig. 2. The powders of the five compounds of the CaO–Al₂O₃ system, shown above, were sandwiched using a bar of ferritic stainless steel, and pressed. The synthesis method of the oxide powders was the same as described in the previous section, and a commercial bar of 430 grade stainless steel (diameter, 8 mm) was used. The composition of the bar is shown in Table 2. The sulfur content was analyzed using the combustion method, and the compositions of other elements were based on the Japan Industrial Standard. The bar was cut to about 6 mm in length, and its cross-section was polished to form a smooth surface. The press was conducted using a hot-rolling simulator (Fiji-Dempa: Thermec Master Z). After the evacuation, the sample, in which the oxide powders were sandwiched using the steel bars, was heated to 1373 K, at a heating rate of 10 K/s. In this temperature, the sample was pressed to 50% in height at a strain rate of 0.01/s. After the treatment, the sample was quenched by He gas.

Fig. 2.

Schematic of the hot deformation process for preparing a diffusion couple.

Table 2. Composition of the steel rod.

Element	C	Si	Mn	P	S	Cr
Mass%	≦0.120	≦0.750	≦1.000	≦0.040	0.0015	16.000–18.000

Following that, the sample of the diffusion couple and a Ti foil were sealed in a quartz tube (O.D. 12 mm, I.D. 10 mm), into which high-purity Ar gas was introduced at a pressure of 20 kPa. The Ti foil functioned as a getter to reduce the oxygen partial pressure in the tube. The heat treatment for this specimen was conducted in air, for 72 h at 1473 K. After the heating, the specimen was quenched by water.

The specimen was cut in the vertical direction, perpendicular to the interface of the two steel bars. After polishing, the interface was observed by EPMA.

3. Results

3.1. Sulfide Capacity

The sulfur contents for each compound at 1373 K after different heating times are listed in Table 3 and shown in Fig. 3. The sulfur content of C₁₂A₇ increased with heating up to 24 h and became constant after that. The sulfur contents of CA, CA₂, and CA₆ did not increase with time. On the other hand, C₃A did not contain sulfur after heating for 4 h and 12 h, but the content increased drastically after heating for 24 h and 48 h. This was due to the precipitation of CaS and will be discussed in detail later. Based on these results, the sulfur content after heating for 24 h was determined as the equilibrium content. The effect of temperature on the sulfur content is shown in Table 4. The sulfur content for C₁₂A₇ was much larger than for the other compounds, except C₃A, and the maximal content was about 1.9 mass% for C₁₂A₇, observed at 1373 K. The sulfur content for C₁₂A₇ decreased with increasing the temperature, and the value at 1673 K was significantly lower than for the other conditions. Compared with C₁₂A₇, CA, CA₂, and CA₆ did not contain sulfur. In the case of C₃A, relatively large values were observed at 1373 and 1473 K.

Table 3. Sulfur content of different solid compounds, at different heating times (mass%).

Time/h	C3A	C12A7	CA	CA2	CA6
4	0.24	0.22	0.21	0.22	0.22
12	0.26	0.94	0.26	0.23	0.26
24	4.14	1.93	0.02	0.00	0.00
48	7.00	1.77	0.03	0.02	0.01

Fig. 3.

Sulfur content vs. heating time (at 1373 K).

Table 4. Sulfur content of different solid compounds, at different temperatures (mass%).

