A study on reaction kinetics and development of silicide coatings on Nb–1Zr–0.1C alloy by molten salt technique

Abstract

In this study a molten salt technique has been used to produce silicide coating on Nb–1Zr–0.1C alloy using a NaCl–KCl–NaF–Na₂SiF₆–Si melt. Molten baths having different concentrations of Si in the metallic state produced silicide coating whereas those molten baths which were devoid of metallic Si could not produce any coating. Structural and chemical characterization of the coated samples by scanning electron microscopy (SEM) and X-ray diffraction (XRD) have shown that the uniform coating had NbSi₂ as a major phase. Depending upon the composition of the salts, post experiment examination by X-ray photoelectron spectroscopy (XPS) technique and ion chromatography revealed the presence of varying relative concentrations of various states of silicon, F⁻ and the etched Nb. Based on the analysis of these results it was elucidated that the presence of Si²⁺ is a prerequisite for silicide coating on the Nb alloy. A mechanism of silicide coating formation on Nb alloy by the molten salt technique has been proposed in the present study.

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Introduction

The Nb–1Zr–0.1C alloy is proposed as a candidate material for structural applications in the next generation high temperature reactors owing to its good metallurgical properties, such as high temperature strength and high melting temperature.^1–6

This alloy, however, exhibits poor oxidation resistance at elevated temperatures (T > 673 K) which limits its application.⁷ Therefore, to use this material for high temperature applications it is required to protect it against oxygen invasion. For this purpose, two common approaches taken are either alloying or applying a coating on the surface which restricts direct contact of oxygen with the substrate.⁸ Generally, in the case of Nb alloys, most of the alloying elements which enhance their oxidation resistance, reduce the melting point of the alloy and thereby adversely affect the high temperature properties of the alloy.⁹ The approach of providing oxidation resistance by surface coating has its own merit. For example, surface coating does not alter the mechanical and thermal properties of the substrate. In this regard, silicide coating stands out among all other coatings, as it has nearly the same thermal expansion coefficient value as Nb which suffices the material property to undergo thermal cycling. Also, silicide coating forms an impervious oxidation resistive layer of oxides of Si. The layer of Si oxides generally exhibits good self healing characteristics.⁷ These qualities make silicide coating an attractive choice for surface protection for Nb alloys.

Various coating techniques, like, pack cementation, dip coating and molten salt technique, have been widely utilised to provide coating on refractory metals.^8–12 Pack cementation technique is an in situ chemical vapour deposition technique based on depositing Si from a halide gas. Yoon et al.,¹⁰ Li et al.¹¹ and Vishwanadh et al.¹² have used pack cementation technique to produce a layer of NbSi₂ on Nb substrate. However, this coating process requires high operable temperatures (1273–1473 K) and thus limits its application. In addition, difficulty in scaling up of the technology is another major disadvantage of this technique. Dip coating technique generally used for aluminization process, is practically not feasible to provide a silicide coating on metals. The high operational temperature and uncontrolled reaction kinetics owing to the high melting temperature of Si (∼1691 K) makes this technique non-attractive. On the other hand, molten salt technique has been adopted for silicide coatings due to its simplicity in construction and ease of operation. The advantages offered by molten salt technique, such as, low temperature operation and shorter processing time than pack cementation method, makes it a promising technique.

Suzuki et al.¹³ and Gay et al.¹⁴ have applied molten salt technique for siliconizing Mo and Ni respectively. Suzuki et al.¹⁵ have interpreted the formation of silicide coating in terms of disproportional reaction between Si⁴⁺ and Si leading to the formation of siliconized layer on metallic substrate. However, the adverse effect of F⁻ ions and the role of each species formed by subsequent reactions in the bath are not well established in the literature.

In the present study, molten salt technique has been used to provide silicide coating on the Nb–1Zr–0.1C alloy. Fig. 1 schematically represents a typical molten salt bath considered for silicide coating. In most of the cases, for silicide coating, eutectic composition of NaCl–KCl (50 : 50)¹⁶ as supporting salt, Si powder as silicon source and alkali halide, such as NaF, to produce silicon halide species (SiF_x) in molten bath are used.

