AT0100402 420 RM55 B.A. Gnesin et al. 15'" International Plansee Seminar, Eds. G. Kneringer, P. Rddhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 1

Eutectics Me5Si3-MeSi2 in a Triple System Mo-W-Si

B.A. Gnesin, PA Gurjiyants, E.B. Borisenko

Institute of Solid State Physics RAS, Chernogolovka, 142432 Russia

Summary: Refractory metals silicides high- eutectics are of great interest for different high temperature applications: production of composite materials with skeleton, antioxidant protective coatings on carbon materials, brazing of carbon, silicon carbide and refractory metals alloys materials. Phase diagrams Mo-Si and W-Si are compared: diagrams are similar but not in all significant details. Number of possible crystal structures for silicides is at least twice more, than for tungsten and this difference is manifested distinctly for composite samples with different W-Mo ratio after high-temperature tests. In tests of new silicon carbide-refractory metal silicides composites materials (REFSIC) with 10-20 seconds heating time up to 1700°C and 20-40 seconds time of cooling silicides with molybdenum prevalence were not so steady as tungsten based silicides. Experimental data concerning eutectic temperature dependence on W-Mo ratio, X-ray diffraction data, scanning electron and optical microscopy structure investigations results and some properties are discussed.

Keywords: Eutectics, refractory metals silicides, composites, directional solidification, brazing, silicon carbide, X-ray texture and phase analysis

1. Introduction:

Development of a new structural materials for high temperature applications remain one of the most important task for material science during many decades. Intentions to increase an efficiency of engines working due to the energy of burning fuel and to propose materials for various heating devices working in an air atmosphere conditions and many other purposes force to continue difficult and expensive investigations. Refractory metals silicides had involved attention due to their high antioxidant properties above 1000°C already about 100 years ago. One of the most practically important applications is connected with their use as high-temperature electric heaters B.A. Gnesin et al. RM 55 421

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[1-5]. Last decade silicides cause large interest for application as materials for high-temperature antioxidant structural composite matrix materials, including aerospace engineering. Silicon carbide is considered to be the most suitable strengthening phase for such materials. Successful experience of working with silicon carbide - refractory metal silicide materials also has long history [6-8]. Such materials had demonstrated high antioxidant properties and good high-temperature strength up to 1550-1800°C with appreciable stability in thermal shock conditions. Modern time investigators hope to invent new materials [9-11] with high antioxidant properties and high-temperature strength. Among many other well known refractory metal silicides molybdenum disilicide MoSi2 is often considered as the main candidate for such purpose. But it is necessary to note that at temperatures above 1600°C Mo5Si3 can better resist to gaseous corrosion of oxygen than MoSi2 [12,13] and that tungsten silicides are not worse [12] in this respect than molybdenum ones in analogous temperature conditions. Therefore attempts of application in high temperature materials of Mo5Si3 as well as tungsten silicides seems to be pertinent. Actually eutectic E(Mo5Si3 - MoSi2) for a long time is used in antioxidant coatings on refractory metals and alloys and in materials for high-temperature electric heaters. Application of E(Mo5Si3 - MoSi2) for refractory metals brazing is already known [14] also. Eutectic structures obtained with a help of directional solidification for system Mo5Si3-MoSi2 were shown to be essentially modified due to Eu alloying: grain sizes were diminished significantly [15]. Eutectic E(Mo5Si3 - MoSi2) was used for wetting and impregnation of silicon carbide ingots [16] and some materials with encouraging properties were obtained. Eutectics E(Mo5Si3-MoSi2) and E(W5Si3-WSi2) seems to be even more perspective, [17]. This short report represents the trial to discuss some experimental results concerning quasi binary system E(Mo5Si3 - MoSi2) —E(W5Si3 - WSi2), in which the relative atomic concentration of Mo and W varies within the limits of 0 — 100 %. Information about proposed family of composite materials REFSIC, [17], produced with the help of such eutectics is discussed also.

