Atomic transport in ultra-high temperature carbides
Abstract
,Ultra-high temperature ceramics (UHTC?s) constitute one of the promising classes of hypersonic materials. Comprising primarily group,s IV and V transition metal carbides, nitrides and borides, UHTCs are characterized by high melting temperatures exceeding 3300 K, l,arge elastic moduli and hardness, good thermal resistance, and low chemical reactivity. The refractory carbides TaC and HfC have the, highest melting temperatures among all UHTCs. Recent years have seen an increased interest in mixed TaC?HfC ceramics, seeking opti,mization of properties by taking the best from each monocarbide. The injection of Hf atoms into the Ta sublattice increases the latt,ice constant and the thermal expansion coefficient, offering a way to control thermal stresses caused by misfit strains. The hardnes,bides are notoriously difficult to sinter due to the strong covalent bonding and low diffusion coefficients. For viable technologica,l applications, scalable synthesis and processing routes must be developed to achieve the highest possible density and chemical homo,geneity of the material. The rates of pore healing and the formation of a homogeneous multicomponent solution during the sintering a,re kinetically controlled by diffusive mass transport. Thus, the knowledge of diffusion coefficients in TaC, HfC, and TaxHf1-xC is e,ssential for optimizing the synthesis and processing routes, and for predicting creep deformation rates. Experimental measurements o,f the diffusivities are scarce and indirect. Direct diffusivity measurements would be extremely difficult to accomplish, especially,at the high temperatures relevant to technological applications. Under the circumstances, calculations offer the only realistic opti,on for obtaining the diffusivities in the TaxHf1-xC carbides and perhaps in all other carbides of the UHTC family.The principal goal, of the proposed research is to create a novel methodology for computational prediction of diffusion coefficients in high-temperatur,e carbides and to demonstrate the new methodology by computing the self-diffusion and interdiffusion coefficients in TaxHf1-xC carbi,des as a function of temperature and chemical composition. The proposed methodology will be general enough to be extendable to other, UHTCs in the future. This goal will be achieved by a hierarchical multi-scale integration of first-principles density-functional-th,eory (DFT) calculations, large-scale molecular dynamics (MD) and Monte Carlo (MC) simulations, and diffusion kinetics theory. Given,that the interdiffusion coefficients include both the atomic mobility information and the thermodynamic factor, the methodology will, combine both diffusion kinetics and thermodynamics of the material. The key step that will allow us to upscale the DFT calculations, to the diffusive time and length scales will be the development of a physically-informed machine-learning interatomic potential tra,ined on a massive DFT database. The potential will accelerate the energy and atomic-force calculations by orders of magnitude while,providing a near-DFT accuracy in energy/force predictions. This will unlock the full power of the statistical-mechanical methods of,ion factors. The new mechanistic insights and the results of the DFT/MD/MC calculations will be fed into the diffusion kinetics mode,ls predicting the self-diffusion, intrinsic diffusion, and interdiffusion coefficients. Transferability of this work to other UHTCs,will be ensured by releasing a suite of computer programs implementing the proposed predictive methodology to the research and devel,opment community. Approved for public release
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Aug 05, 2022
- Source ID
- N000142212645
Entities
People
- Y. Mishin
Organizations
- George Mason University
- Office of Naval Research
- United States Navy