Project
Modelling the metastable phase formation of compositionally complex face-centered cubic (TiVNbTa)1-xAlxN
This project addresses the fundamental thermodynamic and mechanical properties of compositionally complex transition metal nitrides, focusing on the systems (Ti,V,Al)N, (Ti,V,Nb,Al)N, (Ti,V,Ta,Al)N, and (Ti,V,Nb,Ta,Al)N. These materials constitute a new class of high-performance coatings in which multi-element interactions govern phase stability, lattice distortion, and elastic responses. The overarching objective is to establish predictive correlations between Al content, structural stability, and mechanical resilience. This is central to guiding synthesis strategies for advanced hard coatings. Within the system, several subsystems were selected to investigate the effect of increasing compositional complexity; namely, (TiV)1-xAlxN, (TiVNb)1-xAlxN, (TiVTa)1-xAlxN, and (TiVNbTa)1-xAlxN.
Additional projects that were addressed in this project: (1) Prediction of thermal stability of fcc-(Ti,Al)Nx; The objective is to unravel the effect of point defects on the thermal stability of fcc-(Ti,Al)Nx, explicitly accounting for the intrinsic variability of chemical environment-sensitive diffusion activation energies. (2) Quantum mechanical rationalization of point defects; Point defects and impurities significantly influence the structural and functional properties of MAX phases; however, their direct experimental identification remains challenging. Ab initio calculations can provide additional rationalization and guidance in such cases, enabling indirect quantification of defects. (3) Defect energetics in Al-containing transition metal diborides; Experimental data show that boron-rich (Ti,Al)By coatings with B contents exceeding ∼67 at.% exhibit significantly reduced oxidation resistance, whereas (Hf,Al)By coatings retain high oxidation resistance even at B concentrations above this threshold. The superior performance of (Hf,Al)By coatings was rationalized using density functional theory (DFT) calculations. (4) Phase formation in MAB phases Ti2AlB2 and Cr2AlB2; MAB phases are a group of ternary nanolaminated metal borides with a combination of metallic- and ceramic-like bonding, with applications as precursors for 2D materials and as precursors for oxidation-resistant ceramics. To understand the thermodynamics governing phase formation/decomposition in these systems, DFT calculations were performed. (5) Thermal stability in VAlN vs. VAlON and CrAlN vs. CrAlON; DFT calculations were employed to investigate the mechanical and thermal stability of (V,Al)N and (V,Al)(O,N) coatings, with a focus on the effect of oxygen incorporation.
Project Details
Project term
November 8, 2024–November 7, 2025
Affiliations
RWTH Aachen University
Institute
Materials Chemistry
Principal Investigator
Methods
For the work focuses on four alloy families–(TiV)1-xAlxN, (TiVNb)1-xAlxN, (TiVTa)1-xAlxN, and (TiVNbTa)1-xAlxN–that systematically increase in compositional complexity and with Al composition range 0 ≤ x ≤ 1, an envelope approach was used to capture the influence of local chemical environments using DFT calculations of vacancy formation and migration barrier energies. Metal-sublattice disorder is represented by special quasirandom structure supercells (64 or 216 atoms), with an equiatomic distribution among all non-Al metals and a stoichiometric N sublattice throughout.
For the additional work, a similar calculation framework has been used. Additionally, significant resources are devoted to lattice dynamics calculations.
Results
From total energies of all structures, formation energies determined the maximum metastable solubility of AlN in the fcc phase across (TiV)1-xAlxN, (TiVNb)1-xAlxN, (TiVTa)1-xAlxN, and (TiVNbTa)1-xAlxN subsystems, yielding distinct AlN fractions of 0.77, 0.70, 0.73, and 0.67 that quantify Al incorporation limits before hexagonal phase transformation becomes thermodynamically favorable. On this thermodynamic landscape, three representative compositions per subsystem—x(Al) = 0, x(Al) = 0.5, and equiatomic metal sublattice distribution—were selected to investigate vacancy-mediated diffusion kinetics in bulk and near-surface regions. Vacancy formation energies combined with Nudged Elastic Band migration barriers yielded activation energies for mass transport. For equiatomic metal compositions, DFT-computed elastic tensors revealed a systematic decrease in Young’s modulus from 427 to 375 GPa with increasing chemical complexity, in contrast to experiments, which report nearly composition-independent moduli that are approximately 92 GPa higher.