Fig. 1 Schematic representation of formation and deposition of Si²⁺ ions in molten salt for silicide coating on Nb alloy substrate.¹⁵

One of the main aims of this paper is to understand mechanism of silicide coating on the Nb–1Zr–0.1C alloy. For this purpose, a systematic study has been carried out to identify active species involved in the coating. To decipher the role of various species in the formation of coating mechanism, set of experiments have been carried out to determine the diffusing species of Si. XPS and chemical analysis of various salts subsequent to the experiments and detailed characterization of coating formed on the Nb alloy have been carried out.

Experimental

For coating experiments annealed samples of Nb–1Zr–0.1C alloy (composition given in Table 1) were used. In order to prepare samples for coating, specimens were sliced in form of 10 × 8 × 3 mm³ blocks. Surfaces of these samples were mechanically polished and final mirror finished surfaces were obtained using colloidal silica. Later samples were ultrasonically cleaned using an aqueous solution of H₂O₂ and H₂O.

Table 1 Chemical composition of as-solidified Nb alloy

Zr (wt%)	C (wt%)	Impurities	Nb (wt%)
0.9–1.2	0.1–0.13	H – 4 ppm	Balance
		N – 41 ppm
		O – 132 ppm

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Eutectic salt mixtures comprising NaCl–KCl–NaF–Na₂SiF₆–silicon powder were used for silicide coating. Two sets of experiments have been carried out for the same. Initially, one set of experiments were carried out to study the thickness variation of Nb–Si coating as a function of deposition time and temperature. It was found out that the optimum time and temperature to produce maximum thickness (∼12 μm) of silicide coating onto Nb alloy was 4 h at 1073 K (800 °C). Details of these experiments can be found in literature.¹⁷

For the purpose of identifying the diffusing species out of various Si sources, three sets of experiments with different salt compositions, S1, S2, S3 (given in Table 2) were carried out at 1073 K (800 °C) for a time period of 4 h. Al₂O₃ crucible under argon atmosphere has been used to melt these salt mixtures. The temperature of the furnace was gradually increased to 1073 K (800 °C) with a heating rate of ∼3 K min⁻¹ and furnace was kept at this temperature for half an hour for the stabilization of the temperature. Samples of the Nb alloy were suspended and dipped in molten salts with the help of Nb wires. After removing from the bath, coated samples were cooled in air. To remove the adhered salt, samples were ultrasonically cleaned in water.

Table 2 Composition of molten salts used in the study

Salt	Composition (mol%)
	NaCl	KCl	NaF	Na2SiF6	Si
S1	36.58	36.58	21.95	4.89	—
S2	30.05	30.05	18.05	—	21.85
S3	28.58	28.58	17.15	3.84	21.85

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The surface morphologies of the coated samples were examined under optical microscope. X-ray diffraction (XRD) patterns taken from the surface of the samples were analyzed to identify different Nb–Si phases formed. Also, Rietveld analysis of the XRD data was carried out to determine the volume percentage of the phases present in the coated samples (Table 3). The cross-section specimens were prepared from the coated samples. Cross sectional microstructures were examined under field emission scanning electron microscope (FESEM) equipped with energy dispersive X-ray (EDS) analyzer.

Table 3 Volume percentage of phases present in samples dipped in various salts estimated by Rietveld analysis

Salt	% Nb present	% NbSi2 present
S1	100	—
S2	12.3	87.7
S3	4.3	95.7

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X-ray photoelectron spectroscopy (XPS) measurements were carried out on a self-supporting sample wafer (8 mm diameter, ∼1 mm thick) adhered to a double sided carbon tape employing ESCA and AES system (SPECS Surface Nano Analysis GmbH, Germany), using Al K_α X-ray source (1486.74 eV) operating at a voltage of 13.85 kV (385 W) and PHOIBOS 150 hemispherical energy analyser and at pass energy of 50 eV.

As an internal reference for the absolute binding energy, the C-1s peak (284.5 eV) was used. In addition, ion chromatography was used to determine the concentration of F⁻ ions and Nb in the molten salt after the experiments (Table 4).

Table 4 Nb and F⁻ ion concentration in various molten salts estimated by ion chromatography

Salt	F^− ion (mg g^−1)	Nb (μg g^−1)	SiO2 (mg g^−1)	Ratio of F^−/SiO2
S1	287	2367	100	2.87
S2	51.1	49.4	40	1.27
S3	86	17.9	160	0.54

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Thermochemical calculation software, FactSage 7.0, was used to examine the feasibility and applicability of the all possible reactions involving during coating at 1073 K (800 °C). Due to unavailability of thermodynamic data of some of the metastable species, thermodynamic analysis of some of the reactions involving these species could not be completed.

