Investigation on the High-Temperature Oxidation Resistance of Ni-(3~10) Ta and Ni-(3~10) Y Alloys

: Ni-(3~10) Ta and Ni-(3~10) Y alloys were fabricated by vacuum arc melting. The oxidation resistance of the alloys was studied by cyclic and isothermal oxidation tests at 800 ◦ C in static air. The present work focused on the investigation of the effects of the alloying elements (Ta and Y) on the oxidation behavior of Ni-based alloys. The oxidation behavior of alloys was evaluated by mass gain, composition, as well as the microstructure of oxidized products. The experimental results indicated that Ta at a low content (3 wt %) had a positive role in enhancing oxidation resistance by decreasing the oxygen vacancy concentration of the oxide layer to prevent the inward diffusion of oxygen during oxidation, and the mass gain decreased from 2.9 mg · cm − 2 to 1.7 mg · cm − 2 (800 ◦ C/200 h), while Y (3~10 wt %) degraded the oxidation resistance. However, it is worth mentioning that the pinning effect of Y 2 O 3 increased the adhesion between the substrate and oxide layer by changing the growing patterns of the oxide layer from a plane growth to ﬁbrous growth. Among the results, the bonding of the substrate and oxide layer was best in the Ni-3Y alloys.


Introduction
Ni-based alloys have remarkable mechanical properties and excellent oxidation resistance at high temperatures. These alloys are used extensively for high-temperature industrial applications, such as aerospace systems, gas turbine blades, and interconnects of solid oxide fuel cell (SOFC) [1][2][3]. In these applications, with the working temperature of SOFC decreasing to an intermediate temperature (600-800 • C), metallic materials (Fe-based, Ni-based, and Cr-based alloys) as interconnects become feasible [4][5][6]. However, Cr in Ni-based alloys can result in Cr volatilization and deposition on the cathode, which can significantly degrade SOFC performance [7,8]. Because of the oxidation product, Al 2 O 3 , Al is also known to enhance the oxidation resistance [3]. However, the high-temperature conductivity of Al 2 O 3 is extremely poor, which would reduce electrical conductivity. Interconnects must work at high temperatures without a decline in electrical properties and oxidation resistance. To overcome this issue, our previous work completed the evaluation of TiN/Ni cermet as interconnects for solid oxide fuel cells [9]. To further improve the high-temperature oxidation resistance, we considered strengthening the oxidation resistance of metal matrix Ni in the cermet by adding Ta or Y alloying elements.
The composition of Ni-based alloys is a significant factor in controlling oxidation resistance [10,11]. Previous researchers have studied oxidation behavior and tried to find ways to improve the oxidation Granular metal particles (Ni, Ta, and Y) with an average size of φ3 mm × 4 mm (99.9% in purity, Yipin Chuancheng Technology Co., Ltd., Beijing, China) were used for melting stock in the experiments. The chemical compositions of the Ni and Ni-based alloys selected for this study are shown in Table 1. The samples were prepared by using vacuum arc melting (DHL-300, Keyi Co., Ltd., Shenyang, China), with argon gas as the protective gas and a working electric current of about 220 A, melting them several times to improve homogeneity. After melting, the ingots were cut into several bars with a size of 20 mm × 4 mm × 4 mm using a CNC wire cutting machine. The bar surfaces were polished with SiC sandpaper to 1200 # to remove the surface oxide and keep the surfaces smooth. Afterwards, Al 2 O 3 abrasive paste with different particle sizes was used to buff them, and then P-1-type polishing machine (Shanghai Electric Machinery Co., Ltd., Shenyang, China) was used to polish them and make flat mirror surfaces. Finally, the samples were cleaned with acetone for 30 min using an ultrasound method and then dried for the characterization and oxidation tests.

