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Microstructure and mechanical properties of uncoated Nb-18.7Si and Nb-18.7Si-5Ti alloys and their improved oxidation resistance after application of silicide coating
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ScienceDirect Materials Today: Proceedings 15 (2019) 36–43
www.materialstoday.com/proceedings
FCCM-2018
Microstructure and mechanical properties of uncoated Nb-18.7Si and Nb18.7Si-5Ti alloys and their improved oxidation resistance after application of silicide coating K.Geethasree1,2,3*, Md Zafir Alam2,G. Brahma Raju3, V.V. Satya Prasad1 1 Electroslag Refining Group, Defence Metallurgical Research Laboratory, Hyderabad 500 058 India High Temperature Coatings Group, Defence Metallurgical Research Laboratory, Hyderabad 500 058 India 3 Department of Metallurgical and Materials Engineering, National Institute of Technology, Warangal 506 004 India 2
Abstract The near-eutectic Nb-Si alloys are promising refractory alloys for strategic high temperature applications. In the present study, the microstructure of cast Nb-18.7Si and Nb-18.7-5Ti (in at.%) alloys is examined. The effect of Ti on the compression properties and fracture toughness of the cast binary alloy is evaluated at room temperature. Ti addition caused about 120 MPa increase in strength and improvement in the fracture toughness by ~ 4.5 MPa.m0.5. The oxidation resistance, ascertained in air at 1400°C, is extremely poor for both the alloys. Protective Fe-Cr modified slurry silicide coating prevents extensive dimensional degradation of the alloys during cyclic oxidation for at least 4-5 h. The oxidation resistance of the coating is ascribed to the formation of the oxide layer which spreads on the surface as well as heals the coating cracks. Therefore, the silicide coated near eutectic Nb-Si alloys hold the potential for use in hostile and extreme environments. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of Frontiers in Corrosion Control of Materials, FCCM-2018. Keywords:Nb-Si alloys; Compression; Fracture toughness; Oxidation; Coating
*Corresponding author. Tel.: +91 9490411286 Email address: [email protected] 2214-7853© 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of Frontiers in Corrosion Control of Materials, FCCM-2018.
K.Geethasree et al. / Materials Today: Proceedings 15 (2019) 36–43
37
1. Introduction The development of advanced high-efficiency gas turbine and air-breathing engines has necessitated the research on materials having superior temperature capabilities than that of the conventional Ni base superalloys which are current being used at the wedge of their melting temperatures [1]. In the recent times, the refractory Nb-Si alloys have gained major attention as alternatives to Ni superalloys because of their higher melting temperatures, excellent creep resistance, good high temperature strength, and lower density [1]. The Nb-Si alloys consist of the high strength silicide phase (Nb3Si and/or Nb5Si3) along with the toughening Nbss phase. However, despite their promising high temperature mechanical and thermo-physical properties, these alloys exhibit poor oxidation resistance which limits their practical application [1]. Therefore, efforts have been directed towards improving the inherent oxidation resistance of the Nb-Si alloys by additions of Ti, Al, Cr, Sn, Mo without comprising the high temperature strengh and creep properties [2-6]. Al and Cr additions form a protective oxide layer on the surface, which restricts the diffusion of oxygen and improves the oxidation resistance [2,3]. Addition of Sn promotes the formation of Nb5SiSn2 phase which reduces the internal oxidation of the alloy [4]. Mo addition improves the oxidation resistance by decreasing the anion vacancy which reduces the diffusivity of oxygen anions in Nbss phase [5]. Ti addition enhances the oxidation resistance by the formation of TiO2 scale on the surface as well as improves the creep properties of the alloy [6]. The microstructure, mechanical properties, and oxidation behavior of the alloyed Nb-Si needs to be extensively studied and only a handful of literature is available in the open domain. In the present research, the microstructure of as-cast Nb-18.7Si and Nb-18.7Si-5Ti alloys has been studied. The compression and the fracture toughness properties are evaluated at room temperature (RT). The mechanical properties at RT were crucial from the engineering point of view because the Nb-silicides are known to fail by brittle fracture at low temperatures [6]. The cyclic oxidation resistance of the as-cast alloys has also been examined in air at 1400°C. Future directions on the potential use of the Nb-18.7Si and Nb-18.7Si-5Ti alloys after application of the oxidation resistant Fe, Cr modified silicide coating are also demonstrated. 2.
