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20SYNTHESIS AND APPLICATION OF SHS COMPOSITE POWDERS OF THE TITANIUM BORIDE-TITANIUM SYSTEM
G. A. Pribytkov", M. G. Krinitcyn*"A, V. V. Korzhova", and I. A. Firsina"
aInstitute of Strength Physics and Materials Science, SB, RAS, Tomsk, 634055 Russia bNational Research Tomsk Polytechnic University, Tomsk, 634050 Russia *e-mail: [email protected]
DOI: 10.24411/9999-0014A-2019-10133
Metal matrix composites (MMCs) "dispersed hardener-metal matrix (binder)" have a unique combination of hardness and toughness, which provides them with widespread use as bulk materials and coatings on parts working under conditions of intense wear. The use of titanium as a binder allows obtaining materials with high specific strength. Solid particles of refractory compounds usually act as a hardener: TiN, TiC, TiB, TiB2 [1-10]. Of particular interest as a dispersed hardening phase in titanium matrix composites are TiC and TiB compounds bordering on solid solutions based on titanium on double equilibrium diagrams, which ensures good compatibility of the hardening phase with the titanium matrix. These compounds have excellent thermodynamic and chemical stability, high hardness (about 30 GPa), as well as close thermal expansion coefficients a: Ti = 8.2 x 10-6/°C; TiB = 7.2 x 10-6/°C; TiC = 7.9 x 10-6/°C [3-7].
The properties of the MMCs depend not only on the properties of the elements of the composition but also on the structure, the main characteristics of which are: volume fraction, average size, and morphology of solid particles of the strengthening phase. Inclusions with rounded particles (TiC) increase the hardness and wear resistance of composites under dry friction. Phases that form non-axial inclusions (for instance whiskers of TiB) increase the creep resistance [8]. Different methods for producing composite materials can significantly change the morphology and distribution of solid particles in the structure of metal-matrix composites.
It is of interest to use powder obtained from synthesis products in powder mixtures of titanium and boron, and for an additive during surfacing, instead of mechanical mixtures, and an electron beam as a heat source. In this regard, the purpose of this work was to find out the possibility of using SHS composite powders TiB + Ti for electron beam surfacing of coatings and compare the structure and properties of the obtained coatings with previously published results on coatings obtained by laser welding and melting.
Composite powder was obtained from the SHS specs synthesized in the layer-by-layer burning mode of cylindrical compacts with a diameter of 35 mm from powder mixtures of titanium TPP-8 (99.5%, < 160 p,m) and boron amorphous technical grade "A" (93%) in two reaction media (argon and air). The pressing pressure of the reaction mixtures was selected to achieve an initial porosity of the compacts of 40-45%. The burning was initiated by heating the molybdenum helix that ignites the pellet. The synthesis was carried out in two modes: in a sealed reactor in an argon atmosphere with an overpressure of about 0.5 atm followed by slow cooling in the reactor and in the air with rapid cooling in water. Compounds with the estimated content of the titanium bond from 20 to 60 vol % were synthesized with the measurement of the maximum combustion temperatures by the thermocouple method. To measure the temperature, junctions of tungsten-rhenium thermocouples with a diameter of thermoelectrodes of 100 p,m were placed in holes drilled in compacts. Porous SHS specimens obtained by synthesis in argon were crushed and a fraction of 56-200 |im necessary for surfacing was selected by sieving.
Electron beam coatings from composite powders were deposited on a 4-mm titanium VT1-0 substrate in a vacuum using a setup consisting of an electron source, a scanning electron
ÏSHS2019
Moscow, Russia
beam control system, a powder feeder, and a manipulator for moving the substrate relative to the scanning electron beam [11]. Structural studies of the products of synthesis and deposited coatings were carried out using the equipment of the Nanotech Center for Collective Use of the Institute of Strength Physics and Materials Science, SB, RAS by optical metallography (AXIOVERT-200MAT, Zeiss, Germany), scanning electron microscopy (EVO 50, Zeiss, Germany), and X-ray diffraction analysis (DRON-3 diffractometer, Bourevestnik, Russia). ASTM database was used for XRD data decoding.
