Oxidation Behavior of Quaternary Co-20Re-25Cr-3Si Alloy at Laboratory Air

The iso-thermal oxidation of the quaternary Co-20Re-25Cr-3Si alloy at 1,000–1,200 C under laboratory air for 24 h has been studied. The oxidation kinetics at 1,000–1,200 C follow parabolic law perfectly and the value of oxidation activation energy is 220.74 kJ/mol. Unlike the experimental condition of 0.1 MPa pure oxygen, under laboratory air Co-20Re-25Cr-3Si alloy presents a good oxidation resistance due to the formation of a dense and continuous layer of Cr2O3 in contact with the alloy. A 79% drop in oxygen pressure has changed the oxidation mode from catastrophic oxidation to mild oxidation. The thickness of the protective Cr2O3 layer decreases when temperature increases. Therefore, Co-20Re-25Cr-3Si alloy shows the best oxidation resistance at 1,000 C. Moreover, from another perspective, compared with Co-17Re-23Cr-3Si alloy, the addition of about 2% Cr can also improve the oxidation resistance of the alloy greatly by forming the protective Cr2O3 layer.


INTRODUCTION
Superalloys are widely used in many fields such as aero engines, automobile engines, gas turbines, nuclear power, petrochemical industry, etc., and play an important role in economic development by virtue of its excellent oxidation and thermal corrosion resistance (Beran et al., 2016;Gorunov, 2017;Vorobiov et al., 2014;Shi and Zhong, 2010;Wanderka et al., 2016). The temperature in service of iron based superalloys can generally reach up to 750-780 C, while that of nickel based and refractory metal based alloys may be much higher. In fact, the application of nickel-based superalloys is limited to below 1,000 C due to their melting point (Perepezko, 2009;Gorr et al., 2011). In recent years, people pay more attention to low energy consumption and environmental protection, and pursue higher energy efficiency and less CO 2 emissions. Therefore, they hope to develop superalloys that can serve at higher temperatures (Mukherji et al., 2012;Gorr et al., 2014a;Gorr et al., 2014b). In particular, cobaltbased alloys have received extensive attention in recent decades due to their high temperature strength, good thermal corrosion resistance and oxidation resistance (Gorr et al., 2012;Wang et al., 2015a).
Adding high melting point metal Re to Co based alloy can greatly improve the melting point of Co based alloy, and the high temperature strength of the alloy can also be adjusted conveniently by adding appropriate amount of Re or other elements (Rösler et al., 2007;Gorr et al., 2014a;Beran et al., 2018;Strunz et al., 2018). Rösler et al. (Rösler et al., 2007) first proposed Co-Re-based superalloys because of their potential applications over 1,200 C. However, a significant disadvantage of metal Re is that at high temperatures the element Re may evaporate in the form of ReO 3 , resulting in mass loss of the alloy (Gorr et al., 2011). Adding a certain amount of Cr can form a protective Cr 2 O 3 layer on the alloys, which limits the evaporation of rhenium oxides (Gorr et al., 2011;Mukherji et al., 2012). It was found that the oxide film Cr 2 O 3 gradually lost its protective effect as the temperature increased, due to the rapid transformation of the oxide film Cr 2 O 3 into gaseous CrO 3 at high temperature (Gorr et al., 2014a;Gorr et al., 2012). The present of Si can provide nucleation site for the formation of Cr 2 O 3 under high temperature environment, thus significantly improving the oxidation resistance of the alloy (Gorr et al., 2011;Gorr et al., 2012;Xu et al., 2020). There are a few reports about the oxidation behavior of the quaternary Co-Re-Cr-Si alloys under the condition of laboratory air or 0.1 MPa pure O 2 (Gorr et al., 2012;Wang et al., 2013;Xu et al., 2020). The oxidation behavior of Co-17Re-23Cr-xSi (x 1, 2, 3) and Co-17Re-30Cr-2Si in laboratory air at 1,000 C has been investigated by Gorr B et al. (Gorr et al., 2012). The microstructure of the scales of the quaternary Co-17Re-23Cr-xSi (x 1, 2, 3) alloys and the ternary Co-17Re-xCr (x 23, 30) alloys are similar to some extent. Co-17Re-23Cr-xSi (x 1, 2, 3) alloys have suffered a mass loss because the Cr content is insufficient to form a continuous Cr 2 O 3 scale. The oxidation behavior of similar Co-20Re-25Cr-3Si quaternary alloy under the condition of 0.1 MPa pure oxygen at 1,000-1,200 C has been systematically investigated by our group recently (Wang, 2019). The result showed that the alloy suffered continuous mass loss during the oxidation process. However, when the oxidation atmosphere changes, the oxidation behavior of the alloy may change greatly. Therefore, high temperature oxidation behavior of the quaternary Co-20Re-25Cr-3Si alloy under laboratory air has been studied in this paper. Moreover, a special attention is focus on the difference of oxidation behavior between the Co-17Re-23Cr-3Si and Co-20Re-25Cr-3Si alloys, which are of a similar composition.