Experimental details
The alloys of composition Nb-18.7Si and Nb-18.7Si-5Ti (in at.%) were prepared using the non consumable vacuum arc remelting (VAR) of high purity Nb, Si and Ti powders, in a water cooled Cu mould. The melting process was repeated for five times by periodical reversal of the pancake after each melt for better homogenity. The alloy pancake was 8-9mm in thickness and had a diameter of 80mm. For the microstructure analyses, samples of dimensions 5×5×8 (mm) were extracted using wire electro-discharge machining (EDM). For compression testing, cylindrical samples of dimensions: length =7.5 and dia. = 5(mm) were extracted such that the length of the cylinders was along the solidification direction, i.e. along the thickness of the pancake. For the 3 point bend fracture toughness tests, cuboidal samples having dimensions: length = 30, width = 6 and thickness = 3 (mm) were excised by EDM. Subsequently, a notch of depth 3 mm and width 0.15 mm was machined by EDM such that the length of the notch was along the solidification direction. All microstructure characterization of the samples was done using a Philips X-ray diffractometer, Zeiss Supra 55 SEM coupled with EDS, and a CAMECA SX100 EPMA. The compression and fracture toughness tests were carried out on the uncoated samples using BISS 200 kN servo hydraulic testing machine and Instron 5500R universal test machine, respectively. The compression test was carried out an initial strain rate of 0.001 s-1. For the fracture toughness test, the cross head speed was 0.1mm/min and the span length of the samples was 20mm. Microhardness measurements were carried out using a Leica hardness tester. For the coating and oxidation studies, 12 x 10 x 2 mm samples were used. The coating formation process invloved acid pickling, application of a slurry comprising of Fe:Cr:Si powders in the ratio of 1:1:3 on all the sample surfaces, and diffusion heat treatment in vacuum at 1400°C for 2h. The details on the coating process can be obtained from Ref. 7. Cylic oxidation testing of the coated and uncoated samples was carried out in air using an automatic thermal cycling furnace. Each cycle comprised of soaking the sample at 1400˚C for 15 min. and cooling
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K.Geethasree et al. / Materials Today: Proceedings 15 (2019) 36–43
outside the furnace for 15min., and the process was repeated. The weight of the samples, i.e. nett weight of the intact sample excluding the oxide scales, was recorded after each oxidation cycle. Subsequently, thermogravimetric plots were generated for assessing the oxidation behavior of the samples.
3.
Results and discussion
3.1.
Microstructure of the cast alloys:
The microstructure of the as-cast Nb-18.7Si alloy consists of two phases, i.e. the Nb solid solution (Nbss) is dispersed in a matrix of Nb3Si (Fig. 1(a)), as reported in earlier studies [6,8]. The Nb3Si phase can be further categorised into the primary and eutectic variants. The primary Nb3Si phase is relatively devoid of the Nbss phase, whereas the eutectic Nb3Si contains a dispersion of fine Nbss, typical of divorced eutectic phase transformation (Fig.
Fine spherical Eutectic mixture Nbss/Nb3Si
Nbss
Nbss Eutectic mixture Nbss/Nb3Si
Primary Nb3Si Fine irregular Eutectic mixture Nbss/Nb3Si Coarse eutectic mixture Nbss/Nb3Si
Primary Nb3Si
(a)
(b) Fig. 1: Microstructure of the as cast alloys (a) Nb-18.7Si and (b) Nb-18.7Si-5Ti. Table 1: EPMA quantitative analysis of the phases in cast alloy Alloy Nb-18.7Si
Nb-18.7Si-5Ti
Phases Nbss Primary Nb3Si Eutectic Nb3Si Nbss Primary Nb3Si Eutectic Nb3Si
Nb (at.%) 98.3 ±0.14 75.3 ±0.17 75.6 ±0.20 91.07 ±0.60 71.70±0.26 69.73±0.34
Si (at.%) 1.7±0.14 24.7 ±0.17 24.4 ±0.20 2.98±0.16 24.92 ±0.15 24.90±0.10
Ti (at.%) --5.94 ±0.63 3.33±0.14 4.36±0.29
1(a)). The Nbss phase in the eutectic mixture of Nbss/Nb3Si exhibits fine spherical, fine irregular, and coarse morphologies, as indicated in Fig. 1(a). The formation of the Nb3Si phase is confirmed from the presence of ~75 at.% of Nb and 25 at.% of Si, i.e. 3:1 atomic ratio of Nb:Si (Table 1). Minor amount of Si (~2 at.%) is also present in the Nbss phase (Table 1). The relative amount of the Nbss and the overall Nb3Si is 22 and 78 %, respectively.
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The phase constituents in the as-cast Nb-18.7Si-5Ti alloy (Fig. 1(b)) are similar to that of the binary alloy (Fig. 1(a)). However, the Nbss is relatively coarser and its distribution is more uniform and continuous in the Nb18.7Si-5Ti alloy (Fig. 1(b)) compared to the Nbss phase in the binary alloy (Fig. 1(a)), as reported in other studies [6,9]. From Table 1, it is evident that the Si content in the Nbss (~ 3 at.%) is siginificantly low than that in the Nb3Si phase (~25 at.%); i.e. the Si is primarily present in the silicide phase. On the other hand, Ti is present in both the Nbss and Nb3Si phases in almost equal amounts, i.e. in the range of 4-6 at.% (Table 1). The Ti partitioned in both the Nbss and Nb3Si, and formed the substitutional solid solution in these phases [10].Though the Si content (~25at.%) is similar in the Nb3Si phase in both the Nb-18.7Si and Nb-18.7Si-5Ti alloys, the corresponding Nb content is lower (Table 1) which indicates that Ti replaces the Nb atoms in the Nb3Si phase. Therefore, the Nbss and Nb3Si phases in the Nb-18.7Si-5Ti alloy can be represented as (Nb,Ti)ss and (Nb,Ti)3Si, respectively [10]. 3.2.
