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The oxidation behavior of high Cr and Al containing Nb-Si-Ti-Hf-Al-Cr alloys at 1200 and 1250 °C
Accepted Manuscript The oxidation behavior of high Cr and Al containing Nb-Si-Ti-HfAl-Cr alloys at 1200 and 1250°C
Linfen Su, Lina Jia, Kaiyong Jiang, Hu Zhang PII: DOI: Reference:
S0263-4368(17)30262-7 doi: 10.1016/j.ijrmhm.2017.08.006 RMHM 4494
To appear in:
International Journal of Refractory Metals and Hard Materials
Received date: Revised date: Accepted date:
28 April 2017 6 August 2017 6 August 2017
Please cite this article as: Linfen Su, Lina Jia, Kaiyong Jiang, Hu Zhang , The oxidation behavior of high Cr and Al containing Nb-Si-Ti-Hf-Al-Cr alloys at 1200 and 1250°C, International Journal of Refractory Metals and Hard Materials (2017), doi: 10.1016/ j.ijrmhm.2017.08.006
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ACCEPTED MANUSCRIPT The Oxidation behavior of High Cr and Al Containing Nb-Si-Ti-Hf-Al-Cr alloys at 1200 and 1250 oC Linfen Sua,c *, Lina Jiab, Kaiyong Jianga,c, Hu Zhangb* a.
College of Mechanical Engineering and Automation, Huaqiao University, Xiamen 361021
b. School of Materials Science and Engineering, Beihang University, Beijing 100191 c.
Fujian Key Laboratory of Special Energy Manufacturing, Huaqiao University, Xiamen 361021
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*Corresponding author. E-mail: [email protected]
Abstract: The oxidation behavior of two alloys containing different content of Al and
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Cr from the Nb-Si-Ti-Hf-Al-Cr system has been evaluated at 1200 and 1250 oC. The alloy compositions in atomic percent are Nb-24Ti-16Si-2Hf-2Al-10Cr (B1), and
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Nb-24Ti-16Si-2Hf-6Al-17Cr (B2). The oxidation kinetic of B1 alloy at 1200 and 1250 oC followed a mixed parabolic-linear law, while the oxidation kinetic of B2
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alloy at 1200 and 1250 oC followed a parabolic law. The weight gain of B2 alloy was
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18.9 mg/cm2 after oxidation at 1200 oC for 100 h, which was a seventh of the value of that of B1 alloy. Besides, oxidation became more severe as temperature increased to 1250 oC. The oxide scales of B2 alloy consisted of CrNbO4, TiNb2O7 and SiO2, which
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were relatively compact and protective. In addition, the oxidation mechanism of
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Nb-Si based alloys were also discussed. Keywords: High temperature oxidation; Nb-Si based alloy; XRD; SEM; EPMA;
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1. Introduction
Considerable achievements have been devoted to enhance the capabilities of high
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temperature materials in order to develop higher efficiency engines and advanced aerospace systems. It is urgent to develop a novel material which could substitute Ni-based superalloys at temperature higher than 1200 oC. Nb-Si based alloys have been extensively studied as high temperature structural materials because they have some advantages, such as high melting point, low density and relatively high strength at elevated temperature [1]. A multi-component Nb-Si-Ti-Hf-Cr-Al system has been developed by General Electric Co., which could achieve a balance of properties for structural application such as high temperature strength and acceptable oxidation resistance [2,3]. In this system, the phase constitutions are silicides, Cr2Nb and Nb 1
ACCEPTED MANUSCRIPT solid solution (NbSS) [2]. NbSS could provide room temperature fracture toughness, and silicides and Laves phase could offer the high temperature strength and oxidation resistance [4-9]. Recently, Nb-Si based alloys have been designed to satisfy the engineering application. Jia et al. develop a high Cr containing Nb-Si based alloy
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(Nb-12Si-24Ti-10Cr-2Al-2Hf, at%) with a refined microstructure by directional solidification [10]. Guo et al. produce a high Si containing Nb-Si based alloy
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(Nb-18Si-24Ti-2Cr-2Al-2Hf, at%) with a refined microstructure by selective laser melting [11]. Both methods can strengthen alloys by refining the microstructure
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without losing toughness.
However, the high temperature oxidation resistance of Nb-Si based alloys is still
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one of the major difficulties [12-13]. Applying coatings on Nb-Si based alloys is reported to be an effective method to improve the oxidation resistance [14-15]. Even
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so, investigation of the oxidation resistance of Nb-Si based alloys without coatings is still crucial because it is necessary to understand the oxidation behavior of Nb-Si
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based alloys in case of the failure of the coating. As indicated in the literatures
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[5,9,10], the poor oxidation resistance of Nb-Si based alloys can be ascribed to the unprotected oxide products (Nb2O5, CrNbO4, Ti2Nb10O29 and Nb2O5·TiO2) at high temperature environment. The presence of Nb2O5 creates a large volume expansion
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(169 %) upon the alloy [16]. This leads to cracking and spalling of the oxide scale. Thus the metal is unprotected and would expose to further oxidation. In recent
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decades, alloying elements such as Cr, Al, Ti, Sn, Ge, B and Hf are introduced in the Nb-Si based alloy, which have improved the oxidation resistance of the alloy [17-22]. However, it is still difficult to form a protective oxide scale on the alloy. The presence of Cr in Nb-Si alloys could promote the formation of Cr2Nb, which possesses good properties such as low creep rate, high oxidation resistance and high melting point of 1720 °C[1,3,7,12]. As reported by Chan [23], the thermal cycling oxidation resistance of a high Cr containing Nb-Si based alloy (35.8Nb-22.5Ti-4.0Hf -17.3Si-4.8Ge-15.6Cr (at.%)) at 1100 °C is improved when CrNbO4 is formed instead of Nb2O5 and Nb2O5·TiO2. Besides, CrNbO4 can also improve the adherence between 2
ACCEPTED MANUSCRIPT the oxide scale and the substrate as suggested by Qu et al. [24]. Moreover, Cr is a substitutional solute that interacts more strongly with oxygen than niobium, and therefore it could act as a trap to reduce oxygen diffusivity [9]. The presence of Al in Nb-Ti-Si-Al-Cr alloys reduces the pesting susceptibility at 800 °C as reported by Zelenitsas [9]. Besides, Al improves the oxidation resistance of
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silicides as studied by Zheng [25]. In the study of Murakami et al., a selective layer of Al2O3 as protective scale on oxidation is formed when the Nb-47Si-20Al (at.%) is
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oxidized at 1200 °C [26]. Varma et al. suggest that the addition of Al is beneficial at high temperatures only beginning at 1000 °C, and Al2O3 particles form abundantly
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close to the surface of the alloy when the Nb-20Cr-10Si-5Al (at.%) is oxidized at
achieve the formation of the Al2O3 layer.
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1100 °C [17]. Therefore, it is possible that higher content of Al may be necessary to
In this study, Nb-24Ti-16Si-2Hf-6Al-17Cr (at.%) alloy (contained higher content
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of Al and Cr) as well as Nb-24Ti-16Si-2Hf-2Al-10Cr (at.%) alloy were investigated in this study. The short term oxidation behavior and long term oxidation behavior of
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two alloys at 1200 and 1250 oC were investigated. Besides, the oxidation mechanism
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of Nb-Si based alloys were discussed. 2. Material and methods
Alloys
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2.1. Specimen preparation with
atomic
composition
of
Nb-24Ti-16Si-2Hf-2Al-10Cr,
and
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Nb-24Ti-16Si-2Hf-6Al-17Cr are hereafter referred to as B1 and B2 respectively. They were prepared by arc melting. To ensure composition homogeneity and element partition, the ingots were inverted and remelted four times. Oxidation specimens with a size of 8×8×5 mm3 were cut from the ingots by electron discharge machine and all surfaces were mechanically ground with 800-grit SiC paper and ultrasonically cleaned. 2.2. Oxidation testing The oxidation tests were performed in an open-ended tube furnace at 1200 and 3
ACCEPTED MANUSCRIPT 1250 oC. Each specimen was placed in a separate alumina crucible during the test. The specimens were removed from the furnace at the intervals of 10, 20, 40, 60, 80 and 100 hours and weighed together with the crucible using a precision analytical balance (Model CPA225D, Germany) with an accuracy of 0.00001 g. 2.3. Analyzing methods
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X-ray diffraction (XRD) (Model O/MAX-2200, Japan) patterns of the oxidation products were investigated to identify the main phases. The surface morphologies of
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oxidized specimens were analyzed in a scanning electron microscopes (SEM) (Model
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CS-3400, Japan) equipped with energy-dispersive spectroscopy (EDS). The cross-sectional microstructure of oxidized specimens were analyzed in electron probe
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micro-analyzer (EPMA, Model JXA-8230, Japan) equipped with wave-dispersive spectroscopy (WDS) using backscattered scanning electron (BSE) and X-ray mapping
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modes. Nb Lα, Si Kα, Cr Kα, Ti Kα, Al Kα, Hf Lα, O Kα were analyzed with PETJ, TAP, LIFH, PETJ, LIFH, PETJ and LDE2H crystals, respectively. The ZAF corrected
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EPMA was calibrated by pure standards supplied by the manufacture for different
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operating conditions and probe sizes. Before making observations, the cross sections were cut and ground with 1500-grit SiC paper, and then polished on a tightly woven cloth with 1 μm diamond paste.