Results and discussion

Fig. 2 shows surfaces of samples subjected to silicide coating using various salt compositions. For the purpose of comparison, image of bare Nb alloy sample is also included (Fig. 2(a)). Surface morphologies of the samples which were dipped in S1 salt showed formation of large number of pits (indicated by white arrows) on the surface (Fig. 2(b)) whereas the samples dipped in S2 and S3 showed presence of relatively smooth coating surfaces (Fig. 2(c) & (d)). Typical diameters of pits were close to 50 μm. It may be noted that S1 salt does not contain any metallic Si. Therefore, etching effect of F⁻ ions on the specimen is more pronounced in this case. In fact, observation of material loss of ∼89.59 g m⁻² (Table 5) in samples dipped in salt S1 is a clear indicative of the etching effect of F⁻ ions. It is worth noting here that the role of F⁻ ion is essential to remove the inherent oxide layer adhered to Nb alloy so that fresh surface of Nb is exposed to Si ions to form coating on the surface. Ion chromatographic analysis of post experiment salts showed highest concentration of Nb in the salt S1 (Table 4). Source of such high concentration of Nb in the salt could be large number of pits on the sample of the Nb-alloy. Detailed investigations on the surface of samples dipped in salt S1 failed to show the presence of Si coating. Therefore, it was concluded that Na₂SiF₆ as a source of Si was not able to provide coating on the surface.

Fig. 2 Optical images showing surface morphology of Si coated Nb alloy produced when samples were dipped in salt (a) bare Nb alloy (b) S1, (c) S2 (white arrows show location of pits), (d) S3.

Table 5 Summary of the thickness of the coating phases present and weight gain in various salts

Salt	Characterization technique			Weight gain per unit area (ΔM) g m^−2 of the dipped samples
	Optical	XRD	EDS/SEM
S1	Larger pits	Nb	No coating	−89.59
S2	Smaller and lesser pits	NbSi2	∼7 μm	20.23
S3	Smaller and lesser pits	NbSi2	∼12 μm	28.9

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Such large pits were not observed on the samples dipped in salts, S2 and S3. In place, entire surfaces were covered with Si coating in these samples. It may be noted that in both the molten salts (S2 and S3) Si was present in the metallic form. The surface morphology of the coating obtained from molten salt S3 is smoother in comparison to the coating surfaces of the samples obtained from salt S2 (compare Fig. 2(c) & (d)). In the case of samples dipped in salts S2, isolated pits (∼15 μm) were observed (Fig. 2(c)) whereas for the samples dipped in salt S3, the size of the pits as well as their number density has considerably reduced (Fig. 2(d)). In order to examine the thickness of the coating, cross-section samples were examined under SEM (Fig. 3(a)–(c)). No coating was observed in the case of samples dipped in salt S1 (Fig. 3(a)). This observation is consistence with the results reported in the literature.¹⁸ The coating thickness of ∼7 μm was observed when the samples were dipped into salt (S2) comprising pure Si powder as a Si source. In comparison, coating thickness was found to increase to ∼12 μm when the samples were dipped into salt (S3) comprising pure Si powder and Na₂SiF₆ in the supporting salt.

Fig. 3 SEM micrographs of cross sectional view of Nb–Si coating of samples dipped in salt (a) S1, (b) S2, (c) S3.

In the latter two cases, the composition profile obtained from EDS across the coating thickness showed that Nb : Si atomic ratio is 67 : 33(±0.5), which is close to the ideal ratio of NbSi₂ (Fig. 4(a) & (b)). Therefore, the phase could be assigned as NbSi₂. In order to confirm the presence of this phase, XRD patterns from the coated surfaces of the samples were obtained (Fig. 5(a)–(c)). The XRD patterns obtained from the coated surface of the samples dipped into the salt S1 showed presence of only metallic-Nb, as all high intensity peaks could be indexed in terms of bcc structure (JCPDS no. 35-0789). This observation along with the observation of the cross-section microstructure confirmed that no coating has been developed on samples dipped into salt S1. The XRD profiles of the samples dipped in molten salts S2 and S3, on the other hand, showed the presence of peaks pertaining to NbSi₂ reflections (Fig. 5(b) & (c)) (JCPDS no. 72-1032) and thereby corroborating the EDS results. Rietveld analysis of the XRD data was carried out to determine the volume fraction of the phases present in the samples dipped in various salts. The results obtained are presented in Table 3. Based on this analysis, it is established that NbSi₂ has a hexagonal structure with lattice parameters a = 4.80 Å, c = 6.59 Å and Nb has bcc structure with a = 3.30 Å. These lattice parameters match closely with the values reported in the literature (JCPDS no. 72-1032, JCPDS no. 35-0789).