Compression and fracture toughness properties:
The RT compression curves for the as cast Nb-18.7Si and Nb-18.7Si-5Ti alloys is shown in Fig. 2(a). Both the alloys fail in a brittle manner. The samples show limited yielding and plastic deformation, and the total strain to failure is ~3%. For the Nb-18.7Si alloy, the average values for the compressive yield strength (CYS) and the ultimate compressive strength (UCS) are 727 and 735 MPa, respectively (Table 2). The average values of CYS and UCS for the Nb-18.7Si-5Ti alloy is 845 and 854 MPa, respectively (Table 2). Therefore, it is evident that Ti addition improves both the CYS and UCS of the Nb-18.7Si alloy by ~120 MPa, respectively. The load (P)-displacement () plots for the 3-pt. bend samples is shown in Fig. 2(b) and the onset of crackpropagation in the notched samples can be ascertaind from pop-in (load drop) in the P- plot (Fig. 2(b)). The detectable pop-in is observed at a strain of ~0.2% for the Nb-18.7Si-5Ti sample, whereas that for the Nb-18.7Si sample occurs at a lower strain of ~0.15%. The maximum failure for the Nb-18.7Si-5Ti alloy is 340 N, which is about 150 N higher than that of the binary Nb-18.7Si alloy. The value of fracture toughness (KQ) can be calculated using the equation [11]: K Q= ቆ
PQ.S
3 ቇ .f( BW ൗ2
aൗ ) eqn. (1) W
The parameters‘PQ’, ‘B’, ‘S’, ‘W’ and ‘a’ denote the fracture load, specimen thickness, span length, specimen width, and crack length, respectively. The value of the geometrical factor f( aൗW ) was taken as 2.7 [11]. As indicated in Table 2, the KQ for the Nb-18.7Si-5Ti is 10.26 MPa.m0.5, which is about 4.5 MPa.m0.5 higher than that of the binary Nb-18.7Si. The above KQ values are similar to that reported for Nb-Si and Nb-Si-Ti alloys [6,9].
(a) (b) Fig. 2: Room temperature plots for (a) compression and (b) 3 pt.-bend tests. The arrows in (b) indicate the onset of detectable load drop/pop-ins.
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On the other hand, the Nbss phase being relatively more continuous and coarser in morphology, provides improved toughening in the Nb-18.7Si-5Ti alloy by effecting in deflection of the deformation cracks and increasing the fracture energy [9]. Table 2: Room temperature compression and fracture toughness properties Alloy composition
CYS (MPa)
UCS (MPa)
Failure strain (%)
Fracture toughness (MPa.m1/2)
Nb-18.7Si Nb-18.7Si-5Ti
727±145 845±185
735±140 854±190
3.3±0.31 3.32±0.41
5.84±0.38 10.26 ± 0.15
The presence of Ti in solid solution in the Nbss and Nb3Si phases also contributes to the improved strength and toughness. The increased strength of the Nb3Si phase in the Nb-18.7Si-5Ti alloy is also reflected from its higher microhardness value of 990 ± 40 VHN as compared to the corresponding value of 930 ± 55 VHN in the binary alloy. Concomitantly, Ti addition is also known to improve the ductility of Nb phase by enhancing dislocation mobility and plastic flow [6,9]. Therefore, the combination of: (i) higher strength of the (Nb,Ti)3Si phase, and (ii) the increased ductility of the (Nb,Ti)ss as well as its coarse-continuous morphology, causes improvement in the strength and fracture toughness in the Nb-18.7Si-5Ti alloy. However, detailed transmission electron microscopy studies are required for assessing the relative inherent effects of the above on the deformation characteristics of the alloy, which will be reported in our future studies. 3.3.
Need for oxidation resistant coating
The uncoated Nb-18.7Si and Nb-18.7Si-5Ti samples get heavily oxidized and undergo extensive dimensional degradation after just 15 min. (1 cycle) of oxidation, as shown inFig. 3(a). Such rapid oxidation of the uncoated alloys is consistent with that reported earlier for a Nb-18Si-24Ti alloy [12]. Therefore, the protective and oxidation resistant Fe-Cr modified silicide coating is applied on the sample. Fig. 4(a) shows the typical cross-section microstructure of the as-deposited coating. The coating is uniform and ~250 µm in thickness. Based on the phase contrast, three layers can be identified in the coating. The outer layer (OL) contains the equiaxed NbSi2 phase. The intermediate layer (IL) contains columnar grains of complex Nb, Fe, Cr-silicide phases. The inner interdiffusion zone (IDZ) contains fine equiaxed grains of NbSi2 and Nb5Si3 phases. The coating also contains fine throughthickness cracks which form due to thermal stresses caused by the mismatch in CTE of the coating and the substrate. The microstructure of the coating is consistent with that reported earlier [13,14]. Uncoated Nb-18.7Si-5Ti alloy Sample
(a) Before oxidation
After oxidation for 15min (1 cycle)
Coated Nb-18.7Si-5Ti alloy
(b) Before oxidation
10 mm After oxidation 465min(30 cycles)
for
(c) Fig.3: (a,b) Photographs for the un-coated and coated Nb-18.7Si-5Ti samples before and after oxidation at 1400°C. (c) Weight change plots for the coated Nb-18.7Si and Nb-18.7Si-5Ti alloys during cyclic oxidation at 1400˚C in air.