EXPERIMENTAL
The actual composition of Co-20Re-25Cr-3Si (atomic percentage, %) alloy prepared by repeated melting appropriate amounts of the four pure (99.9%) components of Co, Re, Cr and Si in a nonconsumable vacuum arc melting furnace is 2.84 at.% Si, 24.82 at.% Cr, 50.18 at.% Co, 22.16 at.% Re. The two-phase alloy were subsequently annealed for 24 h at 900 C in a vacuum atmosphere to achieve a better equilibration. Figure 1 shows the microstructure of the quaternary Co-20Re-25Cr-3Si alloys after annealing at room temperature. It can be seen that the alloy consists of a continuous Co-rich phase (dark phase, A) and a dispersed Re-rich phase (light phase, B). Table 1 shows the elemental composition of different phases of Co-20Re-25Cr-3Si alloy. There are more Re but no Si present in the Re-rich phase. Samples with a size of 10 mm × 10 mm × 1 mm were cut from the alloy ingots by wire cut electrical discharge machining. A hole with 1 mm diameter for hanging the samples was drilled. To avoid typical edge effects on the oxidation process, rounded edges were specially prepared. The surface of samples was subsequently ground using SiC waterproof paper down to 2000 grit. All samples were thoroughly cleaned by ethanol in an ultrasonic bath before the oxidation process. The surface area of each sample was measured to relate mass change to the initial surface area. Samples were oxidized separately under 0.1 MPa laboratory air (oxygen partial pressure decreased to 21% of 0.1 MPa) at 1,000-1,200°C.
Continuous mass changes of samples for 24 h were measured by thermogravimetric analyzer (Setaram Setsys Evo). The alloy microstructure and the constitution of oxides formed oxides were characterized by X-ray diffraction (XRD, Shimadzu XRD-6100) and scanning electron microscopy (FE-SEM, Zeiss Sigma) combined with energy-dispersive X-ray spectroscopy (EDS, Oxford INCA).

Oxidation Kinetics
Figures 2A,B present the oxidation kinetics and the corresponding parabolic plots of the Co-20Re-25Cr-3Si alloy under 0.1 MPa laboratory air at 1,000-1,200°C, respectively. It can be seen that during the whole oxidation process, the oxidation kinetics at 1,000-1,200°C follow parabolic law perfectly, with the values of rate constants equal to 2.9 × 10 −12 g 2 cm −4 ·s −1 , 1.6 × 10 −11 g 2 cm −4 ·s −1 and 4.4 × 10 −11 g 2 cm −4 ·s −1 , respectively ( Table 2). The mass gain of the alloy increases with temperature, with the corresponding value of 0.45 mg/cm 2 , 1.12 mg/cm 2 and 1.82 mg/cm 2 at 1,000-1,200°C respectively after 24 h oxidation. Co-20Re-25Cr-3Si alloy shows the best oxidation resistance at 1,000°C. According to the previous results by our group, Co-20Re-25Cr-3Si alloy shows very poor FIGURE 1 | Microstructure of the quaternary Co-20Re-25Cr-3Si alloys after annealing at room temperature. Light phase: Re-rich phase; dark phase: Co-rich phase. high temperature oxidation resistance under the experimental condition of 0.1 MPa pure oxygen, as shown in Figure 3 (Wang, 2019). The Cr 2 O 3 layer formed on the alloy is discontinuous and can not prevent the evaporation of Re element in the form of ReO 3 . Therefore, the alloy presents obvious mass loss, with the corresponding value of −4.54 mg/cm 2 , −5.84 mg/cm 2 and −8.81 mg/cm 2 at 1,000-1,200°C respectively after 24 h oxidation (Wang, 2019). However, when the oxygen pressure decreases from 0.1 to 0.021 MPa, the oxidation of Co-20Re-25Cr-3Si alloy changes greatly, from continuous mass loss to mass gain. A 79% drop in oxygen pressure causes the alloy to change from catastrophic oxidation to mild oxidation.