Fig. 4 Composition profiles of Si coated samples by EDS dipped in salt (a) S2, (b) S3.

Fig. 5 XRD patterns showing formation of NbSi₂ phase formed when samples were dipped into salt (a) S1, (b) S2, (c) S3.

In order to determine the oxidation state of the Si in salts, XPS spectrum was obtained from each salt collected subsequent to coating experiments. Fig. 6(a)–(c) show the Si-2p core level for the molten salt bath with three different salt compositions where XPS peaks are observed at 101.9 eV, 101.25 eV, 100.9 eV for the salts – S1, S2 and S3, respectively. Fig. 7 represents de-convoluted spectrum of Si-2p for the three different samples where Shirley function for the base line correction and the Gaussian function for the deconvolution of the peaks were used. Upon deconvolution Si-2p XPS result from salt S1 could be best fitted with single peak at 102.7 eV. The data from salt S2 sample was fitted into three peaks with value of 101.3, 104.5, 96.9 eV while deconvolution of data from salt S3 gave rise to peaks at 103.1, 100.9 and 97.6 eV respectively. Progressive shift in the position of peak observed in different salts was an indicator of the fact that Si in these salts was present in the different electronic environment.

Fig. 6 Si-2p XPS spectra of the molten salt bath (a) S1, (b) S2, (c) S3.

Fig. 7 De-convoluted figures for the Si-2p XPS spectra of (a) S1, (b) S2, (c) S3.

Fig. 8 shows the core level peaks XPS spectra of F-1s. The binding energy of F-1s obtained from all the three salts was estimated at 684.6 eV which is in agreement with the literature value of F-1s.

Fig. 8 F-1s XPS spectra of the molten salt bath (a) S1, (b) S2, (c) S3.

The major findings of the results could be summarized as:

(i) The molten salt comprising NaCl–KCl–NaF–Na₂SiF₆ without metallic Si (salt-S1) could not produce any silicide coating on the samples, whereas pits were observed on the surface of the samples. Loss of weight was also observed in these samples (Table 5). Peaks in the XRD pattern were indexed in terms of single bcc Nb phase (Fig. 5(a)).

(ii) The salt comprising NaCl–KCl–NaF–Si powder salt and not the Na₂SiF₆ compound (salt S2), where metallic Si powder was the only source of Si, produced a coating of ∼7 μm with weight gain in the sample, ∼20.23 g m⁻². The XRD patterns obtained from the coated surface of the samples showed presence of the NbSi₂ phase (Fig. 5(b)). The surface morphology of the coated sample showed the presence of smaller pits.

(iii) A thick NbSi₂ layer (∼12 μm) with the weight gain of 28.9 g m⁻² was obtained on the samples dipped into the salt (S3) comprising NaCl–KCl–NaF–Na₂SiF₆–Si powders. Addition of two Si sources (Na₂SiF₆–Si powder) ensured high Si concentration in the bath for the same time period. The XRD patterns obtained from the surface of the coated samples showed the presence of the NbSi₂ and Nb phase (Fig. 5(c)). The surface morphology showed the presence of smaller and fewer pits than the samples dipped in salt S1 and S2.

These results are summarized in Table 5.