X-ray phase analysis showed that for all the formulations studied by us, the synthesis products are multiphase. According to the estimated content of phases by the sum of the areas under the peaks, the main phase in the synthesis products is titanium monoboride (ASTM 5-700). Also in the products of synthesis other phases were identified. During air synthesis, a significant portion of titanium is consumed in reactions with atmospheric gases to form titanium dioxide and the TiN0.3 phase (ASTM 41-1352), which is isomorphic to hexagonal a-Ti, therefore, this phase is a solid solution of nitrogen in titanium. In products synthesized in argon, titanium dioxide TiO (ASTM 9-240) is formed instead of titanium dioxide (ASTM 21-1276), and its content increases with increasing titanium content in the reaction mixtures. The phase TiN0.3 was not detected in the synthesis products in argon. Titanium diboride is also present in all products of the synthesis (ASTM 35-741). Due to the formation of these side phases (nontarget), the content of unbound titanium, forming a metal binder, is reduced compared to the target values. In composites synthesized in air, almost all titanium, not participating in the formation of monoboride, goes into side phases. The morphology (Figs. 1a, 1c) and microstructure (Figs. 1b, 1d) of composite powders synthesized in argon depend on the composition of the reaction mixtures.
(c) (d)
Fig. 1. (a, c) Morphology and (b, d) microstructure of SHS powders synthesized in argon. The content of the titanium ligament (vol %): (a, b) 20%; (c, d) 50%.
The SHS product containing 20% of the binder is very brittle, therefore, when crushing, many small fragments are formed (Fig. 1a). This is due to the brittleness of TiB, which occupies
80% of the volume in the SHS product. Due to the limited volume of the titanium ligament, the growth of boride whiskers in the melt during the synthesis process takes place in cramped conditions. Therefore, the microstructure of the SHS product consists of relatively small randomly oriented boride whiskers (Fig. 1b). The SHS product with 50 vol % titanium binder has considerable plasticity; therefore, when crushing the SHS cakes, few small fragments are formed among the fragmentation granules (Fig. 1c). It is difficult to interpret the microstructure of the SHS granules of the TiB + 50% Ti product using optical micrographs (Fig. 1d), since according to the results of X-ray diffraction analysis, in addition to the target phases (titanium and titanium monoboride) it contains titanium diboride (22.4%) and titanium oxide. The structure of the granules is highly heterogeneous (perhaps due to insufficient homogeneity of the reaction mixture), and the size of TiB whiskers in the areas where they occur is much larger than in the SHS product with 20% titanium binder. Composite powders of two compositions with calculated titanium binder content of 20 and 50 vol % were synthesized in argon for electron-beam surfacing. Therefore, the main phase in the deposited coatings is titanium (ASTM 5-682). Titanium monoboride is the second phase in coatings. The content of individual phases in surfacing with mixtures with titanium binder content in SHS powders of 20 and 50% is different, despite the identical elemental composition of the mixtures. This difference does not qualitatively affect the microstructure of the deposited coatings (Fig. 2).
(c) (d)
Fig. 2. Structure of the deposited coatings: (a, b) TiB + Ti (20 ^ 80)%; (c, d) TiB + Ti (50 ^ 80)%.
In the structure of the coatings, the coarse needle-like inclusions of the primary titanium monoboride stand out against the background of the etched ligament, which is a dispersed TiB + Ti eutectic. Fine TiB whiskers in the structure of the eutectic are not resolved because of the small increase. The volume fraction of primary borides is visually larger in surfacing with a mixture with composite powder TiB + 20% Ti than in surfacing with a mixture with powder TiB + 50% Ti, which is consistent with the results of X-ray diffraction analysis.
ISHS 2019 Moscow, Russia
This work was supported by the Russian Science Foundation (Grant
no. 17-19-01425).
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