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3. Results and discussion
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3.1. Microstructural characterization of as-cast alloys Fig. 1 demonstrates the XRD patterns and microstructures of B1 and B2 alloys. As shown in Fig.1, the microstructures of B1 and B2 alloys consist of primary Nb5Si3, eutectic (NbSS+ Nb5Si3) and Cr2Nb. Table 1 shows the volume fraction of constituent phases in B1 and B2 alloys. It could be found that B2 alloy contains relatively higher volume fraction of Cr2Nb and Nb5Si3 as compared to B1 alloy. 3.2. Long term oxidation resistance Fig. 2a shows the weight gain per unit area as a function of the exposure time at 4
ACCEPTED MANUSCRIPT 1200 and 1250 oC. B2 alloy exhibits better oxidation resistance with the weight gain of 18.9 mg/cm2 after oxidation at 1200 oC for 100 h, which is a seventh of the value of that of B1 alloy. Besides, the weight gains of B1 and B2 alloys are 217.8 and 32.8 mg/cm2 after oxidation at 1250 oC for 100 h respectively, indicating that oxidation become more severe as temperature increased. In general, oxidation kinetics is formulated by the following relationship [20,27]:
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m (k t ) n S
(1)
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where ∆m is the weight gain in g, S is the surface area of specimen in cm2, k is the
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oxidation rate coefficient, t is the oxidation time in h and n is the rate exponent. By fitting the thermal gravimetric data of B1 alloy at 1200 and 1250 °C to
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equation (1), n are determined to be 0.76 and 0.90, respectively. Since they are between 0.5 and 1, it is speculated that the oxidation kinetics of B1 alloy at 1200 and
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1250oC follow a mixed parabolic-linear law. This oxidation behavior suggests that both diffusion and interface reaction contribute to the oxidation of B1 alloy. In addition, the rate exponents of B1 alloy at 1200 and 1250 °C are more close to 1 than
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0.5, suggesting that the interface reaction is the dominant rate-determining step for the
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oxidation and the oxide scales do not provide an effective barrier to oxygen ingress towards the alloy as oxidation progressed. For B2 alloy at 1200 and 1250 °C, n are determined to be 0.51 and 0.57, respectively. Since they are close to 0.5, suggesting
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that the oxidation kinetics of B2 alloy at 1200 and 1250 oC follow the parabolic law. Fig. 2b shows the representation of the weight gain versus the square root of time for
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B2 alloy oxidized in air. The parabolic rate constants (g2 cm−4 s−1) calculated according to equation (1) for B2 alloy after oxidation at 1200 and 1250 °C for 100 h are equal to 5.27 × 10−10 and 8.95 × 10−10 respectively. Fig. 3 shows XRD patterns of oxides formed on B1 and B2 alloys after oxidation at 1200 and 1250 oC for 100 h. The oxides formed on B1 alloy are Nb2O5, TiO2, CrNbO4 and TiNb2O7. The oxides formed on B2 alloy are TiO2 and CrNbO4, which are expected to be the oxidation resistant oxides. Besides, the ratio of the intensity,
I CrNbO4 , of the CrNbO4 peak at 2θ = 27.3 deg to the intensity, I Nb2O5 , at 2θ = 23.9 deg 5
ACCEPTED MANUSCRIPT can be used as a measure of the relative amounts of CrNbO4 and Nb2O5 in the oxidation products [23]. It could be found that the ratio of B2 is higher than that of B1. Therefore, it could be deduced that the oxidation products of B1 alloy mainly consist of Nb2O5, while the oxidation products of B2 alloy mainly consist of CrNbO4. Surface morphologies of B1 and B2 alloys after oxidation at 1200 and 1250 oC
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for 100 h are shown in Fig. 4. The surfaces of B2 alloy are basically intact, but the surfaces of B1 alloy suffer severe spallation. As shown in Fig. 4, the oxide scales
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formed on B1 alloy are rough and porous. For B2 alloy, the appearance of sheet-like oxides make the oxide scales more compact, which can reduce the growing stress of
slightly coarsened after oxidation at 1250 oC.
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oxide scale and prevent the spalling of oxide scales. Besides, the oxide scale is
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The cross-sectional microstructure of B1 and B2 alloys after oxidation at 1200 and 1250 oC are presented in Fig. 5. The oxidation products are identified based on
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the results from the XRD analysis, the EDS analysis and BSE contrast. As shown in Fig. 5a and Fig. 5b, the residual oxide scales formed on the surface of B1 alloy are
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porous and loose with cracks distributed in it, which may lead to fast transportation of
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oxygen through it. The EDS analysis in Fig. 5a reveals that the light region has a composition of 60.09O-0.30Al-7.60Ti-1.33Cr-30.66Nb-0.01Hf (at.%), indicating that it is Nb2O5. The phases distributed in the grey region is very fine (smaller than 1 μm)
them
by
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(as shown in the amplified image of Fig.5a), therefore it is difficult to distinguish the
EDS
analysis.
This
region
has
a
composition
of
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58.75O-13.96Si-8.33Ti-2.64Cr-16.79Nb-0.53Hf (at.%). Combined with XRD results, it could be deduced that this region consists of TiO2, CrNbO4 and TiNb2O7. For B2 alloy, a uniform and relatively compact oxide scale is formed at 1200 and 1250oC (as shown in Fig. 5c and Fig. 5d), which could depress the inter-diffusion of O. The oxide scale does not peel during the cooling, suggesting that the oxide scale has good adherence to the substrate. It can be seen that the oxide scale mainly contains two layers, i.e. outer and inner layers. The outer layer of the scale is relatively compact and continuous while its corresponding inner layer is relatively loose. In the outer layer, the dark region in Fig. 5c has a composition of 6
ACCEPTED MANUSCRIPT 58.08O-4.21Al-20.53Si-5.59Ti-3.12Cr-8.43Nb-0.05Hf (at.%) and the light region in Fig. 5c has a composition of 55.80O-2.92Al-18.07Ti-9.15Cr-13.53Nb-0.53Hf (at.%). Combined with XRD results shown in Fig. 3, it indicates that the dark region is SiO2 and while the light region is the mixture of TiO2, CrNbO4 and TiNb2O7. It could be noticed that SiO2 is observed in the image of cross-sectional microstructure but the
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XRD pattern of the oxide scale shows no sharp peaks of SiO2, which indicates that SiO2 has the structure of amorphous glass. SiO2 is fluid at high temperature, thus it
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could heal the pores and cracks during oxidation. However, no SiO2 is observed in the inner layer. EDS analysis indicates that the inner layer has the similar composition,
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the region shown by the arrow in Fig. 5c has a composition of 49.03O-2.17Al16.93Ti-11.76Cr-19.01Nb-1.10Hf (at.%), which could be composed of TiO2, CrNbO4
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and TiNb2O7. It could be concluded that the inner layer is relatively loose due to lack of SiO2.
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In addition, all samples are fully affected by internal oxidation to generate dispersed particles of HfO2 with white contrast and TiO2 with black contrast. Oxygen
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would react with Ti and Hf, owing to their high affinities with oxygen. Besides, the
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in-depth diffusion of oxygen is typically through Nbss phases, while silicides and Cr2Nb have excellent resistance to oxidation due to their low diffusion rate of oxygen within them compared to NbSS [2, 28].