Post experiment XPS analysis of the salts provided an insight about the different electronic environments in which Si was present in the molten salt bath which could be correlated with the formation of the silicide (NbSi₂) coating. XPS results showed that Si in the salt S1 was present in the Si⁴⁺ state (Table 6). A single peak fitting for the salt S1 sample at 102.7 eV was obtained. It corresponds mainly to Si⁴⁺ state. This value is lower than the value of binding energy of Si⁴⁺ in the standard state, which is 103.4 eV.^19,20 Lowering of the binding energy value could be attributed to the presence of several SiF_x (x = 4, 3, 2) forming due to thermal dissociation of the Na₂SiF₆ compound.²¹ The concentration of each of the SiF_x species is very low, peaks of these species was not observed in XPS spectra but being lower in electronic state, presence of these species shifts Si⁴⁺ peak towards lower binding energy value of 102.7 eV.^19,20 This argument is supported by the fact that the full width at half maximum (FWHM) for the F-1s for the salt S1 possess a value of 3.31 eV which is higher than the values of FWHM of F-1s in others two salts. These values of FWHM are 2.78 eV and 2.66 eV for salts S2 and S3 respectively. Difference in the FWHM in the case of salt S1 indicates that though all fluorine is present in F⁻¹ state yet they are in the vicinity of Si with differential electronic density. This would lead to the formation of a wider FWHM for the salt S1 as compared to that of S2 and S3.

Table 6 Post experiment salt analysis

Sample	Peak position	FWHM	Area	% present	Si state identified
Salt-S1	102.7	3.39	2460	100	Si^4+
Salt-S2	96.9	2.13	5901	13.57	Si^0
	101.3	3.2	36 665.3	84.38	Si^2+
	104.5	1.7	889.9	2.05	Si^4+
Salt-S3	97.6	1.93	1945	1.87	Si^0
	100.9	2.58	94 866	91.36	Si^2+
	103.1	2.25	7021	6.77	Si^4+

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The XPS signal obtained from the analysis of salt S2 could be best fitted into three peaks with value of 104.5, 101.3, 96.9 eV respectively which primarily are for Si⁴⁺, Si²⁺, Si⁰. This is in parity with the earlier reports of Si²⁺ found from the Si-2p peak.^19,20 The salt S3 has three deconvoluted peaks at (103.1, 100.9 and 97.6) eV respectively. Therefore, there is a definite presence of the Si⁴⁺ (103.1 eV), Si²⁺ (100.9) and Si⁰ (97.6) in this salt too. The FWHM, peak area and probable percent of these peaks are described in the Table 6. The maximum value was observed for 100.9 eV peak representing Si²⁺ state. However the negative shift observed in the Si-2p spectra (Fig. 6) hints towards the fact that both the salts, S2 and the S3 possess Si in lower oxidation states as compared to that of S1.

Mechanism for the formation of silicide coating

In the present study it has been observed that whenever the concentration of F⁻ ions increases with respect to the Si ions, etching effect of F⁻ ions on the surface of the substrate as well as on newly formed silicide coatings was realised. For example, samples dipped in S1 salt, where concentration of F⁻ is maximum (Table 4) and ratio of concentration of F⁻ with respect to Si is high, presence of large pits (∼50 μm) (Fig. 2(b)) has been observed. Extending this argument for the case of samples dipped in S3, which is having two sources of Si, namely metallic Si and Na₂SiF₆, as well as high concentration of F⁻, least numbers of pits were observed. Sizes of these pits were also smallest among all the three cases. Samples dipped in S2, where concentration of F⁻ and Si ions is minimum, on the other hand, showed more numbers of pits as compare to samples dipped in S3. In our recent publication¹⁷ we have shown that the thickness of silicide layer attains a maximum which upon extending time period of coating start reducing the thickness. Reason for reduction in thickness with extending time could be explained on the basis of etching effect of F⁻ concentration due to consumption of Si ions as well as Nb layer.¹⁷ This observation is also in agreement with the present findings.

To understand the role of Si present in Na₂SiF₆ and in powder form, three extreme cases were considered; (i) Na₂SiF₆ only (S1), (ii) Si powder only (S2) and (iii) both Si powder and Na₂SiF₆ (S3). It has been observed in present study that Na₂SiF₆ alone could not produce coating (Fig. 3(a)), whereas metallic Si alone could produce silicide coating on the samples (Fig. 3(b)). This observation indicates that metallic Si powder plays an important role in coating. Post experiment XPS analysis of the salts help to understand this difference. In salt S1 most of the Si was present in Si⁴⁺ state whereas in the other two cases it was mostly present in Si²⁺ state. Therefore, it can be inferred that for the silicide coating, formation of Si²⁺ ionic species in the molten bath is a prerequisite. This observation is in agreement with those reported by Suzuki et al.¹⁸

