The Oxidation Resistance of Nb-Si-Based Alloys at Intermediate and High Temperatures

The oxidation behavior of three Nb-Si-based alloys were evaluated at intermediate (800 °C) and high (1250 °C) temperatures for 100 h in air. At 800 °C, the Nb-24Ti-15Si-13Cr-2Al-2Hf (at. %) alloy suffered from catastrophic pest oxidation. This pest phenomenon was suppressed by the addition of Sn. However, Ta addition protected the Nb-Si-based alloys from pest oxidation for a short time. At 1250 °C, Sn could enhance the oxidation resistance of Nb-Si-based alloys due to the formation of a Sn-rich layer. In addition, the oxidation mechanisms of Nb-Si-based alloys at intermediate and high temperatures were discussed.

the pest phenomenon due to the selective oxidation of alloying elements at the grain boundaries [26]. In the Nb-Ti-Si-Al-Cr system, Al reduces the pest susceptibility at 800 • C [27]. In the Nb-Ti-Si-Cr-Al-Mo system, 5 at. % of Sn can eliminate pest oxidation behavior at 800 • C [2]. Knittel et al. suggest that Nb-Si-based alloys show better oxidation resistance at 800 • C with increasing Sn concentration, but that a brittle phase (Nb,Ti) 3 (Sn,Ti) presents when the content of Sn is higher than 3 at. % [28].
In our previous study, a Nb-Si-based alloy consisting of Nb SS , Nb 5 Si 3 and Cr 2 Nb showed acceptable high-temperature oxidation resistance [21], but its intermediate-temperature oxidation resistance was unclear. In this study, it was selected as the base alloy. Furthermore, to enhance both intermediate-and high-temperature oxidation resistance, 2 at. % of Sn and 2 at. % of Ta were added in the base alloy. The effects of Sn and Ta on the oxidation resistance of an Nb-Si-based alloys at intermediate (800 • C) and high (1250 • C) temperatures were investigated. In addition, the oxidation mechanisms of Nb-Si-based alloys at intermediate and high temperatures were discussed.

Material and Methods
Three Nb-Si-based alloys with different compositions were prepared.
The oxidation tests were conducted in an open-ended tube furnace in air. The oxidation temperatures were 800 and 1250 • C respectively. Each sample was placed in a separate alumina crucible. The samples were removed from the furnace at the intervals of 10, 20, 40, 60, 80 and 100 h and weighed together with the crucible using a precision analytical balance (Model CPA225D, Sartorius, Gottingen, Germany) with an accuracy of 0.00001 g.
The phases of the three alloys and the oxide products were determined by X-ray diffraction (XRD, CuKa-radiation, X'Pert Pro, Panalytical, Almelo, Holland) in the range of 20-90 • at a 2θ scanning rate of 6 • /min. Cross-sections of the samples were grinded on wet SiC paper, starting with 800 grit and increasing to up to 4000 grit, and polished with diamond polishing paste (1 µm). Micrographs of cross-sections and surface morphologies of oxidized specimens were observed through a scanning electron microscope equipped with an energy-dispersive X-Ray spectroscopy system (Sigma 500, Zeiss, Oberkochen, Germany).

Microstructural Characterization of As-Cast Alloys
The XRD patterns of the base alloy, 2Sn alloy and 2Ta alloy are shown in Figure 1. Each of the three alloys consisted of Nb SS (JCPDS card No. 35-0789), Nb 5 Si 3 (JCPDS card No. 30-0875) and Cr 2 Nb (JCPDS card No. 47-1638) phases. The XRD results indicated that the constituent phases did not change, as the base alloy alloyed with 2 at. % of Ta or 2 at. % of Sn. Figure 2 demonstrates the microstructures of the three Nb-Si-based alloys. The microstructure of the base alloy consisted of primary Nb 5 Si 3 , eutectic (Nb SS + Nb 5 Si 3 ) and Cr 2 Nb. The addition of 2 at. % of Sn enlarged the size of the Nb SS dendrites. The addition of 2 at. % of Ta refined the size of both the Nbss dendrites and the Nb 5 Si 3 blocks.   Figure 3 demonstrates the oxidative weight-gain curves and photographs of the oxidized alloys at 800 • C for 100 h. As shown in Figure 3, the base alloy showed a linear oxidation behavior over the first 20 h, then accelerated oxidation behavior was observed after 40 h; after 100 h at 800 • C, the weight gain was 36.6 mg/cm 2 . In the 2Ta alloy, the accelerated oxidation behavior was observed after 80 h; after 100 h at 800 • C, the weight gain was 6.58 mg/cm 2 . Noteworthily, the weight gain of the 2Sn alloy was 2.78 mg/cm 2 , which was only one sixth of that of the base alloy. As shown in Figure 1, the base alloy and the 2Ta alloy both degraded into powder, suggesting that catastrophic pest oxidations had occurred. The oxide scale of the 2Sn alloy was tightly adherent, indicating that the sample was protected upon oxidation. These results suggest that Sn plays a crucial role in suppressing the pest oxidation phenomenon of Nb-Si-based alloys at intermediate temperatures.