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Fig. 6 shows the elemental X-ray mapping of the B2 alloy oxidized at 1250 oC for 100 h, obtained using WDS attached on EPMA. The results clearly display the
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presence of O, Nb, Ti, Si, Al, Hf, and Cr in the oxides. The color scale is given in the bottom right corner: red represents the maximum concentration, yellow and green represent moderate concentrations, while blue represents a low concentration. Considerable Nb, Ti, Al and Hf are detected in the mixed oxide regions composed of TiNb2O7 and CrNbO4. Si is mainly detected in the black particles, i.e. amorphous SiO2 mentioned above. In addition, Al (Fig. 6g) and Cr (Fig. 6h) depleted regions are found in the oxide-metal interface, suggesting that the oxidation occurs not only by inward diffusion of O but also by outward diffusion of Al and Cr. Besides, O is detected in the oxides as well as the alloy, suggesting that the alloy is suffer internal 7
ACCEPTED MANUSCRIPT oxidation. The alloys studied in this work possess relatively better oxidation resistance in comparison with reported Nb-Si-Ti-Hf-Al-Cr alloys [5, 10, 24, 28]. This is because higher content of Cr and Al are introduced in Nb-Si based alloy, which can promote the formation of a relatively compact oxide scale consisting of CrNbO4 ,TiNb2O7 and
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SiO2. Besides, B2 alloy possesses higher volume fraction of oxidation resistance phases Nb5Si3 and Cr2Nb, therefore the oxidation process of B2 alloy is slower than
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that of B1 alloy.
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3.3. Short term oxidation resistance
In order to reveal the initial oxidation of B1 and B2 alloys, the oxidation is
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conducted at 1200 and 1250 oC for 1 h. Fig.7 shows XRD patterns of oxides formed on B1 and B2 alloys. As shown in Fig.7, the oxide scales formed on B1 alloy mainly
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consist of Nb2O5, TiO2, CrNbO4 and TiNb2O7, while the oxide scales formed on B2 alloy mainly consist of TiO2 and CrNbO4.
Fig.8 shows SEM images of the surface morphologies of B1 and B2 alloys after
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oxidation at 1200 and 1250 oC for 1 h. Some holes are found on B1, but not observed on B2 alloy, suggesting that the oxide film grown on B2 alloy is more compact and protective. As shown in Fig. 8a, the oxide scale formed on B1 alloy consists of
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bar-like and sheet-like oxide. EDS analysis reveals that the bar-like phase in Fig. 8a has a composition of 68.44O-1.40Si-5.90Ti-24.27Nb (at.%), indicating that it is Besides,
the
sheet-like
phase
region
has
a
composition
of
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Nb2O5.
32.78O-19.16Si-17.48Ti-4.66Cr-22.02Nb-3.91Hf (at.%), indicating that it is mixture oxide of TiNb2O7, CrNbO4, TiO2 and SiO2. Interestingly, the oxide scale formed on B2 alloy is relatively compact and consists of a grey glassy region with some short stick phase distributed on it. EDS analysis reveals that the short sticks in Fig. 8c has a composition of 69.76O-4.39Al-2.04Si-19.52Ti-4.30Cr (at.%), indicating that it is TiO2. And the
grey glassy region in Fig. 8c has a composition of
48.87O-2.88Al-4.74Si-20.30Ti-12.88Cr-10.33Nb (at.%), indicating that it is mixture oxide of CrNbO4, TiNb2O7 and TiO2. 8
ACCEPTED MANUSCRIPT The cross-sectional microstructure of B1 and B2 alloys after oxidation at 1200 and 1250 oC for 1 h are presented in Fig. 9. It could be found the oxide scale formed on B1 alloy (Fig. 9a) after oxidation for 1 h exhibit similar microstructure as compared to the oxide scale for 100 h (Fig. 6a). The light region consists of Nb2O5 while the grey region consists of SiO2, CrNbO4 and TiNb2O7. For B2 alloy, the oxide
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scale only consists of only one layer after oxidation for 1 h, which is different as compared to the oxide scale for 100 h (Fig. 6c). In this layer, the grey region is
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confirmed to be the mixture oxides consisting of CrNbO4 and TiNb2O7 while dark region is confirmed to be SiO2. It should be noticed that SiO2 distributes in the entire
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oxide layer. However, SiO2 only distributes in the outer layer of oxide scale after oxidation for 100 h. Therefore, it could be deduced that SiO2 would gather in the
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outer layer as the prolonged exposure of Nb-Si based alloy, leading to a relatively
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compact outer layer and relatively loose inner layer. 3.4. Oxidation mechanism
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As mentioned above, the oxide scale formed on B1 alloy is porous mainly
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consisting of Nb2O5. In contrast, the oxide scale formed on B2 alloy is relatively compact mainly consisting of CrNbO4. The integrity of the oxide scale depends on the Pilling Bedworth Ratio (PBR) of oxide, which is the ratio of oxide volume produced
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to metal volume consumed. If there is a large difference between the two, then the oxide has a tendency to spall. The PBR values of Nb2O5, SiO2, Cr2O3, TiO2 and Al2O3
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are reported to be 2.74, 1.72, 2.02, 1.73 and 1.28 respectively [29]. Thus it could be deduced that the PBR values of CrNbO4 and TiNb2O7 are lower than that of Nb2O5. Therefore, B1 alloy exhibit worse oxidation performance because its oxide scale mainly consists of Nb2O5, which would suffer higher stress, leading to the serious spallation of the oxide scale. For B2 alloy, the oxide scale mainly consists of CrNbO4, therefore the oxide scale suffer lower stress. Besides, the oxide scale formed on B2 alloy contains SiO2, which could heal the pores and cracks during oxidation. Therefore, the oxide scale integrity is improved. According to the ΔG0-T plots of various oxides [30], the ascending order of ΔG0 9
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G0 HfO2 G0 Al2O3 G0TiO2 G0 SiO2 G0Cr2O3 G0 Nb2O5
(2)
Thus, the descending order of stability is HfO2> Al2O3 > TiO2 > SiO2> Cr2O3> Nb2O5. Accordingly, HfO2 and TiO2 are most likely formed, owing to their lower ΔG value and high oxygen affinity. Besides, Nb2O5 is easily formed on the surface,
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mainly due to the high niobium content in Nb-Si alloy as well as its poor oxidation resistance. Moreover, the simple oxides (TiO2, Nb2O5, Cr2O3) from all the alloying
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elements would combine together to produce CrNbO4 and TiNb2O7 via solid-state
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reaction according to reaction (3) and (4) [30-31]. However, no Al2O3 is detected according to the microstructure and the XRD results, which can be ascribed to the low
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initial Al content [30].
(3)
Cr2O3+Nb2O5→CrNbO4
(4)
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2TiO2+ 5Nb2O5→TiNb2O7
Based on the discussion above, the schematic illustration of oxidation processes
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of B1 and B2 alloys are shown in Fig.10(a, b, c) and Fig.10(a’, b’, c’) respectively. At
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the initial stage of oxidation, the rapid inward diffusion of oxygen in the alloy occurs via the NbSS. After the dissolution of oxygen, dispersed particles of HfO2 and TiO2 are generated first (as shown in Fig. 10a and Fig. 10a’). After this stage, O2 reacts with
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NbSS, Nb5Si3 and Cr2Nb, generating Nb2O5, TiO2, Cr2O3 and SiO2 on the surface of the specimen. Besides, cracks and pores form due to the stress and the volume
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expansion of oxide (as shown in Fig. 10b and Fig. 10b’). Then, prolonged exposure of Nb-Si based alloys lead to the solid state reactions occurred among the oxides producing CrNbO4 and TiNb2O7. For B1, the oxide scale (consisting of SiO2, TiO2, Nb2O5, CrNbO4 and TiNb2O7) peels off continuously during oxidation due to the larger volume of Nb2O5, resulting in the loose and porous oxide scales (as shown in Fig. 10c). For B2 alloy, the solid state reaction occurs completely as its oxide scale adhered on the substrate during the oxidation, and the glass phase SiO2 is retained and distributed mainly in the outer layer (as shown in Fig. 10c’). In this study, although a selective layer does not form on the alloy, it should 10
ACCEPTED MANUSCRIPT noticed that a relatively compact oxide scale consisting of CrNbO4, TiNb2O7 and SiO2 is formed by alloying the higher content of Al and Cr element. This kind of oxide scale could decrease the stress of the scale, improve the oxide scale integrity and depress the inter-diffusion of O. Conclusions:
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The oxidation behavior of Nb-24Ti-16Si-2Hf-2Al-10Cr(at.%) (B1) and Nb-24Ti16Si-2Hf-6Al-17Cr (at.%) (B2) alloys has been evaluated at 1200 and 1250 oC:
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1. The microstructure of B1 and B2 alloys consisted of Nb5Si3, NbSS and Cr2Nb. B2
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alloy contained relatively higher volume fraction of Cr2Nb and Nb5Si3 as compared to B1 alloy.
