High-temperature oxidation behavior of Nb_67-xW₁₅Si₁₈Hf_x(x=0, 5 and 10) alloys

JIANG Rong-li(姜荣丽)¹, LIU Dong-ming(刘东明)², SHA Jiang-bo(沙江波)¹, MA Yue(马 岳)¹

1. School of Materials Science and Engineering,

Beijing University of Aeronautics and Astronautics, Beijing 100083, China

2. School of Materials Science and Engineering, South Campus, Shandong University, Ji'nan 250061, China

Received 28 July 2006; accepted 15 September 2006

Abstract: The oxidation behaviors of Nb_67-xW₁₅Si₁₈Hf_x (x=0, 5, 10) alloys were studied at 1 250 ℃ in air. It is found that the Nb₆₇W₁₅Si₁₈ alloy has the best oxidation resistance among the three alloys; and Hf addition is harmful to the oxidation resistance of the Nb₆₇W₁₅Si₁₈ alloy. The oxides formed on the Nb₆₇W₁₅Si₁₈ alloy are mainly Nb₁₂WO₃₃ and NbO₂, and that on the Nb₆₂W₁₅Si₁₈Hf₅ and Nb₅₇W₁₅Si₁₈Hf₁₀ alloys is Nb₂O₅. Effect of Hf on the oxidation behavior of the Nb_67-xW₁₅Si₁₈Hf_x alloys has been discussed based on microstructures and kinetics of oxidation.

Key words: Nb-Si based alloys; Ni_67-xW₁₅Si₁₈Hf_x alloys; high-temperature oxidation; oxidation resistance; oxide

1 Introduction

Niobium-silicide based composites are potential candidates for aircraft engine components because of their high melting temperature, low density and relatively high strength at elevated temperatures[1-5]. Significant work on mechanical properties has been focused on optimizing the room-temperature fracture toughness and high-temperature strength[6-10]. In the previous work[11], a typical Nb-Si alloy, Nb₆₇W₁₅Si₁₈, showed excellent high-temperature strength; and its 0.2% yield compressive strength was as high as 880 MPa at  1 200 ℃ and 570 MPa at 1 500 ℃. Its room temperature fracture toughness can be improved by Hf addition, from 5.1 MPa·m^1/2 to 7.2 MPa·m^1/2, when Hf content changes from 0% to 15%(mole fraction). However, the oxidation behaviors of the Nb_67-xW₁₅Si₁₈Hf_x (x=0, 5, 10) alloys are unknown. In the present paper, the Nb_67-xW₁₅Si₁₈Hf_x (x=0, 5, 10) alloys were oxidized at 1 250 ℃ in air to investigate the oxidation behavior and mechanism and probe the balance tendency among strength, toughness and oxidation behavior of Nb-Si based alloys.

2 Experimental

Three alloys, Nb_67-xW₁₅Si₁₈Hf_x (x=0, 5, 10), were

prepared by arc melting in a vacuum from high purity metals Nb(99.6 %), W(99.9 %), Si(99.999 %) and Hf (99.9 %). The ingots were arc-melted five or six times to homogenize the compositions. Then the ingots were annealed at 1 750 ℃ for 50 h in a vacuum of 1×10^-4 Pa followed by furnace cooling to room temperature. The specimens for oxidation were cut into sizes of 6 mm×5 mm×3 mm from the ingots, and ground to an 800 grit polisher and washed in acetone.

Isothermal static oxidation experiment was conducted at 1 250 ℃ for 1 h, 3 h, 5 h and 7 h in air. Oxidation rates were obtained by measuring the mass gains of the samples. The mass gains were weighed by an electronic balance with an accuracy of 0.1 mg. A scanning electron microscope (HITACHI S-3500N) with energy-dispersive spectrometer (Oxford INCA) was used to analyze the surface morphology and cross section of the specimens after oxidation test. The compositions of the oxides were analyzed by X-ray diffraction(XRD) technique.

