High temperature oxidation behavior of W–Cr–Nb Alloys in the Temperature Range of 800–1200 °C

Int. Journal of Refractory Metals and Hard Materials 38 (2013) 47–59

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High temperature oxidation behavior of W–Cr–Nb Alloys in the Temperature Range of 800–1200 °C Suresh Telu, Rahul Mitra, Shyamal Kumar Pabi ⁎ Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur-721302, India

a r t i c l e

i n f o

Article history: Received 20 August 2012 Accepted 23 December 2012 Keywords: W–Cr alloys W–Cr–Nb alloys Sintering Oxidation resistance

a b s t r a c t A comparative study of high temperature oxidation behavior of the (W1−xCrx)90Nb10 (x=0.3, 0.5 and 0.6) alloys has been carried out at 800 °C, 1000 °C and 1200 °C in static air. Microstructural study of the alloy samples conventionally sintered from the nanocrystalline elemental powders of W–Cr–Nb alloys shows the presence of a mixture of W-solid solution (Wss) and Cr2Nb phases. Comparison of the results of isothermal and cyclic oxidation tests shows the damage to be greater during the latter tests. The oxidation resistance during exposure at 800 °C, 1000 °C and 1200 °C is found to be the highest for the (W0.4Cr0.6)90Nb10 alloy. Formation of WO3 appears to be responsible for poor oxidation resistance of (W0.7Cr0.3)90Nb10 alloy at 1200 °C. On the other hand, growth of continuous Cr2WO6 scale appears to have significant role in protection against oxidation for the other two alloys at 1200 °C. The formation of oxide scales containing Cr2WO6 + NbWO5.5 and Cr2WO6 + Nb2O5.3WO3 is found to be responsible for protection against oxidation at 800 °C and1000 °C, respectively. © 2012 Elsevier Ltd. All rights reserved.

1. Introduction Tungsten (W) is one of the prospective refractory materials for elevated temperature applications in future fusion reactors, defense and space aviation technologies. All these applications require a structural material to operate at temperatures up to 1700 °C or more. In addition to very high melting point (3420 °C), pure W also has impressive values of both hardness (≈9.75 GPa) and elastic modulus (≈407 GPa), as well as low vapor pressure [1]. However, the applications of W and its alloys are somewhat limited at moderate temperatures because of their susceptibility to oxidation accomplished with the formation of volatile WO3 [2–4]. The most accepted method for triggering the formation of a protective oxide scale on the W-based alloys would be addition of elements, which oxidize preferentially and form thermodynamically stable as well as adherent oxide scales. Weissgaerber et al. [5] have identified chromium (Cr), silicon (Si) and yttrium (Y) as suitable alloying elements to be added to W. Initial studies on the W–Cr alloys containing low melting sinter-activators such as palladium (Pd) and nickel (Ni) [6,7] have shown that the oxidation resistance of the W–Cr–Pd alloys is superior to that of the W–Cr–Ni alloys. Furthermore, Lee [8] has reported about mass loss in case of W–46.7Cr–1.1Pd and W–58.8Cr–0.99Pd alloys (in at.%), after a few cycles of exposure at 1000 °C or 1200 °C. However, addition of elements with relatively low melting points such as Pd (1552 °C) and Ni (1453 °C) to the W-based alloys needs to be restricted in order to avoid deterioration of their high temperature capabilities. A recent study in this laboratory has reported about consolidation of W1−xCrx (x= 0.3, 0.5 and 0.6) alloys with relative densities of >96% ⁎ Corresponding author. Tel.: +91 3222 283272; fax: +91 3222 282280. E-mail address: [email protected] (S.K. Pabi). 0263-4368/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijrmhm.2012.12.008

through conventional sintering (pressure less sintering) of nanostructured elemental powder blends at 1700 °C for 5 h [9]. It has also been indicated that the high temperature oxidation resistances of the W–Cr alloys are superior to that of pure W at both 800 °C and 1000 °C. However, the top surface of the oxide scales formed on these alloys has shown the presence of cracks as well as evidence of substantial spallation during cyclic oxidation. In the present investigation, niobium (Nb) has been added as a ternary alloying element to the W–Cr based alloys. In the W–Cr–Nb system, Cr and Nb can form a Cr-rich inter-metallic phase (Cr2Nb) [10], which is known to possess high strength as well as excellent creep and oxidation resistance at elevated temperatures [11]. Hence, the W–Cr–Nb system has the potential to be considered as one of the prospective materials to be developed further for high temperature structural applications. However, very little information is available in the literature about the oxidation behavior of W–Cr–Nb alloys. Hence, the present investigation has been undertaken with the following major objectives: (i) to study high temperature oxidation behavior of (W1−xCrx)90Nb10 (x=0.3, 0.5 and 0.6; compositions are in at.%), and (ii) to analyze the oxidation mechanisms of these alloys, based on experimental evidences and comparison with theoretical predictions.

2. Experimental approach 2.1. Materials and processing The (W1−xCrx)90Nb10 (x= 0.3, 0.5 and 0.6) alloys were consolidated by conventional sintering route in a continuous pusher-type furnace (FHD Furnace Limited, Wexham, UK) at either 1700 °C or 1790 °C for

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5 h in reducing atmosphere of hydrogen (H2) with dew point of −70 °C. Prior to sintering, the elemental powder blends of W, Cr and Nb, each having at least 99.5% purity, were made nanostructured by high energy milling in a Fritsch Pulverisette 5 (Fritsch GmbH, Idar-Oberstein, Germany) planetary ball mill. The phases present in the as-sintered alloys were identified using a high resolution X-ray diffractometer (Philips PW 3040/60, Panalytical B.V., Almelo, Netherlands). Furthermore, the microstructures of the sintered alloys were characterized by scanning electron microscope (SEM) (Zeiss Evo 60, Carl Zeiss NTS GmbH, Oberkochen, Germany), where chemical compositions of the constituent phases were examined using energy dispersive spectroscopy (EDS). The densities of the sintered specimens were evaluated by the Archimedes principle using a microbalance based density measurement kit (ME235P, Sartorius, Hamburg, Germany). The microstructures were observed by SEM, and the images were recorded using both secondary electron (SE) and back scattered electron (BSE) modes. 2.2. Oxidation tests

Fig. 1. XRD patterns (Cokα radiation) obtained from the W–Cr–Nb alloys sintered at 1700 °C for 5 h.