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2. The oxidation kinetic of B1 alloy at 1200 and 1250 oC followed a mixed parabolic-linear law, while the oxidation kinetic of B2 alloy at 1200 and 1250 oC
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followed a parabolic law. B2 alloy showed better oxidation resistance with weight gain of 18.9 mg/cm2 after oxidation at 1200 oC for 100 h, which was a seventh of the
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value of that of B1 alloy.
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3. The oxide scale formed on B2 alloy was relatively compact and protective, consisting of CrNbO4, TiNb2O7 and SiO2. Besides, the scale coarsened as temperature increased.
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Acknowledgment
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The authors gratefully acknowledge the financial support by National Nature Science Foundation of P.R. China (51571004) and Natural Science Foundation of Fujian Province (2017J05082). References: [1] J.H. Perepezko, The hotter the engine, the better, Science 326 (2009) 1068-1069. [2] B.P. Bewlay, M.R. Jackson, J.C. Zhao, P.R. Subramanian, A review of very-high temperature Nb-silicide-based composites, Metall. Mater. Trans. A 34 (2003) 2043-2052. [3] B.P. Bewlay, M.R. Jackson, H.A. Lipsitt, The balance of mechanical and 11
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[11] Y. Guo, L.N. Jia, S.B. Sun, B. Kong, J.H. Liu, Rapid fabrication of Nb-Si based alloy by selective laser melting: Microstructure, hardness and initial oxidation behavior, Mate. Des. 109 (2016) 37-46. [12] J.R. DiStefano, B.A. Pint, J.H. DeVan, Oxidation of refractory metals in air and low pressure oxygen gas, Int. J. Refract. Met. Hard Mater. 18 (2000) 237-243. [13] E. Dokumaci, I. Özkan, M.B. Özyigit, B. Önay, Effect of boronizing on the oxidation of niobium, Int. J. Refract. Met. Hard Mater. 41 (2013) 276-281. [14] P. Rödhammer, W. Knabl, C. Semprimoschnig, R. Storf, M.Witwer, H. Clemens, et al., Protection of Nb- and Ta-based alloys against high temperature oxidation, Int. J. 12
ACCEPTED MANUSCRIPT Refract. Met. Hard Mater. 12 (1993-1994) 283-293. [15] J.C. Cheng, S.H. Yi, J.S. Park, Oxidation behavior of Nb-Si-B alloys with the NbSi2 coating layer formed by a pack cementation technique, Int. J. Refract. Met. Hard Mater. 41 (2013) 103-109. [16] D. Alvarez, S.K. Varma, Characterization of microstructures and oxidation
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behaviour of Nb-20Si-20Cr-5Al alloy, Corros. Sci. 53 (2011) 2161-2167. [17] N. Esparza, V. Rangel, A. Gutierrez, B. Arellano, S. K. Varma, A comparison of
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the effect of Cr and Al additions on the oxidation behaviour of alloys from the Nb-Cr-Si system, Materials at High Temperatures 33 (2016) 105-114.
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[18] K.S. Thomas, S.K. Varma, Oxidation response of three Nb-Cr-Mo-Si-B alloys in air, Corros. Sci. 99 (2015) 145-153.
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[19] Y. Li, W.F. Zhu, Q. Li, S.K. Qiu, J.Y. Zhang, Phase equilibria in the Nb-Ti side of the Nb-Si-Ti system at 1200°C and its oxidation behavior, J. Alloy Compd. 704 (2017)
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311-321.
[20] J. Zheng, X. Hou, X. Wang, Y. Meng, X. Zheng, L. Zheng, Isothermal oxidation
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mechanism of a newly developed Nb-Ti-V-Cr-Al-W-Mo-Hf alloy at 800-1200 °C, Int.
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J. Refract. Met. Hard Mater. 54 (2016) 322-329. [21] L.F. Su, L.N. Jia, J.F Weng, Z. Hong, C.G. Zhou, H. Zhang, Improvement in the oxidation resistance of Nb-Ti-Si-Cr-Al-Hf alloys containing alloyed Ge and B.
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Corro. Sci. 88 (2014): 460-465.
[22] J. Geng, P. Tsakiropoulos, G. Shao, Oxidation of Nb-Si-Cr-Al in situ composites
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with Mo, Ti and Hf additions, Mater. Sci. Eng. A 441 (2006) 26-38. [23] K.S. Chan, Cyclic oxidation response of multiphase niobium-based alloys, Metall. Mater. Trans. A 35 (2004) 589-597. [24] S.Y. Qu, Y.F. Han, J.X. Song, Y.W. Kang, Effects of Cr and Al on high temperature oxidation resistance of Nb-Si system intermetallics, Materials Science Forum 546-549 (2007) 1485-1488. [25] H. Zheng, S. Q. Lu, J.Y. Zhu, G.M. Liu, Effect of Al additions on the oxidation behavior of Laves phase NbCr2 alloys at 1373 K and 1473 K, Int. J. Refract. Met. Hard Mater.27 (2008) 659-663. 13
ACCEPTED MANUSCRIPT [26] T. Murakami, S. Sasaki, K. Ichikawa, A. Kitahara, Oxidation resistance of powder compacts of the Nb-Si-Cr system and Nb3Si5Al2 matrix compacts prepared by spark plasma sintering, Intermetallics, 9 (2001) 629-635. [27] R.X. Shi, J. Ding, Y.Q. Cao, A.Y. Zhang, P. Yang, Oxidation behavior and kinetics of Al2O3-TiC-Co composites, Int. J. Refract. Met. Hard Mater. 36 (2013)
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130-135. [28] S. Zhang, X. Guo, Alloying effects on the microstructure and properties of Nb-Si
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based ultrahigh temperature alloys, Intermetallics, 70 (2016) 33-44.
[29] T.F. Li, The High-Temperature oxidation and hot Corrosion of Metal, Chemical
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Industry Press, Beijing, 2003.
[30] J. Zheng, X. Hou, X. Wang, Y. Meng, X. Zheng, L. Zheng, Isothermal oxidation
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mechanism of Nb-Ti-V-Al-Zr alloy at 700-1200 °C: Diffusion and interface reaction, Corro. Sci. 96 (2015) 186-195.
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[31] W. Wan, C.G. Zhou, Hot corrosion behaviour of Nbss/Nb5Si3 in situ composites
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in the mixture of Na2SO4 and NaCl melts, Corros. Sci. 74 (2013) 345-352.
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ACCEPTED MANUSCRIPT List of Figures
Fig.1 (a) Microstructure of B1 alloy; (b) Microstructure of B2 alloy; (c) XRD patterns of the specimens. Fig.2 (a) Weight gain versus time for B1 and B2 alloys oxidized in air; and (b) representation of
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the weight gain versus the square root of time for B2 alloy oxidized in air.
Fig.3 XRD patterns of oxides obtained after oxidation at 1200 and 1250 oC for 100 h.
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Fig.4 Surface morphologies of Nb-Si based alloy after oxidation at 1200 and 1250 oC for 100 h: (a)
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B1 alloy, 1200 oC; (b) B1 alloy, 1250 oC; (c) B2 alloy, 1200 oC; (d) B2 alloy, 1250 oC. Fig.5 Cross-sectional microstructure of Nb-Si based alloy after oxidation at 1200 and 1250 oC for
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100 h: (a) B1 alloy, 1200 oC; (b) B1 alloy, 1250 oC; (c) B2 alloy, 1200 oC; (d) B2 alloy, 1250 oC. Fig.6 Elemental X-ray mappings of B2 alloy after oxidation at 1250 oC for 100 h obtained using
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WDS attached on EPMA (a) Cross-sectional microstructure; (b) O mapping; (c) Nb mapping; (d) Si mapping; (e) Ti mapping; (f) Hf mapping; (g) Al mapping; (h) Cr mapping.
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Fig.7 XRD patterns of oxides obtained after oxidation at 1200 and 1250 oC for 1 h.
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Fig.8 Surface morphologies of Nb-Si based alloy after oxidation at 1200 and 1250 oC for 1 h: (a) B1 alloy, 1200 oC; (b) B1 alloy, 1250 oC; (c) B2 alloy, 1200 oC; (d) B2 alloy, 1250 oC. Fig.9 Cross-sectional microstructure of Nb-Si based alloy after oxidation at 1200 and 1250 oC for
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1 h: (a) B1 alloy, 1200 oC; (b) B1 alloy, 1250 oC; (c) B2 alloy, 1200 oC; (d) B2 alloy, 1250 oC. Fig. 10 Schematic illustration of oxidation process of Nb-Si based alloys:(a, b, c) B1 alloy;
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Fig.1 (a) Microstructure of B1 alloy; (b) Microstructure of B2 alloy; (c) XRD patterns of the specimens.