Calculation of Oxidation Activation Energy
The oxidation behavior of Co-20Re-25Cr-3Si alloy under 0.1 MPa laboratory air at 1,000-1,200°C conforms to the parabolic law. The square of the oxidation mass gain per unit area of the alloy is proportional to the time, as expressed by the following formula: Where y is the mass gain per unit area of the sample, K T is the oxidation rate constant of the alloy, and C is a constant.
Based on the experimental data, the relation diagram of (Mass Gain) 2 and oxidation time is made, as shown in Figure 4. By linearly fitting a straight line, the slope and intercept of the line are the values of K T and C respectively. The oxidation kinetics equation of the alloy at 1,000-1,200°C are shown in Table 3.
Since the oxidation behavior of Co-20Re-25Cr-3Si alloy under 0.1 MPa laboratory air can be described by parabolic law, it conforms to the Arrhenius equation and the oxidation rate constant satisfies the following relation: Where A is the pre-factor, Q is the oxidation activation energy, R is the gas constant, and T is the thermodynamic temperature. By taking the logarithm of both sides of (2), the following formula can be obtained: Substituting the K T value and the thermodynamic temperature value of the three temperatures into the Equation 3, the relationship diagram of ln K T as a function of 1/T shown in It can be found that the activation energy of the oxidation reaction of the Co-20Re-25Cr-3Si alloy is 220.74 kJ/mol.

Scale Microstructure and Composition
The scales grown on Co-20Re-25Cr-3Si after 24 h oxidation under laboratory air at 1,000-1,200°C (Figures 6, 7) are similar to some extent, including three-layer structure. The outermost dark gray layer of CoO is non-protective and easy to peel off. The following darker layer is probably composed mainly of CoCr 2 O 4 spinel, as shown by the XRD (Figure 8) and EDS results. The innermost darkest layer consists mainly of a dense and continuous layer of Cr 2 O 3 in contact with the alloy, providing a good oxidation resistance for the alloy. The thickness of the protective Cr 2 O 3 layer decreases when temperature increases. Therefore, Co-20Re-25Cr-3Si alloy shows the best oxidation resistance at 1,000°C. An internal oxidation zone of Si under the oxide layer is present at all temperatures. The precipitation of SiO 2 and its lateral growth and cross-linking may result in the formation of a continuous SiO 2 layer in local places (as shown in Figure 7B.), which will block the outward migration of cation and thus reducing oxidation rate of the alloy (Wang et al., 2015b). The presence of Si may also act as a nucleation point in the oxidation process, reducing the Cr 2 O 3
Frontiers in Materials | www.frontiersin.org July 2021 | Volume 8 | Article 706489 nuclear distance and promoting the rapid formation of a continuous Cr 2 O 3 protective layer (Gorr et al., 2012). Compared with the experimental condition of 0.1 MPa pure oxygen (Wang, 2019), the oxidation mode of Co-20Re-25Cr-3Si alloy under laboratory air changes drastically. A 79% drop in oxygen pressure promotes the formation of a continuous dense protective Cr 2 O 3 layer on the alloy, which prevents the evaporation of Re element in the alloy. Gorr B et al. studied the oxidation of Co-17Re-23Cr-xSi (x 1, 2, 3) in laboratory air at 1,000°C (Gorr et al., 2012). The scales formed on Co-17Re-23Cr-xSi (x 1, 2, 3) are similar to the ternary Co-17Re-xCr (x 23, 30) alloy. Co-17Re-23Cr-xSi (x 1, 2, 3) alloys have undergone mass loss because the Cr content is insufficient to form a continuous Cr 2 O 3 scale. Therefore, from another perspective, compared with Co-17Re-23Cr-3Si alloy, the addition of about 2% Cr causes the Co-20Re-25Cr-3Si alloy to change from catastrophic oxidation to

CONCLUSION
1) The oxidation behavior of Co-20Re-25Cr-3Si alloy at constant temperature for 24 h in laboratory air conforms to the parabolic law. The parabolic rate constants at 1,000-1200°C are 2.92 × 10 −12 g 2 ·cm −4 ·s −1 , 1.63 × 10 −11 g 2 ·cm −4 ·s −1 and 4.38 × 10 −11 g 2 ·cm −4 ·s −1 , respectively. The oxidation activation energy of the alloys is 220.74 kJ/mol. 2) The scales grown on Co-20Re-25Cr-3Si at 1,000-1,200°C are similar and composed of three layers. The outermost CoO layer is non-protective and easy to peel off. The second darker layer consists mainly of CoCr 2 O 4 spinel. The innermost darkest layer consists mainly of a dense and continuous Cr 2 O 3 layer in contact with the alloy, which provides a good protection for the alloy. 3) A 79% drop in oxygen pressure promotes the formation of a continuous dense protective Cr 2 O 3 layer on the alloy, which prevents the evaporation of Re element in the Co-20Re-25Cr-3Si alloy. Moreover, compared with Co-17Re-23Cr-3Si alloy, the addition of about 2% Cr causes the Co-20Re-25Cr-3Si alloy to change from catastrophic oxidation to mild oxidation under laboratory air.

DATA AVAILABILITY STATEMENT
The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding authors.