Intermediate Temperature Oxidation Resistance
In general, oxidation kinetics of 2Sn and 2Ta alloys are calculated by the following formula [17,29]: where ∆m, S, t and n are the mass variation, the total surface area of the sample, the oxidation rate coefficient and the oxidation duration rate exponent, respectively. The oxidation duration rate exponent (n) of the 2Sn alloy and the 2Ta alloy at 800 • C were determined to be 0.84 and 0.83 respectively, by fitting the thermal gravimetric data according to Formula (1). Therefore, the oxidation kinetics of both the 2Sn and the 2Ta alloys at 800 • C followed a parabolic-linear law. The oxidation duration-rate exponents of 2Sn and 2Ta alloys were close to 1, suggesting that the surface reactions were the dominant rate-determining step for the oxidation; that is, the rate of the interfacial reaction of O 2 with Nb-Si-based alloys. This oxidation behavior suggests that the oxidation products had a slight effect on the oxidation rate.   Figure 5d) was thin, due to serious spallation. Although the oxide scale of the 2Sn alloy (as shown in Figure 5e) was intact, the oxide scale of the 2Ta alloy (as shown in Figure 5f) was cracked, which may have been due to metallographic preparation. Moreover, some cracks formed on the Nb 5 Si 3 near the interface of oxide and substrate in the base and 2Ta alloys. However, only a few cracks formed on the Nb 5 Si 3 in the 2Sn alloy. The inward diffusion of oxygen led to the volume expansion of Nb SS , inducing tensile strains to silicides at intermediate temperatures [30]. Therefore, cracks were formed on Nb 5 Si 3 after oxidation.  To reveal the short-term oxidation behavior of Nb-Si-based alloys at 800 • C, oxidation tests were conducted at 800 • C for 10 h. Figure 6 shows the surface and cross-sectional microstructure of the three alloys after oxidation at 800 • C for 10 h. The images indicate that the oxide scales of all three alloys remained basically intact. The rod-like oxide and glassy oxide were formed from Nb SS and Nb 5 Si 3 respectively. The thickness of oxide scales of the base alloy, 2Sn alloy and 2Ta alloy are 5, 6 and 8 µm respectively. To indicate the elemental analysis in the scale, the X-ray mapping of the 2Sn alloy after oxidation at 800 • C for 10 h is shown in Figure 7. The results clearly showed the presence of O, Nb, Ti, Si, Al, Hf, Sn and Cr in the oxides. Nb, Al, Ti, Cr and Hf were almost uniformly distributed at the scale. A Sn-rich layer was observed between the oxide scale and substrate. Furthermore, Si was enriched in the outer layer of the oxide scale, suggesting that a SiO 2 layer had formed. A similar SiO 2 layer was also observed in the 2Ta alloy.  Some cracks were observed in the surface of the base alloy (as shown in Figure 6a) after oxidation at 800 • C for 10 h. This would have provided more sites for rapid inward diffusion of oxygen, resulting in the higher oxidation rate. Therefore, the pest oxidation of the base alloy may have been due to the generation of cracks in the brittle Nb 5 Si 3 . The formation of cracks increases the oxygen intake rate and leads to catastrophic oxidation behavior. As suggested by Mathieu et al., the crack formation mechanism of Nb-Si-based alloys at medium temperatures is due to the progressive volume expansion of oxides in Nb SS during oxidation [30]. Due to this mechanism, pesting can be eliminated by limiting the inward diffusion of oxygen in Nb SS . In the 2Sn and 2Ta alloys, a SiO 2 layer developed after oxidation (Figure 7). This silica layer reduced the inward oxygen diffusion rate, contributing to the enhancement of the oxidation resistance [23,31]. We therefore deduced that the interdiffusion rate of oxygen in the 2Sn and 2Ta alloys were lower than that of the base alloy. The formation of cracks was consequently suppressed, leading to enhanced oxidation resistance. In addition, the oxidation rate of the 2Ta alloy was higher than that of the 2Sn alloy. The 2Ta alloy suffered from accelerated oxidation beyond 80 h, suggesting that the addition of Ta protects Nb-Si-based alloys from oxidation for a short time.
In addition, the Sn-rich layer in the 2Sn alloy acted as a diffusion barrier against oxygen. Sn accumulated at the region between oxide scale and substrate due to its very low affinity for oxygen, compared to the other constitutive elements of Nb-Si-based alloys [28]. Due to the SiO 2 and Sn-rich layers, the interdiffusion rate of oxygen in the 2Sn alloy was lowest. Therefore, the pest phenomenon was suppressed by the addition of Sn.  According to Formula (1), oxidation duration rate exponents of the base alloy, 2Sn alloy and 2Ta alloy at 1250 • C were calculated to be 0.82, 0.69 and 0.86, respectively. Thus, the oxidation behavior of all three of the alloys followed a mixed parabolic-linear law at 1250 • C. This behavior suggested that the oxidation was governed by both interface reaction and diffusion. Furthermore, the oxidation duration rate exponents of the base, 2Sn and 2Ta alloys were close to 1, thus surface reaction was the dominant rate-determining step. Figure 9 demonstrates XRD patterns of oxidized products formed on the three Nb-Si-based alloys after oxidation at 1250 • C. The oxidized products were TiNb 2 O 7 , CrNbO 4 , Nb 2 O 5 and TiO 2 phases. The oxide products were in good agreement with the oxidation of a Nb-24Ti-2Hf-6Cr-6Al-16Si (at. %) at 1250 • C obtained by TEM and selective-area diffraction. Figure 10 shows the residual oxide scale morphologies after oxidation at 1250 • C for 100 h. The oxide scales formed on the three Nb-Si-based alloys were rough and porous.  To reveal the short-term oxidation behavior of Nb-Si-based alloys at 1250 • C, oxidation tests were conducted at 1250 • C for 10 h. Figure 11 demonstrates the surface morphologies and cross-sectional microstructures of the three alloys after oxidation at 1250 • C for 10 h. These images indicate that the oxide scales of the three alloys remained basically intact. The thickness of oxide scales of the base alloy, 2Sn alloy and 2Ta alloy were about 20, 40 and 120 µm respectively. Cracks formed in the 2Ta alloy, which may have been due to the growing stress of oxide scale. This cracking may have led to the spalling of oxide scales. As shown in Figure 11, the thickness of the 2Ta alloy was three times that of the 2Sn alloy after oxidation for 10 h (Figure 11e,f), which suggests that the addition of Ta led to faster inward transportation of oxygen. Therefore, Ta addition has detrimental effects on the oxidation resistance of Nb-Si-based alloys. An X-ray mapping of the 2Sn alloy after oxidation at 1250 • C for 10 h is shown in Figure 12. The results clearly demonstrated the presence of O, Nb, Ti, Si, Al, Hf, Sn and Cr in the oxides. Nb, Al, Ti, Cr and Hf were almost uniformly distributed at the scale. A Sn-rich layer formed between the oxide scale and substrate. This layer acted as a diffusion barrier against oxygen, leading to a lower inward diffusion rate of oxygen [28]. Therefore, the 2Sn alloy showed the best oxidation resistance in this study. Furthermore, the three alloys were fully affected by internal oxidation just after oxidation at 1250 • C for 10 h. The internal oxides TiO 2 with black contrast and HfO 2 with white contrast mainly distributed in Nb SS and the interface between Nb SS and Nb 5 Si 3 (Cr 2 Nb). At high temperatures, the inward diffusion of oxygen mainly occurred through Nb SS , due to the faster transport of oxygen in Nb SS than that in Cr 2 Nb and Nb 5 Si 3 [4,10].