Alloys
Ni Ta Y

Oxidation and Characterization
Phase analysis was conducted by XRD using a Guinier-Hagg camera (Jungner Instrument, Expectron XDC-1000, Solna, Sweden) with Cu kα radiation. Microstructures were analyzed using a scanning electron microscope (SEM, S-4800, HITACHI, Tokyo, Japan) equipped with an energy dispersive spectrometer (EDS). The cyclic oxidation experiments were performed by repeatedly exposing the samples to static air at 800 • C for a total of 200 h. An electronic microbalance (Shanghai Shunyu Hengping Scientific Instrument Co., Ltd., Shanghai, China), with an accuracy of 10 −4 g, was used to weigh the mass gain of samples every 20 h until 200 h. Figure 1 shows the XRD patterns of Ni-based alloys obtained by vacuum arc melting. As the image shows, all the samples with different compositions include two individual phases without pure Ta or Y phases, Ni and Ni x Me y (Me = Ta or Y), indicating that the doping of Ta and Y was uniform and fully melted.

Oxidation and Characterization
Phase analysis was conducted by XRD using a Guinier-Hagg camera (Jungner Instrument, Expectron XDC-1000, Solna, Sweden) with Cu kα radiation. Microstructures were analyzed using a scanning electron microscope (SEM, S-4800, HITACHI, Tokyo, Japan) equipped with an energy dispersive spectrometer (EDS). The cyclic oxidation experiments were performed by repeatedly exposing the samples to static air at 800 °C for a total of 200 h. An electronic microbalance (Shanghai Shunyu Hengping Scientific Instrument Co., Ltd., Shanghai, China), with an accuracy of 10 −4 g, was used to weigh the mass gain of samples every 20 h until 200 h. Figure 1 shows the XRD patterns of Ni-based alloys obtained by vacuum arc melting. As the image shows, all the samples with different compositions include two individual phases without pure Ta or Y phases, Ni and NixMey (Me = Ta or Y), indicating that the doping of Ta and Y was uniform and fully melted.   Figure  2 only shows the microstructures of the two samples. As can be concluded from the SEM images of the Ni-3Ta alloys (Figure 2a), the Ni matrix was distributed with irregular dendrites phases, which EDS analysis identified as Ni2Ta. As for the Ni-3Y alloys (Figure 2b), discontinuous island phase A and irregular particle phase B were dispersed in the Ni matrix. According to EDS analyses, area A and B were made up of Ni4Y and Ni3Y, respectively.   Figure 2 only shows the microstructures of the two samples. As can be concluded from the SEM images of the Ni-3Ta alloys (Figure 2a), the Ni matrix was distributed with irregular dendrites phases, which EDS analysis identified as Ni 2 Ta. As for the Ni-3Y alloys (Figure 2b), discontinuous island phase A and irregular particle phase B were dispersed in the Ni matrix. According to EDS analyses, area A and B were made up of Ni 4 Y and Ni 3 Y, respectively. The Ta and Y were 2.85 wt % and 2.92 wt %, respectively, in the Ni-3Ta and Ni-3Y alloys by using X-ray fluorescence analysis, which showed that the loss was low during smelting, and the elements were fully doped. The loss was also low for a high Me addition. The melting point of the Ni-based alloys was determined by differential scanning calorimetry ranges, from 1200 °C to 1410 °C. Figure 3 presents the evolution of weight gain as a function of time for the Ni-based alloys with different compositions, and all results satisfy the parabolic kinetics law [23] described by:

High-Temperature Oxidation Resistance of the Ni-Based Alloys
where ΔW is the mass gain, t is oxidation time, A is sample surface area, and K is the oxidation rate constant, which reflects the speed of the oxidation process. Figure 4 shows the squares of mass gains as functions of time at 800 °C for the Ni-based alloys. The parabolic rate constant (K) calculated from the slope of the linear fitting in Figure 4, coefficient of determination (r), and mass gain of Ni-based alloys oxidized at 800 °C for 200 h are listed in Table 2. The coefficients of determination (r) are all better than 0.99, which shows that the oxidation of the alloys followed a parabolic mechanism. As for the Ni-(3~10) Ta alloys (Figure 4a), it can be clearly seen that the K for pure Ni is higher than for Ni-3Ta alloys, and lower than for Ni-6Ta and Ni-10Ta alloys, which