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It is evident from Figs. 3 (b) that the coating prevents oxidation and dimensional degradation of the sample even after 450 min. (30 cycles) of oxidation (Fig. 3(c)), which is unlike the rapid dimensional loss of the uncoated Oxide scale OL (NbSi2)
Coating
Coating
OL (NbSi2) IL (Complex silicides of Fe, Cr) Through thickness crack IDZ (Nb5Si3)
IL (Complex silicides of Fe, Cr) Cracks healed IDZ (Nb Si ) with oxide 5
Substrate (a)
3
Substrate
(b) Cracks healed with the oxide
Oxide layer
(c) Fig. 4: (a) Cross section microstructure of the as deposited coating on Nb-18.7Si-5Ti alloy, (b) Cross section microstructure of the oxidized Nb-18.7Si-5Ti alloy after oxidation at 1400˚C for 450min. The magnified view in (c) shows the healing ability of the coating, i.e. the formation of the protective oxide on the surface as well as within the cracks.
Nb2O5 SiO2
(a)
(b)
Fig. 5: (a) Surface morphology and (b) XRDspectrum of the oxide scale formed on the coating surface after oxidation at 1400˚C for 450min.The bright and dark phases in (a) correspond to Nb2O5 and SiO2, respectively.
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sample (Fig. 3(a)). The coated sample shows a parabolic oxidation behavior and the weight change remains positive for ~ 400-435 min. (25-29 cycles), as evident from Fig 3(c). The weight gain in the coated sample can be ascribed to the formation of the protective oxide scale which forms on the surface (Fig. 5(a)) as well as heals the cracks in the coating (Figs. 4(b,c)). The oxide scale contains a mixture of the Nb2O5 and SiO2 phases (Fig. 5(b)) [14]. The oxide scale provides a barrier against the inward diffusion of oxygen, which is also evident from the parabolic oxidation characteristics and weight gain in the oxidation plot (Fig. 3(c)) [14]. Therefore, the protective oxide scale and the coating prevent the direct reaction of oxygen with that of the substrate alloy, and hence, the overall oxidation resistance of the coated alloy is significantly improved (Fig. 3). 5.
Conclusions
The Nb-18.7Si and Nb-18.7Si-5Ti alloys exhibit a two phase microstructure consisting of Nbss and Nb3Si. Ti partitions to both the Nbss and Nb3Si phases. The addition of Ti increases the compression strength and fracture toughness of the alloy at RT. Despite the presence of Ti, which is an oxidation inhibitor, the uncoated Nb-18.7Si-5Ti alloy exhibits extensive oxidation and severe dimensional degradation from the initial stages of oxidation at 1400˚C. On the other hand, the Fe, Cr modified silicide coating provides good oxidation protection and the coated sample retains its dimensional integrity for 6-7 hours and beyond. The oxidation resistance of the coating is ascribed to the formation of the protective surface oxide scale during oxidation. The application of the coating is indispensible for strategic high temperature applications of the near eutectic Nb-18.7Si and Nb-18.7Si-5Ti alloys. Acknowledgements The authors thank the EMG, NNSG and MBG groups of DMRL. They thank DMRL-DRDO for the financial support and Director, DMRL for the permission to publish this research work. References 1.
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. 2. K. S. Chan, Cyclic-Oxidation Resistance of Niobium-Base in situ Composites: Modelling and Experimentation, Oxid. Met., 61(3/4)(2004) 165-194. 3. Linsen Su, Lina Jia, Kaiyong Jiang, Hu Zhang, The oxidation behaviour of high Cr and Al containing Nb-Si-TiHf-Al-Cr alloys at 1200 and 1250°C, International Journal of Refractory Metals & Hard Materials 69 (2017) 131–137. 4. S. Knittel,S. Mathieu, L. Portebois, M. Vilasi, Effect of tin addition on Nb-Si-based in situ composites. Part II: Oxidation behavio r, Intermetallics 47 (2014) 43-52, 5. K. Chattopadhyay, R. Mitra, and K.K. Ray, Non isothermal and isothermal oxidation behaviour of Nb-Si-Mo alloys, Metall. Mater. Trans., 39A (2008) 577-592. 6. N. Sekido, Y. Kimura, S. Miura and Y. Mishima, Solidification process and mechanical behaviour of the Nb/Nb5Si3 two phase alloys in the Nb-Ti-Si system, Mater. Trans., 45 (2004) 3264-71. 7. M.Z. Alam, S. Sarin, M.K. Kumawat, D.K. Das, Microstructure and oxidation behaviour of Fe–Cr–silicide coating on a niobium alloy, Mater. Sci. Technol., 32(18) (2016) 1826-1837. 8. S. Kashyap, C.S. Tiwary, K. Chattopadhyay, Microstructure and mechanical properties of oxidation resistant suction cast Nb–Si–Al alloy, Mater. Sci. Eng. A, 559 (2013) 74–85. 9. N. Sekido, Y. Kimura, S. Miura, F.G. Wei, Yoshinao Mishima, Fracture toughness and high temperature strength of unidirectionally solidified Nb–Si binary and Nb–Ti–Si ternary alloys, J. Alloys Compd., 425 (2006) 223–229. 10. B.P. Bewlay, M.R. Jackson, W.J. Reeder and H.A. Lipsitt, Microstructure and properties of DS in-sity composites of Nb-Ti-Si alloys, Mat. Res. Soc. Symp. Proc., 364 (1995) 943-948.
