CC BY
11iSHS 2019
Moscow, Russia
SYNTHESIS OF COMPOSITE MATERIALS IN THE Ti-Cr-B SYSTEM FROM MIXTURES BASED ON CALCIUM CHROMATES
P. A. Miloserdov*", V. A. Gorshkov", O. M. Miloserdova", and O. A. Golosova"
aMerzhanov Institute of Structural Macrokinetics and Materials Science, Russian Academy of
Sciences, Chernogolovka, Moscow, 142432 Russia *e-mail: yu_group@ism.ac.ru
Borides of titanium and chromium are used for the manufacture of heat-resistant, refractory and wear-resistant alloys and as the basis for cutting high-temperature materials, in cermets for nuclear engineering, for the manufacture of immersion thermocouple covers, etc. Chromium boride powder is used in various heat-resistant alloys of the type of boronite with significant loads and high temperatures, as well as to create wear-resistant surfacing alloys (BH-2, KBH). Borides of titanium and chromium have unlimited mutual solubility. The Ti-Cr-B system has a hardness higher than individual compounds and, more recently, has been intensively studied as a promising material for nuclear power engineering.
To create these materials, the most widely used methods are melting and high-temperature consolidation (sintering and hot pressing) from mixtures of metal powder and pure powders of boron or soot in vacuum at temperatures of 1800-2200°C. Ceramics based on chromium borides, especially CrB2, has unique properties: high hardness (20-22 GPa), high melting point (2200°C), high modulus of elasticity (211 GPa), good oxidation resistance, high thermal conductivity, low thermal coefficient expansion, high wear resistance and chemical inertness [1, 2].
The most promising way to obtain such materials is the one-step method of self-propagating high-temperature synthesis (SHS) [3]. One of the directions of this method is metallothermic SHS using initial mixtures consisting of metal oxides, a reducing metal (aluminum) and a non-metal (carbon, boron, silicon). The combustion temperatures of such mixtures exceed, as a rule, the melting points of the initial reagents and final products obtained in the combustion wave in a liquid-phase (cast) state. In early studies, the authors used initial mixtures containing chromic anhydride (CrO3), which is hygroscopic, thermally unstable, and toxic [4], which limits the practical implementation of the method.
To prepare a composite material, the following chemical conversion schemes were investigated:
The study of mixtures 1 depending on the boron content in the initial mixture showed that the mixtures are capable of burning; as a result, the cast target product consisting of a mixture of chromium borides (&3B4, CrB2) and aluminum is formed [5].
Thermodynamic analysis of mixtures 1 and 2 showed that with increasing a, where a = [M2/M1 + M2)] x 100%, M1 is the mass of the mixture 1, and M2 is the mass of the mixture 2, the content of the "metallic" phase of products (a) increases. The combustion temperature gradually decreases from 2656 to 2590 K at a = 80%, then there is a sharp drop to 2450 K (Fig. 1).
DOI: 10.24411/9999-0014A-2019-10098
CaCrO4 + 2Al + 2B = CrB2 + AhO3 +CaO 3TiO2 +4Al + 6B = 3TiB2 +2Al2O3
(1) (2)
XV International Symposium on Self-Propagating High-Temperature Synthesis
T „ K
ad 2650260025502500245024002350 0
20
40 60
a, %
80
a, %, wt
50
48
46
44
42
40
-38
36
-34
32
30 100
Fig. 1. The results of thermodynamic analysis of the system CaCrO4 + TiO2 + Al + B.
Experiments carried out using the reactor with V = 3.5 l showed (Fig. 2) that the mixtures burn in the range of a = 0-20%. With increasing a, the burning rate decreases from 11 to 7 mm/s, the pressure increase in the reactor AP also decreases from 13.5 to 8 atm. The yield of the target product decreases with increasing a and the phase separation limit occurs at a = 15%.
12
10-
6-
4-
n.,%
-
\
10
15
20
a
Fig. 2. The influence of a on the burning rate U and the increase in pressure AP in the reactor (a), on the yield of the target product n and the spread of combustion products ^2.
To extend the phase separation limit, the experiment was performed using the mixture with 20% highly exothermic CaO2 + Al at a = 20%. As a result, the boride ingot with a diameter of 1 to 5 mm was poorly separated from oxide layer. Figure 3 shows that the product consists of titanium-chromium boride Cr0.5Ti0.5B2 and chromium borides CrB2 and C3B4.
800-1
w 600
c
o o
£ 400-
c
Ü ' " 200-
□ CrB2 • CrB
□
20
30
40 50
20, deg
60
70
80
(a)
(b)
Fig. 3. (a) XRD pattern and (b) microstructure of the sample CaCrO4 + TiO2 + Al + B. EDS analysis data (wt %): 1 and 2 73 Cr, 3 Ti, 23 B, 1 Al; 3 and 4 33 Cr, 26 Ti, 40 B, 1 Al; 5-5 25 Cr, 36 Ti, 38.5 B, 0.5 Al.
° Cr0.5 0.5B2
Cr0.85 0.15B2
0
ISHS 2019 Moscow, Russia
Conclusions
(1) It is shown that that replacing CrO3 in the initial mixture with slightly hygroscopic stable CaCrO4 allows to retain the high energy of the initial mixture and the ability of the mixture to burn, as well as to obtain cast refractory chromium borides.
(2) The introduction of titanium oxide into the mixture leads to a noticeable decrease in the burning rate of the mixture and the yield of the target product.
(3) The use of highly exothermic additives extends the limits of combustion and phase separation of the mixture. The prepared cast product consists of titanium-chromium boride and chromium borides.
The research was supported by the Russian Foundation for Basic Research (project no. 18-08-00804).
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5. P.A. Miloserdov, V.I. Yukhvid, V.A. Gorshkov, T.I. Ignat'eva, V.N. Semenova, A.S. Shchukin, Combustion and autowave chemical transformations of a highly exothermic CaCrO4/Al/B mixture, Combust. Explos. Shock, 2017, vol. 53, no 6, pp. 665-668.
