A TIME-RESOLVED DIFFRACTION STUDY OF THE TA-C SOLID COMBUSTION SYSTEM

The formation of TaC and Ta2C by combustion synthesis from their elemental constituents has been studied by time-resolved x-ray diffraction (TRXRD) using synchrotron radiation. The reactions have been followed with a time resolution down to 50 ms. Since the adiabatic temperatures for both reactions are well below any liquidus temperature in the Ta-C phase diagram, no melting occurs and these combustion reactions occur purely in the solid state. The phase transformations associated with these reactions are followed by monitoring the disappearance of reactant and appearance of product powder diffraction peaks in real time as the reaction front propagates through the combusting specimen. In the synthesis of TaC, the results show the formation of the subcarbide (Ta2C) phase as an intermediate. In the synthesis of Ta2C, the reaction proceeds directly to the product with no discernible intermediate Ta-C phase within a 50 ms time frame. The chemical dynamics associated with the combustion synthesis of TaC may be described by an initial phase transformation to hexagonal Ta2C arising from carbon diffusion into the Ta metal lattice. As more carbon is available this intermediate subcarbide phase, which has one-half of its octahedral interstices occupied by the carbon, further transforms to the cubic TaC final product, in which all octahedral sites are now occupied. The time-resolved data indicate that the rate of formation of Ta2C is a factor of two faster than that of TaC.