Temperature/K	C3A	C12A7	CA	CA2	CA6
1373	4.14	1.93	0.02	0.00	0.00
1473	1.46	1.87	0.02	0.01	0.01
1573	0.02	1.67	0.03	0.00	0.00
1673	0.00	0.36	0.00	0.00	0.00

The XRD spectra before and after the heating for 24 h are shown in Fig. 4(a) for C₃A and in Fig. 4(b) for C₁₂A₇. No spectral changes were observed for C₁₂A₇. In the case of C₃A, the weak peaks that indicate the formation of CaS and C₁₂A₇ after heating. This suggests that the following reaction occurs and a part of C₃A changes to C₁₂A₇ and CaS:   

$$\begin{array}{l}7\left(3\text{CaO}\cdot {\text{Al}}_2{\text{O}}_3\right)\left(\text{s}\right)+4.5{\text{S}}_2\left(\text{g}\right)=12\text{CaO}\cdot {\text{Al}}_2{\text{O}}_3\left(\text{s}\right)\\ +9\text{CaS}\left(\text{s}\right)+4.5{\text{O}}_2\left(\text{g}\right)\\ \mathrm{\Delta }{G}^0=-2\mathrm{\hspace{0.17em} }269\mathrm{\hspace{0.17em} }461+849.28T\end{array}$$(1)

Fig. 4.

Results of the XRD analysis of oxides, before and after the heat treatment ((a) C₃A, (b) C₁₂A₇).

The element-mapping images obtained using the EPMA are shown in Fig. 5(a) for C₃A and in Fig. 5(b) for C₁₂A₇, after heating at 1473 K. In the case of C₁₂A₇, the distribution of sulfur was mostly uniform. In the case of C₃A, an area with an extremely high content of sulfur was observed. Therefore, C₁₂A₇ can dissolve sulfur without changing the lattice structure, but the high sulfur content in C₃A at lower temperature is caused by the formation of CaS.

Fig. 5.

Results of the element-mapping analysis using the EPMA, after the heat treatment at 1473 K ((a) C₃A, (b) C₁₂A₇). (Online version in color.)

The sulfide capacity was calculated by Eq. (2), where (mass%S)_oxide(s) indicates the sulfur content in the solid oxide in mass%:   

$$C_{\text{S}}={\left(\text{mass\%S}\right)}_{\text{oxide}\left(\text{s}\right)}{\left(\frac{P_{{\text{O}}_2}}{P_{{\text{S}}_2}}\right)}^{1/2}$$(2)

The calculated values are summarized in Table 5. In this table, as the formation of CaS was confirmed, the sulfide capacities of C₃A at 1373 and 1473 K were not calculated. Figure 6 shows the variation of the sulfide capacity with temperature, for the different compounds, compared with that for the liquid oxide.⁸⁾ The sulfide capacity of C₁₂A₇ was much higher than for the other compounds, and increased with increasing the temperature. Furthermore, the value for C₁₂A₇ was higher than that measured for the liquid oxide of the same composition.

Table 5. Sulfide capacities of the different compounds, at different temperatures.

Temperature/K	Sulfide capacity: Cs (×10−3)
	C3A	C12A7	CA	CA2	CA6
1373	／	1.93	0.02	0.00	0.00
1473	／	6.33	0.05	0.03	0.03
1573	／	23.59	0.47	0.00	0.00
1673	／	27.61	0.00	0.00	0.00

Fig. 6.

Comparison of the sulfide capacities of the different compounds and liquid oxides.

The bonding state of sulfur in the compounds was investigated using the Raman spectroscopy. As described in the previous paper, C₁₂A₇ is known to be an electrically conductive oxide ceramic.^16,17,18) Its crystal has a unique sub-nano-porous structure composed of 12 positively charged cages, including two free oxygen ions, which maintain electrical neutrality. The free oxygen ions can be exchanged with an anion X (i.e., fluorine or chlorine). As shown in Fig. 7, comparing the spectra before and after the heat treatment, the O²⁻ band around 1128 cm^{−1 19,20)} disappeared after heating at 1373 K, 1473 K, and 1573 K. In addition, the band at 770 cm⁻¹ slightly shifted to 772 cm⁻¹, indicating that the O₂²⁻ band (768 cm^{−1 19,20)}) decreased and the broad band changed to the Al–O band (tetrahedron, 772 cm^{−1 20)}). The above band changes validate that the number of free oxygen ions trapped in the sub-nano-porous structure of C₁₂A₇ decreased after the heat treatment. On the other hand, S₂O₅²⁻ bands (635 cm⁻¹ and 1085 cm^{−1 21,22)}) and vS₃⁻ band (470 cm^{−1 23,24)}) emerged after the heat treatment. This suggests that sulfur-containing ions were captured by the sub-nano-porous structure after the free oxygen ions were removed. Overall, the following reaction can be deduced from the Raman spectroscopy analysis:   