indicated that, only under appropriate additional concentration, Ta had a positive role in enhancing the oxidation resistance of alloys. As Ta content decreases from 10 wt % to 3 wt %, the weight gain reduces from 3.8 mg•cm −2 to 1.7 mg•cm −2 . However, as with Ni-(3~10) Y alloys, the K for Ni-Y alloys with different compositions are always higher than for pure Ni. Moreover, after oxidation for 200 h, the mass gain of Ni-(3~10) Y alloys is approximately three and six times higher than of pure Ni samples. As shown in Table 2, the K value for Ni-3Ta alloys was approximately one and two orders of magnitude lower than for the Ni-(3~10) Y alloys. The addition of Y in the Ni-based alloys did not inhibit but promoted the oxidation process. The Ta and Y were 2.85 wt % and 2.92 wt %, respectively, in the Ni-3Ta and Ni-3Y alloys by using X-ray fluorescence analysis, which showed that the loss was low during smelting, and the elements were fully doped. The loss was also low for a high Me addition. The melting point of the Ni-based alloys was determined by differential scanning calorimetry ranges, from 1200 • C to 1410 • C. Figure 3 presents the evolution of weight gain as a function of time for the Ni-based alloys with different compositions, and all results satisfy the parabolic kinetics law [23] described by:

High-Temperature Oxidation Resistance of the Ni-Based Alloys
where ∆W is the mass gain, t is oxidation time, A is sample surface area, and K is the oxidation rate constant, which reflects the speed of the oxidation process. Figure 4 shows the squares of mass gains as functions of time at 800 • C for the Ni-based alloys. The parabolic rate constant (K) calculated from the slope of the linear fitting in Figure 4, coefficient of determination (r), and mass gain of Ni-based alloys oxidized at 800 • C for 200 h are listed in Table 2. The coefficients of determination (r) are all better than 0.99, which shows that the oxidation of the alloys followed a parabolic mechanism. As for the Ni-(3~10) Ta alloys (Figure 4a), it can be clearly seen that the K for pure Ni is higher than for Ni-3Ta alloys, and lower than for Ni-6Ta and Ni-10Ta alloys, which indicated that, only under appropriate additional concentration, Ta had a positive role in enhancing the oxidation resistance of alloys. As Ta content decreases from 10 wt % to 3 wt %, the weight gain reduces from 3.8 mg·cm −2 to 1.7 mg·cm −2 . However, as with Ni-(3~10) Y alloys, the K for Ni-Y alloys with different compositions are always higher than for pure Ni. Moreover, after oxidation for 200 h, the mass gain of Ni-(3~10) Y alloys is approximately three and six times higher than of pure Ni samples. As shown in Table 2, the K value for Ni-3Ta alloys was approximately one and two orders of magnitude lower than for the Ni-(3~10) Y alloys. The addition of Y in the Ni-based alloys did not inhibit but promoted the oxidation process. Metals 2019, 9, x FOR PEER REVIEW 5 of 10          (Figure 5d-f), the oxidation products are NiO and Y2O3. This proves that the change in Ta or Y content has no effect on the composition of the oxidation product. Comparing Figure 5a with Figure 5b,c, we can find that with the extension of oxidation time, Ta2O5 is little in the Ni-3Ta alloys but more in the Ni-6Ta and Ni-10Ta alloys. Chicard et al. have reported that when the additional amount of Ta was about 1 wt % in the Ti(C, N)-based cermets, no corresponding oxide was detected during the oxidation, and Ta was mainly doped into the oxide in the form of solid    (Figure 5d-f), the oxidation products are NiO and Y 2 O 3 . This proves that the change in Ta or Y content has no effect on the composition of the oxidation product. Comparing Figure 5a with Figure 5b,c, we can find that with the extension of oxidation time, Ta 2 O 5 is little in the Ni-3Ta alloys but more in the Ni-6Ta and Ni-10Ta alloys. Chicard et al. have reported that when the additional amount of Ta was about 1 wt % in the Ti(C, N)-based cermets, no corresponding oxide was detected during the oxidation, and Ta was mainly doped into the oxide in the form of solid solution phase [16]. In addition, the oxidation process was discovered to be determined by the outward diffusion of Ni ions and inward diffusion of O atoms [24]. Therefore, it can be deduced