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Fig.2 (a) Weight gain versus time for B1 and B2 alloys oxidized in air; and (b) representation of the weight gain versus the square root of time for B2 alloy oxidized in air.
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Fig.3 XRD patterns of oxides obtained after oxidation at 1200 and 1250oC for 100 h.
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Fig. 4 Surface morphologies of Nb-Si based alloy after oxidation at 1200 and 1250 oC for 100 h:
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Fig. 5 Cross-sectional microstructure of Nb-Si based alloy after oxidation at 1200 and 1250 oC for
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Fig.6 Elemental X-ray mappings of B2 alloy after oxidation at 1250 oC for 100 h obtained using WDS attached on EPMA (a) Cross-sectional microstructure; (b) O mapping; (c) Nb mapping; (d) Si mapping; (e) Ti mapping; (f) Hf mapping; (g) Al mapping; (h) Cr mapping.
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Fig.7 XRD patterns of oxides obtained after oxidation at 1200 and 1250 oC for 1 h.
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Fig.8 Surface morphologies of Nb-Si based alloy after oxidation at 1200 and 1250 oC for 1 h: (a) B1 alloy, 1200 oC; (b) B1 alloy, 1250 oC; (c) B2 alloy, 1200 oC; (d) B2 alloy, 1250 oC.
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Fig. 9 Cross-sectional microstructure of Nb-Si based alloy after oxidation at 1200 and 1250oC for
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Fig. 10 Schematic illustration of oxidation process of Nb-Si based alloys:
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Table 1 Volume fraction of constituent phases in B1 and B2 alloys (%).
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ACCEPTED MANUSCRIPT Table 1 Volume fraction of constituent phases in B1 and B2 alloys (%) Eutetic （Nb5Si3+NbSS）
Nb5Si3
Cr2Nb
B1
70.7
20.5
8.8
B2
44.5
34.8
9.7
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Sepecimen
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ACCEPTED MANUSCRIPT Highlights
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1. Oxidation behavior of Nb-24Ti-16Si-2Hf-6Al-17Cr (at.%) followed a parabolic law. 2. A compact oxide scale consisting of CrNbO4, TiNb2O7 and SiO2 was formed. 3. The oxidation processes of Nb-Si-Ti-Hf-Al-Cr alloys were discussed.
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Linfen Su, Lina Jia, Kaiyong Jiang, Hu Zhang PII: DOI: Reference:
S0263-4368(17)30262-7 doi: 10.1016/j.ijrmhm.2017.08.006 RMHM 4494
To appear in:
International Journal of Refractory Metals and Hard Materials
Received date: Revised date: Accepted date:
28 April 2017 6 August 2017 6 August 2017
Please cite this article as: Linfen Su, Lina Jia, Kaiyong Jiang, Hu Zhang , The oxidation behavior of high Cr and Al containing Nb-Si-Ti-Hf-Al-Cr alloys at 1200 and 1250°C, International Journal of Refractory Metals and Hard Materials (2017), doi: 10.1016/ j.ijrmhm.2017.08.006
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ACCEPTED MANUSCRIPT The Oxidation behavior of High Cr and Al Containing Nb-Si-Ti-Hf-Al-Cr alloys at 1200 and 1250 oC Linfen Sua,c *, Lina Jiab, Kaiyong Jianga,c, Hu Zhangb* a.
College of Mechanical Engineering and Automation, Huaqiao University, Xiamen 361021
b. School of Materials Science and Engineering, Beihang University, Beijing 100191 c.
Fujian Key Laboratory of Special Energy Manufacturing, Huaqiao University, Xiamen 361021
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*Corresponding author. E-mail: [email protected]
Abstract: The oxidation behavior of two alloys containing different content of Al and
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Cr from the Nb-Si-Ti-Hf-Al-Cr system has been evaluated at 1200 and 1250 oC. The alloy compositions in atomic percent are Nb-24Ti-16Si-2Hf-2Al-10Cr (B1), and
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Nb-24Ti-16Si-2Hf-6Al-17Cr (B2). The oxidation kinetic of B1 alloy at 1200 and 1250 oC followed a mixed parabolic-linear law, while the oxidation kinetic of B2
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alloy at 1200 and 1250 oC followed a parabolic law. The weight gain of B2 alloy was
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18.9 mg/cm2 after oxidation at 1200 oC for 100 h, which was a seventh of the value of that of B1 alloy. Besides, oxidation became more severe as temperature increased to 1250 oC. The oxide scales of B2 alloy consisted of CrNbO4, TiNb2O7 and SiO2, which
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were relatively compact and protective. In addition, the oxidation mechanism of
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Nb-Si based alloys were also discussed. Keywords: High temperature oxidation; Nb-Si based alloy; XRD; SEM; EPMA;
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1. Introduction
Considerable achievements have been devoted to enhance the capabilities of high
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temperature materials in order to develop higher efficiency engines and advanced aerospace systems. It is urgent to develop a novel material which could substitute Ni-based superalloys at temperature higher than 1200 oC. Nb-Si based alloys have been extensively studied as high temperature structural materials because they have some advantages, such as high melting point, low density and relatively high strength at elevated temperature [1]. A multi-component Nb-Si-Ti-Hf-Cr-Al system has been developed by General Electric Co., which could achieve a balance of properties for structural application such as high temperature strength and acceptable oxidation resistance [2,3]. In this system, the phase constitutions are silicides, Cr2Nb and Nb 1
ACCEPTED MANUSCRIPT solid solution (NbSS) [2]. NbSS could provide room temperature fracture toughness, and silicides and Laves phase could offer the high temperature strength and oxidation resistance [4-9]. Recently, Nb-Si based alloys have been designed to satisfy the engineering application. Jia et al. develop a high Cr containing Nb-Si based alloy
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(Nb-12Si-24Ti-10Cr-2Al-2Hf, at%) with a refined microstructure by directional solidification [10]. Guo et al. produce a high Si containing Nb-Si based alloy
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(Nb-18Si-24Ti-2Cr-2Al-2Hf, at%) with a refined microstructure by selective laser melting [11]. Both methods can strengthen alloys by refining the microstructure
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without losing toughness.
However, the high temperature oxidation resistance of Nb-Si based alloys is still
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one of the major difficulties [12-13]. Applying coatings on Nb-Si based alloys is reported to be an effective method to improve the oxidation resistance [14-15]. Even
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so, investigation of the oxidation resistance of Nb-Si based alloys without coatings is still crucial because it is necessary to understand the oxidation behavior of Nb-Si
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based alloys in case of the failure of the coating. As indicated in the literatures
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[5,9,10], the poor oxidation resistance of Nb-Si based alloys can be ascribed to the unprotected oxide products (Nb2O5, CrNbO4, Ti2Nb10O29 and Nb2O5·TiO2) at high temperature environment. The presence of Nb2O5 creates a large volume expansion
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(169 %) upon the alloy [16]. This leads to cracking and spalling of the oxide scale. Thus the metal is unprotected and would expose to further oxidation. In recent
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decades, alloying elements such as Cr, Al, Ti, Sn, Ge, B and Hf are introduced in the Nb-Si based alloy, which have improved the oxidation resistance of the alloy [17-22]. However, it is still difficult to form a protective oxide scale on the alloy. The presence of Cr in Nb-Si alloys could promote the formation of Cr2Nb, which possesses good properties such as low creep rate, high oxidation resistance and high melting point of 1720 °C[1,3,7,12]. As reported by Chan [23], the thermal cycling oxidation resistance of a high Cr containing Nb-Si based alloy (35.8Nb-22.5Ti-4.0Hf -17.3Si-4.8Ge-15.6Cr (at.%)) at 1100 °C is improved when CrNbO4 is formed instead of Nb2O5 and Nb2O5·TiO2. Besides, CrNbO4 can also improve the adherence between 2
ACCEPTED MANUSCRIPT the oxide scale and the substrate as suggested by Qu et al. [24]. Moreover, Cr is a substitutional solute that interacts more strongly with oxygen than niobium, and therefore it could act as a trap to reduce oxygen diffusivity [9]. The presence of Al in Nb-Ti-Si-Al-Cr alloys reduces the pesting susceptibility at 800 °C as reported by Zelenitsas [9]. Besides, Al improves the oxidation resistance of
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silicides as studied by Zheng [25]. In the study of Murakami et al., a selective layer of Al2O3 as protective scale on oxidation is formed when the Nb-47Si-20Al (at.%) is
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oxidized at 1200 °C [26]. Varma et al. suggest that the addition of Al is beneficial at high temperatures only beginning at 1000 °C, and Al2O3 particles form abundantly
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close to the surface of the alloy when the Nb-20Cr-10Si-5Al (at.%) is oxidized at
achieve the formation of the Al2O3 layer.