2. Materials and experimental procedure

Samples were obtained in a special device for directed solidification of silicides in an atmosphere of cleaned Ar at 1900-2050°C Melted metals possessed purity of commercial Mo and W powders for traditional applications, used silicon was of semi-conductor technology purity. 422 RM 55 B.A. Gnesin et al.

15lh International Plansee Seminar, Eds. G. Kneringer, P. Rddhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 1

Phase structure investigations and microstructure estimations of the resulting materials had been carried out on samples, prepared with the help of consecutive grinding and polishing by diamond powders and pastes. X-ray diffractometer DRON-3 with MoKa, radiation incident-beam graphite monochromator, step-scan mode with a step size of 0.05° and a counting time 20 s in every point in an interval of double Braggs angle 10-28°, it corresponds to 1.58-4.07 angstrom interval was used. Microstructure was investigated in optical microscope "Neophot-32" (most distinct contrast was shown to be in polarised light) and in scanning electron microscopes DSM-960 and JSM-25S (best contrast in backscattered electrons) Melting temperatures were measured with a help of spectral pyrometer. Error in melting temperature determination was proved on samples with well defined melting temperature and was find not to exceed 10-15° C.

3. Comparison of binary phase diagrams with silicides eutectics E(Mo5Si3- MoSi2) and E(W5Si3- WSi2)

Diagrams for binary systems Mo-Si and W -Si (Fig. 1) looks out quite alike but there are some significant for us distinctions between them: 1) Atomic concentrations of silicon in eutectic point are appreciably different: In a case of Mo- Si binary system — 54 at. % In a case of W- Si binary system — 59.5 at. % 2) Volumes fractions of Me5Si3 and MeSi2 phases at room temperature differs appreciably, much more than atomic contents of silicon differs for eutectic point. It is easy to estimate these volume fractions using densities from JCPDS cards (cards numbers - in brackets, point group, cell parameters and syngony a shown also). 3 • Mo5Si3 (34-371) tetragonal; a=9.6483;c=4.9135; 8.19 g/cm 3 • MoSi2 (41-0612) tetragonal; a=3.2047(2); c=7.8449(8); 6.28 g/cm 3 • W5Si3 (16-261) tetragonal; a=9.601; c=4.972;14.54 g/cm 3 • WSi2 (11-195) tetragonal; a=3.211; c=7.829; 9.88 g/cm

Tetragonal silicides of tungsten and molybdenum are isomorphous with cell dimensions very close to each other. B.A. Gnesin et al. RM55 423

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0 10 20 30 40 50 60 70 80 90 100 Mo Atomic Percent Silicon si a)

2400- !«°C •'' •'

2200- V' aiS3°c ,

y' 2000- \ / /

1800- /

7- 1600- l/ 1400- 1/ |M?

1200- 1 „ t ,. 1, J11 l.., ( , , 0 10 20 30 40 50 60 70 80 90 100 Si Atomic Percent Tungsten w

b) Fig1. Binary phase diagrams for Mo-Si and W -Si systems (a, b respectively at. %, [18]).

The weight contents of silicon in Mo5Si3, eutectic point_E(Mo5Si3- MoSi2) and MoSi2 are, [18], 16, 26 and 37 w. % respectively. So there are 52.4 w. % of Mo5Si3 and 47.6 w. % MoSi2 for eutectic point. Volume fractions are 45.5 v.% for MosSia and 54.5 v.% for MoSJ9. 424 RM 55 B.A. Gnesin et al.

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Similarly, the weight contents of silicon in W5Si3, eutectic point and WSi2 are 8, 18 and 23.5 w. % respectively. So there are 35.5 w. % of W5Si3 and 64.5 w. % WSi2 for eutectic point. Volume fractions are 27.2 v.% for WsSia and 72J5 v.%forWSi?.