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11. ASTM E399-90, Standard test methods for plane-strain fracture toughness of metallic materials, West Conshohocken, PA, Reprinted from the Annual Book of ASTM Standards. 12. K. Zelenitsas, P. Tsakiropoulos. Effect of Al, Cr and Ta additions on the oxidation behaviour of Nb–Ti–Si in situ composites at 800 ◦C, Mater. Sci. Eng. A, 416 (2006) 269–280. 13. M. K. Kumawat, M.Z. Alam, D.K. Das, Effect of cyclic oxidation on the tensile behavior of a Fe-Cr-Si coated Nb base alloy, Corros. Sci., 131 (2018) 174–186. 14. M.K. Kumawat, M.Z. Alam, A. Kumar, K. Gopinath, S. Saha, V. Singh, V. Srinivas, DK. Das, Tensile behavior of a slurry Fe-Cr-Si coated Nb-alloy evaluated by Gleeble testing, Surf. Coat. Technol., 39 (2018) 695–706.
ScienceDirect Materials Today: Proceedings 15 (2019) 36–43
www.materialstoday.com/proceedings
FCCM-2018
Microstructure and mechanical properties of uncoated Nb-18.7Si and Nb18.7Si-5Ti alloys and their improved oxidation resistance after application of silicide coating K.Geethasree1,2,3*, Md Zafir Alam2,G. Brahma Raju3, V.V. Satya Prasad1 1 Electroslag Refining Group, Defence Metallurgical Research Laboratory, Hyderabad 500 058 India High Temperature Coatings Group, Defence Metallurgical Research Laboratory, Hyderabad 500 058 India 3 Department of Metallurgical and Materials Engineering, National Institute of Technology, Warangal 506 004 India 2
Abstract The near-eutectic Nb-Si alloys are promising refractory alloys for strategic high temperature applications. In the present study, the microstructure of cast Nb-18.7Si and Nb-18.7-5Ti (in at.%) alloys is examined. The effect of Ti on the compression properties and fracture toughness of the cast binary alloy is evaluated at room temperature. Ti addition caused about 120 MPa increase in strength and improvement in the fracture toughness by ~ 4.5 MPa.m0.5. The oxidation resistance, ascertained in air at 1400°C, is extremely poor for both the alloys. Protective Fe-Cr modified slurry silicide coating prevents extensive dimensional degradation of the alloys during cyclic oxidation for at least 4-5 h. The oxidation resistance of the coating is ascribed to the formation of the oxide layer which spreads on the surface as well as heals the coating cracks. Therefore, the silicide coated near eutectic Nb-Si alloys hold the potential for use in hostile and extreme environments. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of Frontiers in Corrosion Control of Materials, FCCM-2018. Keywords:Nb-Si alloys; Compression; Fracture toughness; Oxidation; Coating
*Corresponding author. Tel.: +91 9490411286 Email address: [email protected] 2214-7853© 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of Frontiers in Corrosion Control of Materials, FCCM-2018.
K.Geethasree et al. / Materials Today: Proceedings 15 (2019) 36–43
37
1. Introduction The development of advanced high-efficiency gas turbine and air-breathing engines has necessitated the research on materials having superior temperature capabilities than that of the conventional Ni base superalloys which are current being used at the wedge of their melting temperatures [1]. In the recent times, the refractory Nb-Si alloys have gained major attention as alternatives to Ni superalloys because of their higher melting temperatures, excellent creep resistance, good high temperature strength, and lower density [1]. The Nb-Si alloys consist of the high strength silicide phase (Nb3Si and/or Nb5Si3) along with the toughening Nbss phase. However, despite their promising high temperature mechanical and thermo-physical properties, these alloys exhibit poor oxidation resistance which limits their practical application [1]. Therefore, efforts have been directed towards improving the inherent oxidation resistance of the Nb-Si alloys by additions of Ti, Al, Cr, Sn, Mo without comprising the high temperature strengh and creep properties [2-6]. Al and Cr additions form a protective oxide layer on the surface, which restricts the diffusion of oxygen and improves the oxidation resistance [2,3]. Addition of Sn promotes the formation of Nb5SiSn2 phase which reduces the internal oxidation of the alloy [4]. Mo addition improves the oxidation resistance by decreasing the anion vacancy which reduces the diffusivity of oxygen anions in Nbss phase [5]. Ti addition enhances the oxidation resistance by the formation of TiO2 scale on the surface as well as improves the creep properties of the alloy [6]. The microstructure, mechanical properties, and oxidation behavior of the alloyed Nb-Si needs to be extensively studied and only a handful of literature is available in the open domain. In the present research, the microstructure of as-cast Nb-18.7Si and Nb-18.7Si-5Ti alloys has been studied. The compression and the fracture toughness properties are evaluated at room temperature (RT). The mechanical properties at RT were crucial from the engineering point of view because the Nb-silicides are known to fail by brittle fracture at low temperatures [6]. The cyclic oxidation resistance of the as-cast alloys has also been examined in air at 1400°C. Future directions on the potential use of the Nb-18.7Si and Nb-18.7Si-5Ti alloys after application of the oxidation resistant Fe, Cr modified silicide coating are also demonstrated. 2.