that Ta could positively enhance the oxidation resistance of Ni-3Ta alloys by forming Ta 5+ in oxide layer (NiO) to reduce the oxygen vacancy concentration, and further suppress the inward diffusion of O. However, as for Ni-6Ta and Ni-10Ta alloys (Figure 5b,c), the oxidation products consist of Ta 2 O 5 and NiO after oxidizing for 200 h. The XRD patterns of Ni-(3~10) Y alloys oxidized at 800 • C for up to 200 h are presented in Figure 5d-f, Y is easily oxidized to form Y 2 O 3 in the oxidation layer. After melting, the XRD of the Ni-based alloys with the addition of Y consisted of Ni, Ni 3 Y, and Ni 4 Y phase, which is consistent with the EDS analysis in Figure 2. solution phase [16]. In addition, the oxidation process was discovered to be determined by the outward diffusion of Ni ions and inward diffusion of O atoms [24]. Therefore, it can be deduced that Ta could positively enhance the oxidation resistance of Ni-3Ta alloys by forming Ta 5+ in oxide layer (NiO) to reduce the oxygen vacancy concentration, and further suppress the inward diffusion of O. However, as for Ni-6Ta and Ni-10Ta alloys (Figure 5b,c), the oxidation products consist of Ta2O5 and NiO after oxidizing for 200 h. The XRD patterns of Ni-(3~10) Y alloys oxidized at 800 °C for up to 200 h are presented in Figure 5d-f, Y is easily oxidized to form Y2O3 in the oxidation layer. After melting, the XRD of the Ni-based alloys with the addition of Y consisted of Ni, Ni3Y, and Ni4Y phase, which is consistent with the EDS analysis in Figure 2. The SEM micrographs of samples oxidized in static air at 800 °C for 200 h are presented in Figure  6. For a better analysis of the oxidation process of Ni-(3~10) Ta and Ni-(3~10) Y alloys. The SEM solution phase [16]. In addition, the oxidation process was discovered to be determined by the outward diffusion of Ni ions and inward diffusion of O atoms [24]. Therefore, it can be deduced that Ta could positively enhance the oxidation resistance of Ni-3Ta alloys by forming Ta 5+ in oxide layer (NiO) to reduce the oxygen vacancy concentration, and further suppress the inward diffusion of O. However, as for Ni-6Ta and Ni-10Ta alloys (Figure 5b,c), the oxidation products consist of Ta2O5 and NiO after oxidizing for 200 h. The XRD patterns of Ni-(3~10) Y alloys oxidized at 800 °C for up to 200 h are presented in Figure 5d-f, Y is easily oxidized to form Y2O3 in the oxidation layer. After melting, the XRD of the Ni-based alloys with the addition of Y consisted of Ni, Ni3Y, and Ni4Y phase, which is consistent with the EDS analysis in Figure 2. solution phase [16]. In addition, the oxidation process was discovered to be determined by the outward diffusion of Ni ions and inward diffusion of O atoms [24]. Therefore, it can be deduced that Ta could positively enhance the oxidation resistance of Ni-3Ta alloys by forming Ta 5+ in oxide layer (NiO) to reduce the oxygen vacancy concentration, and further suppress the inward diffusion of O. However, as for Ni-6Ta and Ni-10Ta alloys (Figure 5b,c), the oxidation products consist of Ta2O5 and NiO after oxidizing for 200 h. The XRD patterns of Ni-(3~10) Y alloys oxidized at 800 °C for up to 200 h are presented in Figure 5d-f, Y is easily oxidized to form Y2O3 in the oxidation layer. After melting, the XRD of the Ni-based alloys with the addition of Y consisted of Ni, Ni3Y, and Ni4Y phase, which is consistent with the EDS analysis in Figure 2. solution phase [16]. In addition, the oxidation process was discovered to be determined by the outward diffusion of Ni ions and inward diffusion of O atoms [24]. Therefore, it can be deduced that Ta could positively enhance the oxidation resistance of Ni-3Ta alloys by forming Ta 5+ in oxide layer (NiO) to reduce the oxygen vacancy concentration, and further suppress the inward diffusion of O. However, as for Ni-6Ta and Ni-10Ta alloys (Figure 5b,c), the oxidation products consist of Ta2O5 and NiO after oxidizing for 200 h. The XRD patterns of Ni-(3~10) Y alloys oxidized at 800 °C for up to 200 h are presented in Figure 5d-f, Y is easily oxidized to form Y2O3 in the oxidation layer. After melting, the XRD of the Ni-based alloys with the addition of Y consisted of Ni, Ni3Y, and Ni4Y phase, which is consistent with the EDS analysis in Figure 2. The SEM micrographs