3). For tungsten silicides eutectics both W5Si3 and WSi2, according to today data, are daltonide phases, that means ratios of atoms for metal and silicon to be precise constant in temperature intervals of solid phases existence: 5:3 and 1:2, respectively. Only in a case of molybdenum silicide Mo5Si3 there is, [18], an appreciable width (about 3 at. %) for phase concentrations limits. This circumstance is very important for investigations of equilibrium achievement kinetics as well as for diffusion processes studies, in a case of protective film growth during high-temperature gas corrosion, for example. For small width of phase concentration limits, diffusion of Me or Si components through this phase is complicated due to very little possible concentrations gradients within the phase: rate of diffusion is limited mainly by point defects concentration in phases with very narrow concentrations limits.

4). Tungsten silicides W5Si3 and WSi2 are known only as tetragonal phases, otherwise molybdenum disilicide may be hexagonal also, usually above 1900°C or after silicon diffusion. Due to carbon contamination of silicides hexagonal Nowotny phase Mo4.8 i3C0.6 (43-1199) is formed. In early studies this phase was erroneously considered as hexagonal "Mo5Si3-.

4. Experimental results and discussion

With a help X-ray diffraction and local microanalysis it was established that tungsten and molybdenum silicides easily forms solid solution of isomorphous tetragonal phases after crystallisation from melt. Introduction in liquid phase of silicon, molybdenum and tungsten in proper quantities allows to obtain after crystallisation eutectic-like mixtures of silicides-solid solutions of phases for any relative concentration of each metal component - from 0 up to 100 %. As it has become clear after X-ray diffraction experiments (more than 50 samples), molybdenum and tungsten usually substitute each other in isomorphic tetragonal silicides. New phases were not revealed, but sometimes phase content was not established distinctly. The problem concerning distribution of molybdenum and of tungsten between silicides Me5Si3 and MeSi2, including dependence on initial condition of B.A. Gnesin et al. RM55 425 15* International Plansee Seminar, Eds. G. Kneringer, P. Rodhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 1

materials, time and temperature of melt overheating above melting point is not yet clear enough. It is desirable to study it out the frames of present work. To obtain melts containing suitable amounts of Si, Mo and W we have used evaluation procedure described as follows. For the given ratio Mo/W in a quasi binary eutectic E(Mo5Si3- MoSi2) + E(W5Si3- WSi2) we have used linear approximation of expected atomic silicon concentration between extreme points (pure tungsten - 59.5 at. % Si, pure molybdenum - 54 at. % Si), thus determine silicon quantity for an appropriate melt. It was established that melts with such a way determined content are remelted after crystallisation in sufficiently narrow temperature interval about 10°C. Special tests were accomplished for pure tungsten and for pure molybdenum eutectics and it was established almost exact coincidence with temperatures for these eutectics given in [18]. In the Table 1 there are shown data concerning melting points of some «eutectic» melts received in this work for different W-Mo ratios. Table 1 Melting temperatures for quasi binary eutectic E(Mo5Si3- MoSi2) + E(W5Si3- WSi2)

Ratio W-Mo Melting Data from [18], °C (W, at.%) temperature, °C spectral pyrometry 0 1900 1900 33 1920 - 50 1930 - 75 1960 - 83 1975 - 100 2010 2010

As it follows from obtained results, an augmentation of tungsten concentration cause infinite increase of melting temperature: about 30°C for first 50 at. % of tungsten and about 80°C for the next ones. Observed increase of melting point temperature with substitution of molybdenum by tungsten causes some important consequences. Protective antioxidant coatings may be received as multi-layered and brazing and coating may be accomplished in any sequence. There arise the only one condition to be fulfilled: subsequent operation has to be carried out at lower temperature than the previous and without significant overheating. As our practice shows, 426 RM 55 B.A. Gnesin et al,