Experimental details
The alloys of composition Nb-18.7Si and Nb-18.7Si-5Ti (in at.%) were prepared using the non consumable vacuum arc remelting (VAR) of high purity Nb, Si and Ti powders, in a water cooled Cu mould. The melting process was repeated for five times by periodical reversal of the pancake after each melt for better homogenity. The alloy pancake was 8-9mm in thickness and had a diameter of 80mm. For the microstructure analyses, samples of dimensions 5×5×8 (mm) were extracted using wire electro-discharge machining (EDM). For compression testing, cylindrical samples of dimensions: length =7.5 and dia. = 5(mm) were extracted such that the length of the cylinders was along the solidification direction, i.e. along the thickness of the pancake. For the 3 point bend fracture toughness tests, cuboidal samples having dimensions: length = 30, width = 6 and thickness = 3 (mm) were excised by EDM. Subsequently, a notch of depth 3 mm and width 0.15 mm was machined by EDM such that the length of the notch was along the solidification direction. All microstructure characterization of the samples was done using a Philips X-ray diffractometer, Zeiss Supra 55 SEM coupled with EDS, and a CAMECA SX100 EPMA. The compression and fracture toughness tests were carried out on the uncoated samples using BISS 200 kN servo hydraulic testing machine and Instron 5500R universal test machine, respectively. The compression test was carried out an initial strain rate of 0.001 s-1. For the fracture toughness test, the cross head speed was 0.1mm/min and the span length of the samples was 20mm. Microhardness measurements were carried out using a Leica hardness tester. For the coating and oxidation studies, 12 x 10 x 2 mm samples were used. The coating formation process invloved acid pickling, application of a slurry comprising of Fe:Cr:Si powders in the ratio of 1:1:3 on all the sample surfaces, and diffusion heat treatment in vacuum at 1400°C for 2h. The details on the coating process can be obtained from Ref. 7. Cylic oxidation testing of the coated and uncoated samples was carried out in air using an automatic thermal cycling furnace. Each cycle comprised of soaking the sample at 1400˚C for 15 min. and cooling
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K.Geethasree et al. / Materials Today: Proceedings 15 (2019) 36–43
outside the furnace for 15min., and the process was repeated. The weight of the samples, i.e. nett weight of the intact sample excluding the oxide scales, was recorded after each oxidation cycle. Subsequently, thermogravimetric plots were generated for assessing the oxidation behavior of the samples.
3.
Results and discussion
3.1.
Microstructure of the cast alloys:
The microstructure of the as-cast Nb-18.7Si alloy consists of two phases, i.e. the Nb solid solution (Nbss) is dispersed in a matrix of Nb3Si (Fig. 1(a)), as reported in earlier studies [6,8]. The Nb3Si phase can be further categorised into the primary and eutectic variants. The primary Nb3Si phase is relatively devoid of the Nbss phase, whereas the eutectic Nb3Si contains a dispersion of fine Nbss, typical of divorced eutectic phase transformation (Fig.
Fine spherical Eutectic mixture Nbss/Nb3Si
Nbss
Nbss Eutectic mixture Nbss/Nb3Si
Primary Nb3Si Fine irregular Eutectic mixture Nbss/Nb3Si Coarse eutectic mixture Nbss/Nb3Si
Primary Nb3Si
(a)
(b) Fig. 1: Microstructure of the as cast alloys (a) Nb-18.7Si and (b) Nb-18.7Si-5Ti. Table 1: EPMA quantitative analysis of the phases in cast alloy Alloy Nb-18.7Si
Nb-18.7Si-5Ti
Phases Nbss Primary Nb3Si Eutectic Nb3Si Nbss Primary Nb3Si Eutectic Nb3Si
Nb (at.%) 98.3 ±0.14 75.3 ±0.17 75.6 ±0.20 91.07 ±0.60 71.70±0.26 69.73±0.34
Si (at.%) 1.7±0.14 24.7 ±0.17 24.4 ±0.20 2.98±0.16 24.92 ±0.15 24.90±0.10
Ti (at.%) --5.94 ±0.63 3.33±0.14 4.36±0.29
1(a)). The Nbss phase in the eutectic mixture of Nbss/Nb3Si exhibits fine spherical, fine irregular, and coarse morphologies, as indicated in Fig. 1(a). The formation of the Nb3Si phase is confirmed from the presence of ~75 at.% of Nb and 25 at.% of Si, i.e. 3:1 atomic ratio of Nb:Si (Table 1). Minor amount of Si (~2 at.%) is also present in the Nbss phase (Table 1). The relative amount of the Nbss and the overall Nb3Si is 22 and 78 %, respectively.