Cyclic oxidation behavior of the as-sintered alloys was studied at 800 °C, 1000 °C and 1200 °C in static air using a muffle furnace (Okay furnace with super Kanthal heating element, Bysakh & Co., Kolkata, India). The sintered cylindrical specimens with approximate dimensions of the order of 10 mm × 3 mm were polished to a 400 grit finish, and ultrasonically cleaned in a bath of acetone prior to loading in the furnace. Each of the specimens was then exposed to the test temperature for a period of 0.5 h, and subsequently cooled to room temperature in static air. This process was repeated for 30 cycles, such that the total time of exposure at each temperature was 15 h. After each cycle, the weight change was measured with an accuracy of ±0.1 mg using a microbalance (CPA 224S, Sartorius, Goettingen, Germany). Furthermore, for the purpose of comparison with the results of cyclic oxidation, the investigated alloys were also exposed isothermally for 15 h (as a single cycle) at all three temperatures. The oxidized specimens were then analyzed using XRD for identification of the phases present in the oxide scales. Subsequently, theoxidized samples were cross-sectioned and analyzed using the SEM and energy dispersive spectroscopy (EDS) (Oxford instruments, AZtech Energy, Oxford shire, UK) for examining the oxide scales formed during the oxidation tests.

1700 °C, those sintered at 1790 °C show the presence of Cr2Nb phase at Wss grain boundaries. From this observation, it is apparent that sintering of the W–Cr–Nb alloys at 1790 °C is promoted by the presence of molten Cr2Nb phase (melting point≈ 1730 °C), and therefore higher densification is achieved on sintering at this temperature compared to that at 1700 °C. More details about nanostructure formation of the W–Cr–Nb elemental powder blends and characterization of these alloys sintered at 1790 °C for 5 h have been reported elsewhere [12]. The significant results from Ref. [12] are summarized as follows (Table 1):

3. Results and discussion

3.2. Oxidation behavior

3.1. Density and pre-oxidation microstructure

Cyclic oxidation behavior of the (W1−xCrx)90Nb10 alloys has been studied in terms of the variation in mass change per unit surface area (Δm/S) with time (t). The results of cyclic oxidation tests involving exposure at 800 °C, 1000 °C and 1200 °C for 15 h (30 cycles) are displayed in Figs. 5, 6 and 7, respectively. The net Δm/S values after exposure for 15 h are presented in Table 2. Also, the results of cyclic oxidation tests carried out for pure W and W–Cr alloys [9] have been included in the figures for the purpose of comparison. Adjacent to these plots, the oxidized surface morphologies of the representative samples exposed for 15 h (30 cycles) are also shown in Figs. 5 to 7. The results of cyclic oxidation study carried out at 800 °C and 1000 °C as shown in Figs. 5(a) and 6(a), respectively, indicate that the Δm/S of the pure W [9] is more than one order of magnitude higher than that of the investigated ternary alloys. This observation suggests that the oxide scales formed on pure W are not protective. More or less similar oxidation behavior has been reported in the past for pure W at these temperatures [3,4]. Furthermore, the Δm/S values obtained in the present study for the W–Cr–Nb ternary alloys are 2 to 9 times lower than that of the corresponding binary W–Cr alloys (W1 − xCrx, where x = 0.3, 0.5 and 0.6) oxidized under similar oxidation conditions [9]. Thus, it appears that the second phase, Cr2Nb in the investigated W–Cr–Nb alloys contributes to superior protection against oxidation at elevated temperatures as compared

Average relative densities measured for the (W1−xCrx)90Nb10 alloy samples sintered at 1700 °C have been found to be in the range of 92.1–93.5% of their theoretical densities. The XRD patterns from the (W1−xCrx)90Nb10 alloy sintered at 1700 °C, as shown in Fig. 1, provide evidence for a dual phase microstructure composed of W-rich solid solution (Wss) and Cr2Nb phase having C15 structure. The SEM (BSE) images depicting the microstructures of the (W1−xCrx)90Nb10 alloy, as shown in Fig. 2(a) through (c), reveal the presence of pores and more or less spheroidal-shaped particles appearing dark gray and dispersed in the W-solid solution (Wss) matrix. The EDS analysis of the dispersed particles indicates the enrichment of both Cr and Nb, as is evident from the spectrum shown in Fig. 2(d), and the X-ray elemental maps presented in Fig. 3(a) through (d). Therefore, based on the results of both XRD and EDS analyses, it appears that these dispersed particles have the composition of Cr2Nb. In contrast to the alloy samples sintered at 1700 °C, those sintered at 1790 °C have shown significantly higher sinter density of 97.5–98.5%. The microstructures of the W–Cr–Nb alloys sintered at 1790 °C are shown in Fig. 4(a) through (c), and the corresponding XRD patterns are shown in Fig. 4(d). Unlike the morphology of the dispersed phase observed in the microstructure of the alloy samples sintered at

(i) The volume fraction of second phase (Cr2Nb) in the microstructure of (W1−xCrx)90Nb10 alloys sintered at 1790 °C is found to increase with increase in Cr content. (ii) The densification of the products sintered at 1790 °C is found to increase as the amount of Cr2Nb in the sintered alloys increases, whereas the grain sizes of the Wss matrix decreases with the increase in the Cr2Nb content. Based on the results of the aforementioned study, the alloy samples sintered at 1790 °C have been chosen for further study of the oxidation behavior because of their higher sintered density compared to that obtained by sintering at 1700 °C.

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Fig. 2. SEM (BSE) images of (a) (W0.7Cr0.3)90Nb10, (b) (W0.5Cr0.5)90Nb10 and (c) (W0.4Cr0.6)90Nb10 alloys after sintering at 1700 °C for 5 h; (d) EDS spectrum of darker phases in subpanel (c).

to pure W or W–Cr alloys, and therefore further investigation is warranted to get an insight into the operating mechanism of oxidation resistance. Based on the results shown in Table 2, the values of Δm/S recorded for the (W0.7Cr0.3)90Nb10 alloy subjected to cyclic exposure at 800 °C for 15 h (30 cycles) is found to be ~ 2 and 3 times higher than those of (W0.5Cr0.5)90Nb10 and (W0.4Cr0.6)90Nb10 alloys, respectively. More or

less similar trends have been observed for cyclic exposure of these alloys at 1000 °C for 15 h (30 cycles), with the net Δm/S being nearly 1/7th of that found for the alloys exposed at 800 °C for similar duration. Among the investigated alloys, the (W0.4Cr0.6)90Nb10 alloy has exhibited the least value of Δm/S at 800 °C and 1000 °C, which suggests that the oxidation resistance of this alloy is superior to those of the other two ternary alloys.