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the available choice of melting temperatures for quasi eutectics in 1900- 2010°C interval appears to be sufficient for this purpose. In contrast with possible powder technology variant for protective coatings production with a help of diffusion processes, directional solidification of high- temperature silicides eutectics allows to receive very well textured coatings. We have established, that rate of gaseous corrosion is very sensitive to crystallographic orientation of silicides surfaces in protective coatings. In comparison with usual samples, samples with significant volume fraction of tetragonal phases (Mo,W)Si2 and/or MoSi2 and/or WSi2 grains in a case when crystallographic planes {001} were preferably parallel a sample surface had demonstrated in several times weaker corrosion rate. Silicides crystallographic texture was studied with a help of pole figures {002} in characteristic Mo Ka X-ray radiation on texture d iff racto meter. Counter with 4 mm width slit was fixed at 10,2-10,4° double Braggs angle, and this had allowed to register X-rays diffraction from any above mentioned tetragonal disilicides phases. With a help of directional solidification of a protective coating it was possible to obtain it with quite strong crystallographic texture {001} for disilicides. As example pole figure {002} (Schuiz tilt method up to 65°) for such a sample is shown on Fig.2. There is a high peak in a centre of pole figure and it corresponds to the fact that crystallographic planes {001} of tetragonal disilicides are preferably parallel to sample's surface. Intensity of diffraction fells more then 10 times for the planes tilting on 15° to a surface and more than 20 times for the planes tilted up to 25° in comparison with a case of planes {001} parallel to samples surface (it was maximum). Pole figure levels of corrected on defocusing and background corresponds to 0.1 and 0.5 of this maximum. Considered here substitution of molybdenum by tungsten in silicides eutectics cause several experimentally observable and important consequences for producing of composite materials, protective coatings and brazing connections: • number of phases found in such samples at once after producing or after thermocyclic tests is reduced significantly because of reducing of hexagonal disilicides volume fraction and other silicides known only for pure molybdenum or molybdenum with low level of tungsten alloying cases • thermal expansion coefficient is reduced significantly for tungsten containing silicides materials B.A. Gnesin et al. RM55 427

15lh International Plansee Seminar, Eds. G. Kneringer, P. Rodhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 1

phase stability after thermal shock and thermocyclic tests arises with tungsten concentration

& =65° max

Fig.2 Pole figure {002} for REFSIC protective coating on carbon material. It is necessary to note, that in X-ray diffraction phase analysis of above described silicides eutectics serious difficulties are possible. Especially in a case of samples with high molybdenum content after quick cooling tests or samples that were changed chemically during testing and especially in a case of hexagonal Nowotny phase formation due to carbon contamination during production or testing the sample. The experimentally observable lines in X-ray diffraction experiments not always suppose unequivocal interpretation due to a plenty of overlapping in diffraction peaks. Information on contrast observable in scanning electron microscope can be rather useful: silicides Me5Si3 and MeSi2 have well differing ability to scatter electrons due to different electronic density (element contrast). It seems to be rather perspective to use channelling patterns contrast of backscattered electrons for to distinguish tetragonal and hexagonal silicides.

Eutectics of silicides E(Me5Si3 - MeSi2) with molybdenum and tungsten described above are very interesting for development of composite materials 428 RM55 B.A. Gnesin et al.

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containing refractory metals and alloys as well as carbon materials and silicon carbide materials due to following circumstances: • these eutectics are capable to wet alloys of refractory metals, first of all tungsten alloys, carbon and silicon carbide materials with quality sufficient for brazing • these eutectics possesses thermal expansion coefficient (tec) in an interval (4-7) *1O"6 1/degree, that is close to tec for all above mentioned materials and this can stabilise at fast change of temperature conditions • there arises an opportunity to adjust chemical and phase structure, eutectic composition to select most suitable brazing composition for connection of above mentioned materials Last decade in ISSP RAS was developed the new family of composite materials REFSIC= REFractory metal Sllicide+ Silicon Carbide (REFSIC), just patented in Russia [19] and claimed abroad [17]. Among the most important properties making perspective directions for REFSIC application are high corrosion resistance in oxidising environments at temperatures 1100-1900°C, high level of high-temperature strength, high endurance in attitude to thermal shocks. High resistance to abrasive wear is also interesting. Among the prime technological application of REFSIC materials supposed to be their using for high-temperature electric heaters manufacturing (first step - up to 1600-1700°C) in laboratory and industrial furnaces. The basic areas of their applications embrace needs of , refractory materials and porcelain productions. We would like to replace some heaters from molybdenum disilicide or from silicon carbide in already available furnaces. In other areas of engineering the materials REFSIC are suggested for manufacturing of details for devices and tools with high hardness and wear resistance, which can work at high temperatures in oxidising environments and thermal shocks conditions. Thermal expansion coefficient can be controlled in a range of (4-7)*10"6 1/°C. Parts and details made of REFSIC materials are of high stability in size under thermal cycling conditions. In tests of new silicon carbide-refractory metal silicides composites materials (REFSIC) with 10-20 seconds heating time up to 1700°C and 15-20 seconds time of cooling silicides with molybdenum prevalence were not so steady as tungsten based silicides. B.A. Gnesin et al. RM55 429

