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20EXOTHERMIC SYNTHESIS AND CONSOLIDATION OF Ta4ZrC5 COMPOSITE
V. A. Shcherbakov*", A. N. Gryadunov", S. G. Vadchenko", and M. I. Alymov"
aMerzhanov Institute of Structural Macrokinetics and Materials Science Russian Academy of Sciences, Chernogolovka, 142432 Russia *e-mail: [email protected]
DOI: 10.24411/9999-0014A-2019-10155
An ultra-high temperature (UHT) composite with melting point of 4000°C, synthesized on the basis of the TaC-ZrC system [1], is interesting for use in an airspace industry for production products having a long service life at high temperature under conditions of erosion wear.
The composites were obtained by multistage methods, including synthesis of refractory TaC and ZrC compouds, grinding to micron particle size, next synthesis of complex carbides (Ta4ZrC5), their re-grinding and subsequent sintering or hot pressing. In order to reduce the sintering temperature, single-phase nanocrystalline powders of complex carbides were prepared by the sol-gel method [2]. Powder of single-phase composites Ta4ZrC5 was obtained by self-propagating high-temperature synthesis (SHS) using a mixture of tantalum, zirconium and soot powders mechanically activated for 5 min in a ball mill [3]. After grinding the SHS product was consolidated by electrospark sintering. The disadvantages of described above methods for producing ultra-high temperature composite materials are multistage process, high energy costs, low productivity.
An effective method combining exothermic synthesis in the electrothermal explosion (ETE) mode and consolidation of a hot product under quasi-isostatic compression has developed at ISMAN [4]. This paper presents experimental results on production of Ta4ZrC5 ultra-high temperature composite by exothermic synthesis and consolidation in one technological stage. It allowed exclude intermediate stages of preparation and grinding the refractory compounds. The aim of this work is to study the possibility of preparation of Ta4ZrC5 single-phase UHT composite by one-stage technique combining ETE of tantalum, zirconium, and carbon black powders mixture and consolidation of the final product in conditions of quasi-isostatic compression.
The exothermic synthesis of the UHT composite was carried out according to the scheme:
4(Ta + C) + (Zr + C) => Ta4ZrC5 (1)
Reaction mixtures were prepared from tantalum (TAM brand), zirconium (PCRK-1 brand), and carbon black (P 804-T brand) powders. The mixture was mixed in a planetary activator AGO-2C at acceleration of 90 g. Mechanical activation of powders was carried out in two stages. The first one produced an activated mixture of tantalum and zirconium powders for 0 to 30 min. On the second, carbon black was added into the resulting mixture of activated metal powders, the obtained mixture was mixed for four min.
Cylindrical samples with a height h = 12 mm and a diameter d = 15 mm were pressed from the prepared mixture to a relative density of 0.5. The test sample was placed in a reaction mold. The laboratory set-up for synthesis of a UHT composite by the ETE under pressure is described in detail in [4]. In the experiments, the sample was compressed at pressure of 100 MPa and heated with Joule heat until the exothermic interaction of the reagents in the thermal explosion mode.
ÏSHS2019
Moscow, Russia
The microstructure of the composites was studied using an autoemission scanning electron microscope of ultra-high resolution Zeiss Ultra plus. The phase composition of SHS-composites was studied with the diffractometer "DRON-3", using monochromatic CrKa radiation, the computer program "Crystallographica Search Match" and the base for diffraction data Power Diffraction File (PDF-2, ICDD, USA, Release 2011). For the study, the sections of synthesized samples were used.
Figure 1 shows the microstructure of the composite obtained by the ETE under pressure of the non-activated mixture. It is seen that the composite has a heterogeneous microstructure. It consists of tantalum carbide (light phase) and solid solution based on zirconium carbide (gray phase). The particle size of the TaC is changed in the range of 20-100 ^m; of the solid solution (Zr,Ta)C, in the range of 10-15 |im. It is important to note that the macropores (up to 10 |im) are in the center of coarse TaC particles. Formation of macropores is associated with the mechanism of high-temperature formation of TaC particles during ETE.
Fig. 1. SEM images of the Ta4ZrC5 composite prepared with using non-activated reactant mixture.
The results obtained are consistent with the data of microstructural analysis of the synthesized composite. The results of the XRD analysis of the UHT composite obtained by ETE of non-activated mixture of tantalum, zirconium, and carbon black powders are presented in Fig. 2 (curve 1). It is seen that the final product contains TaC and a solid solution based on (Zr,Ta)C. The diffraction peaks of the solid solution are shifted relative to the position of the peaks of the phase ZrC.
To reduce the effect of particle size, the powder mixture had been subjected to mechanical activation in the planetary ball mill AGO-2. Activated mixing of tantalum and zirconium powders was carried out for 10 to 30 min.
Figure 3 shows the XRD patterns of mechanically activated powder mixture. It is seen that after 10 min of activated mixing the diffraction peaks of zirconium completely disappeared. For tantalum, on the contrary, after 30 min of activated mixing, the peaks did not disappear, despite the fact that their intensity decreased significantly. This indicates a significant reduction of particles and destruction of crystal lattices of zirconium and tantalum as a result of the formation of defects.
Figure 4 shows the microstructure of the composite prepared with using the mixture mechanically activated in two-stage mixing of tantalum, zirconium, and carbon black powders (the first stage: 20 min without carbon black, the second one: 4 min after the addition of carbon black). It is seen that the single-phase composite Ta4ZrC5 with a particle size of 2-3 |im was formed during the ETE. The single-phase composition of the composite Ta4ZrC5 is confirmed by the results of XRD analysis (Fig. 3, curve 2).
Fig. 2. XRD patterns of the composites obtained using 1 non-activated and 2 activated reactant mixture.
Fig. 3. XRD patterns of the 4Ta+Zr powder mixtures obtained by mechanical activation for 0 (1), 10 (2), 20 (3), and 30 (4) min.
Fig. 4. SEM images of the Ta4ZrC5 composites obtained with using activated reactant mixture.
Thus, it was developed the efficient single-stage method combining the exothermic synthesis in a mode of electrothermal explosion (ETE) and consolidation of a hot product in terms quasi-isostatic compression. The dense Ta4ZrC5 UHT composite was prepared by this technique. The condition for the synthesis of a single-phase composite is the use of a mixture of tantalum, zirconium and carbon powders, mechanically activated in two stages: the first stage produces an activated mixture of tantalum and zirconium powders for 20 min, the second stage includes carbon black and mixing for 4 min. The Ta4ZrC5 single-phase composite with a particle size of 2-3 |im was formed under a pressure of 100 MPa in during few seconds. The residual porosity of the ultra-melting composite is 8-10%.
The work was supported by the Russian Foundation for Basic Research (project no.19-08-01085 A) using the equipment of the Center of Shared Serves of ISMAN.
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