CC BY
28SHS METALLURGY OF COMPOSITE MATERIALS BASED ON REFRACTORY METALS
V. I. Yukhvid*", D. E. Andreev", Yu. S. Vdovin", S. L. Silyakov", N. V. Sachkova",
and I. D. Kovalev"
aMerzhanov Institute of Structural Macrokinetics and Materials Science, Russian Academy of
Sciences, Chernogolovka, Moscow, 142432 Russia
*e-mail: yukh@ism.ac.ru
DOI: 10.24411/9999-0014A-2019-10195
Refractory metals (W, Mo, and Nb) and related composite materials (CMs) are widely used in engineering. Thus, tungsten carbides are the basis of hard alloys for steel processing and oil industry, fabrication of surfacing materials for protective coatings [1]. CMs based on Nb-Si and Mo-Si [2, 3] have great potential for implementation in the aircraft engine industry. The melting points of cast Nb-Si- and Mo-Si-based CMs exceed 1750 and 1950°C, respectively. These cast CMs can have a higher operating temperature than nickel superalloys. Niobium-silicide and molybdenum-silicide CMs are currently being developed for hot section turbines for both aircraft engines and ground turbines. Due to the high melting point, CMs based on W, Mo, and Nb are most often produced by powder metallurgy methods. In this paper, the possibility of obtaining cast CMs based on W, Mo, and Nb by SHS metallurgy was considered.
In experiments, the burning velocity (u), the relative mass loss during combustion (^1), and the relative yield of the "metal" phase into the ingot (^2) were determined: u = h/t, = [(m1 - W2)/m1] x 100%, ^2 = (^3/^1) x 100%, where m1 is the weight of the starting mixture, m2 is the weight of the combustion products, m3 is the weight of the desired product "metal ingot", ( wt %). Metal ingot were investigated by analytical chemistry, XRD analysis, and electron microscopy.
SHS-metallurgy of W-C-Ni. Powders WO3, NiO, Al, granules of Ca and graphite were used as components of the initial mixtures for the production of cast CMs with different ratios of W-C and Ni. 30-g prepared mixtures were placed in graphite shells with an internal diameter of 20 mm and a height of 60 mm and were compacted. The synthesis was carried out in a laboratory reactor under a nitrogen (P) pressure of 5 MPa. The ratio of WO3/Al/Ca/C (0.71/0.09/0.16/0.04) and NiO/Al (0.81/0.19), (a) in the initial mixture was varied. The optimal ratio of reagents in the composition 1 was determined in previous experiments [4]. The composition 2 was calculated from the stoichiometric equation 3NiO + 2Al ^ 3Ni + AhO3. The calculated adiabatic combustion temperature of the mixture at P = 5 MPA ("Thermo" program) in the range 0 < a < 1 is above 3000 K, which significantly exceeds the liquidus temperature of the WC-Ni alloy.
In experimental studies, it was shown, that the mixture burns in the entire range a. Cast combustion products are divided into 2 layers (metal and oxide). The loss of the mixture during combustion does not exceed 5% (Fig. 1). According to XRD data in Fig. 2, cast CM contains WC, W2C, Ni3W10C3.4, and Ni3Al aluminide. EDS data and SEM image presented in Fig. 3 show that WC and W2C are formed as plates, Ni3W10C3.4 is lace-shaped grains, and nickel aluminide (Ni3Al) forms the intergranular boundaries.
SHS-metallurgy of Mo-Si-B. Mo-Si-B was synthesized using a mixture consisting of high exothermic mixture MoO3/Al/Si/B (71.6/26.5/1.4/0.5) and low exothermic mixture Mo/Si/B (96.2/2.8/1.0). 20-g prepared mixtures were placed in quartz shells with an internal diameter of 25 mm and a height of 70 mm and were compacted. According to thermodynamic calculation
XV International Symposium on Self-Propagating High-Temperature Synthesis
in a wide range of ratios of compositions (a), the combustion temperature exceeds 3000 K. The combustion products contain a significant amount of gaseous products (metal vapor and suboxides). In order to suppress the sputtering of the mixture during combustion, experiments were carried out in centrifugal plants under the influence of gravity (a/g) of 1-400 g. Experiments showed that combustion products are formed as a two-layer ingot: the bottom layer was Mo-Si-B (target product) and the upper layer was AhO3 (slag product). The value of a was varied from 0 to 40% wt, a/g was 1 to 400.