$$\begin{array}{l}{\left[{\text{Ca}}_{24}{\text{Al}}_{28}{\text{O}}_{64}\right]}^{4+}\left(2{\text{O}}^{2-}\right)+{\text{S}}_2\to \\ {\left[{\text{Ca}}_{24}{\text{Al}}_{28}{\text{O}}_{64}\right]}^{4+}\left({\text{S}}_3^{-},\mathrm{\hspace{0.17em} }\hspace{0.17em}{\text{S}}_2{\text{O}}_5{}^{2-}\right)+{\text{O}}_2\end{array}$$(3)

Fig. 7.

Raman spectra of C₁₂A₇, before and after the heating at 1473 K.

The maximal sulfur content by the complete exchange of free oxygen ions was estimated to be 2.28 mass% using the mass balance calculation, as shown in the previous paper. Compared with that value, the highest sulfur content after heating was 1.93 mass% in this experiment, which means that about 85% of the free oxygen ions were exchanged by sulfur. The exchange did not reach 100% because some oxygen remained by the formation of S₂O₅²⁻ ions.

In addition, all of the bands were weaker for the sample heated at 1673 K, compared with those for the other samples. As the melting point of C₁₂A₇ is about 1686 K, the crystal structure is likely to be damaged by heating at 1673 K for a long time. As a result, the sulfur content increased as shown in Table 4, even though the O²⁻ band at 1128 cm⁻¹ disappeared, indicating that the amount of free oxygen ions decreased due to the destruction of the crystal structure, and the exchange by sulfur did not occur much.

3.2. Diffusion Couple

The cross-section of the diffusion couple before heating is shown in Fig. 8. The oxide particles, spread on the surface of the rod, were uniformly distributed at the interface, and the boundary of the steel rod could not be identified, except for the edge of the sample.

Fig. 8.

Cross-section of the diffusion couple before heating. (Online version in color.)

As the sulfur content of the steel rod was very low, the concentration distribution in the steel was difficult to quantify by element mapping. Therefore, line scanning was conducted across the interface of the steel and the oxide, using the EPMA. The results for C₁₂A₇, CA, and CA₂ are shown in Figs. 9(a), 9(b), and 9(c). In the case of C₁₂A₇, the content of Ca and Al in the oxide was uniform, and the intensity of sulfur in the oxide was high. This indicates that the diffusion of sulfur from the solid steel to the oxide occurred by the heating, and the sulfur was dissolved the oxide particle. The formation of CaS was not observed. Compared with this, in the cases of CA and CA₂, the points exhibiting extremely high intensities of sulfur with Ca were found near the oxide particles. This suggests the formation of CaS at this position. The intensity of sulfur in the oxides was much lower than that for C₁₂A₇. The formation of CaS occurred as a decrease in the solubility product of Ca and S with decreasing the temperature. In addition, as the Al and Ca contents in the oxide were not uniform, the decomposition of the oxide during heating was inferred. The result for C₃A is shown in Fig. 10. The intensity of Al and sulfur varied widely, but the intensity of Ca was almost uniform. This result shows the decomposition of C₃A to the high sulfur phase and low Al phase. As described in the previous section, C₃A is decomposed into CaS and C₁₂A₇; thus, the present results validate this phenomenon.

Fig. 9.

Results of the line analysis using the EPMA: (a) C₁₂A₇, (b) CA, (c) CA₂.

Fig. 10.