Fig. 3. High magnification SEM (BSE) images of (a) (W0.4Cr0.6)90Nb10 alloy after sintering at 1700 °C for 5 h; the corresponding X-ray elemental maps of (b) W, (c) Cr and (d) Nb.

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Fig. 4. SEM (BSE) images of (a) (W0.7 Cr0.3 ) 90Nb 10 , (b) (W 0.5 Cr0.5) 90 Nb 10 and (c) (W 0.4 Cr0.6) 90 Nb 10 alloys after sintering at 1790 °C for 5 h; (d) the corresponding XRD patterns (Cu kα radiation) [12].

The plots representing the cyclic oxidation behavior at 1200 °C show almost similar mass change characteristics up to ~ 4 h, following which mass loss has been observed in the (W0.7Cr0.3)90Nb10 alloy in contrast to mass gain in the case of other two alloys (Fig. 7). This observation indicates that the oxidation resistance of the (W0.7Cr0.3)90Nb10 alloy is inferior to that of other two ternary alloys studied at temperatures more than 1000 °C. Furthermore, other two ternary alloys have shown > 5 times higher net Δm/S than that of the alloys exposed at 1000 °C for 15 h (30 cycles). The net Δm/S values after isothermal exposure for 15 h at 800 °C, 1000 °C and 1200 °C are shown in Table 2. From these results, it is apparent that the Δm/S values obtained upon exposure under cyclic and isothermal conditions are considerably different from each other. The outcome from these results can be summarized as follows: (i) The net Δm/S values obtained after isothermal exposure for 15 h at 800 °C and 1000 °C are found to be respectively, 5–10 and 15–25 times, lower than that of the alloys examined under cyclic oxidation conditions at identical temperatures. (ii) There is no evidence of mass loss in the investigated ternary alloys during isothermal oxidation at 1200 °C, whereas in case of cyclic oxidation at 1200 °C the alloy with the least Cr content [(W0.7Cr0.3)90Nb10] has shown mass loss. (iii) The difference between the net Δm/S values after 15 h of exposure under cyclic and isothermal conditions at a given

Table 1 Densification, grain size of Wss phase and area % of Cr2Nb phase values of the W–Cr–Nb alloys after sintering at 1790 °C for 5 h. Alloy

Densification, %

Grain size of Wss, nm

Area % of Cr2Nb

(W0.7Cr0.3)90(Nb)10 (W0.5Cr0.5)90(Nb)10 (W0.4Cr0.6)90(Nb)10

97.6 98.2 98.8

10.1 7.62 2.46

7.63 13.5 28.8

temperature is relatively lower for higher amount of Cr or Cr2Nb in the investigated alloy. 3.3. Oxide scale The photographs of the top surface of the test specimens exposed for 15 h (30 cycles) at 800 °C, 1000 °C and 1200 °C are shown in Figs. 5(b)–(d), 6(b)–(d) and 7(b)–(d), respectively. The oxide scales appear as yellowish in all three alloys after 15 h of cyclic exposure at 800 °C and 1000 °C, whereas, those formed at 1200 °C appear to be dark blue. The appearance of the oxide scales on the samples subjected to isothermal exposure at these temperatures has been found to be more or less similar. Furthermore, in the (W0.5Cr0.5)90Nb10 and (W0.4Cr0.6)90Nb10 alloys, the oxide scales formed at all three temperatures are found to be compact and adherent to the alloy surface, whereas similar oxidation tests on W–Cr alloys have shown poor interfacial adhesion due to spallation and crack formation [9]. On the other hand, for the alloy with 30 at.% Cr [(W0.7Cr0.3)90Nb10], the oxide scale formed at 1200 °C is found to contain pores and cracks, which appear to be responsible for its poor oxidation resistance. The differences in the oxide scale morphology can be attributed to the nature of the constituent phases, and this has been discussed in detail in Sections 3.3.1 and 3.3.2, respectively. 3.3.1. Phase identification by XRD The XRD patterns of the oxide scales formed on the investigated W–Cr–Nb alloys after cyclic or isothermal exposure for 15 h at 800 °C, 1000 °C and 1200 °C are shown in Figs. 8, 9 and 10, respectively. The phases identified are presented in Table 3. From these results, it is apparent that the phases present in the oxide scales formed on isothermal and cyclic exposure are more or less identical in nature. However, these phases are different from those found in the oxide scales of the W–Cr alloys, which are reported to be WO3, Cr2O3 and Cr2WO6 as the main oxidation products [9].

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Fig. 5. (a). Plots of Δm/S against t obtained from cyclic oxidation tests carried out in air on pure W, W–Cr and W–Cr–Nb alloys for duration of 15 h (30 cycles) at 800 °C. Outer appearance of oxidized alloys: (b) (W0.7Cr0.3)90Nb10, (c) (W0.5Cr0.5)90Nb10 and (d) (W0.4Cr0.6)90Nb10.