It was found that u and ni decrease, and n2 increases with increasing a from 0 to 40% wt (Fig. 4) and reaches 50 % of the calculated value. At a > 60%, the completeness of the target product yield into the ingot decreases sharply and at a = 70 reaches zero (the limit of gravitational separation is reached). At a = 80% combustion is stopped (reached limit of combustion). According to XRD analysis, cast Mo-Si-B contains 3 phases: Mo, MosSiB2, and Mo3Si (Fig. 5). Figure 6 shows that coarse grains of Mo-Si solution are surrounded by Mo-Si-Al. Inclusions of Mo-Si-B-Al take place in the boundary bulk. SHS-metallurgy of Nb-Si-Hf-Ti. Nb2O5, Al, Si, Hf, HfAb, and Ti mixtures were used for the synthesis of cast Nb/Si/Hf/Ti. The prepared mixtures were placed in quartz shells (15 g) or graphite molds (100 g) and were compacted. The experiments were carried out under artificial gravity (1-500 g). Combustion of mixtures of Nb2O5, Al, Si, Hf (powder), HfAb (granules), and Ti in a centrifugal setup proceeds differently. In the first case, combustion proceeds in an explosive mode with a complete release of the mixture from the reaction form; in the second case, in a stationary mode at a velocity of 0.8-1.5 cm/s and with small losses of mixture during combustion. In the main series of experiments, HfAl3 granules of the following fractions were used: 0-40, 40-100, 100-160, and 160-300 |im. Granules were manufactured from ingots HfAl3 obtained by SHS-metallurgy. In experiments, it was shown that with an increase in the a/g from 50 to 500, the burning velocity of mixture and the completeness of output of alloy into ingot increase from 0.8 to 1.5 cm/s and from 70 to 100 % wt, respectively. The spread of the mixture does not exceed 10 wt % and slightly reduced (Fig. 7). The Hf content in CM decreases markedly, Ti increases. The content of all other target elements in the alloy varies slightly. Figure 8 shows that XRD pattern of CM produced using a large amount of HfAl3 particles contains 3 phases: Nb (base), Nb5Si3. and a small amount of Nb3Si. No phases based on Hf, Ti, and Al were detected. Microstructure of Nb-Si-Hf-Ti is presented in Fig. 9.
Fig. 1. Effect of a on u, ni, and Fig. 2. XRD pattern of ^2. W/C/Ni/Al.
u, cm/s 5
%
100 90 80 70 60 50 40 30 20 10 0
50 20
Fig. 4. Effect of a on u, ni, and Fig. 5. XRD pattern of n2. a/g = 40. Mo/Si/B.
Fig. 3. Microstructure of W/C/Ni/Al.
Fig. 6. Microstructure of Mo/Si/B.
4000
0 - Mo Si
- MoSiB
3500
A- Mo
3000
2500
2000
500
000
500
0
20
30
40
60
70
80
0
20
40
60
80
100
Fig.7. Effect of a/g on u, ni, Fig. 8. XRD pattern of and n2. Nb/Si/Hf/Ti.
Fig. 9. Microstructure of Nb/Si/Hf/Ti.
Two scheme of chemical stage of transformation of compounds Nb2O5/Al/Si/Hf/Ti and Nb2O5/Al/Si/HfAl3/Ti in the combustion wave was suggested from the analysis of experimental results.
(1) For Nb2O5/Al/Si/Hf/Ti mixture, the leading stage determining the combustion mode and regularities is Nb2Os/Hf ^ Nb-HfO2. All other stages: Nb2Os/Al ^ Nb-AhO3, Nb2O5/Ti ^ Nb-Ti2O3, etc. are in spatial mode. Due to the high activity of hafnium, combustion occurs in an explosive mode and is accompanied by the release of combustion products from the reaction form ("champagne effect").
(2) The stage Nb2Os/Al ^ Nb-AhO3 is the leading stage when you replace Hf on HfAb. All other stages, Nb2O5/HfAb ^ Nb-HfO2-AhO3, Nb2Os/Ti ^ Nb-Ti2O3, etc. are carried out spatial mode in with the leading stage. Due to the lower activity of Al and HfAb than hafnium, combustion occurs stationary at a velocity of 0.8-1.5 cm/s and small losses of the mixture during combustion, which leads to the increase in the content of Hf in CM.
The study was supported by the Russian Foundation for Basic Research (project no. 18-08-00228).
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