Microstructural formations and phase transformation pathways in hot isostatically pressed tantalum carbides

A series of XTa:(1 - X)C (0.5 < X < 1) compositions have been fabricated by hot isostatic pressing (HIP) of Ta and TaC powder blends. Depending upon the targeted stoichiometry, single- or multiple-phase microstructures formed. The single-phase microstructures of both TaC and Ta(2)C had equiaxed grain morphologies. The multiphase microstructures had either equiaxed TaC grains with a crisscross pattern of Ta(4)C(3) laths or acicular grain morphologies with rafts of TaC, Ta(4)C(3) and Ta(2)C laths running parallel to the major axis of the grains. The effect of phase transformations on the microstructure of these specimens is discussed and compared to those microstructures seen in a reaction diffusion couple formed between Ta and TaC powders processed under the same HIP conditions. This couple revealed the depletion of carbon from the TaC phase and its reaction with the tantalum metal to form the various Ta-rich carbide phases. The precipitation sequence was found to be paramount in controlling the grain morphology. A close-packed plane and direction orientation relationship was seen between all the phases. The crisscross pattern of Ta(4)C(3) precipitation in TaC was a consequence of TaC's multiple variant {111} orientations and had little or no effect on the grain morphology. In contrast, the single variant close-packed plane {0001} in Ta(2)C resulted in the parallel alignment of the precipitated phases within its grain and an anisotropic growth direction that facilitated the acicular grain morphology. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.