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1100 °C [17]. Therefore, it is possible that higher content of Al may be necessary to
In this study, Nb-24Ti-16Si-2Hf-6Al-17Cr (at.%) alloy (contained higher content
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of Al and Cr) as well as Nb-24Ti-16Si-2Hf-2Al-10Cr (at.%) alloy were investigated in this study. The short term oxidation behavior and long term oxidation behavior of
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two alloys at 1200 and 1250 oC were investigated. Besides, the oxidation mechanism
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of Nb-Si based alloys were discussed. 2. Material and methods
Alloys
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2.1. Specimen preparation with
atomic
composition
of
Nb-24Ti-16Si-2Hf-2Al-10Cr,
and
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Nb-24Ti-16Si-2Hf-6Al-17Cr are hereafter referred to as B1 and B2 respectively. They were prepared by arc melting. To ensure composition homogeneity and element partition, the ingots were inverted and remelted four times. Oxidation specimens with a size of 8×8×5 mm3 were cut from the ingots by electron discharge machine and all surfaces were mechanically ground with 800-grit SiC paper and ultrasonically cleaned. 2.2. Oxidation testing The oxidation tests were performed in an open-ended tube furnace at 1200 and 3
ACCEPTED MANUSCRIPT 1250 oC. Each specimen was placed in a separate alumina crucible during the test. The specimens were removed from the furnace at the intervals of 10, 20, 40, 60, 80 and 100 hours and weighed together with the crucible using a precision analytical balance (Model CPA225D, Germany) with an accuracy of 0.00001 g. 2.3. Analyzing methods
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X-ray diffraction (XRD) (Model O/MAX-2200, Japan) patterns of the oxidation products were investigated to identify the main phases. The surface morphologies of
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oxidized specimens were analyzed in a scanning electron microscopes (SEM) (Model
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CS-3400, Japan) equipped with energy-dispersive spectroscopy (EDS). The cross-sectional microstructure of oxidized specimens were analyzed in electron probe
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micro-analyzer (EPMA, Model JXA-8230, Japan) equipped with wave-dispersive spectroscopy (WDS) using backscattered scanning electron (BSE) and X-ray mapping
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modes. Nb Lα, Si Kα, Cr Kα, Ti Kα, Al Kα, Hf Lα, O Kα were analyzed with PETJ, TAP, LIFH, PETJ, LIFH, PETJ and LDE2H crystals, respectively. The ZAF corrected
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EPMA was calibrated by pure standards supplied by the manufacture for different
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operating conditions and probe sizes. Before making observations, the cross sections were cut and ground with 1500-grit SiC paper, and then polished on a tightly woven cloth with 1 μm diamond paste.
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3. Results and discussion
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3.1. Microstructural characterization of as-cast alloys Fig. 1 demonstrates the XRD patterns and microstructures of B1 and B2 alloys. As shown in Fig.1, the microstructures of B1 and B2 alloys consist of primary Nb5Si3, eutectic (NbSS+ Nb5Si3) and Cr2Nb. Table 1 shows the volume fraction of constituent phases in B1 and B2 alloys. It could be found that B2 alloy contains relatively higher volume fraction of Cr2Nb and Nb5Si3 as compared to B1 alloy. 3.2. Long term oxidation resistance Fig. 2a shows the weight gain per unit area as a function of the exposure time at 4
ACCEPTED MANUSCRIPT 1200 and 1250 oC. B2 alloy exhibits better oxidation resistance with the weight gain of 18.9 mg/cm2 after oxidation at 1200 oC for 100 h, which is a seventh of the value of that of B1 alloy. Besides, the weight gains of B1 and B2 alloys are 217.8 and 32.8 mg/cm2 after oxidation at 1250 oC for 100 h respectively, indicating that oxidation become more severe as temperature increased. In general, oxidation kinetics is formulated by the following relationship [20,27]:
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m (k t ) n S
(1)
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where ∆m is the weight gain in g, S is the surface area of specimen in cm2, k is the
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oxidation rate coefficient, t is the oxidation time in h and n is the rate exponent. By fitting the thermal gravimetric data of B1 alloy at 1200 and 1250 °C to
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equation (1), n are determined to be 0.76 and 0.90, respectively. Since they are between 0.5 and 1, it is speculated that the oxidation kinetics of B1 alloy at 1200 and
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1250oC follow a mixed parabolic-linear law. This oxidation behavior suggests that both diffusion and interface reaction contribute to the oxidation of B1 alloy. In addition, the rate exponents of B1 alloy at 1200 and 1250 °C are more close to 1 than
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0.5, suggesting that the interface reaction is the dominant rate-determining step for the
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oxidation and the oxide scales do not provide an effective barrier to oxygen ingress towards the alloy as oxidation progressed. For B2 alloy at 1200 and 1250 °C, n are determined to be 0.51 and 0.57, respectively. Since they are close to 0.5, suggesting
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that the oxidation kinetics of B2 alloy at 1200 and 1250 oC follow the parabolic law. Fig. 2b shows the representation of the weight gain versus the square root of time for
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B2 alloy oxidized in air. The parabolic rate constants (g2 cm−4 s−1) calculated according to equation (1) for B2 alloy after oxidation at 1200 and 1250 °C for 100 h are equal to 5.27 × 10−10 and 8.95 × 10−10 respectively. Fig. 3 shows XRD patterns of oxides formed on B1 and B2 alloys after oxidation at 1200 and 1250 oC for 100 h. The oxides formed on B1 alloy are Nb2O5, TiO2, CrNbO4 and TiNb2O7. The oxides formed on B2 alloy are TiO2 and CrNbO4, which are expected to be the oxidation resistant oxides. Besides, the ratio of the intensity,
I CrNbO4 , of the CrNbO4 peak at 2θ = 27.3 deg to the intensity, I Nb2O5 , at 2θ = 23.9 deg 5
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for 100 h are shown in Fig. 4. The surfaces of B2 alloy are basically intact, but the surfaces of B1 alloy suffer severe spallation. As shown in Fig. 4, the oxide scales
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formed on B1 alloy are rough and porous. For B2 alloy, the appearance of sheet-like oxides make the oxide scales more compact, which can reduce the growing stress of
slightly coarsened after oxidation at 1250 oC.
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oxide scale and prevent the spalling of oxide scales. Besides, the oxide scale is
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The cross-sectional microstructure of B1 and B2 alloys after oxidation at 1200 and 1250 oC are presented in Fig. 5. The oxidation products are identified based on
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the results from the XRD analysis, the EDS analysis and BSE contrast. As shown in Fig. 5a and Fig. 5b, the residual oxide scales formed on the surface of B1 alloy are
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porous and loose with cracks distributed in it, which may lead to fast transportation of
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oxygen through it. The EDS analysis in Fig. 5a reveals that the light region has a composition of 60.09O-0.30Al-7.60Ti-1.33Cr-30.66Nb-0.01Hf (at.%), indicating that it is Nb2O5. The phases distributed in the grey region is very fine (smaller than 1 μm)
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(as shown in the amplified image of Fig.5a), therefore it is difficult to distinguish the
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analysis.