of samples oxidized in static air at 800 • C for 200 h are presented in Figure 6. For a better analysis of the oxidation process of Ni-(3~10) Ta and Ni-(3~10) Y alloys. The SEM micrographs of pure Ni samples (Figure 6a) is used for comparative analysis. Comparing Figure 6a with Figure 6b-e, the effect of Ta on the oxidation resistance of Ni-based alloys can be summarized as follows. Firstly, the average grain size of surface oxide of pure Ni is greater than of the Ni-3Ta alloys, and close to that of the Ni-6Ta and Ni-10Ta alloys. The low addition of Ta contributes to enhancing the oxidation resistance of Ni-based alloys by refining the size of NiO oxide grain to form more dense oxide layer. Secondly, Figure 6c is a partial topography of the Ni-3Ta alloys (Figure 6b). The surface of the oxide layer is flat and dense, and the grains are uniform, which helps to suppress further oxidation. Lastly, as for the Ni-6Ta and Ni-10Ta alloys (Figure 6d,e), there were some pores on the surface, which were related to the formation of Ta 2 O 5 oxides. The loose and mixed oxide layer of Ta 2 O 5 and NiO could not prevent the diffusion of oxygen atoms, but promoted the oxidation process.
As for the Ni-(3~10) Y alloys (Figure 6f-h), the addition of Y had no effect on the surface topography of the oxide layer. As the added amount of Y increased from 3 wt % to 10 wt %, the mass gain of the Ni-(3~10) Y alloys ranged from 7.7 mg·cm −2 to 13.8 mg·cm −2 (listed in Table 2), all of which were higher than that of the pure Ni. As Y content increased from 3 wt % to 10 wt %, the irregular bonding surface became more and more obvious. According to the EDS and XRD analyses, Y 2 O 3 mainly concentrated in the junction of the matrix and oxide layer. It can be explained that the addition of Y can strengthen the adhesion between the matrix and the oxide layer by changing the growth mode of the oxide layer on account of the pinning effect of Y 2 O 3 . The adhesion of the substrate and oxide layer was best under the addition of 3 wt % Y in the Ni-based alloys. However, the growth mode changing from plane growth to fibrous growth also increased the contact area of the substrate with air, and then promoted oxidation.
Metals 2019, 9, x FOR PEER REVIEW 7 of 10 micrographs of pure Ni samples (Figures 6a and 7a) is used for comparative analysis. Comparing Figure 6a with Figure 6b-e, the effect of Ta on the oxidation resistance of Ni-based alloys can be summarized as follows. Firstly, the average grain size of surface oxide of pure Ni is greater than of the Ni-3Ta alloys, and close to that of the Ni-6Ta and Ni-10Ta alloys. The low addition of Ta contributes to enhancing the oxidation resistance of Ni-based alloys by refining the size of NiO oxide grain to form more dense oxide layer. Secondly, Figure 6c is a partial topography of the Ni-3Ta alloys (Figure 6b). The surface of the oxide layer is flat and dense, and the grains are uniform, which helps to suppress further oxidation. Lastly, as for the Ni-6Ta and Ni-10Ta alloys (Figure 6d,e), there were some pores on the surface, which were related to the formation of Ta2O5 oxides. The loose and mixed oxide layer of Ta2O5 and NiO could not prevent the diffusion of oxygen atoms, but promoted the oxidation process.
As for the Ni-(3~10) Y alloys (Figure 6f-h), the addition of Y had no effect on the surface topography of the oxide layer. As the added amount of Y increased from 3 wt % to 10 wt %, the mass gain of the Ni-(3~10) Y alloys ranged from 7.7 mg•cm −2 to 13.8 mg•cm −2 (listed in Table 2), all of which were higher than that of the pure Ni. As Y content increased from 3 wt % to 10 wt %, the irregular bonding surface became more and more obvious. According to the EDS and XRD analyses, Y2O3 mainly concentrated in the junction of the matrix and oxide layer. It can be explained that the addition of Y can strengthen the adhesion between the matrix and the oxide layer by changing the growth mode of the oxide layer on account of the pinning effect of Y2O3. The adhesion of the substrate and oxide layer was best under the addition of 3 wt % Y in the Ni-based alloys. However, the growth mode changing from plane growth to fibrous growth also increased the contact area of the substrate with air, and then promoted oxidation.