High Temperature Oxidation Resistance
As revealed by the XRD results (Figure 9), the oxide scales that formed on the base alloy, 2Sn alloy and 2Ta alloy at 1250 • C mainly consisted of TiO 2 , Nb 2 O 5 and TiNb 2 O 7 and CrNbO 4 . The formation of Nb 2 O 5 induced extensive compressive stress in the oxide scale, leading to the formation of cracks in oxide scales [20]. Moreover, the glassy SiO 2 resulting from the decomposition of silicides was limited due to its insufficient volume. Thus, the glassy SiO 2 phase could not heal all the cracks. Resultantly, the unprotective oxide scale led to the continuous diffusion of oxygen through the Nbss phases, thus the substrate suffered from a mixed parabolic-linear degradation.
In addition, the difference in the thermal expansion of the Nb-Si-based alloys and oxides generated thermal stress during cooling. The thermal stress also induced the cracking of oxide scale. Moreover, exposure to the cyclic temperatures generated more stresses than isothermal exposure. Unfortunately, according to Figure 11, the addition of Ta or Sn did not significantly reduce stress generation in the oxide scales.

Conclusions
The effects of Sn and Ta on the oxidation behavior of Nb-Si-based alloys were investigated at 800 • C and 1250 • C.

1.
The microstructures of the base alloy, 2Sn alloy and 2Ta alloy consisted of Nb 5 Si 3 , Nb SS and Cr 2 Nb. 2.
The base alloy suffered from catastrophic pest oxidation at 800 • C. The addition of Sn suppressed the pest phenomenon. However, the addition of Ta protected the Nb-Si-based alloy from pest oxidation for a short time.

3.
At 1250 • C, 2 at. % of Sn could enhance the oxidation resistance of Nb-Si-based alloys due to the formation of a Sn-rich layer. However, 2 at. % of Ta facilitated the a faster transportation of oxygen, resulting in worse oxidation resistance.

Conflicts of Interest:
The authors declare no conflict of interest.