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10FORMATION OF Mo/Nb/Si/B CAST COMPOSITE BY SHS IN CONDITIONS OF ARTIFICIAL GRAVITY
D. E. Andreev", Yu. S. Vdovin*", and V. I. Yukhvid"
aMerzhanov Institute of Structural Macrokinetics and Materials Science, Russian Academy of Sciences, Chernogolovka, Moscow, 142432 Russia *e-mail: vdovin-us@ism.ac.ru
DOI: 10.24411/9999-0014A-2019-10009
Mo-Si-based composition materials (CMs) have great potential for realization in industry [1]. The introduction of boron into Mo-Nb-Si allows to form dense borosilicate glass, which protect CMs from oxidization [1, 2]. CMs based on Mo/Nb/Si/B and related parts are produced by casting, powder metallurgy, and additive technologies. In this paper, the cast Mo/Nb/Si/B-based composites produced by SHS in conditions of artificial gravity are studied.
In experiments, the combustion rate (u), the material loss during combustion (^1), and the yield of the "metal" phase into ingot (^2) were determined in the experiments:
u = h/t = [(m1 - W2)/W1] x 100% ^2 = (TO3/TO1) x 100%
where m1 is the mass of green mixture, m2 is the mass of combustion product, and m3 is the mass of target metallic ingot. SHS-prepared ingots were characterized by analytical chemistry, X-ray diffraction analysis, and scanning electron microscopy.
Mo-Nb-Si-B composites were synthesized from green mixture consisting of high exothermic mixture MoO3/Nb2O5/Al/Si/B and low exothermic mixture Mo/Nb/Si/B (Table 1).
Table. 1. Green mixture used in experiments.
Mo Si Nb B MoO3 Nb2O5 Al Mixture 1 - 1.5 - 0.5 68.9 2.4 26.7 Mixture 2 92.5 3.0 3.4 1.1 - - -
40 g of the prepared mixtures were placed in quartz shells with an internal diameter of 25 mm and a height of 70 mm. According to thermodynamic calculation in a wide range of ratios of compositions (a), the combustion temperature exceeds 3000 K. The combustion products contain considerable amounts of gaseous products (metal vapor and suboxides). In order to suppress the sputtering of the mixture during combustion, experiments were carried out under the action of gravity forces a = 1-400 g. As a result, the combustion products are separated into two layers: Mo-Nb-Si-B target product and AhO3 oxide layer (slag product). It was found that n decreases and n increases within the range a = 0-40 wt % weight. At a > 60%, the completeness of the target product yield into ingot decreases sharply and at a = 70 reaches zero (the limit of gravitational separation is reached). At a = 80% combustion stopped.
The influence of the ratio of compositions 1 and 2 in the greenl mixture (a) on the chemical composition of cast CM is shown in Fig. 1. With increasing a from 0 to 40 % Mo content varies slightly, boron is almost unchanged (1 wt %). Content of Si, Al ,and O decreases. It should be noted that for small a, Si content is significantly greater than the calculated value. The appearance of excess Si in combustion products is apparently associated with the melting of the
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surface layer of the quartz cup and with the transition of Si into the composition of СM. With an increase in a/g from 1 to 400, a weak increase in the Mo content and a weak drop in all other elements take place.
Fig. 1. The influence of the ratio of compositions 1 and 2 in the green mixture (a) on the chemical composition of cast CM, a = 40 g.
XRD pattern of cast CM is seen in Fig. 2 to contain 3 phases: Mo(Nb), (Mo, Nb)5SiB2, and (Mo, Nb)3Si. XRD analysis and SEM studies (Fig. 3) showed that the basis of CM is a solution of niobium in molybdenum, in which the strengthening phases are distributed.
The studies revealed the following sequence of processes occurring after ignition of a mixture of high-exothermic (MoO3/Nb2O5/Al/Si/B) and low-exothermic (Mo/Nb/Si/B) compositions. A combustion front is formed, which spreads through the mixture. In the combustion front, the chemical transformation of the initial mixture into final products takes place. The high combustion temperature of the mixtures leads to melting of the initial reagents and combustion products. Under the influence of gravity forces, melts of metal and oxide phases of combustion products are separated. At the final stage there is a cooling of the two-phase melt, the formation of the phase composition and structure of the CM and slag layer occurs.
Fig. 2. XRD pattern of cast Mo/Nb/Si/B Fig. 3. Microstructure of cast Mo/Nb/Si/B composite (a = 40 g). composite (a = 40 g).
Analysis of the processes occurring at the stage of combustion makes it possible to present the following model of combustion of a mixture of «(MoO3/Nb2O5/Al/Si/B) + w(Mo/Nb/Si/B). According to thermodynamic calculations and experimental results obtained in a wide range of ratios MoO3/Nb2O5/Al/Si/B and Mo/Nb/Si/B, the combustion temperature exceeds 3000 K. In
this case, in the zone of heating the combustion wave, the components of the mixture undergo the following transformations:
- melting of MoO3 at T = 1070 K and subsequent boiling (without decomposition) at T = 1480 K;
- a vapour of MoO3 is filtered into the zone of chemical transformations;
- at 2300 K, after melting of protective film (AhO3) on the surface of Al particles, ignition followed by combustion of Al occur;
- high-temperature combustion products enter into heat and mass transfer with other "cold" components of the mixture, which after heating enter into chemical interactions.
The scheme of transformations in reaction cells can be represented as:
The work was supported by RFBR (project no.18-08-00228).
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2. H.-P. Martinz, B. Nigg, J. Matej, M. Sulik, H. Larcher, A. Hoffmann, Properties of the sibor oxidation protective coating on refractory metal alloys. Int. J. Refract. Met. Hard Mater., 2006, vol. 24, pp. 283-291.
MoO3 + Al ^ Mo + AI2O3 Nb2O5 +Al ^ Nb + AI2O3 Mo + xNb+_ySi + zB ^ MoNbxSiyB.
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(1) (2) (3)