As shown in Figs. 8–10 and Table 3, the XRD patterns from all the alloys exposed at 800 °C reveal the presence of cubic NbWO5.5 (ref ICDD code: 00-040-0508) and tetragonal Cr2WO6 (ref ICDD code: 01-072-2060) phases in their oxide scales. On the other hand, the samples exposed at 1000 °C and 1200 °C show the presence of tetragonal Nb2O5·3WO3 (ref ICDD code: 00-016-0792) in addition to the Cr2WO6 phase in their oxide scales. The intensities of Cr2WO6 peaks are found to be higher in the XRD patterns of higher Cr/Cr2Nb containing alloys or from samples exposed at higher temperatures. Comparison of Figs. 8–10 indicates that the peaks representing Cr2WO6 are the most predominant in the XRD patterns obtained from the oxide scales formed on (W0.5Cr0.5)90Nb10 and (W0.4Cr0.6)90Nb10 alloys exposed at 1200 °C. In contrast, the (W0.7Cr0.3)90Nb10 alloy exposed at 1200 °C shows the presence of peaks corresponding to WO3 in addition to those of Cr2WO6. The volatile nature of WO3 at 1200 °C is most likely responsible for the continuous mass loss observed during cyclic oxidation study of the (W0.7Cr0.3)90Nb10 alloy. 3.3.2. Cross-sectional microstructure The SEM (BSE) images of the oxide scale cross-sections, as shown in Figs. 11 through 13, display the alloy–oxide interfaces in the sample of W–Cr–Nb alloys exposed at different temperatures. The contrast variations between different locations within these BSE images indicate

that there are significant differences in composition within the oxide scale. An SEM (BSE) image depicting the alloy-oxide interface in the (W0.5Cr0.5)90Nb10 alloy after cyclic exposure for 15 h at 800 °C is shown at low magnification in Fig. 11(a), while that of the region marked as S in this image is shown at higher magnification in Fig. 11(b). Examination of Fig. 11(a) and (b) indicates the presence of regions having both brighter and darker contrasts. Furthermore, the EDS spectra from different regions of Fig. 11(b) as shown in Fig. 11(c) and (d) reveal that the brighter regions in Fig. 11(b) are enriched in W and Nb, whereas the darker regions are enriched in W and Cr. Based on the XRD analysis (Fig. 8) and the compositional analysis by EDS in Fig. 11(c) and (d), it may be inferred that NbWO5.5 and Cr2WO6 are the main constituents of brighter and darker contrast regions, respectively. More or less similar morphological features have been identified in the oxide scales formed by exposure at 800 °C on the other two alloys as well. Fig. 12(a) through (c) shows the SEM (BSE) images of the alloy– oxide interfaces of all three investigated alloys subjected to cyclic exposure for 15 h at 1000 °C. Similar to the oxide scales of the samples exposed at 800 °C, the oxide scales formed at 1000 °C have also shown the presence of bright and dark regions having Nb2O5·3WO3 and Cr2WO6 phases, respectively. However, the amount of Cr2WO6 formed

Fig. 6. (a). Plots of Δm/S against t obtained from cyclic oxidation tests carried out in air on pure W, W–Cr and W–Cr–Nb alloys for duration of 15 h (30 cycles) at 1000 °C. Outer appearance of oxidized alloys: (b) (W0.7Cr0.3)90Nb10, (c) (W0.5Cr0.5)90Nb10 and (d) (W0.4Cr0.6)90Nb10.

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Fig. 7. (a). Plots of Δm/S against t obtained from cyclic oxidation tests carried out in air on W–Cr–Nb alloys for duration of 15 h (30 cycles) at 1200 °C. Outer appearance of oxidized alloys: (b) (W0.7Cr0.3)90Nb10, (c) (W0.5Cr0.5)90Nb10 and (d) (W0.4Cr0.6)90Nb10.

in the oxide scales of all three alloys is found to be higher at 1000 °C than that at 800 °C. In agreement with the XRD results in Figs. 8 and 9, the amount of Cr2WO6 in the oxide scale is found to increase with increase in Cr/Cr2Nb content of the alloy. Furthermore, from XRD and EDS analyses, it is presumed that the brighter regions in the oxide scales at this temperature, as shown in Fig. 12(a) through (d), are composed of primarily Nb2O5·3WO3. The SEM (BSE) images depicting the morphologies of the alloy– oxide interfaces of (W0.5Cr0.5)90Nb10 [Fig. 13(a)] and (W0.4Cr0.6)90Nb10 [Fig. 13(b)] alloys subjected to cyclic exposure for 15 h at 1200 °C indicate the formation of a layered structure in each of these oxide scales. The oxide scales are found to consist of an inner layer with almost uniform composition of Cr2WO6 and an outer porous layer containing a mixture of Nb2O5·3WO3 and Cr2WO6. The region marked as S in the outer layer of the oxide scale in Fig. 13(a) is shown at higher magnification in Fig. 14 along with EDS X-ray elemental maps of W, Cr and Nb. Examination of the results in Fig. 14 indicates that the whisker shaped particles with long aspect ratios are Nb2O5·3WO3, whereas the particles appearing distinct with more or less spherical shape and having average diameter of ~0.5 μm have the composition of Cr2WO6. Based on the results of cyclic oxidation study (Section 3.2) and examination of the oxide scale constituents by XRD and SEM analyses, it is evident that there is a transition in the mechanism of oxide scale formation as the temperature of exposure is increased from 800 °C to 1200 °C. Therefore, a thorough evaluation of the oxidation mechanisms requires the analysis of thermodynamic feasibility for the formation of different oxide scale constituents at the selected temperatures of exposure, as discussed in Sections 3.4 and 3.5. Typical SEM (BSE) images depicting the alloy–oxide interface in the (W0.5Cr0.5)90Nb10 alloy after isothermal exposure for 15 h at different temperatures are shown in Fig. 15. On examination of these images, it is apparent that the oxide scales of the alloy samples

Table 2 Total weight change per unit surface area (mg/cm2)of W–Cr–Nb alloys subjected to cyclic (30 cycles of 0.5 h each) or isothermal exposure for 15 h at 800 °C, 1000 °C and 1200 °C. Alloy

800 °C

1000 °C

1200 °C

Cyclic Isothermal Cyclic Isothermal Cyclic (W0.7Cr0.3)90(Nb)10 6.06 (W0.5Cr0.5)90(Nb)10 3.85 (W0.4Cr0.6)90(Nb)10 2.16

0.61 0.49 0.42

45.36 36.96 21.54

1.76 1.52 1.05

Isothermal

244.1 38.03 212.2 22.16 −98.18 21.71

exposed under isothermal conditions possess relatively fewer pores and cracks compared to those formed by cyclic exposure. The thickness of oxide scale of each sample after exposing to 800 °C, 1000 °C or 1200 °C for 15 h under cyclic or isothermal condition has been measured by using image analysis software, and the results obtained by averaging five measurements at different locations of each sample are shown in Table 4. As expected, the estimated oxide scale thickness of the sample subjected to isothermal oxidation is lower than that of the samples exposed under cyclic conditions, which is consistent with the trends of mass change characteristics shown in Section 3.2.

