CC BY
23iSHS 2019
Moscow, Russia
SHS OF SOLID SOLUTIONS IN Ta-Zr-C SYSTEM: MACROKINETIC FEATURES, PHASE/STRUCTURE FORMATION,
AND PHASE STABILITY
S. Vorotilo*", E. A. Levashov", K. Sidnov", and E. I. Patsera"
aNational University of Science and Technology MISiS, Moscow, 119049 Russia
*e-mail: [email protected]
DOI: 10.24411/9999-0014A-2019-10188
Two approaches were used for the preparation of the reaction mixtures Ta-Zr-C: mechanical activation (MA) in a planetary ball mill (PBM) and mixing in a ball mill. The composition of reaction mixtures was calculated for the formation of solid solutions TaC-10, 30, and 50 at % ZrC. MA of the Ta-Zr-C mixtures was performed in the PBM AIR-0.015. MA was carried out in the air atmosphere during 5, 10, and 15 min. PBM was equipped with the steel jars and balls, while ball mill was equipped with hard-metal jar and balls. In the case of ball milling of reaction mixtures during 4 h, a macro-homogeneous mixture is formed with a uniform distribution of carbon black on the surface of the metal particles. Metal particles retain their size and form. During the MA of Ta-Zr-C mixtures, intense plastic deformation of the metal particles takes place. Zirconium particles are drastically reduced in size. Formation of composite granules (100^300 |im) takes place, which are comprised of layers of tantalum (25^40 |im), zirconium (1^2 |im), and carbon black.
Although the increase in the MA duration for Ta-Zr-C mixtures led to the accumulation of excess energy in the form of defects of crystal lattice of Ta, increase in the MA duration over 15 min MA is not viable due to emergence of the carbide phases during the MA and substantial increase of inflammability of the MA mixtures. The dependence of heat release on the MA duration was investigated using the mixtures TaC-50% ZrC. When the mixture was prepared in ball bill (MA duration was 0 min) the heat release (Q) was equal to 70 kJ/mol; after 5, 10, and 15 min of MA Q = 70, 75, and 74 kJ/mol, correspondingly.
MA mixtures activated during the 10 min demonstrated the highest reaction activity and were subsequently used for the synthesis of the single-phase solid solutions specimens. However, for the investigation of the macrokinetic parameters and the mechanisms of the structure and phase formation in the combustion wave, it is expedient to use the mixtures prepared in a ball mill because the green specimens prepared from MA mixtures disintegrate during the combustion. For the Ta-Zr-C system, the highest combustion velocity (up to 1.17 cm/s) and combustion temperature (up to 2890°C) were measured for the TaC-10% ZrC mixture. Effective activation energies for the mixtures TaC-10, 30, and 50% ZrC were estimated to be 206, 194, and 213 kJ/mol correspondingly, which is close to the data reported in the literature for the SHS of TaC. The thermograms of combustion for all the measured specimens were characterized by a single peak; combustion occurred in the stationary regime.
To elucidate the mechanisms of the structure and phase formation, the techniques of time-resolved XRD (TRXRD) and quenched combustion front (QCF) were applied for the mixtures TaC-10, 30, and 50% ZrC. TRXRD demonstrated that for the mixtures TaC-10, 30, and 50% ZrC the sequence of phase formation in the combustion front was similar. After 1 s after the beginning of the analysis, the decrease in the intensity of the peaks of initial reagents and emergence of peaks of TaC and ZrC were observed. After 2 s, the peaks of initial reagents vanished, while the intensity of the peaks of TaC and ZrC increased. After 3 s, the displacement of the peaks of TaC and ZrC was observed, suggesting the formation of solid solution (Ta,Zr)C.
XV International Symposium on Self-Propagating High-Temperature Synthesis
Combustion fronts were quenched in a copper wedge. In all the investigated Ta-Zr-C mixtures, in the pre-heating zone, the agglomerates of highly dispersed TaC particles were formed. These agglomerated were located in the close proximity with the Zr particles which retained their initial morphology, suggesting that in the pre-heating zone the formation of TaC occurs via the gas-transport mechanism by Boudoir-Bell reaction. However, while in the case of TaC-10% ZrC and TaC-30% ZrC, TaC in the pre-heating zone formed porous "fractal-like" aggregates, in the case of TaC-50% ZrC the TaC particles in the pre-heating zone had equiaxial shape with a size of up to 500 nm and were distributed uniformly on the surface of tantalum particles.
In the combustion zone, zirconium melted, and tantalum partially dissolved into it. For the mixtures TaC-10, 30% ZrC in the combustion zone the zirconium and tantalum carbides formed nanostructured ring structures (rings width was up to 100 nm, diameter was up to 400-600 nm). The formation of these structures was evidently caused by the capillary spreading of the local micro-volumes of zirconium-based melt on the surface of carbon black with the subsequent crystallization of carbides. Simultaneously. in the located nearby relatively large Ta particles (up to 20 |im). diffusion layers were formed with a gradient of C concentration. Evidently, in the case of Ta particles which did not interact with the zirconium melt, TaC is formed via the gas-phase mechanism. In the case of TaC-50% ZrC mixture, in the combustion zone, equiaxial carbide grains (up to 0.7 |im) were present instead of ring-shaped structures, suggesting that the formation of the ZrC occurs via the dissolution of C in the volume of Zr melt and precipitation of ZrC grains from the oversaturated solution instead of capillary spreading of the Zr melt.
In the case of SHS of Ta-Zr-C mixtures prepared in a ball mill, the formation of singlephase solid solution did not occur due to the relatively high heterogeneity of the reaction mixture. Single-phase carbide solid solutions TaC-10, 30, and 50% ZrC were produced by the MA-SHS corresponding to the following optimized regimes: (1) preparation of the mixture with 1.5 wt % excess C over the stoichiometry; (2) 10 min of MA in PBM AIR in the air atmosphere; (3) SHS of MA mixtures in the "sand mold" with the reaction mixture to "chemical oven" ratio of 1:1.5. The synthesized SHS products were characterized by the porosity of 50%. Ball-milling of the SHS products produced the powders of single-phase carbide solid solutions with the grains size up to 10 |im for the subsequent sintering.
This work was carried out with financial support from the Russian Science Foundation in the framework of project no. 19-19-00117.