Conclusions
The Ni-(3~10) Ta and Ni-(3~10) Y alloys were fabricated by vacuum arc melting, using argon gas as protective gas, and working electric current of about 220 A. The actual content of Ta or Y in the Nibased alloys was confirmed to be close to the theoretical content by using X-ray fluorescence analysis. According to the XRD patterns and SEM images of Ni-Me (Me = Ta or Y) alloys after melting, the doping of Ta and Y was uniform, and fully melted without individual Ta or Y phase. After the cyclic and isothermal oxidation at 800 °C for 200 h, the Ni-based alloys with 3 wt % addition of Ta presented better oxidation resistance by forming the Ta 5+ ions to restrict the inward diffusion of oxygen. The addition of Y decreased the oxidation resistance. However, it was worth noting that the pinning effect of Y2O3 could heighten the adhesion between the matrix and the oxide layer by changing the growth pattern. Through the experimental results, we can know the specific effect of Ta or Y element on the high-temperature oxidation resistance of Ni-based alloys, which will be helpful for us in trying to improve the oxidation resistance of cermet in future research. The investigated alloys are the potential candidates for the matrix of TiN/Ni-Me cermet material, instead of pure Ni. The results obtained within the present work show, that almost all alloys show worse oxidation behavior than pure Ni. It seems to be that only Ni-3Ta can be potentially considered as a candidate for the cermet matrix. We also know that there are some deficiencies in the experiments; we need to design the Ni-based alloys with integrated addition of Ta and Y for comparative experiment. However, it is hard to melt the Ni-

Conclusions
The Ni-(3~10) Ta and Ni-(3~10) Y alloys were fabricated by vacuum arc melting, using argon gas as protective gas, and working electric current of about 220 A. The actual content of Ta or Y in the Ni-based alloys was confirmed to be close to the theoretical content by using X-ray fluorescence analysis. According to the XRD patterns and SEM images of Ni-Me (Me = Ta or Y) alloys after melting, the doping of Ta and Y was uniform, and fully melted without individual Ta or Y phase. After the cyclic and isothermal oxidation at 800 • C for 200 h, the Ni-based alloys with 3 wt % addition of Ta presented better oxidation resistance by forming the Ta 5+ ions to restrict the inward diffusion of oxygen. The addition of Y decreased the oxidation resistance. However, it was worth noting that the pinning effect of Y 2 O 3 could heighten the adhesion between the matrix and the oxide layer by changing the growth pattern. Through the experimental results, we can know the specific effect of Ta or Y element on the high-temperature oxidation resistance of Ni-based alloys, which will be helpful for us in trying to improve the oxidation resistance of cermet in future research. The investigated alloys are the potential candidates for the matrix of TiN/Ni-Me cermet material, instead of pure Ni. The results obtained within the present work show, that almost all alloys show worse oxidation behavior than pure Ni. It seems to be that only Ni-3Ta can be potentially considered as a candidate for the cermet matrix. We also know that there are some deficiencies in the experiments; we need to design the Ni-based alloys with integrated addition of Ta and Y for comparative experiment. However, it is hard to melt the Ni-Ta-Y ternary alloys by vacuum arc melting. To overcome this issue, the TiN/Ni-3Ta cermet with minor Y 2 O 3 addition can be fabricated by spark plasma sintering. Moreover, the novel cermet could combine the excellent effect of Ta and Y to enhance the high-temperature oxidation resistance, and then be used as SOFC interconnects with better properties, which will be the focus of our future research.
Author Contributions: H.D. performed the experiments and wrote the paper; Y.L. and T.L. analyzed the data; H.Z. searched the reference; Z.H. performed the microstructural analysis and edited the paper.

Funding:
The study received financial support from the National Natural Science Foundation of China (Grant No. 51671209).