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Some sort of REFSIC materials proved to be suitable for cutting and polishing with diamond instruments. The materials can be applied both to manufacture massive products and to make high temperature antioxidant protective coating on carbon and/or silicon carbide materials, for instance on C-C composites.

Some properties of materials REFSIC family:

Microhardness at room temperature - 10-32 GPa. Strength at tests in three-dot bending - 80-200 MPa at 20-1400°C. Number of thermal cycles from room temperature up to 1600°C without visually detected destruction of a surface (heating up for 15 sec, cooling up to 200°C for 20 sec) In a supersonic flow of gas1 - up to 25 cycle1s In quiet air (up to 1800 °C) - up to 60 cycles. Wide range of accessible density in this family of materials - from 2 to 14 g/cm3 (low value corresponds to pores containing structures with possible carbon materials comonents) Specific electric resistance - from 10000 to 90000 mkom*mm. The realisation of various kinds of temperature dependence of electric resistance is possible. Table 2 Comparison of some propertied of REFSIC high temperature electric heaters with molybdenum disilicide traditional ones

REFSIC MoSi2 Maximum temperature 1600-1700 <1300 for horizontal heater without support, °C Thermal expansion (4-6) x10'6 8.25x10-6 coefficient Maximum thermal 40 20 specific power, W/cm2 Deformation after 10 <2 mm, ~ 10 mm in vertical thermal cycles up to mainly due deformation - 20 mm in horizontal 1600°C in any direction of a holder direction for 160 mm long heater

1 Experiments in supersonic flow were carried out in Moscow Aviation University (MAI) in common investigation 430 RM55 B.A. Gnesin et al.

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Specific electrical 10 000-90 000 500-1000 resistance at room temperature, mkom*mm Specific electrical 10 000-25 000 3 000-5 000 resistance at 1600°C, mkom*mm Heating -cooling time 40 3000 for1600°C temperature thermal cycles, sec

Some microstructures of REFSIC materials.

Silicon carbon skeleton structures are very effective for high temperature strength maintenance. It is possible to produce silicon carbide structure with different grade of SiC volume fracture, different phase (polytyps) structure and with different level of silicon carbide grains interconnections with each other. Some illustrations (Fig.3) of possible approaches to structure tailoring of REFSIC are demonstrated and discussed here.

a)x1000 Fig.3 a Different silicon carbide skeleton structures in REFSIC materials. B.A. Gnesin et al. RM55 431

15th International Plansee Seminar, Eds. G. Kneringer, P. Rodhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 1

b)x1000

C) x700 Fig.3 b,c. Different silicon carbide skeleton structures in REFSIC materials.

Scanning electron microscope, backscattered electrons, x1000. White phase - Me5Si3 and/or Me5Si3C phase (Nowotny phase), grey phase - MeSi2. Dark 432 RM 55 B.A. Gnesin et al.

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phase is silicon carbide. Well grained particles are of hexagonal polytyps, little smooth - of cubic one. a) silicon carbide component structure and properties may be controlled with a help of adjusting of: • grain size (1-5 mkm for Fig.3b, 10-50 mkm for Fig.3c) • volume fracture • degree of silicon carbide grains interconnections with each other (very little for Fig.3c and high for Fig.3b) • cubic p-silicon carbide (Fig.3b) or other polytypes phase composition (Fig.3a,c)

b) silicides component structure controlling by- • eutectic directional structure (Fig.3a,c) • different phase compositions, for example with Nowotny phase (Fig.3b) It is possible to produce high temperature horizontal electric heater (Fig 4) with very short time of heating -15-25 seconds only.