3.4. Probable oxidation reactions and thermodynamic analysis The possible reaction involved in oxidation of the metallic W or the Wss phase is: ð2=3ÞWðsÞ þ O2 ðgÞ→ð2=3ÞWO3 ðsÞ:

ð1Þ

In addition, the atomic Cr and Nb present in the Wss phase diffuses through the alloy, and oxidizes through the Reactions (2) and (3): ð4=3ÞCrðsÞ þ O2 ðgÞ→ð2=3ÞCr2 O3 ðsÞ

ð2Þ

ð4=5ÞNbðsÞ þ O2 ðgÞ→ð2=5ÞNb2 O5 ðsÞ:

ð3Þ

The variation of the free energy of formation with temperatures for Reactions (1) to (3) is shown in Fig. 16. On examination of this figure, it is evident that in the temperature range of 800–1200 °C, the free energy of formation of Nb2O5 is the lowest, indicating that the tendency of Nb to oxidize is higher than that of Cr or W. Thus, it is appropriate to infer that both Cr and Nb are capable of reducing WO3 through the Reactions (4) and (5), whereas the Nb is capable of reducing Cr2O3 through the Reaction (6), and these reactions are shown as follows WO3 ðsÞ þ ð6=5ÞNbðsÞ→ð3=5ÞNb2 O5 ðsÞ þ WðsÞ

ð4Þ

WO3 ðsÞ þ 2CrðsÞ→Cr2 O3 ðsÞ þ WðsÞ

ð5Þ

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a

a

b

b

Fig. 8. XRD patterns (Cukα radiation) obtained from the oxide scales formed on W–Cr–Nb alloys during (a) cyclic and (b) isothermal oxidation tests carried out in air for duration of 15 h at 800 °C.

Cr2 O3 ðsÞ þ ð6=5ÞNbðsÞ→ð3=5ÞNb2 O5 ðsÞ þ 2CrðsÞ

53

Fig. 9. XRD patterns (Cukα radiation) obtained from the oxide scales formed on W–Cr–Nb alloys during (a) cyclic and (b) isothermal oxidation tests carried out in air for duration of 15 h at 1000 °C.

ð6Þ

On the other hand, the possible oxidation reactions involving Cr2Nb are: ð4=11ÞCr2 Nb þ O2 →ð4=11ÞCr2 O3 þ ð2=11ÞNb2 O5

ð7Þ

ð2=3ÞCr2 Nb þ O2 →ð2=3ÞCr2 O3 þ ð2=3ÞNb

ð8Þ

ð4=5ÞCr2 Nb þ O2 →ð2=5ÞNb2 O5 þ ð8=5ÞCr

ð9Þ

As shown in Fig. 16, the free energy change for the Reactions (7)–(9) are substantially lower than that for the Reactions (1)–(3). Among these reactions, Reaction (7) is found to be the most favorable, which suggests the possibility of preferential oxidation of Cr2Nb in the alloy through this reaction.

In addition to the free energy of formation, the available concentration of the oxide forming element at the air–alloy interface, and the equilibrium oxygen partial pressure required for driving the reaction are also expected to influence the kinetics of oxidation process [13]. Thus, the minimum activity (a) of each element required for the formation of its oxide, and the partial pressure of oxygen (PO2) for the Reactions (1) to (3) have been calculated, and are presented in Table 5. For simplicity, two following assumptions have been made for the calculations: (1) aWO3 = aCr2O3 = aNb2O5 = 1, considering these oxides as pure substances; and (2) the partial pressure of oxygen has been considered to be proportional to its volume fraction in air. Analysis of the data in Table 5 indicates that the minimum activity of Cr required for the formation of Cr2O3 is several orders of magnitude lower than that of W and Nb for the formation of WO3 and Nb2O5, respectively. Therefore, Cr2O3 is expected to form preferentially in the oxide scales formed on the W–Cr–Nb alloys. On the other hand, the values of calculated partial pressures of oxygen for Reactions (1) to (3) shows that it is significantly higher for the

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a

Table 3 Oxide layer composition of W–Cr–Nb alloys upon oxidation at 800 °C, 1000 °C and 1200 °C for 15 h. Alloy

Test type

1000 °C

1200 °C

NbWO5.5, Cr2WO6 Isothermal NbWO5.5, Cr2WO6 Cyclic NbWO5.5, Cr2WO6 Isothermal NbWO5.5, Cr2WO6 Cyclic NbWO5.5, Cr2WO6

800 °C

Nb2O5·3WO3, Cr2WO6 Nb2O5·3WO3, Cr2WO6 Nb2O5·3WO3, Cr2WO6 Nb2O5·3WO3, Cr2WO6 Nb2O5·3WO3, Cr2WO6

Isothermal NbWO5.5, Cr2WO6

Nb2O5·3WO3, Cr2WO6

Nb2O5·3WO3, Cr2WO6 Nb2O5·3WO3, Cr2WO6 Nb2O5·3WO3, Cr2WO6 Nb2O5·3WO3, Cr2WO6 WO3, Nb2O5·3WO3, Cr2WO6 Nb2O5·3WO3, Cr2WO6

(W0.7Cr0.3)90(Nb)10 Cyclic

(W0.5Cr0.5)90(Nb)10

(W0.4Cr0.6)90(Nb)10

Cr2 O3 ðsÞ þ WO3 ðsÞ→Cr2 WO6 ðsÞ:

On the other hand, thermodynamic data is not available for NbWO5.5 and Nb2O5.3WO3 compounds. Moreover, any report mentioning about the formation of these compounds by reactions of Nb2O5 + W, WO3 + Nb and Nb + W with oxygen is not available in the existing literature, to the best of our knowledge. However, Fiegel et al. [17] have reported 33% solid solubility of Nb2O5 in WO3. Furthermore, it has been reported that annealing of Nb2O5 and WO3 powder mixtures at 800 °C and 1200 °C has resulted in the formation of cubic NbWO5.5 and tetragonal Nb2O5·3WO3, respectively [18,19]. Thus, the probable reactions involved in the formation of NbWO5.5 and Nb2O5·3WO3 maybe predicted as:

b

Fig. 10. XRD patterns (Cukα radiation) obtained from the oxide scales formed on W–Cr–Nb alloys during (a) cyclic and (b) isothermal oxidation tests carried out in air for duration of 15 h at 1200 °C.