3 Results and discussion

XRD analysis shows that the phase structures of the three alloys contain Nb solid solution (hereafter referred to as Nb_SS) and (Nb, W, Hf)₅Si₃ silicide (hereafter referred to as M₅Si₃). Fig.1 shows the typical micro- structure of the three alloys, in which the bright and dark areas correspond to the Nb_SS and the M₅Si₃ phases, respectively. With 18%(mole fraction) Si, the Nb₆₇W₁₅- Si₁₈ alloy approaches the eutectic composition, and the microstructure should be composed of eutectoid composition according to the binary Nb-Si phase diagram. However, the Nb₆₇W₁₅Si₁₈ alloy annealed at   1 750 ℃ for 50 h shows more primary Nb_SS than the eutectic Nb-18Si alloy. This result suggests that the W addition may give rise to a shift of the eutectic line towards the Si-rich corner of the phase diagram. EDS results reveal that most of W is dissolved in the Nb_SS, while most of Hf is dissolved in the M₅Si₃. The annealed Nb₆₇W₁₅Si₁₈ alloy shows a microstructure composed of the primary Nb_SS and the eutectic containing the secondary Nb_SS and the M₅Si₃ silicide. Compositions for the phases in the Nb₅₇W₁₅Si₁₈Hf₁₀ alloy annealed at 1 750 ℃ for 50 h are shown in Table 1.

Fig.1 Microstructure (BSE image) of Nb₆₇W₁₅Si₁₈ alloy annealed at 1 750 ℃ for 50 h

Table 1 Compositions for phases of Nb₅₇W₁₅Si₁₈Hf₁₀ alloy annealed at 1 750 ℃ for 50 h(mole fraction, %)

The oxidation kinetics curves of the three alloys at  1 250 ℃ for 7 h are shown in Fig.2. The mass gains of the 5Hf and 10Hf samples are larger than those of the Hf-free sample under the same oxidation conditions. The 10Hf sample shows the highest mass gain of 111.87 mg/cm² after oxidation for 7 h. The oxidation rates of the 0Hf, 5Hf and 10Hf alloys are 5.97 mg/(cm²·h), 11.30 mg/(cm²·h) and 15.98 mg/(cm²·h), respectively. It can be concluded that with increasing Hf content the oxidation rate increases. After oxidation at 1 250 ℃ for 7 h, the oxide layer is light yellow and some oxides scale off the alloys.

Fig.3 shows the surface morphologies of the three oxidized alloys. The microstructures and compositions of the oxide scales were analyzed by SEM/EDS and XRD.

The oxides of the Nb₆₇W₁₅Si₁₈ alloy are cluster rods (average diameter about 1-4 μm), which grow outwardly from the inner parts of the oxide scale. At 5%(mole fraction) Hf content, the oxide surface is covered partially by Hf-rich particles with a size of 1-2 μm. For the 10%(mole fraction) Hf alloy, nearly 90% of the surface is covered by particle phase, as shown in Fig.3(c). The XRD results in Fig.4 indicate that the oxides in the Nb₆₇W₁₅Si₁₈ alloy mainly consist of Nb₁₂WO₃₃ and NbO_2, while those of the 5Hf and 10Hf alloys are Nb₂O₅ and WO₃. It can also be concluded from Fig.3 that the morphology of the 5Hf alloy surface is similar to that of the 0Hf alloy surface except that some Hf-rich particles appear on the oxide rods in the 5Hf alloy. The oxides, Nb₁₂WO₃₃ and NbO₂, in the Nb₆₇W₁₅Si₁₈ alloy, are distributed in the scale. For the 10Hf alloy, the oxide surface is entirely covered by particles with some cracks, which offers channels for the diffusion of oxygen, and then the oxide Nb₂O₅ grows outwardly through the cracks.

