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Eutectics Me 5 Si 3-Mesi 2 in a Triple System Mo-W-Si

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Eutectics Me 5 Si 3-Mesi 2 in a Triple System Mo-W-Si 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-melting point eutectics are of great interest for different high temperature applications: production of composite materials with silicon carbide 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 molybdenum 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 15'" International Plansee Seminar, Eds. G. Kneringer, P. Rodhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 1 [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 15" International Plansee Seminar, Eds. G. Kneringer, P. Rbdhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 1 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?<C -(si) 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. 15'" International Plansee Seminar, Eds. G. Kneringer, P. Rodhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 1 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 %.
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