This
region
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58.75O-13.96Si-8.33Ti-2.64Cr-16.79Nb-0.53Hf (at.%). Combined with XRD results, it could be deduced that this region consists of TiO2, CrNbO4 and TiNb2O7. For B2 alloy, a uniform and relatively compact oxide scale is formed at 1200 and 1250oC (as shown in Fig. 5c and Fig. 5d), which could depress the inter-diffusion of O. The oxide scale does not peel during the cooling, suggesting that the oxide scale has good adherence to the substrate. It can be seen that the oxide scale mainly contains two layers, i.e. outer and inner layers. The outer layer of the scale is relatively compact and continuous while its corresponding inner layer is relatively loose. In the outer layer, the dark region in Fig. 5c has a composition of 6
ACCEPTED MANUSCRIPT 58.08O-4.21Al-20.53Si-5.59Ti-3.12Cr-8.43Nb-0.05Hf (at.%) and the light region in Fig. 5c has a composition of 55.80O-2.92Al-18.07Ti-9.15Cr-13.53Nb-0.53Hf (at.%). Combined with XRD results shown in Fig. 3, it indicates that the dark region is SiO2 and while the light region is the mixture of TiO2, CrNbO4 and TiNb2O7. It could be noticed that SiO2 is observed in the image of cross-sectional microstructure but the
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XRD pattern of the oxide scale shows no sharp peaks of SiO2, which indicates that SiO2 has the structure of amorphous glass. SiO2 is fluid at high temperature, thus it
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could heal the pores and cracks during oxidation. However, no SiO2 is observed in the inner layer. EDS analysis indicates that the inner layer has the similar composition,
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the region shown by the arrow in Fig. 5c has a composition of 49.03O-2.17Al16.93Ti-11.76Cr-19.01Nb-1.10Hf (at.%), which could be composed of TiO2, CrNbO4
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and TiNb2O7. It could be concluded that the inner layer is relatively loose due to lack of SiO2.
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In addition, all samples are fully affected by internal oxidation to generate dispersed particles of HfO2 with white contrast and TiO2 with black contrast. Oxygen
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in-depth diffusion of oxygen is typically through Nbss phases, while silicides and Cr2Nb have excellent resistance to oxidation due to their low diffusion rate of oxygen within them compared to NbSS [2, 28].
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Fig. 6 shows the elemental X-ray mapping of the B2 alloy oxidized at 1250 oC for 100 h, obtained using WDS attached on EPMA. The results clearly display the
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presence of O, Nb, Ti, Si, Al, Hf, and Cr in the oxides. The color scale is given in the bottom right corner: red represents the maximum concentration, yellow and green represent moderate concentrations, while blue represents a low concentration. Considerable Nb, Ti, Al and Hf are detected in the mixed oxide regions composed of TiNb2O7 and CrNbO4. Si is mainly detected in the black particles, i.e. amorphous SiO2 mentioned above. In addition, Al (Fig. 6g) and Cr (Fig. 6h) depleted regions are found in the oxide-metal interface, suggesting that the oxidation occurs not only by inward diffusion of O but also by outward diffusion of Al and Cr. Besides, O is detected in the oxides as well as the alloy, suggesting that the alloy is suffer internal 7
ACCEPTED MANUSCRIPT oxidation. The alloys studied in this work possess relatively better oxidation resistance in comparison with reported Nb-Si-Ti-Hf-Al-Cr alloys [5, 10, 24, 28]. This is because higher content of Cr and Al are introduced in Nb-Si based alloy, which can promote the formation of a relatively compact oxide scale consisting of CrNbO4 ,TiNb2O7 and
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SiO2. Besides, B2 alloy possesses higher volume fraction of oxidation resistance phases Nb5Si3 and Cr2Nb, therefore the oxidation process of B2 alloy is slower than
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3.3. Short term oxidation resistance
In order to reveal the initial oxidation of B1 and B2 alloys, the oxidation is
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conducted at 1200 and 1250 oC for 1 h. Fig.7 shows XRD patterns of oxides formed on B1 and B2 alloys. As shown in Fig.7, the oxide scales formed on B1 alloy mainly
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consist of Nb2O5, TiO2, CrNbO4 and TiNb2O7, while the oxide scales formed on B2 alloy mainly consist of TiO2 and CrNbO4.
Fig.8 shows SEM images of the surface morphologies of B1 and B2 alloys after
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oxidation at 1200 and 1250 oC for 1 h. Some holes are found on B1, but not observed on B2 alloy, suggesting that the oxide film grown on B2 alloy is more compact and protective. As shown in Fig. 8a, the oxide scale formed on B1 alloy consists of
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bar-like and sheet-like oxide. EDS analysis reveals that the bar-like phase in Fig. 8a has a composition of 68.44O-1.40Si-5.90Ti-24.27Nb (at.%), indicating that it is Besides,
the
sheet-like
phase
region
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of
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Nb2O5.
32.78O-19.16Si-17.48Ti-4.66Cr-22.02Nb-3.91Hf (at.%), indicating that it is mixture oxide of TiNb2O7, CrNbO4, TiO2 and SiO2. Interestingly, the oxide scale formed on B2 alloy is relatively compact and consists of a grey glassy region with some short stick phase distributed on it. EDS analysis reveals that the short sticks in Fig. 8c has a composition of 69.76O-4.39Al-2.04Si-19.52Ti-4.30Cr (at.%), indicating that it is TiO2. And the
grey glassy region in Fig. 8c has a composition of
48.87O-2.88Al-4.74Si-20.30Ti-12.88Cr-10.33Nb (at.%), indicating that it is mixture oxide of CrNbO4, TiNb2O7 and TiO2. 8
ACCEPTED MANUSCRIPT The cross-sectional microstructure of B1 and B2 alloys after oxidation at 1200 and 1250 oC for 1 h are presented in Fig. 9. It could be found the oxide scale formed on B1 alloy (Fig. 9a) after oxidation for 1 h exhibit similar microstructure as compared to the oxide scale for 100 h (Fig. 6a). The light region consists of Nb2O5 while the grey region consists of SiO2, CrNbO4 and TiNb2O7. For B2 alloy, the oxide
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scale only consists of only one layer after oxidation for 1 h, which is different as compared to the oxide scale for 100 h (Fig. 6c). In this layer, the grey region is
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confirmed to be the mixture oxides consisting of CrNbO4 and TiNb2O7 while dark region is confirmed to be SiO2. It should be noticed that SiO2 distributes in the entire
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oxide layer. However, SiO2 only distributes in the outer layer of oxide scale after oxidation for 100 h. Therefore, it could be deduced that SiO2 would gather in the
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outer layer as the prolonged exposure of Nb-Si based alloy, leading to a relatively
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As mentioned above, the oxide scale formed on B1 alloy is porous mainly
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consisting of Nb2O5. In contrast, the oxide scale formed on B2 alloy is relatively compact mainly consisting of CrNbO4. The integrity of the oxide scale depends on the Pilling Bedworth Ratio (PBR) of oxide, which is the ratio of oxide volume produced
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to metal volume consumed. If there is a large difference between the two, then the oxide has a tendency to spall. The PBR values of Nb2O5, SiO2, Cr2O3, TiO2 and Al2O3
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are reported to be 2.74, 1.72, 2.02, 1.73 and 1.28 respectively [29]. Thus it could be deduced that the PBR values of CrNbO4 and TiNb2O7 are lower than that of Nb2O5. Therefore, B1 alloy exhibit worse oxidation performance because its oxide scale mainly consists of Nb2O5, which would suffer higher stress, leading to the serious spallation of the oxide scale. For B2 alloy, the oxide scale mainly consists of CrNbO4, therefore the oxide scale suffer lower stress. Besides, the oxide scale formed on B2 alloy contains SiO2, which could heal the pores and cracks during oxidation. Therefore, the oxide scale integrity is improved. According to the ΔG0-T plots of various oxides [30], the ascending order of ΔG0 9
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G0 HfO2 G0 Al2O3 G0TiO2 G0 SiO2 G0Cr2O3 G0 Nb2O5
(2)
Thus, the descending order of stability is HfO2> Al2O3 > TiO2 > SiO2> Cr2O3> Nb2O5. Accordingly, HfO2 and TiO2 are most likely formed, owing to their lower ΔG value and high oxygen affinity. Besides, Nb2O5 is easily formed on the surface,
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mainly due to the high niobium content in Nb-Si alloy as well as its poor oxidation resistance. Moreover, the simple oxides (TiO2, Nb2O5, Cr2O3) from all the alloying
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elements would combine together to produce CrNbO4 and TiNb2O7 via solid-state
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reaction according to reaction (3) and (4) [30-31]. However, no Al2O3 is detected according to the microstructure and the XRD results, which can be ascribed to the low
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initial Al content [30].