Fig.2 Kinetics curves of isothermal static oxidation of Nb_67-xW₁₅Si₁₈Hf_x alloys at 1 250 ℃: 1 Nb₆₇W₁₅Si_18; 2 Nb₆₂- W₁₅Si₁₈Hf_5; 3 Nb₅₇W₁₅Si₁₈Hf₁₀

Fig.3 Surface morphologies (SEM/SE) of Nb_67-xW₁₅Si₁₈Hf_x (x=0, 5, 10) alloys oxidized at 1 250 ℃ for 7 h in air:

(a) Nb₆₇W₁₅Si_18; (b) Nb₆₂W₁₅Si₁₈Hf_5; (c) Nb₅₇W₁₅Si₁₈Hf₁₀

Fig.4 X-ray diffraction patterns of Nb_67-xW₁₅Si₁₈Hf_x alloys oxidized at 1 250 ℃ for 7 h: (a) Nb₆₇W₁₅Si_18; (b) Nb₆₂W₁₅Si₁₈Hf_5; (c) Nb₅₇W₁₅Si₁₈Hf₁₀

The cross sectional morphologies of the oxide scales of the Nb_67-xW₁₅Si₁₈Hf_x alloys are illustrated in Fig.5. There are three distinct layers on the cross section, i.e., oxide layer, transition layer and substrate. The crack along the oxide/transition interface indicates that the oxide and the substrate are not well bonded, and the crack becomes wider with the increase of Hf, so the oxides scale off the substrate after oxidation. The transition on the 0Hf alloy is about 20 μm, and it is larger than 20 μm with the addition of Hf element. Table 2 and Table 3 show the mass fraction of oxygen in the Nb_SS and M₅Si₃ phases. It can be seen that oxygen dissolved in the M₅Si₃ is more than that in the Nb_SS, therefore, the dark areas in the transition layer become thicker than in the substrate. In view of the oxidation thermodynamics, the Gibbs free energy for the oxides Nb₂O₅ and NbO₂ formation is much lower than that for the oxides HfO₂ and SiO₂. Accordingly, when oxidation occurs, the Nb_SS phase is oxidized preferentially and forms Nb₂O₅ and NbO₂ on the surface because of its high content of Nb. In the transition layer, the silicide phase is more than the Nb_SS phase.

Fig.5 SEM/SE images of cross-sectional morphologies of Nb_67-xW₁₅Si₁₈Hf_x alloys oxidized at 1 250 ℃ for 7 h in air:

(a) Nb₆₇W₁₅Si_18; (b) Nb₆₂W₁₅Si₁₈Hf_5; (c) Nb₅₇W₁₅Si₁₈Hf₁₀

Table 2 Mass fractions of elements in transition layer of Nb₆₇W₁₅Si₁₈ alloy(A₁ and B₁ respectively correspond to the Nb_SS and M₅Si₃ areas in Fig.5 (a).)

Table 3 Mass fractions of elements in transition layer of Nb₅₇W₁₅Si₁₈Hf₁₀ alloy(A₂ and B₂ respectively correspond to the Nb_SS and M₅Si₃ areas in Fig.5 (c).)

The fine Pt markers placed on the top of the samples prior to oxidation are seen in the upper parts of the scale after oxidation, as shown in Fig.6. This implies that oxidation proceeds via the inward diffusion of oxygen. The oxide Nb₂O₅ grows continually from the substrate of the oxide layer. For the 10Hf alloy, most of Hf is separated out to the oxide surface and broken up as particles on the upper parts of the scale, as shown in Fig.3(c) and Fig.5, while Nb is oxidized at lower parts of the scale and grows outwardly through the Hf-rich particles.

Fig.6 SEM/SE images of Nb₆₇W₁₅Si₁₈alloy with Pt markers on oxide layer after oxidation at 1 250 ℃ for 3 h in air:

(a) Surface image; (b) Cross-section image

4 Conclusions

1) Hf is harmful to the oxidation resistance of the Nb_67-xW₁₅Si₁₈Hf_x alloys. The oxidation rate is accelerated by increasing Hf content.

 2) In oxidation procedure, oxygen diffuses inwardly form the oxide Nb₂O₅, and Hf diffuses outwardly and conglomerates on the oxide layer in the form of particle. The oxidation mechanism of the Nb_67-xW₁₅Si₁₈Hf_x alloys is internal oxidation.

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(Edited by PENG Chao-qun) 

Foundation item: Project(50671002) supported by the National Natural Science Foundation of China

Corresponding author: JIANG Rong-li; Tel: +86-10-82315989(300); E-mail:apple_j@126.com