Results of the line analysis using the EPMA, for C₃A.

These experiments clarified that only C₁₂A₇ can dissolve sulfur without the precipitation of CaS during the heating of low-sulfur-content stainless steel.

4. Discussion

In this section, the precipitation behavior of CaS from the oxide of the CaO–Al₂O₃ system is discussed, using the results of this study. When the liquid oxide of the CaO–Al₂O₃ system is solidified by decreasing the temperature, the phase separation to C₁₂A₇+C₃A or C₁₂A₇+CA occurs, depending on the composition of the liquid oxide. In this situation, sulfur, dissolved in the liquid oxide, is enriched only to C₁₂A₇ as the other phases cannot dissolve sulfur in the solid state. If the sulfur content in the C₁₂A₇ phase increases and exceeds the solubility limit, CaS is formed.

At first, the sulfur content in the liquid oxide was calculated at 1823 K for 11 mass% Cr containing steel, assuming it was determined by the sulfide capacity (Eq. (2); Cs), the partial pressure of sulfur (P_S2), and the partial pressure of oxygen (P_O2). The sulfide capacity of the liquid oxide, measured by Hino et al.,⁸⁾ was used, and the oxygen activity in the molten steel was calculated by deoxidation equilibrium with Al (Eq. (4)²⁵⁾). The relationships for calculating P_O2 and P_S2 are shown in Eqs. (5)²⁵⁾ and (6),²⁵⁾ respectively.   

$$2\left[\text{Al}\right]+3\left[\text{O}\right]={\text{Al}}_2{\text{O}}_3\hspace{1em}\text{log}{K}_4=\frac{64 \mathrm{\hspace{0.17em} }000}{T}-20.57$$(4)

  

$$\frac{1}{2}{\text{O}}_2=\left[\text{O}\right]\hspace{1em}\text{log}{K}_5=\frac{6\mathrm{\hspace{0.17em} }070}{T}+0.210$$(5)

  

$$\frac{1}{2}{S}_2=\left[S\right]\hspace{1em}\text{log}{K}_6=\frac{6\mathrm{\hspace{0.17em} }535}{T}-0.964$$(6)

The composition of the liquid oxide and its sulfide capacity at 1823 K are listed in Table 6. In this table, the oxide composition was determined for the molar ratios of C₁₂A₇:C₃A or C₁₂A₇:CA set to 8:1, 5:5, or 1:8. The sulfur contents under the equilibrium with various steel compositions of these oxides are listed in Table 7. At 1573 K, sulfur that was dissolved in the liquid oxide at 1823 K was assumed to be concentrated only in the C₁₂A₇ phase, and the sulfur content in this phase was calculated by the mass balance.

Table 6. Calculated compositions of oxides and their sulfide capacities at 1823 K.

Oxide	mass%			mass%		log Cs
	C3A	C12A7	CA	CaO	Al2O3
C3A:C12A7=8:2	80.0	20.0	0.0	59.01	40.99	−2.280
C3A:C12A7=5:5	50.0	50.0	0.0	54.88	45.12	−2.545
C3A:C12A7=2:8	20.0	80.0	0.0	50.75	49.25	−2.779
C12A7:CA=8:2	0.0	80.0	20.0	45.40	54.60	−3.183
C12A7:CA=5:5	0.0	50.0	50.0	41.50	58.50	−3.524
C12A7:CA=2:8	0.0	20.0	80.0	37.60	62.40	−4.047

Table 7. Sulfur contents for the different oxides at 1823 K, for various steel compositions.