(3)
Cr2O3+Nb2O5→CrNbO4
(4)
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2TiO2+ 5Nb2O5→TiNb2O7
Based on the discussion above, the schematic illustration of oxidation processes
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the initial stage of oxidation, the rapid inward diffusion of oxygen in the alloy occurs via the NbSS. After the dissolution of oxygen, dispersed particles of HfO2 and TiO2 are generated first (as shown in Fig. 10a and Fig. 10a’). After this stage, O2 reacts with
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NbSS, Nb5Si3 and Cr2Nb, generating Nb2O5, TiO2, Cr2O3 and SiO2 on the surface of the specimen. Besides, cracks and pores form due to the stress and the volume
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expansion of oxide (as shown in Fig. 10b and Fig. 10b’). Then, prolonged exposure of Nb-Si based alloys lead to the solid state reactions occurred among the oxides producing CrNbO4 and TiNb2O7. For B1, the oxide scale (consisting of SiO2, TiO2, Nb2O5, CrNbO4 and TiNb2O7) peels off continuously during oxidation due to the larger volume of Nb2O5, resulting in the loose and porous oxide scales (as shown in Fig. 10c). For B2 alloy, the solid state reaction occurs completely as its oxide scale adhered on the substrate during the oxidation, and the glass phase SiO2 is retained and distributed mainly in the outer layer (as shown in Fig. 10c’). In this study, although a selective layer does not form on the alloy, it should 10
ACCEPTED MANUSCRIPT noticed that a relatively compact oxide scale consisting of CrNbO4, TiNb2O7 and SiO2 is formed by alloying the higher content of Al and Cr element. This kind of oxide scale could decrease the stress of the scale, improve the oxide scale integrity and depress the inter-diffusion of O. Conclusions:
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The oxidation behavior of Nb-24Ti-16Si-2Hf-2Al-10Cr(at.%) (B1) and Nb-24Ti16Si-2Hf-6Al-17Cr (at.%) (B2) alloys has been evaluated at 1200 and 1250 oC:
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1. The microstructure of B1 and B2 alloys consisted of Nb5Si3, NbSS and Cr2Nb. B2
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alloy contained relatively higher volume fraction of Cr2Nb and Nb5Si3 as compared to B1 alloy.
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2. The oxidation kinetic of B1 alloy at 1200 and 1250 oC followed a mixed parabolic-linear law, while the oxidation kinetic of B2 alloy at 1200 and 1250 oC
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followed a parabolic law. B2 alloy showed better oxidation resistance with weight gain of 18.9 mg/cm2 after oxidation at 1200 oC for 100 h, which was a seventh of the
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3. The oxide scale formed on B2 alloy was relatively compact and protective, consisting of CrNbO4, TiNb2O7 and SiO2. Besides, the scale coarsened as temperature increased.
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Acknowledgment
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The authors gratefully acknowledge the financial support by National Nature Science Foundation of P.R. China (51571004) and Natural Science Foundation of Fujian Province (2017J05082). References: [1] J.H. Perepezko, The hotter the engine, the better, Science 326 (2009) 1068-1069. [2] B.P. Bewlay, M.R. Jackson, J.C. Zhao, P.R. Subramanian, A review of very-high temperature Nb-silicide-based composites, Metall. Mater. Trans. A 34 (2003) 2043-2052. [3] B.P. Bewlay, M.R. Jackson, H.A. Lipsitt, The balance of mechanical and 11
ACCEPTED MANUSCRIPT environmental properties of a multi-element niobium-niobium silicide-based in-situ composite, Metall. Mater. Trans. A 27 (1996) 3801-3808. [4] J. Geng, P. Tsakiropoulos, G.S. Shao, Oxidation of Nb-Si-Cr-Al in situ composites with Mo, Ti and Hf additions, Mater. Sci. Eng. A 441 (2006) 26-38. [5] D.Z. Yao, R. Cai, C.G. Zhou, J.B. Sha, H.R. Jiang, Experimental study and
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ACCEPTED MANUSCRIPT [26] T. Murakami, S. Sasaki, K. Ichikawa, A. Kitahara, Oxidation resistance of powder compacts of the Nb-Si-Cr system and Nb3Si5Al2 matrix compacts prepared by spark plasma sintering, Intermetallics, 9 (2001) 629-635. [27] R.X. Shi, J. Ding, Y.Q. Cao, A.Y. Zhang, P. Yang, Oxidation behavior and kinetics of Al2O3-TiC-Co composites, Int. J. Refract. Met. Hard Mater. 36 (2013)
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ACCEPTED MANUSCRIPT List of Figures
Fig.1 (a) Microstructure of B1 alloy; (b) Microstructure of B2 alloy; (c) XRD patterns of the specimens. Fig.2 (a) Weight gain versus time for B1 and B2 alloys oxidized in air; and (b) representation of
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the weight gain versus the square root of time for B2 alloy oxidized in air.
Fig.3 XRD patterns of oxides obtained after oxidation at 1200 and 1250 oC for 100 h.
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Fig.4 Surface morphologies of Nb-Si based alloy after oxidation at 1200 and 1250 oC for 100 h: (a)
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B1 alloy, 1200 oC; (b) B1 alloy, 1250 oC; (c) B2 alloy, 1200 oC; (d) B2 alloy, 1250 oC. Fig.5 Cross-sectional microstructure of Nb-Si based alloy after oxidation at 1200 and 1250 oC for
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100 h: (a) B1 alloy, 1200 oC; (b) B1 alloy, 1250 oC; (c) B2 alloy, 1200 oC; (d) B2 alloy, 1250 oC. Fig.6 Elemental X-ray mappings of B2 alloy after oxidation at 1250 oC for 100 h obtained using
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WDS attached on EPMA (a) Cross-sectional microstructure; (b) O mapping; (c) Nb mapping; (d) Si mapping; (e) Ti mapping; (f) Hf mapping; (g) Al mapping; (h) Cr mapping.
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Fig.7 XRD patterns of oxides obtained after oxidation at 1200 and 1250 oC for 1 h.
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Fig.8 Surface morphologies of Nb-Si based alloy after oxidation at 1200 and 1250 oC for 1 h: (a) B1 alloy, 1200 oC; (b) B1 alloy, 1250 oC; (c) B2 alloy, 1200 oC; (d) B2 alloy, 1250 oC. Fig.9 Cross-sectional microstructure of Nb-Si based alloy after oxidation at 1200 and 1250 oC for
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1 h: (a) B1 alloy, 1200 oC; (b) B1 alloy, 1250 oC; (c) B2 alloy, 1200 oC; (d) B2 alloy, 1250 oC. Fig. 10 Schematic illustration of oxidation process of Nb-Si based alloys:(a, b, c) B1 alloy;
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Fig.1 (a) Microstructure of B1 alloy; (b) Microstructure of B2 alloy; (c) XRD patterns of the specimens.
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Fig.2 (a) Weight gain versus time for B1 and B2 alloys oxidized in air; and (b) representation of the weight gain versus the square root of time for B2 alloy oxidized in air.
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Fig.3 XRD patterns of oxides obtained after oxidation at 1200 and 1250oC for 100 h.
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Fig. 4 Surface morphologies of Nb-Si based alloy after oxidation at 1200 and 1250 oC for 100 h:
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Fig. 5 Cross-sectional microstructure of Nb-Si based alloy after oxidation at 1200 and 1250 oC for
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Fig.6 Elemental X-ray mappings of B2 alloy after oxidation at 1250 oC for 100 h obtained using WDS attached on EPMA (a) Cross-sectional microstructure; (b) O mapping; (c) Nb mapping; (d) Si mapping; (e) Ti mapping; (f) Hf mapping; (g) Al mapping; (h) Cr mapping.
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Fig.7 XRD patterns of oxides obtained after oxidation at 1200 and 1250 oC for 1 h.
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Fig.8 Surface morphologies of Nb-Si based alloy after oxidation at 1200 and 1250 oC for 1 h: (a) B1 alloy, 1200 oC; (b) B1 alloy, 1250 oC; (c) B2 alloy, 1200 oC; (d) B2 alloy, 1250 oC.
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Fig. 9 Cross-sectional microstructure of Nb-Si based alloy after oxidation at 1200 and 1250oC for
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Fig. 10 Schematic illustration of oxidation process of Nb-Si based alloys:
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Table 1 Volume fraction of constituent phases in B1 and B2 alloys (%).
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ACCEPTED MANUSCRIPT Table 1 Volume fraction of constituent phases in B1 and B2 alloys (%) Eutetic （Nb5Si3+NbSS）
Nb5Si3
Cr2Nb
B1
70.7
20.5
8.8
B2
44.5
34.8
9.7
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Sepecimen
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ACCEPTED MANUSCRIPT Highlights
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1. Oxidation behavior of Nb-24Ti-16Si-2Hf-6Al-17Cr (at.%) followed a parabolic law. 2. A compact oxide scale consisting of CrNbO4, TiNb2O7 and SiO2 was formed. 3. The oxidation processes of Nb-Si-Ti-Hf-Al-Cr alloys were discussed.
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