Additional projects: (1) Calculations showed stoichiometric compositions exhibit the highest stability, indicated by elevated diffusion activation energies from vacancy-concentration-dependent envelopes, linking defect chemistry to diffusion across chemical environments. (2) DFT results indicated V Frenkel pairs are unstable due to rapid recombination, while N Frenkel pairs are stable, confirming V vacancies and N Frenkel defect coexistence; this enables nitrogen-controlled layered architectures with strain-engineered interfaces. (3) Systematically lower formation energies for vacancies and interstitials occurred with increasing local Al coordination; Hf-based systems showed significantly lower overall barriers, indicating (Hf,Al)By readily accommodates excess B via lattice defects. (4) Convex hull depths were −0.427 eV/atom for Cr2AlB2 and −0.762 eV/atom for Ti2AlB2, underscoring the high stability of competing phase mixtures matching Ti2AlB2 MAB composition. (5) Predictions correlated with spatially resolved analyses, where high oxygen vacancy formation energy and limited mobility suppress hexagonal phase formation, stabilizing oxygen-containing fcc solid solutions.
Discussion
High-performance computing resources were used to perform large-scale computationally expensive DFT simulations by distributing individual calculations across multiple compute nodes and cores, enabling the treatment of supercells with hundreds of atoms and dense k-point meshes within feasible wall times. In a batch-queue environment, production jobs were launched using parallelized DFT codes, exploiting MPI and OpenMP to accelerate the self-consistent field cycle and the evaluation of forces and stresses. This setup enabled systematic exploration of composition, defect configurations, and strain states via extensive parameter sweeps and high-throughput workflows that would not be practical on a single workstation. The DFT data are subsequently used to train a machine-learning interatomic potential to reduce computational cost.
Additional Project Information
DFG classification: 302-03 Chemical Solid State and Surface Research, Theory and Modelling
Software: VASP, LAMMPS, Phonopy, Ace/Grace
Cluster: CLAIX
Publications
Sebastian Lellig, Amir Hossein Navidi Kashani, Peter Schweizer, Marcus Hans, Ganesh K. Nayak, Johann Michler, Jochen M. Schneider,
Passivating oxidation behavior of Ti0.12Al0.21B0.67 coatings investigated by scanning transmission electron microscopy and chemical environment dependent density functional theory simulations,
https://dx.doi.org/10.1016/j.actamat.2024.120662, Feburary 2025
Rajib Sahu, Peter J. Pöllmann, Dimitri Bogdanovski, Clio Azina, Ganesh Kumar Nayak, Jochen M. Schneider, Christina Scheu,
Formation of 3D-Cr2(C1-yOy) at Cr2AlC / AlOx interfaces
https://dx.doi.org/10.1016/j.apsusc.2025.163590, October 2025
P. J. Pöllmann, R. Sahu, M. Fečík, C. Scheubc, J. M. Schneider,
Direct formation of MXene domains and compositional defects in magnetron sputtered V2AlC-AlOx heterostructures revealed by theory and experiments,
https://dx.doi.org/10.1039/D5TA01276G, 2025
Pauline Kümmerl, Sebastian Lellig, Amir Hossein Navidi Kashani, Marcus Hans, Peter J. Pöllmann, Lukas Löfler, Ganesh Kumar Nayak, Damian M. Holzapfel, Szilárd Kolozsvári, Peter Polcik, Peter Schweizer, Daniel Primetzhofer, Johann Michler & Jochen M. Schneider ,
Improved oxidation behavior of Hf0.11Al0.20B0.69 in comparison to Hf0.28B0.72 magnetron sputtered thin films,
https://dx.doi.org/10.1038/s41598-024-75854-8, 2024
Hendrik C. Jansen, Amit Sharma, Krzysztof Wieczerzak, Ganesh K. Nayak, Jochen M. Schneider, Jakob Schwiedrzik, Thomas E.J. Edwards, Johann Michler,
On the efficacy of Xe+-pFIB preparation to avoid Ga+-FIB induced phase transformations in Al-Ni alloys
https://dx.doi.org/10.1016/j.scriptamat.2025.116589, April 2025