Microstructural formations and phase transformation pathways in tantalum carbides

Abstract

Transition metal carbides have a large assortment of applications because of their high hardness, chemical resistance, and high melting temperatures. Tantalum carbide (TaC) and its sub-stoichiometric Ta2C and Ta4C3 phases have emerged as candidate materials for ultra-high temperature structural applications. A consequence of the high melting temperature is the limiting methods to fabricate near-net shape, near full density tantalum carbides. In general, hot-isostatic pressing (HIP) and/or arc melting/vacuum plasma spraying (VPS) of powders are the viable means of manufacturing. In HIP'ing, the phase formation is through solid-state reactions whereas arc melting/VPS involves rapid solidification. Additionally, the precipitation of multiple phases generates various orientation relationships that influence the grain morphology. Depending on carbon content, the grains were equiaxed, equiaxed with a cross-hatch pattern of thin laths of secondary phases, to acicular grains. The microstructures were quantified through a series of different 2D and 3D analytical techniques. To understand how these microstructures developed, a series of XTa:(1-X)C (0.5<X<1) atomic compositions have been fabricated by VPS and HIP processes that spanned the single phase TaC, multi-phase TaC+Ta4C3+Ta2C and single phase Ta2C fields. The results revealed that the grain size was constrained by either the larger as-sprayed grain sizes in the VPS process or the largest powder sizes that sintered in the HIP process. The equiaxed grains formed in the single phase materials because they did not have another phase which would dictate a low energy orientation relationship to change the grain morphology. The cross-hatch pattern in the equiaxed grains formed from the precipitation of the lower melting temperature Ta4C3 and Ta2C phases in the TaC matrix on the closed packed planes. Since the B1 TaC structure has multiple variants of these {111} planes, these precipitates formed on these different planes. The acicular grains revealed fine secondary phase laths that were parallel to the major axis of the grain. These laths formed unidirectional as a result of the TaC phase precipitation from a Ta2C matrix, which is a hexagonal closed packed structure and only has one closed packed plane orientation, {0001}. This low energy interface exhibited a preferential growth direction. The formation of oxide inclusions and porosity within the tantalum carbides were also examined. The oxides phases were identified to be Ta2O5 and TaO through selected area electron diffraction. Serial sectioning and 3D reconstruction was used to quantify the globular oxide structure. Finally, a thermo-mechanical testing apparatus has been constructed where an electrical current provides resistive heating and, in the presence of a magnetic field, provides a Lorentz force for the application of a load on a test bar specimen. The electromagnetic Helmholtz coil can be used to independently control the magnetic flux, or load, while adjusting the specimen current for resistive heating of the specimen. The coils and specimen were encased in a stainless steel chamber that controlled the testing environment. The apparatus successfully deformed test bars of γ-TaC at 2600 ºC and 3100 ºC for 30 minutes. The temperature and deflection measurements were simulated using a finite element model. During the thermo-mechanical testing, the equiaxed grains grew isotropic with the intrinsic porosity, observed in pre-tested grain boundaries, providing microstructural markers of the grains initial size, shape and location in the microstructure.