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The phase constituents in the as-cast Nb-18.7Si-5Ti alloy (Fig. 1(b)) are similar to that of the binary alloy (Fig. 1(a)). However, the Nbss is relatively coarser and its distribution is more uniform and continuous in the Nb18.7Si-5Ti alloy (Fig. 1(b)) compared to the Nbss phase in the binary alloy (Fig. 1(a)), as reported in other studies [6,9]. From Table 1, it is evident that the Si content in the Nbss (~ 3 at.%) is siginificantly low than that in the Nb3Si phase (~25 at.%); i.e. the Si is primarily present in the silicide phase. On the other hand, Ti is present in both the Nbss and Nb3Si phases in almost equal amounts, i.e. in the range of 4-6 at.% (Table 1). The Ti partitioned in both the Nbss and Nb3Si, and formed the substitutional solid solution in these phases [10].Though the Si content (~25at.%) is similar in the Nb3Si phase in both the Nb-18.7Si and Nb-18.7Si-5Ti alloys, the corresponding Nb content is lower (Table 1) which indicates that Ti replaces the Nb atoms in the Nb3Si phase. Therefore, the Nbss and Nb3Si phases in the Nb-18.7Si-5Ti alloy can be represented as (Nb,Ti)ss and (Nb,Ti)3Si, respectively [10]. 3.2.
Compression and fracture toughness properties:
The RT compression curves for the as cast Nb-18.7Si and Nb-18.7Si-5Ti alloys is shown in Fig. 2(a). Both the alloys fail in a brittle manner. The samples show limited yielding and plastic deformation, and the total strain to failure is ~3%. For the Nb-18.7Si alloy, the average values for the compressive yield strength (CYS) and the ultimate compressive strength (UCS) are 727 and 735 MPa, respectively (Table 2). The average values of CYS and UCS for the Nb-18.7Si-5Ti alloy is 845 and 854 MPa, respectively (Table 2). Therefore, it is evident that Ti addition improves both the CYS and UCS of the Nb-18.7Si alloy by ~120 MPa, respectively. The load (P)-displacement () plots for the 3-pt. bend samples is shown in Fig. 2(b) and the onset of crackpropagation in the notched samples can be ascertaind from pop-in (load drop) in the P- plot (Fig. 2(b)). The detectable pop-in is observed at a strain of ~0.2% for the Nb-18.7Si-5Ti sample, whereas that for the Nb-18.7Si sample occurs at a lower strain of ~0.15%. The maximum failure for the Nb-18.7Si-5Ti alloy is 340 N, which is about 150 N higher than that of the binary Nb-18.7Si alloy. The value of fracture toughness (KQ) can be calculated using the equation [11]: K Q= ቆ
PQ.S
3 ቇ .f( BW ൗ2
aൗ ) eqn. (1) W
The parameters‘PQ’, ‘B’, ‘S’, ‘W’ and ‘a’ denote the fracture load, specimen thickness, span length, specimen width, and crack length, respectively. The value of the geometrical factor f( aൗW ) was taken as 2.7 [11]. As indicated in Table 2, the KQ for the Nb-18.7Si-5Ti is 10.26 MPa.m0.5, which is about 4.5 MPa.m0.5 higher than that of the binary Nb-18.7Si. The above KQ values are similar to that reported for Nb-Si and Nb-Si-Ti alloys [6,9].
(a) (b) Fig. 2: Room temperature plots for (a) compression and (b) 3 pt.-bend tests. The arrows in (b) indicate the onset of detectable load drop/pop-ins.
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On the other hand, the Nbss phase being relatively more continuous and coarser in morphology, provides improved toughening in the Nb-18.7Si-5Ti alloy by effecting in deflection of the deformation cracks and increasing the fracture energy [9]. Table 2: Room temperature compression and fracture toughness properties Alloy composition
CYS (MPa)
UCS (MPa)
Failure strain (%)
Fracture toughness (MPa.m1/2)
Nb-18.7Si Nb-18.7Si-5Ti
727±145 845±185
735±140 854±190
3.3±0.31 3.32±0.41
5.84±0.38 10.26 ± 0.15
The presence of Ti in solid solution in the Nbss and Nb3Si phases also contributes to the improved strength and toughness. The increased strength of the Nb3Si phase in the Nb-18.7Si-5Ti alloy is also reflected from its higher microhardness value of 990 ± 40 VHN as compared to the corresponding value of 930 ± 55 VHN in the binary alloy. Concomitantly, Ti addition is also known to improve the ductility of Nb phase by enhancing dislocation mobility and plastic flow [6,9]. Therefore, the combination of: (i) higher strength of the (Nb,Ti)3Si phase, and (ii) the increased ductility of the (Nb,Ti)ss as well as its coarse-continuous morphology, causes improvement in the strength and fracture toughness in the Nb-18.7Si-5Ti alloy. However, detailed transmission electron microscopy studies are required for assessing the relative inherent effects of the above on the deformation characteristics of the alloy, which will be reported in our future studies. 3.3.