formation of WO3 than that for either Cr2O3 or Nb2O5 at all temperatures. This implies that there is a possibility of formation of Cr and Nb oxides underneath the external layer of WO3. However, the peaks of WO3, Cr2O3 and Nb2O5 have not been observed in the XRD patterns from the oxide scales (Figs. 8 through 10), except in that representing the (W0.7Cr0.3)90Nb10 alloy exposed at 1200 °C, whereas complex oxides including Cr2WO6, NbWO5.5 and Nb2O5.3WO3, have been found to form on the surfaces of the samples exposed at all the test temperatures. These observations suggest that solid-state reactions occur between different binary oxides to form more complex ternary oxide phases. The equilibrium phases in the ternary W–Cr–O system have been investigated in the past, and are well-documented [14–16]. Based on the results of earlier investigation, the possible chemical reactions leading to the formation of Cr2WO6 can be expressed as follows [15]: ð2=3ÞWðsÞ þ ð2=3ÞCr2 O3 ðsÞ þ O2 ðgÞ→ð2=3ÞCr2 WO6 ðsÞ

ð11Þ

ð10Þ

ð1=2ÞNb2 O5 ðsÞ þ WO3 ðsÞ→NbWO5:5 ðsÞ

ð12Þ

Nb2 O5 ðsÞ þ 3WO3 ðsÞ→Nb2 O5 ·3WO3 ðsÞ:

ð13Þ

Therefore, consumption of Nb2O5, Cr2O3 and WO3 in the formation of Cr2WO6, NbWO5.5 and Nb2O5·3WO3 [through Reactions (10) to (13)] is expected to limit the amount of Nb2O5, Cr2O3 and WO3 present in the oxide scales. In addition to this, both WO3 and Cr2O3 are known to form volatile oxide phases through Reactions (14) and (15), respectively, at temperatures more than 1000 °C [20,21], i.e., 3WO3 ðsÞ→ðWO3 Þ3 ðgÞ

ð14Þ

Cr2 O3 ðsÞ þ O2 ðgÞ→CrO3 ðgÞ:

ð15Þ

Thus, it is apparent that the formation and subsequent vaporization of the aforesaid volatile oxides would have significant influence on the oxidation behavior of the present ternary alloys at 1200 °C. For instance, the mass loss due to volatilization of Cr2O3 pellet during oxidation study at 1100 °C and 1200 °C for 20 h has been reported as 0.6 and 2.1 mg, respectively [22].

3.5. Oxidation mechanisms In the (W1 − xCrx)90Nb10 (x = 0.3, 0.5 and 0.6) alloys, simultaneous oxidation of elemental W, Cr and Nb through Reactions (1)–(3), and that of Cr2Nb through Reactions (7)–(9) appears to occur in the initial stages at all three temperatures, which leads to the growth of WO3,

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Fig. 11. SEM (BSE) images of (W0.5Cr0.5)90Nb10 alloy after exposing to 800 °C for 15 h (30 cycles): (a) alloy–oxide interface and (b) high magnification image of the region marked as S in subpanel a); EDS spectra of Fig. 11(b): (c) brighter and (d) darker contrast regions.

Cr2O3 and Nb2O5 at the top of the alloy surface. Furthermore, with increase in the exposure time, preferential formation of Nb2O5 at the alloy–oxide interface is also possible through reduction of WO3 by Nb. The activities of both Nb and Cr at the alloy–oxide interface are decreased continuously as the reaction progresses, and this is expected to trigger the formation of WO3 as well. In other words,

simultaneous formation of all three oxides is preferred at the alloy– oxide interface. By reacting with each other they form complex oxide compounds, such as Cr2WO6, NbWO5.5 and Nb2O5·3WO3, through Reactions (10) to (13). A previous study on the annealing of NbWO5.5 compound at 900 °C for 12 h, has reported the phase transition from cubic to

Fig. 12. SEM (BSE) images of alloy–oxide interface for (a) (W0.7Cr0.3)90Nb10, (b) (W0.5Cr0.5)90Nb10 and (c) (W0.5Cr0.5)90Nb10 alloys oxidized at 1000 °C for 15 h (30 cycles); (d) high magnification image of subpanel (c).

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Fig. 13. SEM (BSE) images of alloy–oxide interface for (a) (W0.7Cr0.3)90Nb10 and (b) (W0.5Cr0.5)90Nb10 alloys oxidized at 1200 °C for 15 h (30 cycles).

tetragonal structure [18]. Therefore, identification of cubic NbWO5.5 in the oxide scales formed at 800 °C and tetragonal Nb2O5·3WO3 in those formed at ≥1000 °C, respectively, through XRD analysis

is thus consistent with the information available in the existing literature. To confirm the occurrence of such phase transformation at temperatures ≥ 1000 °C, the samples of (W0.5Cr0.5)90Nb10 have been exposed in static air for 5 min at both 1000 °C and 1200 °C. The peaks from the phases present in the top surfaces of the oxide scales formed by such short duration exposures at 1000 °C and 1200 °C are displayed in the XRD patterns shown in Fig. 17, and confirm the preferred formation of cubic NbWO5.5 at 1000 °C and Cr2WO6 at 1200 °C. Thus, it appears that the formation of NbWO5.5 is observed primarily during the initial stage of exposure at 1000 °C; and as the oxidation process progresses, this cubic phase is transformed to tetragonal Nb2O5·3WO3. However, the reaction kinetics for the formation of Cr2WO6 is more favorable at temperatures more than 1000 °C. Furthermore, it is noted from the results in Figs. 8 and 9 that the amount of Cr2WO6 in the oxide scale increases with increase in the amount of Cr or Cr2Nb in the present ternary alloys. This observation is expected, because the formation of Cr2WO6 is dependent on the flux of Cr atoms to the alloy–oxide interface, which in turn depends on the sum of two distinct factors, i.e., (i) the concentration of Cr atoms in the investigated alloy, and (ii) the ease of Cr diffusion. It is well-known that the atomic or ionic diffusion rate is much higher through the grain boundaries as compared to that via lattice. Therefore, the sample with finer grain size or with larger density of grain boundaries is expected to provide much higher flux of Cr atoms to the alloy–oxide interface. Therefore, the rate of diffusion of Cr 3+ for formation of Cr2O3 is expected to be higher in the alloy with higher Cr content and finer grain size, which in turn favors higher reaction kinetics for the formation of Cr2WO6 through Reaction (10) or (11). Considering the observation that the oxidation resistance of the investigated W–Cr–Nb ternary alloys at 800 °C and 1000 °C scales with their Cr content, it appears that the formation of the protective scale involves the preferential growth of the Cr2WO6 layer. It may be pointed out that the mass transport process through micro-cracks or porosities result in higher oxidation rate in the initial stages of oxidation [23]. Therefore, the superior oxidation resistance of

Fig. 14. (a) High magnification SEM (BSE) image of the region marked as S in Fig. 13(a) and the corresponding X-ray maps: (b) W, (c) Cr and (d) Nb.

