Microstructure and Mechanical Properties of Spark Plasma Sintered TaC 0.7 Ceramics

2018

J Am Ceram Soc

This review summarizes the phase stability in the group IVB (Ti‐C; Zr‐C; Hf‐C) and group VB (V‐C; Nb‐C; Ta‐C) transition‐metal carbides. The order parameter functional (OPF) method and density functional theory (DFT) method have been used to predict phase equilibria in these systems. Extensive experimental investigations have attempted to determine both phase stability as a function of composition as well the crystal structures present using X‐ray diffraction, neutron diffraction, electron backscatter detection, and selected area electron diffraction. These investigations have demonstrated that the structures that form are based on the close‐packing of the metal atoms and the arrangement of the carbon atoms in the octahedral interstices. In general, the rocksalt B1 phase is stable for all of the transition‐metal carbides, with their substoichiometry tolerance increasing with temperature; vanadium carbide is the exception due to its negative vacancy formation energy. Vacancy‐ordered M6C5 phases have been predicted and experimentally confirmed in both groups of carbides; however, kinetic limitations often inhibit the formation of vacancy‐ordered phases, which has contributed to controversy in phase identification. The vacancy‐ordered M4C3 phase has been predicted for select carbides and has only been observed in zirconium carbide. In contrast, the stacking fault phase ζ‐M4C3−x has been readily reported in the group VB carbides (but not in the group IVB carbides). The vacancy‐ordered M3C2 has been predicted by DFT for the group IVB carbides but not in the group VB carbides, whereas OPF predicts its stability in both carbides. Vacancy‐ordered M3C2 phases have been experimentally observed in the Ti‐C and Hf‐C systems. Finally, the M2C phase has been predicted in both group carbides, except for hafnium carbide, with an order‐disorder transition with temperature. These factors result in phase diagrams that are similar among all the carbides, but each phase diagram is unique due to subtle differences in bonding that result in slight differences in thermodynamically stable phases.

Section: Experimental Observations Of Specific Crystal Structuresmentioning

confidence: 99%

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, in materials with high‐volume fractions of this phase …”

Section: Experimental Observations Of Specific Crystal Structuresmentioning

confidence: 99%

Review of phase stability in the group IVB and VB transition‐metal carbides

Weinberger

2018

J Am Ceram Soc

“…These properties have enabled them to find potential applications in hypersonics as well as wear resistance abrasives and cutting tools . In recent years, there has been renewed interest in the Ta‐C system because it has been demonstrated that, in the presence of the ζ‐Ta 4 C 3‐x phase, a very high fracture toughness is achieved and is likely a result from the complex microstructure formed from its precipitation with TaC and/or Ta 2 C . Although the zeta phase is routinely observed in the tantalum carbides, and exhibits improvement in mechanical strength for these carbides, there continues to be a long‐standing debate regarding its structure and stability .…”

Section: Introductionmentioning

confidence: 99%

A computational search for the zeta phase in the tantalum carbides

Weinberger

2018

J Am Ceram Soc

A systematic electronic structure density functional theory search for the zeta phase in the tantalum carbon system has been conducted, demonstrating that the zeta phase does indeed lie on the convex hull, establishing low‐temperature thermodynamic stability, and is structurally stable. Even though the zeta phase is most oft reported as a R3¯m ζ‐Ta4C3‐x structure, these new computational results reveal that the perfectly ordered zeta phase occurs at the Ta3C2 composition and belongs to the C2/m space group. Furthermore, these results suggest that the R3¯m space group may be a result of an order‐disorder transition and potential deviations from ideal stoichiometry at higher temperatures. The structure of this Ta3C2 compound can be described as ordered strings of carbon vacancies on a single carbon plane, which is also the preferred plane where additional carbon vacancies or interstitials form. These defects have modest formation energies, which provides further explanation for the small homogeneity range in zeta phase.

“…Between the carbides and nitrides, this phase has been most heavily studied in the tantalum carbides because high volume fractions of ζ-M 4 C 3 has been correlated to a fracture toughness that is approximately twice in either monolithic TaC or Ta 2 C [12,13,14]. Furthermore, the phase stability of this zeta phase is debated [15,16,7].…”

Section: Introductionmentioning

confidence: 99%

“…Since many of these phases either do not form or because they cannot be isolated in experiments to concretely determine their formation enthalpy, we utilize electronic structure density functional theory to compute the enthalpies of formation of these structures using the tantalum-carbon system as our case study. The Ta-C system is an ideal case study system since the stability of the zeta phase is debated [15,16], though current phase diagrams from computational thermodynamics list it as a stable phase [7], the low temperature microstructure of this phase has been studied extensively [10,37], and its presence can dramatically alter mechanical properties [12,13]. The general framework, using Ta-C, will provide insight into the nature of stability and microstructures that form from the existence of the zeta phase and other SFPs in this tantalum carbon as well as other carbide and nitride systems.…”

Section: Introductionmentioning

confidence: 99%

A model for understanding the formation energies of nanolamellar phases in transition metal carbides and nitrides

Guziewski

Modelling Simul. Mater. Sci. Eng.

et al. 2016

Abstract. In this paper we introduce a stacking-fault based model to understand the energetics of formation of the nanolamellar-based metal carbide and nitride structures. The model is able to reproduce the cohesive energies of the stacking fault phases from Density Functional Theory calculations by fitting the energy of different stacking sequences of metal layers. The model demonstrates that the first and second nearest metal-metal neighbor interactions and the nearest metal-carbon/nitrogen interaction are the dominant terms in determining the cohesive energy of these structures. The model further demonstrates that above a metal to non-metal ratio of 75%, there is no energetic favorability for the stacking faults to form a long-range ordered structure. The model's applicability is demonstrated using the Ta-C system as its case study from which we report that the interfacial energy between ζ-Ta 4 C 3 and TaC or Ta 2 C is negligible. Our results suggest that the closed packed planes of these phases should be aligned and that precipitated phases should be thin, which is in agreement with experiments.