Need for oxidation resistant coating
The uncoated Nb-18.7Si and Nb-18.7Si-5Ti samples get heavily oxidized and undergo extensive dimensional degradation after just 15 min. (1 cycle) of oxidation, as shown inFig. 3(a). Such rapid oxidation of the uncoated alloys is consistent with that reported earlier for a Nb-18Si-24Ti alloy [12]. Therefore, the protective and oxidation resistant Fe-Cr modified silicide coating is applied on the sample. Fig. 4(a) shows the typical cross-section microstructure of the as-deposited coating. The coating is uniform and ~250 µm in thickness. Based on the phase contrast, three layers can be identified in the coating. The outer layer (OL) contains the equiaxed NbSi2 phase. The intermediate layer (IL) contains columnar grains of complex Nb, Fe, Cr-silicide phases. The inner interdiffusion zone (IDZ) contains fine equiaxed grains of NbSi2 and Nb5Si3 phases. The coating also contains fine throughthickness cracks which form due to thermal stresses caused by the mismatch in CTE of the coating and the substrate. The microstructure of the coating is consistent with that reported earlier [13,14]. Uncoated Nb-18.7Si-5Ti alloy Sample
(a) Before oxidation
After oxidation for 15min (1 cycle)
Coated Nb-18.7Si-5Ti alloy
(b) Before oxidation
10 mm After oxidation 465min(30 cycles)
for
(c) Fig.3: (a,b) Photographs for the un-coated and coated Nb-18.7Si-5Ti samples before and after oxidation at 1400°C. (c) Weight change plots for the coated Nb-18.7Si and Nb-18.7Si-5Ti alloys during cyclic oxidation at 1400˚C in air.
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It is evident from Figs. 3 (b) that the coating prevents oxidation and dimensional degradation of the sample even after 450 min. (30 cycles) of oxidation (Fig. 3(c)), which is unlike the rapid dimensional loss of the uncoated Oxide scale OL (NbSi2)
Coating
Coating
OL (NbSi2) IL (Complex silicides of Fe, Cr) Through thickness crack IDZ (Nb5Si3)
IL (Complex silicides of Fe, Cr) Cracks healed IDZ (Nb Si ) with oxide 5
Substrate (a)
3
Substrate
(b) Cracks healed with the oxide
Oxide layer
(c) Fig. 4: (a) Cross section microstructure of the as deposited coating on Nb-18.7Si-5Ti alloy, (b) Cross section microstructure of the oxidized Nb-18.7Si-5Ti alloy after oxidation at 1400˚C for 450min. The magnified view in (c) shows the healing ability of the coating, i.e. the formation of the protective oxide on the surface as well as within the cracks.
Nb2O5 SiO2
(a)
(b)
Fig. 5: (a) Surface morphology and (b) XRDspectrum of the oxide scale formed on the coating surface after oxidation at 1400˚C for 450min.The bright and dark phases in (a) correspond to Nb2O5 and SiO2, respectively.
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sample (Fig. 3(a)). The coated sample shows a parabolic oxidation behavior and the weight change remains positive for ~ 400-435 min. (25-29 cycles), as evident from Fig 3(c). The weight gain in the coated sample can be ascribed to the formation of the protective oxide scale which forms on the surface (Fig. 5(a)) as well as heals the cracks in the coating (Figs. 4(b,c)). The oxide scale contains a mixture of the Nb2O5 and SiO2 phases (Fig. 5(b)) [14]. The oxide scale provides a barrier against the inward diffusion of oxygen, which is also evident from the parabolic oxidation characteristics and weight gain in the oxidation plot (Fig. 3(c)) [14]. Therefore, the protective oxide scale and the coating prevent the direct reaction of oxygen with that of the substrate alloy, and hence, the overall oxidation resistance of the coated alloy is significantly improved (Fig. 3). 5.
Conclusions
The Nb-18.7Si and Nb-18.7Si-5Ti alloys exhibit a two phase microstructure consisting of Nbss and Nb3Si. Ti partitions to both the Nbss and Nb3Si phases. The addition of Ti increases the compression strength and fracture toughness of the alloy at RT. Despite the presence of Ti, which is an oxidation inhibitor, the uncoated Nb-18.7Si-5Ti alloy exhibits extensive oxidation and severe dimensional degradation from the initial stages of oxidation at 1400˚C. On the other hand, the Fe, Cr modified silicide coating provides good oxidation protection and the coated sample retains its dimensional integrity for 6-7 hours and beyond. The oxidation resistance of the coating is ascribed to the formation of the protective surface oxide scale during oxidation. The application of the coating is indispensible for strategic high temperature applications of the near eutectic Nb-18.7Si and Nb-18.7Si-5Ti alloys. Acknowledgements The authors thank the EMG, NNSG and MBG groups of DMRL. They thank DMRL-DRDO for the financial support and Director, DMRL for the permission to publish this research work. References 1.
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