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Fig. 16. Variation of the free energy change for selected reactions with temperature.

Fig. 15. SEM (BSE) images of alloy–oxide interface of (W0.5Cr0.5)90Nb10 alloy after isothermal oxidation at (a) 800 °C, (b) 1000 °C and (c) 1200 °C for 15 h.

the alloy with higher Cr or Cr2Nb content [(W0.4Cr0.6)90Nb10] at 800 °C, 1000 °C and 1200 °C can also be attributed to its higher sintered density compared to that of other two investigated alloys with lower Cr content. As expected, the net mass change observed during cyclic oxidation are substantially higher than the corresponding values for the samples exposed isothermally for identical durations in the temperature range of 800 °C to 1200 °C. Based on the observation of micrographs depicting the alloy–oxide interfaces, as shown in Figs. 11 to 15, it is apparent that the presence of relatively higher amount of pores as Table 4 Oxide scale thicknesses (μm) of W–Cr–Nb alloys upon oxidation at 800 °C, 1000 °C and 1200 °C for 15 h. Alloy

800 °C

1000 °C

Cyclic Isothermal (W0.7Cr0.3)90(Nb)10 96.2 (W0.5Cr0.5)90(Nb)10 128.7 (W0.4Cr0.6)90(Nb)10 168.6

61.4 96.2 116.8

1200 °C

Cyclic Isothermal

Cyclic Isothermal

227.8 352.6 409.1

– 668.9 989.1

149.3 220.2 282.7

467.1 563.6 765.2

well as cracks in the oxide scales of the samples subjected to cyclic exposure are responsible for their higher mass gains compared to those recorded for isothermal oxidation tests. During each thermal cycle, defects in the form of pores or microcracks are formed and accumulated in the oxide scale due to residual stresses generated by thermal expansion mismatch between the oxide scale constituents and the base alloy. Thus, cycles of heating the sample to a given elevated temperature followed by air cooling leads to thermal fatigue owing to the repeated application of thermal mismatch strain, and thereby causing growth of the flaws in the oxide scale by linking of pores or microcracks. Formation and growth of the flaws in the oxide scale enhance the ingress of oxygen, and thereby the rate of oxidation is increased on cyclic exposure. In contrast, such defect density has been found to be much lower in the oxide scales of the isothermally exposed samples. Therefore, it is intuitive that the net mass gain observed during isothermal exposure of the investigated alloys is much less than that found after cyclic oxidation for similar duration, as shown in Table 2. At 1200 °C, the alloy with the lowest Cr content [(W0.7Cr0.3)90Nb10] shows net mass loss on subjecting to cyclic oxidation test for 15 h (Table 2), whereas it shows mass gain in course of isothermal exposure for similar period. This observation is quite different from the results obtained for tests at lower temperatures, at which mass gain has been observed for both isothermal and cyclic exposures. The change in trend on exposure at 1200 °C with respect to that at lower temperatures may be attributed to the instability of WO3 and Cr2O3 at 1200 °C Table 5 Standard free energies (ΔG), equilibrium constants (K,) partial pressure of oxygen (Po2) and activities of W, Cr and Nb for the formation of WO3, Cr2O3 and Nb2O5. Thermodynamic parameter

800 °C

1000 °C

1200 °C

ΔG1 =0.66 ΔGf (WO3) (kJ/mol) K1 PO2 (atm) aW ΔG2 =0.66 ΔGf (Cr2O3) (kJ/mol) K2 PO2 (atm) aCr ΔG3 =0.4 ΔGf (Nb2O5) (kJ/mol) K3 PO2 (atm) aNb

−381.3 2.15 × 1027 3.66 × 10−20 1.04 × 10−40 −568.3 2.73 × 1028 4.65 × 10−27 1.52 × 10−21 −572.6 7.53 × 1027 1.33 × 10−28 1.00 × 10−34

−349.3 4.63 × 1015 2.16 × 10−16 3.29 × 10−23 −536.3 9.82 × 1022 1.02 × 10−22 1.83 × 10−17 −539.5 1.37 × 1022 7.30 × 10−23 1.50 × 10−27

−317.3 5.58 × 1011 1.79 × 10−13 2.49 × 10−17 −504.3 1.30 × 1017 7.69 × 10−18 4.71 × 10−13 −508.7 7.96 × 1018 1.26 × 10−19 1.66 × 10−23

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confirm that having an optimum Cr/W ratio in the (W1 − xCrx)90Nb10 alloys is necessary to restrict the formation of a surplus amount of WO3 in the oxide scale during exposure to temperatures more than 1000 °C. Of course, further studies are required to examine the effect of variation in the Nb concentration on oxidation behavior of ternary W–Cr–Nb alloys. 4. Conclusions

Fig. 17. XRD patterns (Cukα radiation) obtained from the oxide scales formed on the (W0.5Cr0.5)90Nb10 alloy on exposing for 5 min at 1000 °C and 1200 °C.

