Experimental investigation into the crack propagation in multiphase tantalum carbide ceramics

A computational search for the zeta phase in the tantalum carbides

“…As alluded to above, the fracture toughness of tantalum carbide composites can be enhanced by the precipitation of the zeta phase as originally reported by Hackett et al The authors demonstrated that when the carbon content of TaC x mixtures was reduced below x = 0.8, the zeta phase precipitates out in either a two phase mixture of TaC and ζ‐Ta 4 C 3‐x or ζ‐Ta 4 C 3‐x and Ta 2 C. More importantly, the fracture toughness rose from 5.3 ± 0.7 MPa√m to 12.7 ± 0.7 MPa√m. The enhanced fracture toughness in tantalum carbide composites have been reproduced by Limeng et al and Schulz et al The origin of the enhanced toughness appears to be a result of the nature of bonding in the materials and the microstructure which act in concert to toughen the material through crack deflection and bridging. Schulz et al demonstrated, using micro and nanoindentation, that cracks propagated preferentially along the laths that form in the ζ‐Ta 4 C 3‐x microstructure.…”

“…The enhanced fracture toughness in tantalum carbide composites have been reproduced by Limeng et al and Schulz et al The origin of the enhanced toughness appears to be a result of the nature of bonding in the materials and the microstructure which act in concert to toughen the material through crack deflection and bridging. Schulz et al demonstrated, using micro and nanoindentation, that cracks propagated preferentially along the laths that form in the ζ‐Ta 4 C 3‐x microstructure. This argument was supported by density functional theory calculations of Yan et al who showed the Ta‐Ta bonded planes are weaker than Ta‐C bonded planes and Yu et al who showed that the carbon depleted planes in ζ‐Ta 4 C 3‐x would be the preferred cleavage path.…”

“…In contrast, the four {111} planes in TaC enables ζ‐Ta 4 C 3‐x to create a crisscross lath microstructure which may appear more random than that in the Ta 2 C/ζ‐Ta 4 C 3‐x composite. However, an increase in high fracture toughness is observed regardless if the zeta phase is found in coexistence of TaC or Ta 2 C …”

“…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 .…”

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

“…To provide further insight into this process and the associated microstructure, Yu et al demonstrated the energetic equivalence of the organized stoichiometric ζ‐Ta 4 C 3 phase and isolated stacking faults in the TaC structure, which helps explain these observed microstructures . The stability of this zeta phase and the associated microstructure in the tantalum carbides has garnered much of its attention from reports that it has a very high fracture toughness, ~15 MPa, in materials with high‐volume fractions of this phase …”