A computational investigation into the microstructures and stability of the zeta phase in transition metal carbides and nitrides

ABSTRACT

A high-volume fraction of the zeta phase in multiphase group VB transition metal tantalum carbides has been shown to dramatically increase fracture toughness. This has been attributed to its unique nanoscale lath-based microstructure. However, what governs the microstructure and how it forms is still not well understood. In this paper, we propose a precipitation model for the formation of these phases and demonstrate that the anisotropic surface energies govern the observed zeta-phase morphology. The energetics and zeta-phase microstructure for other group VB carbides were found to be similar. In contrast, multiphase hafnium nitrides can form both thin-lath-based microstructure as well as large, single zeta-phase grains. The difference between hafnium nitride and the group VB carbides is attributed to the relative bulk free energies and low-temperature stability between the phases.

KEYWORDS:

• UHTCs
• DFT
• carbides
• nitrides
• modelling

Acknowledgements

C. R. Weinberger and H. Yu recognise Air Force Office of Scientific Research grant FA9550-15-1-0217, Dr Ali Sayir programme manager. G.B. Thompson recognises Air Force Office of Scientific Research grant FA9550-15-1-0095, Dr. Ali Sayir programme manager. This work utilised the RMACC Summit supercomputer, which is supported by the National Science Foundation (awards ACI-1532235 and ACI-1532236), the University of Colorado Boulder and Colorado State University. The RMACC Summit supercomputer is a joint effort of the University of Colorado Boulder and Colorado State University. This paper was originally presented at the Ultra-High Temperature Ceramics: Materials for Extreme Environments Applications IV Conference (Windsor, UK) and has subsequently been revised and extended before consideration by Advances in Applied Ceramics.

Disclosure statement

No potential conflict of interest was reported by the authors.

ORCID

Hang Yu http://orcid.org/0000-0003-1205-1588

Additional information

Funding

This work was supported by Directorate for Computer and Information Science and Engineering: [Grant Number ACI-1532235; ACI-1532236]; Air Force Office of Scientific Research (US): [Grant Number FA9550-15-1-0217; FA9550-15-1-0095].