Fig.4 High-temperature horizontal electric heater during testing in open air. 1600°C. Cross section 3.0*3.0 mm, length 160 mm, ~75W/cm2, 225V, 74 A. B.A. Gnesin et al. RM 55 433

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5. Conclusion 1. Melting temperature dependence on tungsten -molybdenum ratio was determined for quasi binary eutectic system E(Mo5Si3- MoSi2) + E(W5Si3- WoSi2). 2. It was shown that in isomorphic tetragonal silicides Mo5Si3 (34-371) - W5Si3 (16-261) and MoSi2 (41-0612) -WSi2 (11-195) tungsten and molybdenum seems to form solid solution substituting each other in 0-100% concentration limits. 3. Strong crystallographic texture {001}may be obtained for tetragonal disilicides and this texture diminish gaseous corrosion rate significantly. 4. A lot of possible structures including silicon carbide materials, carbon materials, refractory metals and alloys may be obtained with a help of wetting ad/or brazing of such materials by eutectic melts. 5. Some possible advantages and characteristics of REFSIC electric heaters are discussed.

6. References: 1. Sweden, Pat. N° 153961, 26.2.1947, E.H.M. Hagglund, N.G. Rehnquist. 2. USA Pat. N° 2 622 304, 2.10.50, L.W. Coffer. 3. Osterreich Pat. Ns179100, 24.8.1951, R. Kieffer, F. Benesovsky, C. Konopitsky. 4 Sweden Pat. N° 155836, 10.6.1953, UK Pat. Ns 574 170, K.H.J. Medin. 5. Switzerland Pat. N° 333119, 10.6.1953, 8b, N.G.Schrewelius, K.H.J. Medin. 6. USA Pat. Pat., Ns 2 412 373, 27.11.1943, A. R. Wejnard. 7. USA Pat., Ns 3 036 017, N.G. 3.6.1954 N.G. Schrewelius. 8. USA Pat, Ns 3 009 886, 10.9.1958, A. R. Wejnard. 9. USA Pat, Ns 4 927 792, 5.6.1989, J.J. Petrovich, D.H. Carter, F. D. Gac. 10. M.J. Maloney, R.J. Hecht, Development of continuous-fiber-reinforced MoSi2 -base composites, Materials Science and Engineering, V.A155, 1992, p.19-31. 11. R.M. Aikin, Jr., Strengthening of discontinuously reinforced MoSi2 composites at high temperatures, Materials Science and Engineering, V.A155, 1992, p.121-133. 12. G.V. Samsonov, L.A. Dvorina, B.M. Rud, Silicides, Moscow, «Metallurgia», 1979, 272 p. (in russian). 13. A.V. Kasatkin , !.V. Matvienko, Antioxidant properties of silicide protective coating on molybdenum and its alloys, Inorganic materials, 1994, v.30, No7, p. 928-931 (in russian). 434 RM55 B.A. Gnesin et al.

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14. G.B. Cherniack, A.G. Elliot, High-temperature behavior of MoSi2 and Mo5Si3, Journal of The American Society, 1965, v.47, No3, pp. 136- 141. 15. R. Gibala, A.K. Ghosh, D.C. Van Aken et al., Mechanical behavior and interface design of MoSi2 - based alloys and composites, Materials Science and Engineering, V.A155, 1992, p. 147-158 16. B.A. Gnesin, P.A. Gurjiyants B.M. Epelbaum. Mo-Si-C composite ceramic prepared by directional solidification , Inorganic materials, 1998, v.34, No2, p.234-240. 17. International application WO N9PCT/RU/99/00221, 5.7.1999, B.A. Gnesin, P.A. Guriyants(in russian). 18. Ed. Massalski B Binary alloy phase diagram, Metals Park: Am. Soc. Met,1986. v.1,2, 2224 p. 19. Russia Pat. N° 2160790, 7.7.1998, B.A. Gnesin, P.A. Gurjiyants.