[20,21]. The higher concentration of W as well as lower volume fraction of the Cr2Nb phase in the (W0.7Cr0.3)90Nb10 alloy causes the formation of a greater amount of WO3 in the oxide scale through the Reaction (1). It has been shown in Section. 3.4 that atomic or ionic W forms complex oxides, such as Cr2WO6 and Nb2O5.3WO3 through the Reactions (10) to (13). However, as the amount of Cr or Cr2Nb in this alloy is less in comparison to that of the other two investigated ternary alloys, the amount of available Cr2O3 or Nb2O5 in the initial stage of oxidation is insufficient for complete conversion of WO3 to Cr2WO6 or Nb2O5·3WO3. This argument could probably be used to explain why the results of XRD analysis have shown the WO3 to be the predominant constituent of the oxide scale formed on the (W0.7Cr0.3)90Nb10 alloy during cyclic oxidation at 1200 °C. Therefore, the residual WO3 most likely vaporizes through Reaction (14) at temperatures more than 1000 °C, which in turn leaves pores or microcracks at the surface of the oxide scale. Formation of these pores or microcracks enhances the ingress of oxygen at oxide–alloy interface, and thereby promotes oxidation of this alloy. It may also be worthwhile to propose that further oxidation of Cr2O3 through Reaction (15) is probably restricted due to its tendency to react with both W and WO3 to form Cr2WO6 [via Reactions (10) and (11)]. Based on the observation that the other two ternary alloys [(W0.5Cr0.5)90Nb10 and (W0.4Cr0.6)90Nb10] do not exhibit any mass loss on exposure at 1200 °C, it is inferred that more than 27 at.% Cr is required in the investigated ternary (W1 −xCrx)90Nb10 alloys to restrict the formation of either WO3 or Cr2O3 during high temperature exposure. 3.6. Impact of this study The results of the present study have shown the oxidation resistance of (W1 − xCrx)90Nb10 alloys to be far superior compared to that of pure W as well as corresponding binary W–Cr alloys. The extent of protection against oxidation in the temperature range of 800– 1200 °C is a function of the concentration of Cr or amount of Cr2Nb in the alloy. The oxidation resistance of the alloy with higher W and lower amount of Cr/Cr2Nb [(W0.7Cr0.3)90Nb10] is prone to continuous mass loss after initial mass gain at 1200 °C, which is attributed to the formation and vaporization of WO3 at this temperature. However, in the other two alloys, formation of volatile oxidation products is restricted due to the formation of other non-volatile as well as stable ternary oxides, such as Cr2WO6 and Nb2O5·3WO3. These observations

The present study reports the results of high temperature oxidation tests carried out on the (W1 − xCrx)90Nb10 (x = 0.3, 0.5 and 0.6) alloys, which were consolidated by sintering of mechanically alloyed nanocrystalline elemental powders. Studies of the microstructures of sintered samples have shown the presence of W solid solution (Wss) phase and Cr2Nb Laves phase after sintering at 1790 °C for 5 h. This study has also shown that the damage due to cyclic oxidation is more severe than that observed on isothermal exposure. For exposure in air at temperatures between 800 °C and 1200 °C, the oxidation resistance of the present ternary alloys is found to scale with their Cr content. At 800 °C, both Wss and Cr2Nb oxidize cooperatively to form a mixture of cubic NbWO5.5 and Cr2WO6. Similar constituents have been identified in the oxide scales formed at 1000 °C; however, phase transformation from cubic NbWO5.5 to tetragonal Nb2O5·3WO3 has been observed. Formation of Cr2WO6 is favored in the alloys, which have higher volume fraction of Cr2Nb and at higher temperatures. At 1200 °C, (W0.5Cr0.5)90Nb10 and (W0.4Cr0.6)90Nb10 alloys have shown formation of a continuous scale comprising Cr2WO6 near the alloy surface. On the other hand, the oxide scales of (W0.7Cr0.3)90Nb10 formed at 1200 °C has shown the presence of a significant amount of WO3, which is non-protective and volatile, thereby contributing to poor oxidation resistance. Acknowledgment The authors are grateful to Dr. T. K. Nandy and Mr. M. Sankarnaryana of the Defence Metallurgical Research Laboratory (DMRL), Hyderabad, India, for extending their sintering facilities for this work. The work is sponsored by the Defence Research Development Organization (DRDO), India, through the project number ERIP/ER/0700335/M/01/1014. References [1] Lassner E, Schubert W-D. Tungsten — properties, chemistry, technology of the element, alloys, and chemical compounds. New York: Kluwer Academic/Plenum Publishers; 1999. p. 7–56 [and 85]. [2] Wriedt HA. The O–W (oxygen–tungsten) system. Bull Alloy Phase Diagr 1989;10: 368–84. [3] Webb WW, Norton JT, Wagner C. Oxidation of tungsten. J Electrochem Soc 1956;103:107–11. [4] Gulbransen EA. Thermochemistry and the oxidation of refractory metals at high temperature. Corrosion 1970;26:19–28. [5] Weissgaerber T, Kloeden B, Kieback B. Self-passivating tungsten alloys. Proc. Powder Metallurgy World Congress & Exhibition, 3; 2010. p. 377–83 [PM2010, Florence, Italy]. [6] Itagaki T, Yoda R. Effect of palladium on the oxidation behavior of sintered W–Cr–Pd alloys. J Jpn Inst Met 1974;38:486–92. [7] Itagaki T, Yoda R. Effects of palladium on oxidation behavior of sintered tungsten– chromium–palladium alloys. Trans Natl Res Inst Met 1975;17:163–9. [8] Lee DB. Oxidation of Mo–W–Cr–Pd alloys. J Less Common Met 1990;163:51–62. [9] Telu S, Patra A, Sankaranarayana M, Mitra R, Pabi SK. Microstructure and cyclic oxidation behavior of W–Cr alloys prepared by sintering of mechanically alloyed nanocrystalline powders. Int J Refract Met Hard Mater0263-4368 2012, http: //dx.doi.org/10.1016/j.ijrmhm.2012.08.015. [10] Venkatraman M, Neumann J. The Cr–Nb (chromium–niobium) system. J Phase Equilib 1986;7:462–6. [11] Hong S, Fu CL. Phase stability and elastic moduli of Cr2Nb by first-principles calculations. Intermetallics 1999;7:5–9. [12] Telu S, Mitra SR, Pabi SK. Densification and characterization of W–Cr–Nb alloys prepared by sintering of mechanically alloyed nanocrystalline powders. Powder Metall 2012, http://dx.doi.org/10.1179/1743290112Y.0000000035. [13] Mitra R, Rao V. Elevated-temperature oxidation behavior of titanium silicide and titanium silicide-based alloy and composite. Metall and Mat Trans A 1998;29: 1665–75.

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