	Case A	Case B	Case C
[mass%Al]	0.005	0.005	0.01
[mass%S]	0.001	0.005	0.001
Oxide	(mass%S) in oxide
C3A:C12A7=8:2	0.077	0.386	0.123
C3A:C12A7=5:5	0.042	0.210	0.067
C3A:C12A7=2:8	0.025	0.123	0.039
C12A7:CA=8:2	0.010	0.048	0.015
C12A7:CA=5:5	0.004	0.022	0.007
C12A7:CA=2:8	0.001	0.007	0.002

The variation of the sulfur content in the C₁₂A₇ phase for each oxide is shown in Fig. 11. Based on the steel composition of each case, the calculated ratio of P_O2/P_S2 for Case B was the smallest, and thus Case B showed the highest sulfur content in oxide and also in the phase of C₁₂A₇. Case C had the largest P_O2/P_S2, and then showed the lowest sulfur content. For each case, when the oxide consisted of C₁₂A₇ and C₃A, the sulfur content in the C₁₂A₇ phase increased sharply with increasing CaO content. This was because the sulfide capacity of the liquid oxide increased, and the phase ratio of C₁₂A₇ at 1573 K decreased with increasing CaO content. On the other hand, when the oxide consisted of C₁₂A₇ and CA, the sulfur content in the C₁₂A₇ phase increased slightly with decreasing CaO content, though both the sulfide capacity of liquid oxide and the phase ratio of C₁₂A₇ decreased. Such results occurred because the decrease in the C₁₂A₇ phase ratio was greater than the sulfur content in liquid oxide. In addition, the sulfur content in the C₁₂A₇ phase showed a minimum value at a CaO content of 48%, when the phase ratio of C₁₂A₇ had a maximum value.

Fig. 11.

Dependence of the sulfur content in the C₁₂A₇ phase on the oxide composition, for different steel compositions in Table 7.

In Fig. 11, the equilibrium sulfur content in the C₁₂A₇ phase with the solid steel is also shown in the horizontal line. In this calculation, the measured value of Cs at 1573 K was used, and P_O2 and P_S2 were assumed to be the same as those at 1823 K. If the sulfur content in C₁₂A₇ would exceed this value, precipitation of CaS would be expected. However, except for the case in which the mole fraction of CaO in the oxide was extremely high, the sulfur contained in the liquid oxide was soluble in the C₁₂A₇ solid phase, and the formation of CaS could be avoided. This discussion clarifies the importance of precipitating C₁₂A₇ in the oxide of the CaO–Al₂O₃ system for suppressing the formation of CaS.

5. Conclusions

In this research, to evaluate the sulfur dissolution capacities of oxide compounds in the solid CaO–Al₂O₃ system, the sulfide capacities of the solid oxides were measured. The compounds 3CaO·Al₂O₃(C₃A), 12CaO·7Al₂O₃(C₁₂A₇), CaO·Al₂O₃(CA), CaO·2Al₂O₃(CA₂), and CaO·6Al₂O₃(CA₆) were synthesized, and the sulfur contents were analyzed after the heating at 1373 K, 1473 K, 1573 K, and 1673 K, for 24 h, under a CO–CO₂–SO₂–Ar gas mixture. The results showed that the sulfide capacity of C₁₂A₇ was much higher than those of the other compounds, and it increased with increasing temperature. The value for C₁₂A₇ was higher than that measured for the liquid oxide of the same composition. Furthermore, the diffusion behavior of sulfur in solid steel to the inclusion of the CaO–Al₂O₃ system was investigated using a diffusion couple. After the heat treatment at 1473 K for 72 h, in the case of C₁₂A₇, the intensity of sulfur in the oxide was high, but the formation of CaS was not detected. Compared with that, in the cases of CA and CA₂, the formation of CaS near the oxide particles was observed. This indicates that the formation of CaS is suppressed around C₁₂A₇ particles by the diffusion of sulfur in the solid oxide.

Acknowledgement

This work was performed under the Research Program “Dynamic Alliance for Open Innovation Bridging Human, Environment and Materials”. The authors sincerely appreciate the support by Prof. Goro Miyamoto and Dr. Elango Chandiran in using the hot deformation simulator and Dr. Sohei Sukenaga for supporting the analysis of the Raman spectra.

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