Based on these results, it can be proposed that Na₂SiF₆ and NaF dissociate into several species (eqn (1) and (2)). Dissociation of NaF (eqn (2)) is not spontaneous as the free energy of the reaction is positive (∼675.4 kJ). Therefore, this reaction involves consumption of energy. However, the reaction can be moved forward by the consumption of F⁻ by SiF₄ to produce SiF₆²⁻ (eqn (3)). Metallic Si reacts with NaF to produce SiF₄ (eqn (4)) and reaction further proceeds as presented in eqn (3). Metallic Si and F⁻ ions in the molten bath react with SiF₆²⁻ species to form SiF₆⁴⁻ ions (eqn (5)). Later, SiF₆⁴⁻ species oxidise on the Nb substrate giving free Si which subsequently diffuses into the substrate to form NbSi₂ layer (eqn (6)). Therefore, the silicide coating formation could be attributed to the inward diffusion of the silicon ions deposited by SiF_x species on the surface of the substrate and the counter effect of the F⁻ ions. The coating mechanism based on the observations could be expressed in terms of the following equations

Na₂SiF₆ → 2NaF + SiF₄ (ΔG = −8.78 kJ) (1)

NaF → Na⁺ + F⁻ (ΔG = +675.39 kJ) (2)

SiF₄ + 2F⁻ → SiF₆²⁻ (ΔG = −124.77 kJ) (3)

Si + 4NaF → SiF₄ + 4Na (ΔG = −325.7 kJ) (4)

Si + SiF₆²⁻ + 6F⁻ → 2SiF₆⁴⁻ (5)

2SiF₆⁴⁻ + Nb(surface) → Nb–Si + 6F⁻ + SiF₆²⁻ (6)

The changes in the Gibbs free energy for eqn (1)–(4) have been calculated using thermochemical software, Factsage 7.0. Due to paucity of data, Gibbs free energy change for eqn (5) and (6) could not be estimated. In case of the salt S1, the reactions shown by eqn (1)–(3) are possible. In the absence of any metallic Si, reactions represented in eqn (4) and (5) are not possible. These reactions will be observed only in the salts S2 and S3 where metallic Si was added as a source of Si. Presence of metallic Si in the salts S2 and S3 lead to a disproportional reaction in the molten salt producing metastable species, SiF₆⁴⁻. This species eventually produces free Si on the Nb substrate which reacts with Nb to produce NbSi₂ coating. In contrast, owing to high stability of the SiF₆²⁻ species, coating could not be observed in the case of salt S1.

Conclusion

In the present study, formation of silicide coating on the Nb–1Zr–0.1C alloy using different sources of Si (Na₂SiF₆ and metallic Si) has been studied. For this purpose NaCl–KCl–NaF–salt was used as supporting medium. Main observations from the study could be summarised in the following points:

(i) Na₂SiF₆ as the source of Si could not produce coating on Nb alloys. Metallic Si and the combination of metallic Si and Na₂SiF₆ could produce coating on the Nb-alloy.

(ii) A coating of 12 μm could be produced in the sample dipped in salt containing both Na₂SiF₆ and metallic Si as silicon sources at 1073 K (800 °C) for 4 h.

(iii) XRD analysis of the samples dipped in salts S2 and S3 showed the formation of single phase hexagonal NbSi₂ at 1073 K (800 °C). EDS analysis of the cross sections of these samples also confirmed that the chemistry of the coating was close to the NbSi₂ phase.

(iv) Post experiments XPS analysis of the salts revealed the presence of Si in various electronic states. Salt, containing only Na₂SiF₆ showed the presence of Si in Si⁴⁺ state only. Whereas other salts, containing metallic Si and combination of Na₂SiF₆ and metallic Si, showed presence of Si⁰, Si²⁺ and Si⁴⁺ states of silicon. Post experiment ion chromatographic analysis showed the presence of etched Nb in all the three salts thereby confirming the etching effect of F⁻ ions.

(v) Based on the, microstructural, chemical and thermodynamic analyses it has been proposed that the formation of SiF₆⁴⁻ state is a prerequisite for the producing the layer of silicide coating. Based on this observation a mechanism of silicide coating has been proposed.

Acknowledgements

Authors would like to thank Dr G. K. Dey, Director, Materials Group, BARC for his invaluable support and guidance. The assistance provided by Mr Rakesh Kumar, G&AMD, BARC during the XRD experiments is gratefully acknowledged. The technical inputs provided by Dr A. K. Tripathi, Chemistry Division, BARC, during the preparation of this manuscript are greatly acknowledged.

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